6.22 A-quantity of 2.00 X 102 mL of O.862 M HC? is mixed ...

'?

QUESTIONS AND PROBLEMS 233

(i,4 Describe the energy conversions that occur in the fol- heating conditions, which metal would take longer to
lowing processes: (a) You throw a softball up into the reach a temperature of 21oC?
air and catch it. (b) You switch on a flashlight. (c) You 6.15 Define calorimetry and describe two cormnonly used
ride the ski lift to the top of a hill and then ski down.
(d) You strike a match and let it burn down. calorimeters.

ENERGY CHANGES IN CHEMK:AL REACTIONS 6.16 In a calorimetric measurement, why is it important
HBview Questions that we know the heat capacity of the calorimeter?

0,5 What is heat? How does heat differ from thermal en- Problems
ergyt Under what condition is heat transferred from
one system to another? 6.17 A piece of silver of mass 362 g has a heat capacity
of 85.7 J/oC. What is the specific heat of silver?
6.6 Explain the following terms: thermochemistry, sys-
tem, surroundings, open system, closed system, iso- 6.18 A 6.22-kg piece of copper metal is heated from
lated system, exothermic process, endotherrnic 20.5oC to 324.3oC. Calculate the heat absorbed (in
kJ) by the metal.
process.
6.19 Calculate the amount of heat liberated (in kJ) from
6.7 Stoichiometry is based on the law of conservation of 366 g of mercury when it cools from 77.OoC to
mass. On what law is thermochemistry based?
12.OoC.
6.8 Describe two exotherrnic processes and two en-
dothermic processes. 6.20 A sheet of gold weighing 10.0 g and at a tempera-
ture of 1 8.OoC is placed flat on a sheet of iron weigh-
ENTHALPY ing 20.0 g and at a temperature of 55.6oC. What is
the final temperature of the combined metals?
Review Questions Assume that no heat is lost to the surroundings. (Hint:
The heat gained by the gold must be equal to the heat
6.9 Write an expression for the enthalpy of a reaction in lost by the iron.)
terms of the enthalpies of products and reactants.
Under what condition is the heat of a reaction equal 6.21 A O.1375-g sample of solid magnesium is burned in
to the enthalpy change of the same reaction?
a constant-volume bomb calorimeter that has a heat
6.10 In writing thermochemical equations, why is it im-
portant to indicate the physical state (that is, gaseous, capacity of 1769 J/oC. The calorimeter contains ex-
liquid, solid, or aqueous) of each substance? actly 300 g of water, and the temperature increases
by l.l26oC. Calculate the heat given off by the burn-
6.11 Explain the following thermochemical equation: ing Mg, in kJ/g and in kJ/mol.

4NH3(g) + 50;i(g) ? 4NO(g) + 6H20(g) 6.22 A-quantity of 2.00 X 102 mL of O.862 M HC? is

AH = -905 J mixed with 2.00 X 102 mL of O.431 M Ba(OH)2 in
a constant-pressure calorimeter that has a heat ca-
6.12 Consider the following reaction: pacity of 453 J/oC. The initial temperature of the HC?
and Ba(OH)2 solutions is the same at 20.48oC. For
2CH30H(}) + 302(g) -> 4H20(!) + 2CO,(J)
the process
,?p =-1452.8 kJ
H+(aq) + OH (aq) -+ H20(I)
What is the value of bH if (a) the equation is multi-
plied throughout by 2, (b) the direction of the reac- the heat of neutralization is -56.2 kJ. What is the fi-
tion is reversed so that the products become the re-
actants and vice versa, (c) water vapor instead of nal temperature of the mixed solution?
liquid water is formed as the product?
STANDARD ENTHALPY OF FORMATION AND REACTION
CALORIMETRY
Review Queslions
Review Questions
6.23 What is meant by the standard-state condition?
6.13 What is the difference between specific heat and heat 6.24 How are the standard enthalpies of an element and of
capacity? What are the units for these two quantities?
Which is the intensive property and which is the ex- a compound deterrnined?
tensive property? 6.25 What is meant by the standard enthalpy of a reaction?
6.26 Write the equation for calculating the enthalpy of a
6.14 Consider two metals A and B, each having a mass of
100 g and an initial temperature of 20oC. The spe- reaction. Define all the terms.
cific heat of A is larger than that of B. Under the same
6.27 State Hess's law. Explain, with one example, the use-
fulness of Hess's law in thermochemistry.

