Modern Concept Science and Technology 10 New

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 347 and their reasonable corrections are given below. 1. Position of Hydrogen: Since hydrogen has the least atomic number, i.e., 1, it is kept in group 1 of the modern periodic table, but its position is still controversial due to its dual characteristics, i.e., characteristics of group 1 and group 17. 2. Position of Isotopes: Isotopes are elements having the same atomic number but different atomic weights, e.g., C, C, C 12 6 13 6 14 6 . So, without any doubt, all isotopes of one element are kept in one place. 3. Position of Lanthanides and Actinides: Elements of Lanthanide series and elements of Actinide series are kept below the main block of the periodic table as they have different properties from other elements. 4. Correction of Periodic Law: Some elements do not obey Mendeleev's periodic law, but when they are arranged according to atomic number they obey the modern periodic law. For example, Ar 39.9 18 is placed before K 39.1 19 , Co 58.9 27 is placed before Ni 58.6 18 . 5. Position of Alkali metals and Coinage metals: In the modern periodic table, alkali metals (Li, Na, K, etc.) are kept in group IA and coinage metals (Cu, Ag, Au) are in group IB. 6. In the modern periodic table, the eighth group is divided into three vertical columns. Differences between Modern Periodic Table and Mendeleev's Periodic Table S.N. Modern Periodic Table S.N. Mendeleev's Periodic Table 1. It is based on increasing atomic number. 1. It is based on increasing atomic weight. 2. In this table, there are eighteen vertical columns. 2. In this table, there are eight vertical columns. 3. In this table, there is fixed position for hydrogen, isotopes, lanthanides and actinides. 3. In this table, there is no fixed position for hydrogen, isotopes, lanthanides and actinides. Position of elements in modern periodic table 1. Position of hydrogen Hydrogen having the least atomic number, i.e., 1 is placed in group 1 of the modern periodic table. But it also shows the properties of group 17 of the periodic table. FACTS WITH REASONS Write any two reasons why hydrogen is kept in group 1 in the periodic table. Two reasons for keeping hydrogen in group 1 in the periodic table are: (i) Hydrogen has one electron in its shell. (ii) Hydrogen can lose one electron like other elements of group 1. Alkali metals are more reactive, why? The atoms of alkali metals have only one electron in their outermost orbit. Therefore, alkali metals are more reactive to obtain octet state.

348 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 349 2. Position of metals Metals are placed on the left side of the modern periodic table. For example, Alkali metals (Li, Na, K, Rb, Cs, and Fr) are kept in group 1(IA). Alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra) are kept in group 2(IIA). The metals of group 1 are called alkali metals because they react with water and produce alkalis. Similarly, the metals of group 2 are called alkaline earth metals because they are found on the surface of the earth in the form of oxides and react with water to produce alkalis. 3. Position of non-metals Non-metals are placed on the left side of the modern periodic table. For example, halogens (F, Cl, Br, I and At) are placed in group 17. The elements of group 17 are called halogens because they react with hydrogen to form acids, from which simple salts like NaCl, KCl, KBr, etc. can be produced. 4. Position of metalloids Metalloids (like Si, Ge, As, Te, etc.) are placed in between metals and non-metals. 5. Position of Lanthanides and Actinides Elements of Lanthanides and Actinides series, having different properties from other elements of the periodic table, are kept in f-Block below the main table in a separate box. The 15 elements from cerium (58Ce) to lutetium (71Lu) along with lanthanum (57La) are called Lanthanides and other 15 elements from thorium (90Th) to lawrencium (103Lr) along with actinium (89Ac) are called Actinides. Lanthanides are rare earth metals, whereas Actinides are radioactive metals. The properties of Lanthanides resemble each other very closely but are different from the rest of the elements due to the preferential filling of f-orbitals. So, they are placed separately to avoid unnecessary sidewise expansion of the periodic table. 6. Position of inert gases Inert gases (He, Ne, Ar, Kr, Xe and Rn) are kept in the zero group (18) at extreme right side of the periodic table. These elements have zero valency as they have complete octet or duplet in their valence shell. Characteristics of Modern Periodic Table 1. Modern periodic table is based on increasing atomic number. 2. In modern periodic table there are seven horizontal rows called periods and 18 vertical columns called groups. 3. In modern periodic table elements from group IA to group VIIA are called normal elements or representative elements. 4. In modern periodic table, metals are kept in left hand side, non-metals are kept in right hand side and metalloids are kept in between them. 5. In modern periodic table inert gases are kept at the extreme right hand side of the table (zero group or 18). 6. In modern periodic table lanthanides and actinides are kept below the main table. 7. In modern periodic table most reactive alkali metals are kept in IA group and most reactive non-metals (halogens) are kept at VIIA group.

350 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 8. Transitional metals (elements from IB to VIIB and VIII ) are kept between metals and non-metals. Periods and Groups of Modern Periodic Table Periods In the modern periodic table, there are seven horizontal rows, which are called periods. In a period, elements having gradual change in properties are placed one after another. According to the number of elements, periods of the modern periodic table are divided into four groups. They are very short, short, long and very long. S.N. Period Number of elements Types of period 1. First 2 Very short period 2. Second 8 Short period 3. Third 8 Short period 4. Fourth 18 Long period 5. Fifth 18 Long period 6. Sixth 32 Very long period 7. Seventh 32 Very long period Groups In the modern periodic table, there are a total of 18 vertical columns (IUPAC system), which are called groups. In a group, elements having similar properties are placed one below the other. In the modern periodic table, representative elements are kept in groups 1, 2, 13, 14, 15, 16 and 17. These elements are called representative elements, or normal elements, as they have only one incomplete outer shell. Noble gas elements are kept in 0 (zero) group of the Modern periodic table. Similarly, transitional elements are kept in groups 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12. Note: The groups are written in two different methods. They are given in the table. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 IA IIA IIIB IVB VB VIB VIIB VIIIB VIIIB VIIIB IB IIB IIIA IVA VA VIA VIIA 0 Differences between Periods and Groups. S.N. Periods S.N. Groups 1. Horizontal rows of the periodic table are called periods. 1. Vertical columns of the periodic table are called groups. 2. In periods, the atomic size of elements decreases while moving from left to right. 2. In groups, the atomic size of elements increases while moving from top to bottom.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 351 3. Valency of elements increases first (upto group IV) and then decreases (upto group 0). 3. Valency of elements remains the same in a group. 4. The elements of the same period have different properties. 4. The elements of the same group have similar properties. Sub-shell After the scientific research, it is found that the main shell is not the ultimate structure of an atom. It has one or more sub-shells. It means that each main shell contains one or more than one sub-shell, which are denoted by s, p, d and f. So, sub-shell is a region of orbit where probability of finding an electron is maximum. The main shells along with their sub-shells are listed in the given box. Main Shells Sub-shells (orbitals) K (n=1) s L (n=2) s and p M (n=3) s, p and d N (n=4) s, p, d and f The maximum number of electrons that can be accommodated by each sub-shell is given below: Sub-shells (orbitals) Maximum number of electrons s (sharp) 2 p (principal) 6 d (diffuse) 10 f (fundamental) 14 The above data shows that K shell (n = 1) contains only one sub-shell (1s) with maximum two electrons. L shell (n = 2) contains two sub-shells (2s and 2p) with maximum eight electrons. M shell (n = 3) contains three sub-shells (3s, 3p and 3d) with maximum eighteen electrons. N shell (n = 4) contains four sub-shells (4s, 4p, 4d and 4f) with maximum thirty two electrons. The last electron present in a sub-shell determines the block of an element. Aufbau Principle This principle was given by Wolfgang Pauli and Niels Bohr in the early 1920s. The different sub-shells of an atom have different energy. Electrons always try to enter the sub-shell which has less energy. Aufbau principle states, "the electrons in an atom are so distributed that they occupy shells in the order of their increasing energy." It means that the shells having low energy are filled faster than the shells having high energy in the following sequence. 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p MEMORY TIPS Aufbau principle cannot properly explain electronic configuration of copper and chromium.

352 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p. Electronic configuration of some elements on the basis of sub-shells (s, p, d, and f) is given below: S.N. Element Atomic no. Electronic configuration based on shells (K, L, M, N) Electronic configuration based on sub-shells (s, p,d,f) Valency 1 Hydrogen 1 1 1s1 1 2 Helium 2 2 1s2 0 3 Lithium 3 2, 1 1s2 , 2s1 1 4 Beryllium 4 2, 2 1s2 , 2s2 2 5 Boron 5 2, 3 1s2 , 2s2 2p1 3 6 Carbon 6 2, 4 1s2 , 2s2 2p2 4 7 Nitrogen 7 2, 5 1s2 , 2s2 2p3 3 8 Oxygen 8 2, 6 1s2 , 2s2 2p4 2 9 Fluorine 9 2, 7 1s2 , 2s2 2p5 1 10 Neon 10 2, 8 1s2 , 2s2 2p6 0 11 Sodium 11 2, 8, 1 1s2 , 2s2 2p6 , 3s1 1 12 Magnesium 12 2, 8, 2 1s2 , 2s2 2p6 , 3s2 2 13 Aluminium 13 2, 8, 3 1s2 , 2s2 2p6 , 3s2 , 3p1 3 14 Silicon 14 2, 8, 4 1s2 ,2s2 2p6 , 3s2 , 3p2 4 15 Phosphorus 15 2, 8, 5 1s2 ,2s2 2p6 , 3s2 , 3p3 3, 5 16 Sulphur 16 2, 8, 6 1s2 ,2s2 2p6 , 3s2 , 3p4 2, 6 17 Chlorine 17 2, 8, 7 1s2 ,2s2 2p6 , 3s2 , 3p5 1 18 Argon 18 2, 8, 8 1s2 ,2s2 2p6 , 3s2 , 3p6 0 19 Potassium 19 2, 8, 8, 1 1s2 ,2s2 2p6 , 3s2 , 3p6 , 4s1 1 20 Calcium 20 2, 8, 8, 2 1s2 ,2s2 2p6 , 3s2 , 3p6 , 4s2 2 Project work Based on sub-shells, show electronic configuration of first 20 elements in a chart paper and display in the class. Classification of Elements Based on Electronic Configuration After the discovery of sub-shells, elements were divided into four groups. They are: i. s - Block elements ii. p - Block elements iii. d - Block elements iv. f - Block elements

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 353 i) s-Block Elements The elements whose last electron enters into s-orbital are called s-Block elements. The elements of groups 1 and 2 are s-Block elements. Either one or two valence electrons are filled in s-orbital of s-Block elements. In first group elements, the general electronic configuration is represented by ns1 and in second group elements the general electronic configuration is represented by ns2 . s-Block is located on the left side of the Modern periodic table. The elements of s-Block form electropositive ions by losing one or two electrons. All the elements of s-Block are metals except H and He. Example: Sodium (Na) is kept in s-Block as its last electron enters the s-orbital, i.e., 11Na: 1s2 , 2s2 2p6 , 3s1 . ii) p-Block Elements The elements whose last electron enters the p-orbital of the outermost shell are called p-Block elements. The elements of groups 13, 14, 15, 16, 17 and elements of zero (0) group or group 18 except Helium are p-Block elements. p-Block is situated on the right side of Modern periodic table. This block consists of metals, metalloids and non-metals. Example: Aluminium (Al) is kept in p-Block as its last electron enters into p-orbital, i.e. 13Al: 1s2 , 2s2 2p6 , 3s2 3p1 . FACTS WITH REASONS Argon atom can exist freely in nature, why? The outermost orbit of Argon atom is filled by having eight electrons. Due to this, it does not take part in chemical reaction. Therefore, argon atom can exist freely in nature. iii) d-Block Elements Elements whose last electron enters the d-orbital of the penultimate shell are called d-Block elements. This block consists of elements of group 3 to 12. d-Block is located between s-Block and p-Block. The elements of d-Block are called transitional metals as they have partially filled d subshell but can form stable cations. Example: Copper (Cu) is kept in d-Block as its last electron enters the d-orbital, i.e., 29Cu: 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 4s1 , 3d10. iv) f-Block Elements Elements whose last electron enters the f-orbital of the ante-penultimate shell are called f-Block elements. The elements of lanthanide and actinide series are kept in f-Block. This block is located at the bottom of Modern periodic table just below the d-Block elements. The properties of f-Block elements are similar to the transition metals of MEMORY TIPS Elements having general electronic configuration ns1 and ns2 are kept in s-block. MEMORY TIPS In the 17 (VIIA) group, the general electronic configuration is represented by ns2 np5

