Chemistry MS2

*CONFIDENTIAL 1
CLASS:………………
NAME: …………………………………………….

KOLEJ TINGKATAN ENAM SHAH ALAM
JALAN TIMUN 24/1

SHAH ALAM, SELANGOR DARUL EHSAN

PENTAKSIRAN SEMESTER 2 STPM 2021 KALI KEDUA

CHEMISTRY 962/2

PAPER 2

DURATION: 1 HOUR 30 MINUTES

Instruction to candidates
DO NOT OPEN THIS QUESTION PAPER UNTIL YOU ARE TOLD TO DO SO

Write your name, I.C number and class in the space provided. There For examiner’s use
are fifteen questions in Section A. For each question, four choices of
answers are given. Choose one correct answer and indicate it on the Section A
multiple-choice answer sheet provided. Answer all questions. 1-15

Answer all the questions in Section B. Write your answers in the Section B
spaces provided. 16

Answer two out of the 3 questions in Section C. Write your answer on 17
your examination test pad. Begin each answer on a new sheet of
paper and arrange your answers in numerical order. Section C

This question paper consists of pages TOTAL

Prepared by: Checked by: Validated by
……………………………
……………………..…. …………………………… Pn Maziah Binti Muhammad Zain
Mas Liana Binti Mansor Pn Marini Binti Abdul Ghani
Head of Department
Head of Chemistry Unit

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*CONFIDENTIAL Section A [15 marks] 2

1. A 2. C 3. D 4. C 5. A
6. A 10. A
11. D 7. D 8. A 9. C 15. A

12. D 13. A 14. A

Section B [15 marks]
Answer all questions in this section

1. (a) Samples of calcium and strontium are placed separately in water at room temperature and left
for one day . In each case, describe what you would see and write a balanced equation for
each reaction.

Elements Observations Equations

i. calcium little / small amount of calcium Ca + 2 H2O ⇌ Ca(OH)2 + H2.
dissolved in water.

strontium dissolve completely Sr + 2 H2O ↔ Sr(OH)2 + H2

ii. strontium

/ almost completely in water

[4 marks]

(b) Strontium nitrate, Sr(NO3)2 undergoes thermal decomposition. [1 mark]
i. Write a balanced equation for this reaction. [2 mark]

2 Sr(NO3)2 → 2 SrO + 4 NO2 + O2

ii. Explain why a high temperature is required to decompose strontium nitrate.

• Ionic character of strontium nitrate is high
• Sr-NO3 is more stable than Sr-O

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*CONFIDENTIAL 3

17 (a) Silicon dioxide is the main component used to make glass. Among the most common and
cheapest glass produced is soda glass which is manufactured by mixing silicon dioxide,
sodium carbonate and limestone.

i. State one of the component in soda glass, and write the equation for the formation of the

component stated. [2 marks]

o sodium silicate (Na2SiO3) or calcium silicate (CaSiO3)

o SiO2 + Na2CO3 → Na2SiO3 + CO2 or
o SiO2 + CaCO3 → CaSiO3 + CO2

ii. State the compound added to silicon dioxide to form borosilicate glass and state one of its

property and its application. [3 marks]

• boron oxide (or B2O3)
• B2O3 + 3 SiO2 → B2(SiO3)3

• high melting point / less brittle / low thermal expansion / resistant to heat
or chemical

(b) Silicones are rubber-like materials used commercially in a wide variety of products.
Commercial silicones have infinite O-Si-O chains with alkyl or aryl groups attached to the
silicon atoms.

i. Draw the structure of chain silicone showing three repeating units with alkyl groups.
[2 marks]

ii. State one property of silicone. [1 mark]

• insoluble in water / non-toxic /stable to heat

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*CONFIDENTIAL 4

Section C [30 marks]
Answer any two of the three questions below

18. The reaction between ethane, C2H6 and chlorine gas, Cl2 to form chloroethane, C2H5Cl and
hydrogen chloride, HCl, can be described in the equation below

C2H6 (g) + Cl2 (g) → C2H5Cl (g) + HCl (g)

(a) Given then enthalpy of atomisation for these compounds are

Species Enthalpy change
Ethane, C2H6 of atomisation
Chlorine, Cl
+ 405 kJ

+ 121 kJ

Chloroethane, C2H5Cl + 418 kJ
Hydrogen chloride, HCl + 184 kJ

i. Define enthalpy change of atomisation. [1 mark]

enthalpy / heat absorbed when 1 mol of substance is changed into gaseous atoms

ii. Calculate the enthalpy change of reaction for the reaction between ethane and chlorine.