6.28 Describe how chemists use Hess's law to detern'iine

the b,H( of a compound by measuring its heat (en-

thalpy) of combustion.

234 THERMOCHEMISTRY

Problems Calculate the heats of combustion of these alcohols
in kJ/mol.
6.29 Which of the following standard enthalpy of forma-
tion values is not zero at 25oC? Na(s), Ne(g), CH4(g), 6.40 The standard enthalpy change for the following re-
Ss(s), Hg(I), H(g).
action is 436.4 kJ:
6.30 The b'[-[7 values of the two allotropes of oxygen, 02
and 03, are O and 142.2 kJ/mol, respectively, at 25oC. H2(.!,) ? H(g) + H(g)
Which is the more stable fom'i at this temperature?
Calculate the standard enthalpy of formation of
6.31 Which is the more negative quantity at 25oC: LH7 for atomic hydrogen (H).
H20(}) or fsl[', for H20(g)? 6.41 From the standard enthalpies of fomiation, calculate
Ag'n for the reaction
6.32 Predict the value of LF]7 (greater than, less than, or
C6H12(I) + 902(g) -+ 6CO2(g) + 6H20(})
equal to zero) for these elements at 25oC: (a) Brz(g);
Br2('), (b) Iz(g): I2(S)- For C6H12(I), ul, = -151.9 kJ/mol.
6.33 In general, compounds with negativevebb'HH'i7vvaalues are
6.42 The first step in the industrial recovery of zinc from
more stable than those with positiivveeAbHH'il values. the zinc sulfide ore is roasting, that is, the conversion
H202(7) has a negative bH7 (see Table 6.-3). Why, of ZnS to ZnO by heating:

then, does H202(I) have a tendency to decompose to 2ZnS(s) + 302(g) -+ 2ZnO(s) + 2SO2(g)
H20(}) and 02(g)? AH',,, = -879 kJ
6.34 Suggest ways (with appropriate equations) that would
allow you to measure the bHy values of Ag20(s) and Calculate the heat evolved (in kJ) per gram of ZnS
CaCl2(s) from their elements. No calculations are
roasted.
necessary.
6.43 Deterrnine the amount of heat (in kJ) given off when
6.35 Calculate the heat of decomposition for this process
1.26 X 104 g of ammonia are produced according to
at constant pressure and 25oC:
the equation
CaCO3(s) -? CaO(s) + CO;i(g)
N2(g) + 3H2(g) -+ 2NH3(g) h"':xn = -92.6 kJ
(Look up the standard enthalpy of formation of the
reactant and products in Table 6.3.) Assume that the reaction takes place under standard-
6.36 The standard enthalpies of formation of ions in aque-
ous solutions are obtained by arbitrarily assigning a state conditions at 25oC.

value of zero to H+ ions; that is, Aq[H+(aq)] '0. 6.44 At 850oC, CaCO3 undergoes substantial decomposi-

(a) For the following reaction tion to yield CaO and CO2. Assuming that the LH7

HC?(g)-+H+(aq)+Cl(aq) AHo= -74.9kJ values of the reactant and products are the same at
850oC as they are at 25oC, calculate the enthalpy
calculate LH7 for the Cl ions. (b) Given that AF]7 change (in kJ) if 66.8 g of CO2 are produced in one

for OH ions is -229.6 kJ/mol, calculate the en- reaction.
thalpy of neutralization when 1 mole of a strong
monoprotic acid (such as HC?) is titrated by 1 mole 6.45 From these data,
of a strong base (such as KOH) at 25oC.
6.37 Calculate the heats of combustion for the following s(rhombic) + 02(g) -+ so2(g) Affixn = -296.06 kJ
reactions from the standard enthalpies of formation S(monoclinic) + 02(g) -+ SO2(g)
listed in Appendix 3:
(a) 2Hz(g) + Oz(g) --> 2HzO(l) h"rxn = -296.36 kJ
(b) 2C2H2(,!') + 50z(g) ? 4CO2(.!,) + 2HzO(!)
6.38 Calculate the heats of combustion for the following calculate the enthalpy change for the transformation
reactions from the standard enthalpies of formation
listed in Appendix 3: S(rhombic) ? S(monoclinic)
(a) C:iHa(g) + 302(,!') '-+ 2CO2(,!,) + 2HzO(})
(b) 2H2S(,!') + 30z(g) ? 2H:?O(I) + 2SO2(,!') (Monoclinic and rhombic are different allotropic
6.39 Methanol, ethanol, and n-propanol are three common forms of elemental sulfur.)
alcohols. When 1.00 g of each of these alcohols is 6.46 From the following data,
burned in air, heat is liberated as follows: (a) methanol
(CH30H), -22.6 kJ; (b) ethanol (C2H50H), C(graphite) + O;i(g) ? CO2(g) Agn = -393.5 kJ
-29.7 kJ; (c) n-propanol (C3H70H), -33.4 kJ.
H:i(g) + 2'Oz(g)?Hz0(I) AFI',n= 285.8kJ