354 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur d-Block. However, their last electron enters f-orbital. Therefore, f-Block elements are kept separately below the d-Block elements or transitional elements. So, f-Block elements are also called inner-transition elements. Characteristics of Periods and Groups 1. Atomic Radius Atomic radius is the distance between the outermost shell and the center of the nucleus of an atom. It determines the size of an atom. It is measured in picometer (pm) [1m = 10-12 pm]. The atomic radius (atomic size) decreases on moving from left to right in a period of the Modern periodic table. Elements of 3rd period Na (2, 8, 1) Mg (2, 8, 2) Al (2, 8, 3) Si (2, 8, 4) P (2, 8, 5) S (2, 8, 6) Cl (2, 8, 7) Atomic radius (pm) 157 136 125 117 110 104 99 Na Mg Al Si P S Cl Atomic size decreases The atomic number of elements increases as we move from left to right in a period. It means that the number of protons and electrons increases, but extra electrons are added into the same shell. As a result, the electrons are pulled towards the nucleus due to more powerful positive charge in the nucleus of the atom. Due to this, the atomic size of elements decreases on moving from left to right in a period. The atomic radius of elements increases gradually on moving from top to bottom in a group of the Modern periodic table. Group 1 Atomic radius (pm) Atomic size increases while moving down in a group Li 123 Li The smallest atom Na 157 Na K 202 K Rb 216 Rb Cs 235 Cs Fr 256 Fr The biggest atom As we move down in a group, a new shell is added to the atom at every step. As a result, the size of the atom increases gradually from top to bottom of the same group. However, as we move down in a group, the nuclear charge also increases, but the nuclear attraction becomes less as compared to the increase in atomic size. The smallest atomic size is found at the top of a group and the biggest atomic size is found at the bottom of the same group.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 355 2. Ionization Energy or Nuclear Power The amount of energy required to remove the outermost loosely bound electron of an isolated gaseous atom is called ionization energy. It increases in a period on moving from left to right but decreases in a group on moving from top to bottom. Period Group Ionization energy decreases from top to bottom in group Ionization energy increases from left to right in a period 3. Valence Electrons The electrons present in the outermost shell (or valence shell) of an atom are called valence electrons. The number of valence electrons remains the same in a group, but it increases continuously in a period. Period Group Number of valence electrons remains the same in a group Number of valence electrons increases from left to right in a period 4. Valency The total number of electron(s) gained, lost or shared by atoms during a chemical reaction is called valency. In a period, the valency increases from 1 to 4 and then decreases to zero (0). It is because the number of valence electrons increases from 1 to 8 on moving from left to right in a period. But in a group, the valency of all elements remains the same as they have equal number of valence electrons in the same group. Period Group Valency of elements remains the same in a group Valency increases from 1 to 4 and then decreases to zero 5. Electronegativity The amount of energy required for an atom to attract the outer electron of another atom is called electronegativity (EN). It increases on moving from left to right in a period and decreases from top to bottom in a group. Period Group Electronegativity decreases from top to bottom in a group Electronegativity increases from left to right in a period

356 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 6. Metallic Character (electro positivity) Metallic character is the tendency of an atom to lose one or more electrons. It increases when we move from top to bottom in a group and decreases from left to right in a period. Period Group Metallic character increases from top to bottom in a group Metallic character decreases from left to right in a period 7. Non-metallic Character Non-metallic character is the tendency of an atom to gain one or more electrons when supplied with energy. It increases in a period on moving from left to right and decreases on moving from top to bottom in a group. Period Group Non-metallic character decreases on moving from top to bottom in a group Non-metallic character increases on moving from left to right in a period Reactivity of Elements Metals have a tendency to lose electrons, and non-metals have a tendency to gain or share electrons. A metal which loses electrons easily is called an active metal and a non-metal which gains electrons easily is called an active non-metal. There are several factors on which the reactivity of elements depend. Among them, some common factors are the atomic size, nuclear charge and number of electrons present in the valence shell. Reactivity of Metals in a Group The chemical reactivity of metals increases on moving down in a group of modern periodic table. When we move from top to bottom in the groups of metals, the atomic size increases gradually. Due to this, nuclear attraction to the valence electron decreases. As a result, the tendency of metal atoms to lose electron/s increases. Therefore, chemical reactivity of metals increases on moving from top to bottom in a group. Example: In group 1, the chemical reactivity of alkali metals increases from lithium (Li) to francium (Fr). Li Least reactive metal Most reactive metal Chemical reactivity of metals increases on moving down in a group Na K Rb Cs Fr Group 1

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 357 FACTS WITH REASONS Lithium and sodium are kept in group 1, why? Both lithium and sodium contain one valence electron, and both of them react with water to produce corresponding alkali and hydrogen gas. So, lithium and sodium are placed in group IA of the modern periodic table. Elements of Group IA, IIA and IIIA (or group 1, 2 and 13) are more reactive as we go down in the group of the periodic table, why? Elements of Group IA, IIA and IIIA (or group 1, 2 and 13) are more reactive as we go down in the group because of the following reasons: (i) Atomic radius of these elements increases as we go down in the group. (ii) Ionization potential decreases as we go down in the group. Elements of group VA, VIA and VIIA (or group 15, 16, and 17) are less reactive as we go down in the group of the Modern periodic table, why? The atomic size of elements increases as we move downwards in groups. The elements of group VA, VIA and VIIA (or group 15, 16 and 17) are non-metals. The chemical reactivity of non-metals decreases when the atomic size of those elements increases. Therefore, the elements of group VA, VIA and VIIA (or group 15, 16 and 17) are less reactive as they go down in the group of the Modern periodic table. Potassium is more reactive than sodium although they both belong to the same group, why? Potassium is more reactive than sodium because the atomic size of potassium atom is larger than that of the sodium. So, the valence electron of potassium can more easily be taken by other reacting atoms as compared to that of sodium. Hence, potassium is more reactive than sodium. Reactivity of Non-metals in a Group The chemical reactivity of non-metals decreases on moving from top to bottom in a group of the modern periodic table. When we move from top to bottom in a group of non-metals, the atomic size increases gradually. Hence, the nuclear attraction for incoming electron/s decreases. As a result, the tendency of the non-metallic atom to gain electron/s decreases. Therefore, the chemical reactivity of non-metals decreases on moving from top to bottom in a group. Most reactive non-metal Least reactive non-metal Reactivity of non-metals decreases on moving from top to bottom in a group F Cl Br I At Group VIIA/17

358 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Example: In group VIIA/17, the chemical reactivity of halogens decreases from fluorine (F) to Astatine (At). Reactivity of elements in a period The atomic size of elements decreases gradually on moving from left to right in a period of the Modern periodic table. So the reactivity of metallic elements decreases and the reactivity of non-metals increases on moving from left to right in a period. Elements of 3rd period Na Mg Al Si P S Cl Chemical reactivity Most reactive Least reactive Most reactive → Reactivity decreases → → Reactivity increases → In the given table, Na is the most reactive metal among Na, Mg, and Al. Si is the least reactive element. Similarly, Cl is the most reactive non-metal among P, S and Cl. FACTS WITH REASONS Why is fluorine more active than chlorine although both lie in the same group? Since the atomic size of fluorine is smaller than that of chlorine, nuclear attraction is more on the valence shell of the fluorine atom. As a result, fluorine can get one electron more easily during chemical reaction. Therefore, fluorine is more reactive than chlorine although both of them belong to the same group. Chlorine is more reactive than bromine, why? Chlorine is more reactive than bromine because the atomic size of chlorine is smaller than that of bromine. Due to the smaller atomic radius of the chlorine atom, nuclear attraction is greater on the valence shell of chlorine than that of bromine. Therefore, chlorine is more reactive than bromine. Variation in periodic properties in periods and groups S.N. Properties Along a period (While moving from left to right) Down in a group (While moving from top to bottom) 1. Atomic size (radius) Decreases Increases 2. Valency 1 to 4 and 4 to zero Remains the same 3. Metallic character Decreases Increases 4. Non-metallic character Increases Decreases 5. Ionization potential Increases Decreases 6. Electron affinity Increases Decreases 7. Electronegativity Increases Decreases

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 359 HOT SKILL HIGHER ORDER THINKING SKILL 1. Despite lots of drawbacks, Mendeleev’s periodic table is very important. Despite lots of drawbacks, Mendeleev’s periodic table is very important because: i. It classified all elements available at that time into a scientific table. ii. It helped to rectify the incorrect atomic weight of some elements. iii. Table could predict new elements. iv. Study of elements got easier. 2. Hydrogen is kept in IA despite not being an alkali metal. Hydrogen is kept in IA despite not being an alkali metal because it is the simplest and the first element with one valance electron. 3. Potassium belongs to the fourth-period, IA group. The electronic configuration of potassium is 2,8,8,1. Since there are four orbits potassium belongs to the fourth period. There is 1 valance electron in the last orbit(N). So, it belongs to the IA group. It means only one oxidation state (no variable valency). 4. Chlorine is considered a non-metal with its valency 1. Chlorine has an electronic configuration 2, 8, 7. Its valance orbit has seven electrons. To be stable by the octet rule, chlorine needs to take one electron from other elements. Since it can take electrons, it is called a non-metal. Since it can take only one electron, its valency is 1. 5. Based on the given electronic configuration answers the following questions. A = 1s2 , 2s2 2p6 , 3s1 B = 1s2 , 2s2 2p6 , 3s2 3p6 , 4s1 C = 1s2 , 2s2 2p5 D = 1s2 , 2s2 2p6 , 3s2 3p5 i. Identify period, group, valency, metallic nature and block of the above elements. Element Period Group Valency Reactivity Block A 3 IA 1 Reactive metal s B 4 IA 1 Reactive metal s C 2 VII A 1 Reactive nonmetal p D 3 VII A 1 Reactive nonmetal p ii. Name the compound formed between A with D and B with C. The compound formed by the reaction between A and D is sodium chloride (NaCl). The compound formed by the reaction between B and C is potassium fluoride (KF) iii. Which is more reactive between a) A and B b) C and D and why? Metals are more reactive if it loses electrons easily. Between metals A and B, B is more reactive as it has a bigger atomic size, less ionization potential and loses electrons easily. Similarly, non-metals are more reactive if they can pull electrons easily. So, between C and D, C is more reactive as it has a smaller atomic size, more electronegativity and gains electrons easily.