[6 marks]

• C2H6 (g) → 2 C (g) + 6 H (g) ∆H = +405 kJ

• Cl2 (g) → Cl (g) ∆H = +242 kJ

• C2H5Cl (g) → 2C(g) +5H(g) + Cl(g) ∆H = +418 kJ

• HCl (g) → H (g) + Cl (g) ∆H = + 184 Kj

• ∆H = (-184) + (-418) + 405 + 242 [3 marks]
• = - 45 kJ (insist '-' and kJ)

iii. Draw the energy level diagram for the reaction
Energy/ kJ
C2H6 (g) + Cl2 (g)

- 45 kJ
C2H5Cl (g) + HCl (g)

Axis [1] Label [1] Exothermic [1]

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*CONFIDENTIAL 5

(b) The enthalpy of solution of lead (II) chloride and sodium chloride are + 95 kJ mol-1 and

+7.0 kJ mol-1 respectively. Explain the difference between the solubilities of lead (II)

chloride and sodium chloride in terms of bonding. [5 marks]

• PbCl2 is less soluble than NaCl [1]
• Since Pb is less electropositive than Na [1]
• Difference of electronegativity between Pb and Cl is smaller than Na and Cl [1]
• NaCl has greater ionic character [1]
• PbCl2 has high covalent character[1]

19 (a) An alkaline fuel cell, as shown in diagram below, is used by astronaut as one of the energy
and electrical sources in outer space.

i. Write the reaction equation at cathode, anode and overall equation for the fuel cell above.

[3 marks]

• Cath : O2 + 2H2O + 4e- → 4OH- [1]

• Anode : H2 + 2 OH- → 2 H2O + 2e- [1]

• Overall : 2 H2 + O2 → 2 H2O [1]

ii. Calculate the voltage for the alkaline fuel cell above. [2 marks]

• Eocell = E0cat - E0ano @ + 0.40 - (-0.83) [1]

• = + 1.23 V [1]

iii. A fuel cell can also works under acidic condition to produce the same voltage under

standard condition. Explain the statement above [2 marks]

• Ano : H2 → 2H+ + 2e E0 = 0.0 V [1]
• Cath :4H+ + O2 + 4e- → 2H2O E0 = +1.23V
• Overall 2 H2 + O2 → 2 H2O Ecell = + 1.23 V

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*CONFIDENTIAL 6
[2 marks]
iv. State two advantages of fuel cell compare to other chemical cell.
• Light
• Does not produce toxic

• No air pollutants

(any two of the answers)

(b) Using suitable standard reduction potentials, describe the chemistry involved in the rusting

of iron under acidic environment [6 marks]

• finding both half equation [1]

Oxi : Fe → Fe2+ + 2e- E0 = + 0.44 V

Red : 4H+ + O2 + 4e- → 2H2O E0 = +1.23V

• Overall : 2Fe + 4H+ + O2 →2Fe2+ + 2H2O [1]

• Ecell = + 1.67 V [1]

• Find both equation [1]

Oxi : Fe2+ → Fe3+ + e– E0 = – 0.77 V

Red : 4H+ + O2 + 4e- → 2H2O E0 = +1.23V

• Overall : 4Fe2+ + O2 + 4H+ → 4Fe3+ + 2 H2O [1]

• E0cell = + 0.46 V [1]

• 2 Fe3+ + 4 H2O → Fe2O3.H2O + 6 H+ [1]

(any 6 of the answers)

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*CONFIDENTIAL 7

20 (a) Describe the reactions that occur when chlorine is bubbled through cold and hot, aqueous sodium
hydroxide. Write equations and explain how the oxidation number of chlorine changes during
these reactions. What observation would make it clear that the chlorine had reacted? [6 marks]

• In cold NaOH : 2 NaOH + Cl2 → NaCl + NaOCl + H2O [1]
• oxidation state changes from 0 → +1 ; -1 [1]
• In hot NaOH : 6 NaOH + 3Cl2 → 5 NaCl + NaClO3 + H2O [1]
• oxidation state changes from 0 → +5 ; -1 [1]
• Chlorine undergoes disproportionation reaction. [1]
• The greenish-yellow colour of chlorine gas disappear makes it clear that the

chlorine had reacted, [1]

(b) Table below list the melting points and boiling points of chlorine, bromine and iodine.

Elements Melting point / oC Boiling point / oC
Chlorine - 101 - 34.6
Bromine - 7.2 58.8
Iodine 114 183

Discuss the trend with regard to the structure and bonding of these elements [3 marks]

• Cl2, Br2, I2 are simple molecule hold by weak Van Der Waals forces [1]
• molecular mass/size increase down to group 17 [1]
• this will increased weak Van Der Waals (or intermolecular) forces [1]

(c) Chlorine is often used as disinfectant and bleaching agent in swimming pool. Explain the role

of chlorine as bleaching agent in swimming pool, and the effect of exposing the mixture

under sunlight, hence write suitable equations involved. [6 marks]

• chlorine is oxidised to form HOCl and reduced to HCl [1]

• Cl2 + H2O → HCl + HclO [1]

• HClO act as bleaching agent [1]

• HClO decomposed upon exposed to UV light [1]

• 2 HClO → 2 HCl + O2 [1]

• Overall : 2 Cl2 + 2 H2O → 4 HCl + O2 [1]

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