2C2H6(,!,) + 70z(g) ? 4CO2(,!,) + 6H20(I)
Ag, = -31l9.6kJ

calculate the enthalpy change for the reaction

1

362 CHEMICAL BONDING l: BASIC CONCEPTS

9.52 Draw two resonance structures for diazomethane, BOND DISS(XIATION ENERGY
CH2N2. Show formal charges. The skeletal structure
Review Questions
of the molecule is
9.65 What is bond dissociation energy? Bond energies Oy
H polyatornic molecules are average values, wherea3
those of diatomic molecules can be accurately detey-
CNN
mined. Why?
H
9.66 Explain why the bond energy of a molecule is us[-
9.53 Draw three reasonable resonance structures for the
OCN ion. Show formal charges. ally defined in terms of a gas-phase reaction. Why
are bond-breaking processes always endotherrnic qj
9.54 Draw three resonance structures for the molecule
bond-forming processes always exothemiic?
N20 in which the atoms are arranged in the order
Problems
NNO. Indicate formal charges.
9.67 From the following data, calculate the average bond
EXCtPTjONS TO THE OCTET RULE
energy for the N-H bond:
Review Questions
NH3(g) -+ NH2(,!,) + H(g) LHo = 435 kJ
9.55 Why does the octet rule not hold for many compounds
containing elements in the third period of the peri- NH2(g) -+ NH(g) + H(,!,) ?o = 381 kJ

odic table and beyond? NH(g) ? N(g) + H(g) AHo = 360 kJ

9.56 Give three examples of compounds that do not sat- 9.68 For the reaction
isfy the octet rule. Write a Lewis structure for each.
O(g)+02(g')?03(g) fsHo= -l07.2kJ
9.57 Because fluorine has seven valence electrons
Calculate the average bond energy in 03.
(2s"2ps), seven covalent bonds in principle could 9.69 The bond energy of F:?(g) is 156.9 kJ/mol. Calculate

form around the atom. Such a compound might be aHi for F(g).
FH7 or FCl7. These compounds have never been pre-
9.70 For the reaction
pared. Why?
9.58 What is a coordinate covalent bond? Is it different 2CzHb(g) + 70zC!') -'-> 4COz(g) + 6H20(,!,)

from a normal covalent bond? (a) Predict the enthalpy of reaction from the average

Problems bond energies in Table 9.4.

9.59 The Al?3 molecule has an incomplete octet around Al. (b) Calculate the enthalpy of reaction from the stan-
dard enthalpies of formation (see Appendix 3) of the
Draw three resonance structures of the molecule in reactant and product molecules, and compare the re-
which the octet rule is satisfied for both the Al and sult with your answer for part (a).

the I atoms. Show formal charges. ADDITIONAL PROBIEMS
9.60 In the vapor phase, beryllium chloride consists of dis-
9.71 Classify the following substances as tonic compounds
crete BeCl2 molecules. Is the octet rule satisfied for or covalent compounds containing discrete mole-
Be in this compound? If not, can you form an octet
around Be by drawing another resonance structure? cules: CH4, KF, CO, SiCl4, BaCl2.
How plausible is this structure?
9.61 0f the noble gases, only Kr, Xe, and Rn are known 9.72 Which of the following are tonic compounds? Which
to form a few compounds with O and/or F. Write are covalent compounds? RbCl, PF5, BrF3, KO2, CI4
Lewis strOctures for the following molecules:
(a) XeF2, (b) XeF4, (c) XeF6, (d) XeOF4, (e) Xe02F2. 9.73 Match each of the following energy changes with one
of the processes given: ionization energy, electron
In each case Xe is the central atom. affinity, bond dissociation energy, and standard en-