360 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 6. Chlorine is smaller than sodium even though both have three orbits. Chlorine is smaller than sodium even though both have three orbits because chlorine has 17 protons and 18 neutrons whereas sodium has 11 protons and 12 neutrons only. Since the nuclear charge of chlorine is greater than that of sodium it holds valance electrons strongly compared to sodium. So, the chlorine atom shrinks and gets smaller than sodium. 7. Helium is kept in the p-block despite its last electron staying in the s sub-shell? Helium is kept in the p block despite its last electron staying in the s subshell because its valency is zero. It does not take part in a chemical reaction similar to inert gases. So, it is placed along with the inert gases of the p block. 8. Compare between electronegativity and electro-positivity. The comparison between electronegativity and electro-positivity are: SN Electronegativity SN Electro positivity 1 Electronegativity is the property of an element or ion by the virtue of which it tends to attract electrons to itself. 1 The electro positivity is the property of an element or ion by the virtue of which it can donate electrons to other atoms. 2 The electronegativity of an atom decreases if we observe from top to bottom in a group. 2 The electro-positivity of an atom increases if we observe from top to bottom in a group. 9. On what basis groups and periods are classified in the modern periodic table. Groups are classified based on the number of valance electrons. Periods are classified based on the number of orbits. For example:Lithium has electronic configuration= 2,1. It has two orbits K and L. So, it belongs to the second period. It has 1 valance electron in the valance shell (L). So, it belongs to the IA group. 10. The reactivity of metals increases down the group but the reactivity of non-metals decreases down the group. Metals are elements that should lose electrons. If an atom is bigger and valance electrons will be farther away from the nucleus then it can lose electron easily. So bigger atoms of metals can lose electrons easily and are more reactive. Since elements down the group are bigger, the reactivity of metals must increase down the group. Non-metals are elements that should take electrons. If an atom is bigger, valance electrons will be farther away from the nucleus and cannot be held properly. So, the atom cannot pull electrons from other elements. So bigger atoms of elements cannot pull electrons effectively and are less reactive. Since elements down the group are bigger, the reactivity of non-metals must decrease down the group. 11. Write some short-comings of modern periodic table. i. Hydrogen is nonmetal but kept in group of alkali metal. ii. Helium is kept in p block even if its last electron enters in s block. iii. Lanthanides and actinides are not kept in main body of periodic table.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 361 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Choose the best answer from the given alternatives. a. In which groups of the Modern Periodic Table, inert gases are placed? i. I A group ii. 18th group iii. 8th group iv. VIII B group b. What are the criteria for the classification of elements in the modern periodic table? i. atomic mass ii. mass number iii. number of protons iv. number of neutrons c. Which group do the element 1s2 ,2s2 2p5 belong to? i. IIA ii. V A iii. VIIA iv. IA d. Which one of the following is the most reactive non-metal? i. fluorine ii. nitrogen iii. oxygen iv. neon e. Which one of the following is the most reactive metal? i. lithium ii. aluminium iii. sodium iv. magnesium f. Which of the following represents the reactivity of metals in the IA group of the modern periodic table? i. Li < Na < K ii. C<N<O iii. Na> Mg > Al iv. Li > Na > K 2. Define the following terms with required examples. a. Classification of elements b. Periodic table c. Mendeleev’s periodic law d. Modern periodic law e. Mendeleev’s periodic table f. Isotope g. Lanthanides h. Actinides i. Periods j. Groups k. Sub-shell l. Aufbau principle m. s-block elements n. p-block elements o. d-block elements p. f-block elements q. Atomic radius r. Ionization energy s. Valance electron 3. Answer the following questions in very short. a. How many periods and groups are there in the Modern periodic table? b. Write down the position of metals and non-metals in the Modern periodic table. c. What is the group of alkali metals, alkaline earth metals and inert gases in the Modern periodic table? d. Which block of the modern periodic table makes acidic oxides?

362 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur e. Write the name of two elements of group 17 of the Modern Periodic Table. f. Write down the number of groups and periods in Mendeleev’s periodic table. g. Which periods are the longest periods in the Modern Periodic Table? How many elements are there in that period? h. What is electronic configuration? i. What is duplet state? Write with one example. j. What are alkaline earth metals? k. What are halogens? Give one example. l. What are inert gases? Give one example. m. What are metalloids? Give one example. n. Name any two elements that give positively charged ions. o. Name any two elements that give negatively charged ions. p. What are transition metals? q. What are the elements called that lie in the group between IIA and IIIA called? r. In which periodic table elements are arranged based on increasing atomic number? s. Write down two factors that determine the reactivity of elements. t. What is the position of lanthanides in the Modern Periodic Table? u. What is the position of halogens in the Modern Periodic Table? v. In which group the most active non-metals are placed in the Modern Periodic Table? STEP2 4. Give reasons. a. What is the valency of halogens? Why? b. Halogens are kept in group 17 of the Modern periodic table. c. Sodium is called an alkali metal. d. Magnesium is called an alkaline earth metal. e. Lithium is less reactive than sodium. f. Argon atoms can exist freely in nature. g. Reactivity of metals increase as we go down in the periodic table. h. The reactivity of elements increases on moving from top to bottom in group 1 of the Modern Periodic Table. i. Elements of groups 15, 16 and 17 are less reactive as we go down in the group of the Modern Periodic Table. j. Potassium is more reactive than sodium although they both belong to group 1. k. Chlorine is more reactive than bromine although both lie in the same group. l. Potassium is more reactive than calcium although both lie in the same period. m. Metallic properties of elements decrease but non-metallic properties of elements increase across the period. n. Argon is kept in the 18th group.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 363 5. Differentiate between the following. a. Modern periodic table and Mendeleev’s periodic table b. Periodd and groups c. IA elements and VII A elements d. s-block elements and p-block elements e. Electro positivity and electronegativity 6. Answer the following questions in short. a. Which one is more reactive between magnesium and calcium, why? b. What difference in chemical reactivity of metals of the second period was observed while moving from left to right in Modern Periodic Table? c. What is the change in chemical reactivity of very active non-metals when their atomic size increases? Describe. d. Write down the distinguishing features of groups and periods of the modern periodic table. e. Which one is more reactive, Fluorine or Chlorine? Why? f. A small portion of the Modern Periodic Table (Group 1) is given. Write down the common name of the elements shown in the given table.Give two reasons for placing hydrogen along with metals in this group. g. What type of elements are kept in p-Block? Write with examples. h. Modern periodic table is less defective than Mendeleev’s periodic table. Give two reasons to justify it. i. Write the electronic configuration of nitrogen and write its position in the modern periodic table. 7. Answer the following questions on the basis of the table given below. Which group of the periodic table does the element ‘A’ belong to? Which element is more reactive in between ‘B’ and ‘C’? Give reason. STEP3 8. Answer the following questions. a. Write down three characteristics of the Modern periodic table. b. The atomic number of elements ‘A’ and ‘B’ are 8 and 13, respectively. Answer the following questions. i. Give the electronic configuration of elements ‘A’ and ‘B’ based on the subshell. ii. Write down the molecular formula of the compound formed by the combination of the above elements. c. Identify the groups of the alkali metals, metalloids and inert gases from the given table. What is the valency of chlorine and neon? Why? 1 2 13 14 15 16 17 18 Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca H Li Na K Element Electronic configuration A 2, 8, 5 B 2, 3 C 2, 8, 3

364 cl aSSification of elementS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur d. Molecular formula of a certain ionic compound is XY2 and ‘X’ is a metal. State group to which elements X and Y belong to the periodic table. Which group of elements of the periodic table are kept in the given table? What happens to the chemical reactivity of elements from top to bottom in the given table? Why? e. A part of the periodic table is given below. Study it and answer the following questions. i. How are these elements arranged? ii. Which one is more active, Li or Na, and why? iii. Write the formula of a compound made from Mg and Cl. f. Give the name and group of an element whose electronic configuration is 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 4s1 . On what factors is the Modern periodic table different from Mendeleev’s periodic table? Write any two points. Which one is more reactive between Fluorine and Chlorine? Why? g. Study the part of a periodic table and answer the following questions. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca i. State the law on which the given periodic table is based. ii. Which element belongs to group 17 and period 3? iii. Which one is more reactive, S or Cl? Write with reason. h. Answer the following questions on the basis of the given table. Name of element Electronic configuration X 1s2 , 2s2 2p6 , 3s1 Y 1s2 , 2s2 2p6 , 3s2 3p5 Z 1s2 , 2s2 2p6 , 3s2 3p6 i. Write the block of the elements X and Z. ii. Write the valency and chemical nature of element Z. iii. Write the balanced chemical equation between elements X and Y. Li Be B C N O F Ne Na Mg Al Si P S Cl Ar

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 365 Key terms and terminologies of the unit 1. Chemical reaction : The combination, decomposition or displacement that occurs in the molecules of matters during a chemical change is called a chemical reaction. 2. Word equation : A chemical reaction expressed by writing the full names of reactants and products is called word equation. 3. Chemical equation : A chemical reaction expressed by writing the symbols and molecular formulae of reactants and products is called a chemical equation. 4. Balanced chemical equation: A chemical equation written by balancing the total number of atoms of each element on the reactant side and product side is called a balanced chemical equation. 5. Reversible reaction : A chemical reaction in which the products can recombine to give back the reactants is called a reversible reaction. 6. Irreversible reaction : A chemical reaction in which the products cannot recombine to give back the reactants is called an irreversible reaction. 7. Exothermic reaction : A chemical reaction which evolves heat during the chemical change is called an exothermic reaction. 8. Endothermic reaction : A chemical reaction which absorbs heat during the chemical change is called an endothermic reaction. 9. Catalyst : A catalyst is a chemical substance which increases or decreases the rate of chemical reaction but itself remains chemically unchanged. 10. Positive catalyst : A catalyst which increases the rate of chemical reaction is called a positive catalyst. 11. Negative catalyst : A catalyst which decreases the rate of chemical reaction is called a negative catalyst. Sequence of Curriculum Issued by CDC  Types of chemical reaction: composition, decomposition, displacement and acid-base reaction.  Rate of chemical reaction  Factors affecting the rate of chemical reaction(heat, pressure, catalyst, light and surface area) UNIT Chemical Reaction 15 Estimated teaching periods Theory Practical 4 2 Amedeo Avogadro is well known for his contribution to the molecular theory known as Avogadro’s law. He was born in Italy on 9th August 1776 and died on 9th July 1856. He is an Italian chemist. He discovered that 1 mole of substances contains 6.02214076 ×1023 entities, known as Avogadro’s constant. Amedeo Avogadro About the Scientist

366 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 12. Addition reaction : A chemical reaction in which two or more reactants combine together to give a single product is called addition reaction. 13. Decomposition reaction : A chemical reaction in which a single reactant is broken down into two or more products is called decomposition reaction. 14. Displacement reaction : A chemical reaction in which an atom or a radical of a compound is displaced by another element is called displacement or replacement reaction. 15. Single displacement reaction :A chemical reaction in which one atom or one radical is displaced by another element is called single displacement reaction. 16. Double displacement reaction: A chemical reaction in which an element or a radical of a compound is mutually displaced by an element or a radical is called double displacement reaction. 17. Acid-base reaction : A chemical reaction in which an acid and a base react together to give salt and water is called acid-base reaction. 18. Rate of chemical reaction : The positive change in the concentration of a reactant or a product per unit time is called the rate of chemical reaction. Introduction Water (H₂O) is made when hydrogen gas (H2) burns in air (O2). In this case, hydrogen and oxygen join together to make water. Calcium carbonate (CaCO₃) breaks down into calcium oxide (CaO) and carbon dioxide (CO₂) when it is heated (CO₂). Similarly, when iron (Fe) powder is kept in copper sulphate solution (CuSO4 ), copper (Cu) and iron sulphate (FeSO4 ) are formed. Here, iron displaces copper from copper sulphate solution. These are some examples of chemical changes. H2 + O2 burn H2 O [Combination] CaCO3 heat CaO + CO2 [Decomposition] Fe + CuSO4 FeSO4 + Cu [Displacement] The combination, decomposition or displacement that occurs in the molecules of matters during a chemical change is called a chemical reaction. It can be represented by an equation. The elements or compounds that take part in a chemical reaction are called reactants, whereas the elements or compounds that are formed as a result of chemical change are called products. Reactants are written on the left side of an arrow and products are written on the right side of the arrow. The direction of the arrow indicates the reactants and the products. Examples: Reactants Products Zinc + Sulphuric acid Zinc sulphate + Hydrogen Zn + H2 SO4 ZnSO4 + H2 Word Equation A chemical reaction expressed by writing the full names of reactants and products is called word equation. MEMORY TIPS The chemical reaction is represented by word equation and formula (chemical) equation.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 367 Examples: Magnesium + Oxygen Magnesium oxide Hydrochloric acid + Sodium hydroxide Sodium chloride + Water Chemical Equation A chemical reaction expressed by writing the symbols and molecular formulae of reactants and products is called a chemical equation. Examples: 2Mg + O2 2MgO HCl + NaOH NaCl + H2 O Balanced Chemical Equation A chemical equation written by balancing the total number of atoms of each element on the reactant side and product side is called a balanced chemical equation. Examples: 2KClO3 ∆ 2KCl + 3O2 H2 SO4 + 2KOH ∆ K2 SO4 + 2H2 O Methods of Writing a Balanced Chemical Equation i. First of all, the chemical reaction should be written correctly in the form of a word equation. Example: Nitrogen + Hydrogen Ammonia ii. The word equation is written in the form of a formula or chemical equation. Example: N2 + H2 NH3 iii. The number of atoms of each element should be made equal by using a suitable coefficient without changing the molecular formula. Example: N2 + 3H2 2NH3 Some More Examples of Balanced Chemical Equations 1. Word equation : Potassium chlorate Heat Potassium chloride + Oxygen Chemical equation : KClO3 ∆ KCl + O2 Balanced equation : 2KClO3 ∆ 2KCl + 3O2 2. Word equation : Hydrogen peroxide Catalyst Water + Oxygen Chemical equation : H2 O2 MnO2 H2 O + O2 Balanced equation : 2H2 O2 MnO2 2H2 O + O2 Information Obtained from a Balanced Chemical Equation A balanced chemical equation provides the following information: i. The names and symbols of reactants and products

368 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur ii. The total number of atoms or molecules of reactants and products iii. The ratio of molecular weight of reactant and product molecules iv. The type of chemical reaction The above information is illustrated by the following example: Information Reactants Product Chemical equation N2 + 3H2 2NH3 Name Nitrogen Hydrogen Ammonia Total molecules 1 3 2 Molecular weight 28 6 34 Type of reaction Addition reaction or combination reaction The above example also states that 28g of nitrogen reacts with 6g of hydrogen to give 34g of ammonia. The ratio of the molecular weight of nitrogen to ammonia is 14:17 and ratio of the molecular weight of hydrogen to ammonia is 3:17. Limitations of a balanced chemical equation A balanced chemical equation cannot provide the following information: i. The physical state of reactants and products ii. The concentration of reactants iii. Conditions required for the reaction like heat, light, pressure, catalyst, etc. iv. The rate of chemical reaction v. Time taken to complete the reaction vi. Whether the reaction is reversible or irreversible vii. Whether the reaction is exothermic or endothermic Modification of Chemical Equation To make a chemical equation more informative the following modifications are done: i. The physical state of the reactants and products is denoted by 's' for solid, 'l' for liquid, 'g' for gas and 'aq' for aqueous solution. ii. Concentration of reactants is denoted by 'dil.' for dilute and 'conc'. for concentrated. iii. The conditions like temperature, pressure, light, catalyst, etc. are written above or below the arrow. iv. For reversible reaction, a double way arrow ( ) and for irreversible reaction, a single way arrow (→) is used. v. The symbol ∆ indicates heat, ↑ indicates gas and ↓ indicates the precipitate. For example, 2Na(s) + 2H2 O (l) 2NaOH (aq) + H2 ↑

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 369 N2 (g) + 3H2 (g) heat / pressure catalyst / promoter 2NH3 ↑ 2HgO (s) 2Hg (l) + O2 ↑ NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl ↓ Reversible Reaction A chemical reaction in which the products can recombine to give back the reactants is called a reversible reaction. For example, H2 + I2 heat 2HI When hydrogen (H2 ) and iodine (I2 ) are heated, hydrogen iodide (HI) is formed. Heating hydrogen iodide in a closed vessel also forms hydrogen and iodine. Therefore, the given reaction is a reversible reaction. Reversible reactions are written by giving a double way arrow between the reactants and products, as follows: H2 + I2 2HI N2 + 3H2 2NH3 2H2 + O2 2H2 O Irreversible Reaction A chemical reaction in which the products cannot recombine to give back the reactants is called an irreversible reaction. For example, CaCO3 ∆ CaO + CO2 ↑ 2Na+2H2 O 2NaOH + H2 ↑ 2KClO3 ∆ 2KCl + 3O2 ↑ Exothermic Reaction A chemical reaction which evolves heat during the chemical change is called an exothermic reaction. For example, C + 2H2 CH4 + Heat C + O2 CO2 + Heat CaO + H2 O Ca(OH)2 + Heat CH4 + 2O2 CO2 + 2H2 O + Heat Zn + 2HCl ZnCl2 + H2 + Heat Endothermic Reaction A chemical reaction which absorbs heat during the chemical change is called an endothermic reaction. For example,

370 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur N2 +O2 Heat 2NO ↑ 2KClO3 Heat 2KCl + 3O2 ↑ CaCO3 Heat CaO + CO2 ↑ NaNO2 + NH4 Cl Heat NaCl + 2H2 O + N2 ↑ Factors Which Affect a Chemical Reaction There are several factors which play an important role during a chemical reaction. Some important factors are discussed below: 1. Heat Heat provides kinetic energy to the reactant molecules. Due to the absorption of heat energy, the frequency of the collision of these molecules increases to give more products. There are certain reactions which do not initiate without heat. So, heat plays an important role as a reaction initiator. Some chemical reactions that occur in the presence of heat are as follows: CaCO3 ∆ CaO + CO2 ↑ 2 KClO3 ∆ 2KCl + 3O2 ↑ 2. Light Certain chemical reactions occur in the presence of sunlight. For example, photosynthesis, chlorination of methane, decomposition of silver bromide, formation of hydrogen chloride, etc. 6H2 O + 6CO2 Sunlight C6 H12O6 + 6O2 ↑ CH4 + Cl2 UV-Rays CH3 Cl + HCl 2AgBr Light 2Ag + Br2 H2 + Cl2 Light 2HCl 3. Pressure The reactant molecules having more volume than product cannot collide easily with each other to give a product. Hence, to accelerate such a reaction, high pressure is needed. For example, when pressure is applied on a mixture of potassium chloride and sulphur, explosion take place. A fire cracker explodes on applying pressure. Similarly, the synthesis of ammonia by Haber's process needs high pressure, i.e., about 200-500 atmospheric pressure. N2 + 3H2 200-500 atm, 600 0 C Fe/Mo 2NH3 ↑

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 371 4. Catalyst A catalyst is a chemical substance which increases or decreases the rate of chemical reaction but itself remaining chemically unchanged. The mass and chemical nature of a catalyst does not change during the chemical reaction. There are two types of catalysts. a. Positive catalyst : A catalyst which increases the rate of chemical reaction is called a positive catalyst. For example, manganese dioxide (MnO2 ) acts as a positive catalyst during the decomposition of hydrogen peroxide. 2H2 O2 MnO2 (catalyst) 2H2 O + O2 ↑ b. Negative catalyst : A catalyst which decreases the rate of chemical reaction is called a negative catalyst. For example, glycerine acts as a negative catalyst and decreases the rate of the given chemical reaction. 2H2 O2 Glycerine 2H2 O + O2 ↑ 5. Solution Acid doesn't show any effect without water. Similarly, salt doesn't give ions without water. This shows that a solution also plays an important role in many chemical reactions. When silver nitrate and calcium chloride are brought in contact, they do not show any reaction. But if they are dissolved in water, they give corresponding ions; and after exchanging ions, products are formed. AgNO3 (aq) + NaCl (aq) NaNO3 (aq) + AgCl↓ 2AgNO3 (aq) + CaCl2 (aq) Ca(NO3 )2 (aq) + 2AgCl↓ 6. Direct Contact No contact, no collision, no reaction. Some active substances react on coming in direct contact with others. For example, when sodium is kept in chlorine solution, it gives sodium chloride (NaCl). 2Na + Cl2 2NaCl 7. Electricity Electricity is also one of the important factors that brings about chemical reaction. When electricity is passed in acidified water, it decomposes and gives hydrogen (H2 ) and oxygen (O2 ) gas. H2 O Electricity 2H2 + O2

372 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Types of Chemical Reaction There are basically four types of chemical reactions on the basis of their nature. 1. Combination or Addition or Synthesis Reaction A chemical reaction in which two or more reactants combine together to give a single product is called addition reaction. This type of chemical reaction occurs either in the presence or absence of heat, light, pressure, electricity, catalyst, etc. Examples: N2 + 3H2 2NH3 C + O2 CO2 C + 2H2 CH4 2 Na + Cl2 2NaCl Fe + S FeS CaO + H2 O Ca (OH)2 CaCO3 + H2 O + CO2 Ca(HCO3 )2 2Mg + O2 2MgO 2. Decomposition or Dissociation or Analysis Reaction A chemical reaction in which a single reactant is broken down into two or more products is called decomposition reaction. Such type of chemical reaction occurs in the presence of heat, light, catalyst, electricity, etc. Examples: CaCO3 ∆ CaO + CO2 2KClO3 ∆ 2KCl + 3O2 CuCO3 ∆ CuO + CO2 2Ag2 O ∆ 4Ag + O2 2HgCO3 ∆ 2Hg + 2CO2 + O2 2Pb(NO3 )2 ∆ 2PbO + 4NO2 + O2 2Cu(NO3 )2 ∆ 2CuO + 4NO2 + O2 2 AgNO3 ∆ 2Ag + 2 NO2 + O2 3. Displacement Reaction or Replacement Reaction A chemical reaction in which an atom or a radical of a compound is displaced by another element is called displacement or replacement reaction. It is of two types. i. Single displacement reaction : A chemical reaction in which one atom or one radical is displaced by another element is called single displacement reaction.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 373 Examples: Zn + 2HCl ZnCl2 + H2 Zn + H2 SO4 ZnSO4 + H2 2KBr + Cl2 2KCl + Br2 2KI + Cl2 2 KCl + I2 Zn + CuSO4 ZnSO4 + Cu Mg + ZnCl2 MgCl2 + Zn Fe + CuSO4 FeSO4 + Cu Cu + 2AgCl CuCl2 + 2Ag ii. Double displacement reaction : A chemical reaction in which an element or a radical of a compound is mutually displaced by an element or a radical is called double displacement reaction. Examples: AgNO3 + NaCl NaNO3 + AgCl CaCl2 + 2AgNO3 Ca(NO3 )2 + 2AgCl HgCl2 + 2KI 2KCl + HgI2 Pb (NO3 )2 + Na2 SO4 PbSO4 + 2NaNO3 FeCl2 + 2NaOH Fe (OH)2 + 2NaCl 4. Acid-base Reaction A chemical reaction in which an acid and a base react together to give salt and water is called acid-base reaction. Here, acidic and basic nature of the compounds is neutralized during the chemical reaction, so it is also called neutralization reaction. However, all acid-base reactions are not neutralization reactions. Examples: Acid + Base Salt + Water HCl + NaOH NaCl + H2 O H2 SO4 + CaO CaSO4 + H2 O H2 SO4 + 2NaOH Na2 SO4 + 2H2 O 2HCl + CuO CuCl2 + H2 O 2HNO3 + Ca(OH)2 Ca(NO3 )2 + 2H2 O H2 SO4 + 2KOH K2 SO4 + 2H2 O