9.62 Write a Lewis stmcture for SbCl5,.- Does this mole- thalpy of formation.
cule obey the octet rule'!
(a) F(g) + e -+F (g)
9.63 Write Lewis structures for SeF4 and SeF6. Is the octet
(b) Fz(g) -? 2F(g)
rule satisfied for Se?
(c) Na(g) -+ Na'(g) + e
9.64 Write Lewis structures for the reaction (d) Na(s) + ,'F2(g) ? NaF(s)
9.74 The formulas for the fluorides of the third-period el-
AIC?':i + Cl --?AlCl4 ements are NaF, MgF2, AlF3, SiF4, PF5, SF6, and
ClF3. Classify these compounds as covalent or tonic.
What kind of bond joins Al and Cl in the product?

QUESTIONS AND PROBLEMS 235

2C(graphite) + 3H2(g) -+ C2H6(g) 6.56 Explain what is meant by a state function. Give two
examples of quantities that are state functions and two
0.47 From the following heats of combustion,
that are not.
CHsOH(}) + 23C%(g) ? CO2(,!,) + 2H;iO(})
6.57 The internal energy of an ideal gas depends only on
AH',,, = -726.4 kJ its temperature. Do a first-law analysis of the fol-
C(graphite) + 02(g) -x CO2(g) Agn = -393.5 kJ lowing process. A sample of an ideal gas is allowed
to expand at constant temperature against atmos-
Hz(g) + 2' Oz(g) ? HzO(}) A?n = 285.8 kJ pheric pressure. (a) Does the gas do work on its sur-
roundings? (b) Is there heat exchange between the
calculate the enthalpy of formation of methanol system and the surroundings? If so, in which direc-
(CH30H) from its elements: tion? (c) What is AE for the gas for this process?

C(graphite) + 2H2(g) + 402(g) -+ CH30H(0 6.58 At constant pressure, in which of the following reac-
tions is work done by the system on the surround-
6.48 Calculate the standard enthalpy change for the reac- ings? By the surroundings on the system? In which

tion of them is no work done?

2Al(s) + Fe203(s) -+ 2Fe(s) + Al203(s) (a) Hg(}) -+ Hg(g)
(b) 30z(g) '? 2C%(g)
given that (c) CuSO4 a 5H20(s) -+ CuSO=i(s) + 5H20(g)
(d) Hz(g) + Fz(g) -+ 2HF(g)
2Al(s)+3HO2(g)-+Al203(s) LHo,n= -160lkJ
Problems
2Fe(s) + y302(g) -? Fe203(s) AP,n = -821 kJ
6.59 A gas expands and does P-V work on the surround-
HEAT OF SOLUTION AND DIIUTION ings equal to 325 J. At the same time, it absorbs 127 J
of heat from the surroundings. Calculate the change
Review Questions in energy of the gas.

6.49 Define the following terms: enthalpy of solution, hy- 6.60 The work done to compress a gas is 74 J. As a result,
dration, heat of hydration, lattice energy, heat of di- 26 J of heat is given off to the surroundings. Calculate
the change in energy of the gas.
lution.
6.61 Calculate the work done when 50.0 g of tin are dis-
6.50 Why is the lattice of a solid always a positive quan-
tity? Why is the hydration of ions always a negative solved in excess acid at 1.00 atm and 25oC:
quantity?
Sn(s) + 2H+(aq) -+ Sn?'+(aq) + H2(g)
6.51 Consider two tonic compounds A and B. A has a
larger lattice energy than B. Which of the two com- Assume ideal gas behavior.
pounds is more stable? 6.62 Calculate the work done in joules when 1.0 mole of