374 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur FACTS WITH REASONS Why is acid-base reaction called neutralization reaction? In acid-base reaction, both acid and base lose their properties during the chemical reaction. As a result, a neutral substance (i.e., salt and water) is formed. So, acid-base reaction is also called neutralization reaction. Rate of Chemical Reaction The positive change in the concentration of a reactant or a product per unit time is called the rate of chemical reaction. It is calculated by the given formula. Rate of chemical reaction = Change in concentration of a reactant or a product Time taken for the change The SI unit of the rate of chemical reaction is moles per liter per second (mol/ls). FACTS WITH REASONS Write down the effect of concentration of reactants on the rate of chemical reaction. The rate of a chemical reaction increases on increasing the concentration of reactants. It is because increasing the concentration of reactants increases the number of reacting molecules. There will be more collision between the reacting molecules and hence the rate of chemical reaction increases. Various factors affect the rate of chemical reaction. Some major factors are: i. Concentration of reactants ii. Temperature iii. Catalyst iv. Chemical nature of reactants v. Physical nature of reactants vi. Surface area of the reactants vii. Pressure i) Concentration of Reactants The rate of chemical reaction increases on increasing the concentration of reactants. It is because increasing the concentration of reactants increases the number of reacting molecules. There will be more collision between the reacting molecules, and hence the rate of chemical reaction increases. ii) Temperature Increase in temperature increases the rate of a chemical reaction, whereas decrease in temperature decreases the rate of chemical reaction. The energy of the reacting molecules increases on increasing the temperature of the reactants. Due to increased energy, the frequency of collision increases, and finally the rate of chemical reaction increases. iii) Catalyst A positive catalyst increases the rate of chemical reaction. In the presence of a positive catalyst, the reaction takes place faster and at low temperature. For example, 2KClO3 360 0 C 2KCl + 3O2 ↑

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 375 2KClO3 240 0 C MnO2 2KCl + 3O2 ↑ From the above example, it becomes clear that potassium chlorate (KClO3 ) can be decomposed into potassium chloride (KCl) and oxygen (O2 ) by heating at about 3600 C, but the same reaction takes place at 240°C by using MnO2 as a positive catalyst. FACTS WITH REASONS Manganese dioxide (MnO2 ) is called a positive catalyst, why? Manganese dioxide (MnO2 ) increases the rate of chemical reaction. So, it is called a positive catalyst. iv) Physical nature of reactants The reaction takes place in the contact area of the reacting molecules. So the rate of chemical reaction can be increased by increasing the area of contact between the reactants. Similarly, the rate of chemical reaction increases by using a common solvent if the reactants are not soluble in one another. The common solvent helps bring the reacting molecules closer, and hence the rate of the reaction increases. v) Chemical nature of reactants Chemical nature of reactants also determines the rate of chemical reaction. Some reactants are more active and some are less reactive. The elements of group IA and VIIA are more reactive than the elements of other groups. Similarly, the reactions between ionic compounds are faster than the reactions between covalent compounds. vi. Surface area of the reactants The surface area of a reactants can affect the rate of a chemical reaction. If surface area of reactants increases, it increases the rate of collisions between reactant molecules and increases the rate of reaction. Similarly, when the surface area of a reactant is decreased, the rate of reaction may decrease as there are fewer chances for successful collisions to occur. vii. Pressure Pressure can affect the rate of a chemical reaction if at least one of the reactants is a gas. When pressure is increased, the gas molecules are compressed and more densely packed. It increases the frequency of collisions between gas molecules and possibly increases the rate of reaction. MEMORY TIPS Examples of positive catalyst are: MnO2, V2O5, Fe, etc. Similarly, examples of negative catalyst are H3PO4,H2O2, glycerine, etc.

376 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur HOT SKILL HIGHER ORDER THINKING SKILL 1. A reaction between hydrochloric acid and sodium hydroxide is called a neutralization reaction. Show chemical equations along with the reason. A reaction between hydrochloric acid and sodium hydroxide is called a neutralization reaction because whenever these two reactants react, they produce neutral substances such as salt (sodium chloride) and water. Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water HCl + NaOH → NaCl + H2 O 2. Powder sodium chloride does not react with powder sliver nitrate but they react when water is added to them. Why? Powder sodium chloride does not react with powder silver nitrate but it can easily react when water is added to it. The reason is the solution state of the reactant. When the sodium chloride and silver nitrate are mixed with water, they will change into a solution. They will ionize and produce radicals. More molecules will come in contact and the reaction will occur faster. 3. Carbon dioxide is a fire extinguisher. What happens if a burning magnesium is put in a gas jar full of carbon dioxide? Write a word equation for the above chemical reaction. Convert it into a balanced chemical equation and mention the type of reaction. Carbon dioxide does not support in burning. It is a fire extinguisher. But if burning magnesium is put in a gas jar full of carbon dioxide, the fire won’t extinguish. The magnesium ribbon keeps burning by replacing carbon from carbon dioxide. As a result, a white powder of magnesium oxide and black particles of carbon are formed. Above chemical reaction is represented as: Word equation : Magnesium + Carbon dioxide → Magnesium oxide + Carbon Unbalanced equation : Mg + CO2→ MgO + C Balanced equation : 2Mg + CO2→ 2MgO + C The above reaction between burning magnesium ribbon and carbon dioxide is a displacement reaction. 4. If an iron nail is placed in blue vitrol solution for a few days, what will happen? What type of reaction is it? Write a balanced chemical equation for the reaction. Since iron is more reactive than copper, iron displaces copper from copper sulphate to produce iron sulphate. Word equation : Copper sulphate + Iron → Iron sulphate + Copper Balanced equation : CuSO4 + Fe → FeSO4 + Cu So when an iron nail is placed in blue vitrol solution for a few days, the iron nail starts to lose iron molecules continuously and copper molecules fill those eroded places. After a few days, the iron nail will change into a copper nail due to a continuous displacement of iron by copper. It is an example of single displacement reaction. 5. What happens when limestone is heated at a higher temperature in a coal furnace? Mention the type of reaction. When limestone is heated at a higher temperature it absorbs heat and breaks down to produce calcium oxide and carbon dioxide.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 377 Limestone →∆ Calcium oxide + Carbon dioxide CaCO3 →∆ CaO + CO2 The reaction where limestone is heated to form calcium oxide and carbon dioxide is a decomposition reaction. 6. Silver nitrate and hydrogen peroxide cannot be stored in a transparent container. Silver nitrate and hydrogen peroxide are sensitive to light. When light enters from a transparent container these chemicals decompose to produce respective products. So these chemicals won’t be useful. Therefore, silver nitrate and hydrogen peroxide cannot be stored in a transparent container. 7. What happens to the rate of chemical reaction, when yeast is added to the hydrogen peroxide? The yeast releases the chemicals that force hydrogen peroxide to decompose. Therefore when yeast is added to hydrogen peroxide, the rate of chemical reaction will increase. 8. Observe the given chemical reaction and answer the following questions. Zn + dil.HCl →ZnCl2 + H2 i. What type of reaction is it? It is a single displacement reaction. ii. Is it an exothermic or endothermic reaction? It is an exothermic reaction. iii. Write the names of reactants and products. Reactants : zinc and hydrochloric acid Products : zinc chloride and hydrogen gas iv. Balance the chemical reaction. Zn + dil.2 HCl → ZnCl2 + H2 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Choose the best answer from the given alternatives. a. What is A in the given chemical reaction? NaOH + A → NaCl + H2 O i. H2 ii. O2 iii. HCl iv. Cl2 b. Which one of the following is a negative catalyst? i. manganese dioxide ii. molybdenum iii. ferrous iv. glycerine c. What is the type of reaction if hydrogen combines with nitrogen under high pressure to form ammonia? i. reversible, combination, exothermic ii. double displacement, reversible, endothermic iii. combination, irreversible, endothermic iv. synthesis, irreversible, exothermic d. What does the arrow (→) mean in a chemical equation? i. give the product ii. reversible reaction iii. double displacement reaction iv. irreversible reaction

378 cHemical Reaction Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur e. What is the chemical reaction between sodium chloride and silvernitrate? i. combination ii. decompostion iii. double decomposition iv. acid base reaction 2. Define the following terms with required examples. a. Chemical reaction b. Catalyst c. Positive catalyst d. Negative catalyst e. Addition reaction f. Decomposition reaction g. Displacement reaction h. Single displacement reaction i. Double displacement reaction j. Acid-base reaction k. Rate of chemical reaction 3. Answer the following questions in very short. a. List the factors which affect the rate of a chemical reaction. b. What are the chemical species that undergo chemical reactions called? c. What are the chemical species added to a solid catalyst to improve the performance of the catalyst called? d. What type of chemical reaction is it? Fe + CuSO4 → FeSO4 + Cu e. What is the type of reaction if barium chloride reacts with sodium sulphate? STEP2 4. Give reasons. a. Acid-base reaction is called a neutralization reaction. b. The rate of a chemical reaction increases when the concentration of reactants is increased. c. The rate of a chemical reaction increases when the temperature is increased. d. The rate of a chemical reaction increases on powdering the reactants. e. Glycerine is called a negative catalyst during the production of oxygen from hydrogen peroxide. 5. Differentiate between the following. a. Reactants and products b. Synthesis reaction and dissociation reaction c. Positive catalyst and negative catalyst d. Single displacement reaction and double displacement reaction 6. Answer the following questions in short. a. In what condition do sodium chloride and silver nitrate react? Write the balanced chemical equation of that reaction. b. Discuss the role of a catalyst with an example. c. What are the four factors that affect the rate of the chemical reaction? d. Why does the water rise up in the glass, when a burning candle on a plate with water is covered by the inverted glass?

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 379 e. Write the name of reactant X in the given chemical equation. KClO3 → 2KCl + X Complete the reaction and balance it. What type of reaction is it? STEP3 7. Answer the following questions. a. Change the given word equation into a balanced chemical equation. What type of chemical reaction is it? What is the role of MnO2 in this reaction? Potassium chlorate MnO2 Potassium chloride + Oxygen b. Give one example of an exothermic chemical reaction. Write any two characteristics of a catalyst. c. Describe an experiment to demonstrate that the rate of chemical reaction increases on increasing the surface area of reactants. d. How does the concentration of sodium thiosulphate affect the rate of the chemical reaction between hydrochloric acid and sodium thiosulphate? e. What happens if magnesium ribbon is burned at high temperatures? Write the name of the product. Write the balanced chemical equation for the reaction. Mention the type of reaction. f. If an iron nail is placed in copper sulphate solution for a few days, what will happen? What type of reaction is it? Write a balanced chemical equation for the reaction. g. Give an example of each four types of reaction. h. Write the word equation of the reaction between very active metal and very active non-metal. Convert it into a balanced chemical equation. Also mention the type of reaction. 8. Write the balanced chemical equation for the following word equations. Also mention the type of reaction. a. Calcium + Chloride →Calcium chloride b. Iron + Oxygen → Iron oxide c. Calcium carbonate →∆ Calcium oxide + Carbon dioxide d. Zinc + Hydrochloric acid →Zinc chloride + Hydrogen e. Sodium chloride + Silver nitrate →Sodium nitrate + Silver chloride f. Sodium hydroxide +Sulphuric acid →Sodium sulphate + Water g. Calcium bicarbonate → Calcium carbonate +Water +Carbon dioxide h. Aluminium + Hydrochloric acid → Aluminium chloride + Hydrogen i. Potassium chlorate → Potassium chloride + Oxygen j. Methane + Oxygen → Crbon dioxide + Water

380 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Key terms and terminologies of the unit 1. Laboratory preparation of ammonia : In the laboratory, ammonia is prepared by heating a mixture of ammonium chloride and calcium hydroxide (slaked lime) in the ratio 2:1. 2NH4 Cl + Ca(OH)2 CaCl2 + 2H2 O + 2NH3 ↑ 2. Chemicals required to prepare ammonia : Ammonium chloride [NH4 Cl] and Calcium hydroxide [Ca(OH)2 ] 3. Haber's process of ammonia manufacturing : In this process, a mixture of nitrogen and hydrogen (1:3) is combined directly at 500 0 C temperature and 200 to 500 atmospheric pressure. For this reaction, finely powdered iron is taken as a catalyst and molybdenum as a promoter. N2 + 3H2 2NH3 4. Promoter : A promoter is a chemical substance that makes a catalyst more active during a chemical reaction, e.g., Molybdenum (Mo). 5. Laboratory preparation of carbon dioxide : In the laboratory, carbon dioxide gas is prepared by the reaction of dilute hydrochloric acid with calcium carbonate (limestone or marble). CaCO3 + 2HCl CaCl2 + H2 O + CO2 ↑ 6. Chemicals required to prepare carbon dioxide :Calcium carbonate [CaCO3 ] and dilute hydrochloric acid [HCl]. 7. Carbogen : The mixture of 95% oxygen and 5%carbon dioxide is called carbogen. It is used in the artificial respiration of pneumonic patients. Sequence of Curriculum Issued by CDC  Laboratory preparation of carbon dioxide and ammonia gases  Physical properties, chemical properties and application  Greenhouse effect: causes, effects and control measures  Acid rain: causes (reaction of carbon dioxide and sulphur dioxide with water to produce respective acid)  Effects and control measures UNIT Some Gases 16 Estimated teaching periods Theory Practical 5 3 Fritz Haber is well known for developing the Haber-Bosch process to synthesize ammonia from nitrogen and hydrogen. He was born in Prussia (now Poland) on 9th December 1868 and died on 29th January 1934. He was a chemist who is known for surface chemistry, the Haber process, Haber’s rule, explosives and fertilizer. He synthesised urea that could be used as fertilizer. He was honoured with the Iron cross and Nobel Prize in Chemistry. Fritz Haber About the Scientist

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 381 8. Dry ice : When carbon dioxide is cooled to below -78°C, it is converted into solid form, which is known as dry ice. It is known as dry ice because it melts without wetting the surface. 9. Fire extinguisher: A fire extinguisher is a protective device which is used to extinguish fire. It has a red metallic vessel containing a sodium bicarbonate solution and a bottle of sulphuric acid. 10. Greenhouse: Green house is either a natural or an artificial structure which can trap heat energy and maintain particular temperature. 11. Artificial greenhouse: An artificial greenhouse is a structure covered with transparent polyethylene (plastic) or glass that traps solar rays inside. 12. Natural greenhouse: The earth is surrounded by several layers of gases called atmosphere that traps the heat from the sun increasing temperature of the earth similar to the glass or plastics house. It is called natural greenhouse. 13. Acid rain: The rain fall containing different types of acids is known as acid rain. 14. Greenhouse effect: The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such a way that it is relatively at a higher temperature is known as the greenhouse effect. 15. Greenhouse gases: The gases which trap the solar rays and heat up the atmosphere are known as the greenhouse gases. A. CARBON DIOXIDE Introduction Carbon dioxide is a gaseous compound made of one atom of carbon and two atoms of oxygen. Carbon dioxide occurs in a free or combined state in nature. In the free state, it is present in the atmosphere, about 0.03% by volume. In the atmosphere, carbon dioxide is released by the burning of fuels like diesel, petrol, kerosene, wood, coal, etc. and by the respiration of living beings. In the combined state, it is present in different types of carbonates, bicarbonates, etc. Since carbon dioxide is heavier than air, it is present at a lower level of the atmosphere and occurs in deep wells, mines, caves, etc. Carbon dioxide has a great role to play in the living world because it is used by green plants for photosynthesis to convert solar energy into chemical energy. This chemical energy is supplied to all animals for their existence. Carbon dioxide is produced by the reaction of limestone (CaCO3 ), dolomite (CaCO3 .MgCO3 ) and magnesite (MgCO3 ) with dilute acids. p+= 8 p+= 6 p+= 8 n0 = 8 n0 = 6 n0 = 8 Molecular structure of CO2 O C O General Methods of the Preparation of Carbon dioxide 1. By the combustion of hydrocarbons like methane, ethane, etc. When hydrocarbons like methane, ethane, etc. are burnt, carbon dioxide is produced.

382 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur CH4 + 2O2 CO2 + 2H2 O 2C2 H6 + 7O2 4CO2 + 6H2 O 2. By the reaction of acid with carbonates and bicarbonates When carbonates and bicarbonates of different metals, like calcium, magnesium, etc. are treated with acid, carbon dioxide is produced. CaCO3 + 2HCl CaCl2 + H2 O + CO2 Na2 CO3 + 2HCl 2NaCl + H2 O + CO2 Ca (HCO3 ) 2 + 2HCl CaCl2 + 2H2 O + 2CO2 3. By burning carbon in plenty of oxygen When carbon burns in plenty of oxygen, carbon dioxide is formed. C + O2 CO2 4. By heating limestone vigorously When limestone or marble is heated vigorously, carbon dioxide is formed. CaCO3 ∆ CaO + CO2 Laboratory Preparation of Carbon dioxide Principle In the laboratory, carbon dioxide is prepared by the reaction of dilute hydrochloric acid with calcium carbonate (limestone or marble). CaCO3 + 2HCl CaCl2 + H2 O + CO2 ↑ Apparatus required Woulfe’s bottle, thistle funnel, gas jar, delivery tube, corks, matchbox, moist blue litmus paper Chemicals required i. Calcium carbonate or marble or limestone pieces (CaCO3 ) ii. Dilute hydrochloric acid (HCl) Dilute hydrochloric acid Delivery tube Carbon dioxide Moist blue litmus paper Gas jar Thistle funnel Cork Woulfe's bottle Lime stone pieces Laboratory preparation of carbon dioxide Procedure i. We should keep some pieces of marble in a Woulfe’s bottle and arrange the apparatus as shown in the figure.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 383 ii. Then we have to pour some dilute hydrochloric acid through the thistle funnel until it covers the lower end of the thistle funnel and the marble pieces. iii. Chemical reaction takes place between the marble and hydrochloric acid, and it produces carbon dioxide. iv. The gas is passed through the delivery tube and collected in the gas jar by the upward displacement of air. FACTS WITH REASONS Dilute sulphuric acid cannot be used in place of dilute hydrochloric acid. Why? Dilute sulphuric acid cannot be used in place of dilute hydrochloric acid because calcium sulphate is formed after the reaction, which covers the remaining part of the marble and stops further reaction. Carbon dioxide is collected in the gas jar by the upward displacement of air. Why? Carbon dioxide is collected in the gas jar by the upward displacement of air because it is heavier than air. The lower end of the thistle funnel should be dipped in the solution. Why? The lower end of the thistle funnel should be dipped in the solution otherwise carbon dioxide can escape. Carbon dioxide cannot be collected in the gas jar passing through water. Why? Carbon dioxide is soluble in water. So it cannot be collected in the gas jar by passing through water. Test of carbon dioxide i. A burning match stick is kept near the mouth of the gas jar. If the burning match stick extinguishes, it proves that the gas jar is filled with carbon dioxide gas. ii. When carbon dioxide is passed through lime water, i.e. calcium hydroxide, it turns milky white due to the formation of insoluble calcium carbonate (CaCO3 ). CO2 + Ca(OH)2 CaCO3 + H2 O When carbon dioxide is passed in lime water for a long time, the milky colour disappears slowly due to the formation of water soluble calcium bicarbonate [Ca(HCO3 )2 ]. CaCO3 + H2 O + CO2 Ca(HCO3 )2 iii. Carbon dioxide is acidic in nature so it turns moist blue litmus paper red. Precautions i. The apparatus should be made air tight. ii. The lower end of the delivery tube should not touch the solution within the Woulfe's bottle. iii. The lower end of the thistle funnel should be dipped in the solution. iv. Carbon dioxide is collected in the gas jar by the upward displacement of air. Industrial Preparation of Carbon dioxide For commercial purpose, carbon dioxide is manufactured by heating calcium carbonate (limestone or marble). During this process, calcium oxide, i.e., lime, or quick lime, is also produced. CaCO3 CaO + CO2 ↑ When calcium oxide reacts with water, it gives calcium hydroxide, which is also known as slaked lime. CaO + H2 O Ca(OH)2

384 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Properties of Carbon Dioxide a. Physical properties i. It is a colourless, odourless and tasteless gas. ii. It is soluble in water and produces carbonic acid when dissolved in water. iii. It is heavier than air. iv. It is an acidic gas. So, it turns moist blue litmus paper red. v. It is neither combustible nor a supporter of combustion. vi. When carbon dioxide is cooled to below -78°C, it is converted into solid form, which is known as dry ice. It is known as dry ice because it directly changes into carbon dioxide. No liquid is formed when it melts. b. Chemical properties i. Reaction with water : Carbon dioxide reacts with water and gives carbonic acid. CO2 + H2 O H2 CO3 (Carbonic acid) ii. Reaction with lime water : Carbon dioxide reacts with lime water and gives insoluble calcium carbonate (CaCO3 ), which makes the solution milky-white. CO2 + Ca(OH)2 CaCO3 + H2 O When carbon dioxide is passed continuously into lime water for a long time, the milky colour disappears slowly due to the formation of water soluble calcium bicarbonate Ca(HCO3 ) 2 . CaCO3 + H2 O + CO2 Ca(HCO3 )2 iii. Green plants convert solar energy into chemical energy by photosynthesis. In this process, carbon dioxide and water react together to give starch and oxygen. 6CO2 + 6H2 O sunlight chlorophyll C6 H12O6 + 6O2 iv. Carbon dioxide is neither combustible nor a supporter of combustion. But a burning magnesium strip burns in carbon dioxide with dazzling light. During this process, white powder (ash) of magnesium oxide and black particles of carbon are produced. 2Mg + CO2 2MgO + C v. Carbon monoxide is formed when carbon dioxide is heated with red hot coke at about 9000 C. CO2 + C 900ºC 2CO vi. Ammonia reacts with carbon dioxide at about 15000 C and certain pressure (30 atm.) to form urea and water. 2NH3 + CO2 1500ºC pressure (30atm.) NH2 – CO – NH2 + H2 O (Urea) MEMORY TIPS A promoter is a chemical substance that makes a catalyst more active during a chemical reaction, e.g., Molybdenum (Mo). MEMORY TIPS Dry ice is used in refrigerators to preserve foods, fruits, vegetables and meat.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 385 vii. Potassium hydroxide absorbs carbon dioxide. 2KOH + CO2 K₂CO₃ + H₂O FACTS WITH REASONS The level of water rises when we keep KOH in the gas jar containing CO2 in kept inverted in the water trough. Why? KOH absorbs carbon dioxide and creates vacuum in the gas jar. So water rises to fill the vacuum. Uses of Carbon dioxide i. Green plants use carbon dioxide to prepare food during photosynthesis. ii. It is used for making soft as well as hard drinks like soda water, coca cola, beer, etc. iii. It is used in the manufacture of fertilizers like urea and washing soda, i.e., sodium carbonate. iv. Solid carbon dioxide, i.e., dry ice is used as a refrigerant to preserve foods, fruits, meat, etc. v. It is used in fire extinguishers. vi. It is used in carbonation process to purify sugarcane juice. Working Mechanism of a Fire Extinguisher A fire extinguisher is a protective device which is used to extinguish fire. It has a red metallic vessel containing a sodium bicarbonate solution and a bottle of sulphuric acid. Knob Nozzle for gas Metallic cylinder solution Concentrated H2 SO4 Saturated sodium bicarbonate External and internal structure of a fire extinguisher When we need carbon dioxide to extinguish a fire, the vessel is inverted. Afterwards, the bottle of concentrated sulphuric acid strikes against the floor. The plug of the acid bottle falls down and acid comes in contact with the sodium bicarbonate solution. When those chemicals react together, they give carbon dioxide. 2NaHCO3 + H2 SO4 (conc.) Na2 SO4 + 2H2 O + 2CO2 ↑ The gas produced in the above reaction comes out through the nozzle at high pressure and puts off the fire, just like putting a blanket over the surface of the flame. MEMORY TIPS The mixture of 95% oxygen and 5% carbon dioxide is called carbogen. It is used in the artificial respiration of pneumonic patients.