6.52 Mg2+ is a smaller cation than Na+ and also carries water vaporizes at 1.0 atm and lOOoC. Assume that
the volume of liquid water is negligible compared
more positive charge. Which of the two species has with that of steam at lOOoC and ideal gas behavior.
a larger hydration energy (in kJ/mol)? Explain.
6.53 Consider the dissolution of an tonic compound such Additional Pmblems
as potassium fluoride in water. Break the process into
the following steps: separation of the cations and an- 6.63 The convention of arbitrarily assigning a zero en-
ions in the vapor phase and the hydration of the ions thalpy value for the most stable form 6f each element
in the aqueous medium. Discuss the energy changes in the standard state of 25oC is a convenient way of
associated with each step. How does the heat of so- dealing with enthalpies of reactions. Explain why this
lution of KF depend on the relative magnitudes of convention cannot be applied to nuclear reactions.
Uhese two quantities? On what law is the relationship
6.64 Consider the following two reactions:
based?
A ? 2B AH',, = LH,
6.54 Why is it dangerous to add water to a concentrated A -+ C hr,, = bH2
acid such as sulfuric acid in a dilution process?
Determine the enthalpy change for the process
FIRST IAW OF THERMODYNAMICS
2B -+ C
Review Questions
6.65 The standard enthalpy change b.Ho for the thermal
6.55 0n what law is the first law of thermodynamics decomposition of silver nitrate according to the fol-
based? Explain the sign conventions in the equation
aE = q + w.

m

750 ENTROPY, FREE ENERGY, AND EQUILIBRIUM

8. Most biological reactions are nonspontaneous. They are driven by the hydrolysis of ATP,

for which LGo is negative.

KEY WORDS Second law of Standard free energy of Third law of
reaction (AGo), p. 738
Entropy (S), p. 727 thermodynamics, p. 731 thermodynamics, p. 734
Free energy (Q), p. 737 Standard entropy of reaction, Standard free energy of
Gibbs free energy ((7), formation (AG7), p. 738
p.731
p. 737

QUESTIONS AND PROBLEMS

SPONTANEOUS PROCESSES AND ENTROPY 18.8 State the third law of thermodynamics and explain its

Review Questions usefulness in calculating entropy values.

18.1Explainwhatismeantbyaspontaneousprocess.Give Problems

two examples each of spontaneous and nonsponta- 18.9 For each pair of substances listed here, choose the

neous processes. one having the larger standard entropy value at 25oC.
The same molar amount is used in the comparison.
18.2 Which of the following processes are spontaneous Explain the basis for your choice. (a) Li(s) or Li(7);
and which are nonspontaneous? (a) dissolving table (b) C2H50H(}) or CH30CH3(I) (Hint: Which mole-
salt (NaCl) in hot soup; (b) climbing Mt. Everest; (c)
spreading fragrance in a room by removing the cap cule can hydrogen-bond?); (c) Ar(g) or Xe(g);
from a perfume bottle; (d) separating helium and neon (d) CO(g) or COz(g); (e) O:i(g) or C%(g); (f) NC%(g)
from a mixture of the gases or NxC%C!')
18.10 Arrange the following substances (1 mole each) in
18.3 Which of the following processes are spontaneous
order of increasing entropy at 25oC: (a) Ne(g), (b)
ta-un;ed'7.which are no--n-s-p-'o-n--ta--n-e-o--u-s-a-t -a ?g'iv=e-n--te-m--p-'e-r-a--

(a) NaNO?,(s)H?,ONaNO,,(aq) saturated soln SO2(g), (c) Na(s), (d) NaCl(s), (e) NH:i(g)- Give the
reasons for your arrangement.

(b) NaNO3(s)Hl?,ONaNO3(aq) unsaturated soln 18.11 Using the data in Appendix 3, calculate the standard
(c) NaNO:i(s) H-?20NaNO,,(aq) supersaturated soln entropy changes for the following reactions at 25oC:
(a) S(S) + O:z(g) -'-> SOz(g)

18,4 Define entropy. What are the units of entropy? (b) MgCO3(s) -? MgO(s) + CO2(g)

18.5 How does the entropy of a system change for each 18.12 Using the data in Appendix 3, calculate the standard

of the following processes? entropy changes for the following reactions at 25oC:
(a) A solid melts. (a) H2(g) + CuO(s) -+ Cu(s) + H20(g)

(b) A liquid freezes. (b) 2Al(s) + 3ZnO(s) -+ AlzO:i(s) + 3Zn(s)

(c) A liquid boils. (C) CHa(g) + 20z(g) -'-> COz(g) + 2HzO(I)

(d) A vapor is converted to a solid. 18.13 Without consulting Appendix 3, predict whether the

(e) A vapor condenses to a liquid. entropy change is positive or negative for each of the

(f) A solid sublimes. following reactions. Give reasons for your predic-

(g) Urea dissolves in water. tions.