386 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur B. AMMONIA Introduction Ammonia is a gaseous compound made up of one nitrogen atom and three hydrogen atoms. Pure ammonia was first prepared by heating the mixture of sal ammoniac (ammonium chloride) and lime (calcium hydroxide). Ammonia occurs in free as well as in combined states. In a free state, is present in air and soil whereas in a combined state, it is found in different ammonium salts like ammonium sulphate [(NH4 )2 SO4 ], ammonium phosphate [(NH4 )3 PO4 ], ammonium chloride [NH4 Cl], etc. Ammonia is also synthesized in nature by nitrifying bacteria. General Methods of Preparation of Ammonia 1. By heating ammonium salts Ammonia can be prepared by heating ammonium salts, like ammonium sulphate [(NH4 )2 SO4 ], ammonium chloride (NH4 Cl), ammonium carbonate, (NH4 )2 CO3 ], etc. (NH4 )2 SO4 ∆ 2NH3 + H2 SO4 NH4 Cl ∆ NH3 + HCl (NH4 )2 CO3 ∆ 2NH3 + H2 O + CO2 2. By heating ammonium salts with strong bases Ammonia can be prepared by the reaction of ammonium salts, like ammonium chloride (NH4 Cl), ammonium sulphate [(NH4 )2 SO4 ], etc. with strong bases like sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. NH4 Cl + KOH ∆ KCl + H2 O + NH3 ↑ (NH4 )2 SO4 + 2NaOH ∆ Na2 SO4 + 2H2 O + 2NH3 ↑ Laboratory Preparation of Ammonia Principle In the laboratory, ammonia is prepared by heating a mixture of ammonium chloride and calcium hydroxide (slaked lime) in the ratio 2:1. 2NH4 Cl + Ca(OH)2 ∆ CaCl2 + 2H2 O + 2NH3 ↑ p+ = 1 n0 = 0 p+ = 7 n0 = 7 p+ = 1 n0 = 0 p+ = 1 n0 = 0 Molecular structure of ammonia (NH3 ) N H H H

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 387 FACTS WITH REASONS Ammonia cannot be collected in a gas jar either by the downward displacement of water or upward displacement of air, why? Ammonia cannot be collected in a gas jar by the downward displacement of water because this gas is highly soluble in water. It forms ammonium hydroxide (NH4OH) when passed in water. Ammonia is lighter than air. So, it cannot be collected in the gas jar by the upward displacement of air. A moist red litmus paper is used to test ammonia gas, why? Ammonium hydroxide is an alkali. So, a wet red litmus paper is used to test if the gas is ammonia or not. Mixture of Ca(OH)2 and NH4 Cl Ammonia gas Lime tower CaO Burner Stand Stand Laboratory preparation of ammonia gas Apparatus required Hard glass test tube, delivery tube, gas jar, Bunsen burner, stands, red litmus paper, cork Chemicals required i. Ammonium chloride [NH4 Cl] ii. Calcium hydroxide [Ca(OH)2 ] Procedure i. Make a mixture of ammonium chloride and calcium hydroxide in the ratio 2:1. ii. Keep the mixture in a hard glass test tube, and arrange the apparatus as shown. Supply heat from the Bunsen burner. iii. After heating, ammonia is produced. It is collected in the gas jar by downward displacement of air. Precautions i. The apparatus should be made air-tight. ii. The hard glass test tube should be slightly slanted. iii. Ammonia is collected by downward displacement of air. Test of ammonia i. Ammonia turns moist red litmus paper into blue because it is basic in nature. ii. It has a strong pungent odour. iii. When a glass rod dipped in conc. hydrchloric acid (HCl) is brought in contact with ammonia, it forms a white fume of ammonium chloride (NH4 Cl).

388 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur FACTS WITH REASONS We bring moist red litmus paper near the mouth of the gas jar, why? We bring moist red litmus paper near the mouth of the gas jar to test whether the gas jar is filled by ammonia or not. Since ammonia is basic in nature, it turns moist red litmus paper blue. Why is the gas jar inverted? The gas jar is inverted because ammonia is lighter than air. So, this gas is collected by the downward displacement of air. Ammonia is not collected by the downward displacement of water similar to hydrogen and oxygen. Why? Ammonia is not collected by the downward displacement of water similar to hydrogen and oxygen because it is highly soluble in water and gives ammonium hydroxide. NH3 + H2O NH4OH (Ammonium hydroxide) A hard glass test tube is slightly inclined during preparation of ammonia, why? A hard glass test tube is slightly inclined during preparation of ammonia, otherwise the steam produced during the reaction exerts pressure, which may crack the test tube. So, the hard glass test tube is slanted to prevent it from cracking. Why is lime tower (CaO) used during preparation of ammonia? Lime tower (CaO) is used to obtain dry and pure ammonia. Industrial Preparation of Ammonia For commercial purpose, ammonia is manufactured by Haber's process. In this process, a mixture of nitrogen and hydrogen (1:3) is combined directly at 500 0 C temperature and 200 to 500 atmospheric pressure. For this reaction, finely powdered iron is taken as a catalyst and molybdenum as a promoter. If the catalyst is not used, the rate of reaction becomes extremely slow, and we need to supply high heat and high pressure. N2 + 3H2 500 0 C/200 to 500 atm. Fe (catalyst) and Mo (promoter) 2NH3 FACTS WITH REASONS Ammonia gas cannot be collected passing through concentrated sulphuric acid, calcium chloride and phosphorus pentachloride because these compounds react with ammonia gas. Conditions required for Haber's process German chemist Fritz Haber, in 1913 AD, discovered the following conditions for the industrial preparation of ammonia. i. Temperature 500°C ii. Pressure 200 to 600 atm. iii. Catalyst Iron powder (Fe) iv. Promoter Molybdenum (Mo)

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 389 Properties of Ammonia a. Physical properties i. It is a colourless and tasteless gas having a strong pungent odour. ii. It lighter than air. iii. It is highly soluble in water. iv. It turns moist red litmus paper blue. v. It is a non-combustible gas. ACTIVITY 1 To demonstrate that ammonia is highly soluble in water and basic in nature A round bottom flask containing ammonia is taken, and a delivery tube is fitted on it as shown in the diagram. The other end of the delivery tube is kept within the water containing some drops of phenolphthalein. The delivery tube carries a jet inside the round bottom flask. When ammonia cools, it creates low pressure inside the flask, so some water drops rise up in the flask and form ammonium hydroxide. As a result, more vacuum is created inside the flask. Hence, water rushes up with high pressure to fill up the vacuum, which looks like a fountain. Afterwards the entire flask appears pink. This experiment proves that ammonia is highly soluble in water and basic in nature. Note : If we take kerosene in place of water, it does not form ammonium hydroxide and does not give a pink color. NH3 Round bottom flask Water Stand b. Chemical properties i. Ammonia dissolves in water and forms an alkali, i.e., ammonium hydrxide. NH3 + H2 O 1500 0 C Pressure NH4 OH ii. Ammonia is a basic gas, so it reacts with acid to give salt. NH3 + HCl NH4 Cl 2NH3 + H2 SO4 (NH4 )2 SO4 NH3 + HNO3 NH4 NO3 iii. Ammonium hydroxide, i.e.,ammonia solution, reacts with acid to give salt and water. 2NH4 OH + H2 SO4 (NH4 )2 SO4 + 2H2 O NH4 OH + HCl NH4 Cl + H2 O NH4 OH + HNO3 NH4 NO3 + H2 O iv. Ammonia and carbon dioxide react together at about 1500 °C and under certain pressure (30 atm.) to give urea. It is a very useful chemical fertilizer rich in nitrogen.

390 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 2NH3 + CO2 NH2 - CO - NH2 + H2 O (urea) v. Ammonia reacts with conc. hydrochloric acid and forms solid particles of ammonium chloride. NH3 + conc. HCl NH4 Cl vi. Ammonia reacts with oxygen and forms nitrogen and water. 4NH3 + 3O2 2N2 + 6H2 O Uses of Ammonia i. It is used in the manufacture of chemical fertilizers, like ammonium nitrate, ammonium phosphate, urea, etc. ii. Liquid ammonia is used in refrigerators and cold storages. iii. It is used in the manufacture of different types of industrial products, like nitric acid, plastics, dyes, nylon, rayon, washing soda, chemical explosives, etc. iv. It is used to develop the blue print of maps. v. It is used to make medicines like ammonium carbonate, ammonium chloride, etc. vi. Liquid ammonia and ammonia solution are used as a laboratory reagent. Greenhouse A greenhouse can either be an artificial (greenhouse made up of transparent glass or plastic) or a natural greenhouse i.e. the earth having dense atmosphere. In the artificial greenhouse, summer plants can be grown even in winter. This is possible as the glass or plastic that covers the greenhouse traps the heat from solar rays maintaining a higher temperature inside. Similarly, the earth also traps the heat of the sun in the same way as an artificial greenhouse. Therefore, it is called a natural greenhouse. Artificial Greenhouse An artificial greenhouse is a structure covered with polyethylene (plastic) or glass that traps solar rays inside. Due to the trapping of the solar rays, it is hotter inside a greenhose than outside. This enables us to grow summer plants or off season plants even in winter season. A greenhouse is called a greenhouse because we mostly grow green plants inside it to maintain their greenery. FACTS WITH REASONS The roofs of greenhouse can be of various shapes but mostly slanted roof is preferred, why? The roofs of greenhouse can be of various shapes but mostly slanted roof is preferred because it allows maximum sun rays to enter even the sun is too low in the horizon. Artificial Greenhouse MEMORY TIPS The green house is either a natural or an artificial structure that can trap solar energy and maintain certain temperature. Artificial green house are glass houses or plastic tunnels. Earth itself is a natural green house.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 391 Phenomenon of heat trapping When the solar rays enter the glass or plastic, most of the short wavelength having higher energy are allowed to enter inside. These rays strike the ground and plants and are absorbed. But when they are emitted, the rays have a long wavelengths having low energy. These emitted rays cannot penetrate the glass or plastic and thus gets trapped inside. Use and importance of artificial greenhouse 1. A greenhouse is used to grow the plants that need higher temperature. 2. It helps to grow fruits, vegetables and flowers commercially in a large quantity even in the off season. 3. It protects the plants from excess frost and dew formation. 4. It helps to protect the plants from air pollution. 5. It prevents the breakage of plant stems during heavy rainfall. 6. It also protects the smaller plants from the winds. The main function of an artificial greenhouse is to preserve the heat of the sun inside its glass or plastic structure. FACTS WITH REASONS It is not the sun due to which we feel hot but mostly the atmosphere, why? It is not the sun due to which we feel hot but mostly the atmosphere because the atmosphere absorbs a large amount of heat from the sun and ground. Without the atmosphere, we would certainly die by being exposed to UV rays but the temperature at night would be reduced below the freezing point. Earth: A Natural Greenhouse If we compare the earth and the moon, which are nearly at same distances from the sun, the earth has suitable temperature, but the moon is very cold during the night and very hot during the day respectively. This is due to the reason that moon doesn’t have atmosphere. So it is not able to trap the solar rays inside it. But the earth is surrounded by several layers of gases called atmosphere that traps the heat from the sun. But how? The answer is due to the greenhouse effect. The Greenhouse Effect The earth receives two kinds of radiation from the sun: shortwave radiation (UV rays and visible light mostly) and long wave radiation (Infrared rays mostly). The short wave radiation can pass easily into the earth’s atmosphere while the long wave radiation is absorbed. When these solar rays are absorbed by the earth’s surface, it starts to reflect infrared rays (heat carrying rays). They are immediately re-absorbed by the atmosphere. After absorption, the Natural greenhouse

392 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur atmosphere also gives heat to all directions. This atmospheric heat is again absorbed by the ground. In this way, there is a continuous exchange of heat between the atmosphere and the earth’s surface due to which the temperature of the earth is maintained warmer. This phenomenon is known as the greenhouse effect. FACTS WITH REASONS Earth acts as a natural greenhouse, why? Earth acts as a natural greenhouse because the atmosphere that covers the earth traps the solar heat inside. The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such a way that it is relatively at a higher temperature is known as the greenhouse effect. The greenhouse effect is mostly advantageous to us as it maintains a constant surface temperature on the earth and prevents it from chilling. But if there is excessive greenhouse effect, it can cause the overall temperature of the earth to rise significantly, which can then be disadvantageous to us. Hence, greenhouse effect is both beneficial as well as harmful to us. FACTS WITH REASONS Refrigerators, air conditions, deodorants etc. are bad for ozone layer, why? Refrigerators, air conditions, deodorants etc. are bad for ozone layer as they release gases called Chlorofluorocarbons which deplete the ozone layer. But some of them contain ammonia and hydrofluorocarbons which do not deplete the ozone layer but contribute to greenhouse effect. Among several gases of the atmosphere, ozone (O3 ), carbon dioxide (CO2 ), water vapour (H2 O), methane (CH4 ), Nitrous oxide (N2 O), chlorofluorocarbon (CFC), etc. have higher ability of absorbing solar rays compared to other gases. These gases which trap the solar rays and hat up the atmosphere are known as the greenhouse gases. Some of the greenhouse gases and their sources are given below: i. Carbon dioxide (CO2 ): burning of fossil fuels, coal, petrochemicals, wood, respiration etc. ii. Water vapour (H2 O): evaporation from seas, oceans, rivers, etc., burning of hydrocarbons etc. iii. Methane (CH4 ): decomposition of organic substances like dung, faeces, waste vegetables, dead organisms, etc. iv. Chlorofluorocarbon (CFC): aerosols and coolants v. Nitrous oxide (N2 O): fertilizers, waste water management, burning of fossil fuels, livestock manure, etc. Adverse impacts of greenhouse effect Some of the adverse impacts of greenhouse effect are given below: 1. Effects on animals i. Greenhouse effect helps to spread different types of diseases like cholera, diarrhoea, malaria, filariasis, etc. ii. It helps to increase the generation of pathogenic insects. 2. Effects on plants i. It affects the cultivation and harvesting periods of crops. ii. Tropical plants are seen at the temperate region due to the effect of global warming

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 393 iii. It helps to increase the disease causing insects. 3. Effects on climate i. Greenhouse effect increases the temperature of the earth. ii. It helps melting of ice in the pola region, which increases the sea level. As a result, low land areas of the earth may submerge. iii. It changes the pattern of rain fall and weather condition. FACTS WITH REASONS It is always warmer inside the green house than surrounding. Why? It is always warmer inside the green house than surrounding because solar heat that enters into green house is trapped due to decrease in wave length of the reflected rays. Ways of controlling greenhouse effect i. Greenhouse effect can be controlled by controlling the production and use of CFCs. ii. It can be controlled by using alternative sources of energy like solar energy, wind energy, bio-gas, etc. iii. It can be controlled by reducing the burning of fossil fuels like petrol, diesel, kerosene, coal, etc. iv. It can be controlled by decreasing deforestation and increasing afforestation. Acid Rain The rain fall containing different types of acids is known as acid ran. Rain water is lightly acidic due to the presence of CO2 , NO and SO2 . But, occasionally its acidity may increase due to the mixing of different acids like Hydrochloric acid (HCl), Sulphuric acid (H2 SO4 ), Nitric acid (HNO3 ), Carbonic acid (H2 CO3 ), etc. These acids are formed when industrial gases like CO2 , SO2 , NOx and Cl2 react with water vapour of the atmosphere. After their formation, these acids mix with rain and fall down as acid rain. The chemical reactions involving their formation are given below: Carbonic acid rain CO2 + H2 O H2 CO3 Sulphuric acid rain 2SO2 +O2 2SO3 SO3 + H2 O H2 SO4 Effects of acid rain i. Acid rain causes respiratory problems, skin and eye irritation in humans. ii. It increases the acidity of soil and decreases the fertility. iii. It kills aquatic and amphibian animals. iv. It affects the aquatic ecosystems. v. Acid rain falling on plant leaves, fruits and flowers or absorbed by soil decreases the productivity. vi. It corrodes the statues, monuments, shrines, metallic architectures and marble works.

394 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Ways of controlling acid rain i. Reduce the use of fossil fuels and promote alternative and clean sources of energy. ii. Adopt techniques of reuse of emitted gases from industries. iii. Develop the technology that controls production of SO₂ and CO₂. HOT SKILL HIGHER ORDER THINKING SKILL 1. Let’s suppose you became the Mayor of your city, what would you do to prevent acid rain? Let’s suppose I became the mayor of my city, I would do the following things to prevent acid rain: i. First of all I would replace the LPG gas stove with an electric induction heater in my city. ii. I would request everyone to use electrical devices over coal-powered devices. iii. I would plant more plants in any available area, footpath and parks in my city. iv. I would prevent the use of old vehicles in my city. v. I would request every industry and factory to resettle far away from the city. vi. I would introduce electric buses, electric trains and other electrical means of transportation in the city. vii. I would manage solid waste from the city properly. 2. We do not use dilute sulphuric acid in place of dilute hydrochloric acid during the preparation of carbon dioxide. When dilute sulphuric acid reacts with calcium carbonate it forms carbon dioxide gas and calcium sulphate. Dil. H2 SO4 + CaCO3→ CaSO4 + CO2 + H2 O Thus, formed calcium sulphate is insoluble in water. It deposits on calcium carbonate and covers it. It prevents further reaction. And hence the production of carbon dioxide will be stopped. 3. Huge cities are more likely to suffer from acid rain. Acid rain is a rainfall which contains higher concentration of acid. Acid rain is formed when industrial gases dissolve with rainwater and form respective acids. Industrial gases are released from industries, factories, LPG gas and vehicles. These are more available in cities. So, the city sky has a huge quantity of industrial gas which can mix with rainfall to form acid rain. 4. Answer the following questions after observing the given figure of laboratory preparation of ammonia. i) Write the principle of laboratory preparation of ammonia. Ammonia can be prepared in the laboratory by heating the mixture of ammonium chloride and calcium hydroxide in a ratio of 2:1. 2NH4 Cl + Ca(OH)2→ CaCl2 + 2H2 O + 2NH3 ii) Write the name of the solids present in the hard glass test tube. The solids present in that hard glass test tube are ammonium chloride and calcium hydroxide. iii) How is the gas tested? This gas is tested by using moist red litmus paper which turns blue on contact with ammonia.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 395 It is also tested by introducing hydrochloric acid which turns into a white cloud of ammonium chloride. iv) Why is the gas collected by the downward displacement of air? This gas (ammonia) is collected by the downward displacement of air because it is lighter than the air. 5. Why is it dangerous and suffocating to go inside deep wells? It is dangerous and suffocating to go inside the deep wells because there could be a higher concentration of carbon dioxide. Since carbon dioxide gas is denser than air, it is collected in deep wells, caves and mines. 6. What happens when ammonia is mixed in water? Write its use. When ammonia is mixed with water it reacts to produce ammonium hydroxide. It is also called liquor ammonia. NH3 + H2 O → NH4 OH Liquor ammonia is used as a cleaning agent and emulsifier. 7. What is formed when carbon dioxide is passed through clean lime water for a short duration? When carbon dioxide is passed through clean limewater for a short duration calcium carbonate will be formed. Since calcium carbonate is insoluble in water, the lime water turns milky. Word equation : Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate + Water Balanced formula equation: Ca(OH)2 + CO2→ CaCO3 + H2 O 8. Differentiate between the natural greenhouse and artificial greenhouse. The comparison between a natural greenhouse and an artificial greenhouse are: SN Natural greenhouse SN Artificial greenhouse 1 Earth itself is a natural greenhouse where solar radiation is trapped by the atmosphere. 1 The artificial greenhouse is a plastic or glass house where the temperature is regulated artificially. 2 It helps to keep the earth warm, regulate weather etc. 2 It helps to grow off-season herbs, flowers and vegetables. 9. Why is the hard glass test tube slightly slanted during the laboratory preparation of ammonia? The hard glass test tube is slightly slanted during laboratory preparation of ammonia gas so that water drops would flow down to the lower parts far away from the hot parts and chemicals. Slanting also helps to reduce steam pressure and prevent cracking of the test tube. 10. Earth acts as a natural greenhouse. How? How has it helped to sustain life on earth? Explain. Earth has an atmosphere. It contains greenhouse gases such as carbon dioxide, methane and water vapour naturally. These greenhouse gases trap solar energy and keep the earth warm. Radiations reflected by the earth cannot leave the earth’s atmosphere due to the ozone layer. As a result, the earth acts like a natural greenhouse. Earth acting as a natural greenhouse helps to support life in the following ways: i. It helps to keep the earth warm. ii. It prevents the earth from freezing. iii. It helps to maintain weather and climate.

396 Some gaSeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Choose the best answer from the given alternatives. a. Why is Haber’s process carried out in an iron container? i. to create 200 to 600 atm pressure. ii. iron can withstand 500°C temperature. iii. Iron container is rigid and strong. iv. iron increases the rate of the combination of nitrogen and hydrogen. b. Hard glass test tube is slanted during laboratory preparation of ammonia. Why? i. to balance the centre of gravity so that the hard glass test tube won’t fall. ii. to maintain uniform heating of reactants iii. to avoid falling of condensed water droplets on the reactants iv. to align the hard glass test tube with the delivery tube. c. Why does phenolphthalein solution turn pink, when ammonia is passed through it? i. Ammonia is base, so it can turn phenolphthalein pink. ii. Ammonia is acid, so it can turn phenolphthalein pink. iii. Ammonia is pink, so it changes everything into pink iv. Ammonia dissolves in water to produce strong acid. d. Which of the following is responsible for the greenhouse effect? i. oxygen ii. nitrogen iii. water vapour iv. sulphur dioxide e. How is carbon dioxide manufactured? i. dissolving limestone in acid ii. heating limestone in the furnace iii. burning coal in excessive oxygen iv. burning coal in minimum oxygen 2. Define the following terms with required examples. a. Promoter b. Carbogen c. Dry ice d. Haber’s process e. Fire extinguisher f. Greenhouse g. Greenhouse gases h. Greenhouse effect i. Artificial greenhouse j. Acid rain 3. Answer the following questions in very short. a. Write down the molecular formula and molecular weight of carbon dioxide. b. What happens when carbon dioxide is cooled to -78°C? c. Write down the molecular formula and molecular weight of ammonia.