Problems (a) 2KClO,,(s) ? 2KClO,,(s) + 02(g)
(b) H20(.!,) -'-> HzO(l)

18.6 Referring to the setup in Figure 18.1(a), calculate the (c) 2Na(s) + 2H20(}) -+ 2NaOH(aq) + H2(g)
probability of all the molecules ending up in the same (d) N2(,!,) -+ 2N(g)
flask if the number is (a) 6, (b) 60, (c) 600. 18.14 State whether the sign of the entropy change expected
for each of the following processes will be positive

THE SECOND LAW OF THERMODYNAMICS or negative, and explain your predictions.
(a) PCl3(I) + Cl2(g) -+ PC?s(s)
Review Questions (b) 2HgO(s) -+ 2Hg(I) + 02(,!,)
(C) H2(,!,) -? 2H(g)
18.7 State the second law of thermodynamics in words and (d) U(s) + 3F2(g) ? UF6(s)
express it mathematically.

+

I

QUESTIONS AND PROBLEMS 751

GIBBS FREE ENERGY 2HzO(g) :? 2H:?(.!') + Oz(g)

Rev;ew Questions 18.27 (a) Calculate bGo and KP for the following equilib-
rium reaction at 25oC. The LG7 values are O for
18.15 Define free energy. What are its units? Cl2(g), -286 kJ/mol for PC?:i(g), and -325 kJ/mol

18.16 Why is it more convenient to predict the direction of for PCl5(g).

awhreaat cctoionndiitniotnesrmcasnofLAGGsyYsSsi'nbseteuasdeodf 'univ? Under PClb(g) ? PC?:t(!') + Clz(g)
spontaneity of a reaction?
to predict the (b) Calculate AG for the reaction if the partial pres-

Problems sures of the initial mixture are Pp,?l5 = 0.0029 atm,

18.17 Calculate bGo for the following reactions at 25oC: Pp(.13 = 0.27 atm, and P(212 = 0.40 aim.
18.28 The -equilibrium constant ;Kp) for the reaction
(a) N2(.!,) + Oz(g) -+ 2NO(g)
Hz(g) + CO2(,!') ':? HzO(g) + CO(,!')
(b) HzO(!) -'-> H20(.!,)
is 4.40 at 2000 K. (a) Calculate bGo for the reaction.
(C) 2CzHz(g) + 502(,!,) -'-> 4CO2(,!,) + 2H;iO(I) (b) Calculate AG for the reaction when the partial
(Hint: Look up the standard free energies of forma-
tion of the reactants and products in Appendix 3.) pressures are PH2 '- 0.25 atm, P(2@2 = 0.78 atm,
i8.l8 Calculate LGo for the following reactions at 25oC: PH20 = 0.66 atm, ffid P(20 = 1.20 atm.
18.29 Co;sider the decomposition of calcium carbonate:
(a) 2Mg(s) + 02(g) -+ 2MgO(s)
CaCO3(s) '? CaO(s) + CO2(g)
(b) 2SOz(g) + Oz(g) -+ 2SO:iC!')
(c) 2CzHb(g) + 702(g) -€ 4CO2(,!,) + 6HzO(I) Calculate the pressure in atm of CO2 in an equilib-
See Appendix 3 for thermodynarnic data. rium process (a) at 25oC and (b) at 800oC. Assume

18.19 From the values of AH and LS, predict which of the that ?o = ?77.8 kJ and LSo = 160.5 J/K for the
following reactions would be spontaneous at 25oC:
temperature range.
Reaction A: AH = 10.5 kJ, AS = 30 J/K; reaction B:
LH = 1.8 kJ, AS = -113 J/K. If either of the reac- 18.30 The equilibrium constant Kp for the reaction

tions is nonspontaneous at 25oC, at what temperature CO(g) + Clz(g) '? COC?;i(g)
might it become spontaneous?
!8.20 Find the temperatures at which reactions with the fol- is 5.62 X 1035 at 25oC. Calculate AG7 for COCl2 at
lowing LH and AS values would become sponta-
neous: (a) LH = -126 kJ, AS = 84 J/K; (b) LH = 25oC.

-11.7 kJ, AS = -105 J/K. 18.31 At 25oC, biGo for the process

FREE ENERGY AND CHEMICAL ECaUlLlBRlUM HzO(I) '=' H20(.!,)

Review Questions is 8.6 kJ. Calculate the "equilibrium constant" for the

18.21 Explain the difference between AG and LGo. process.
18.22 Explain why Equation (18.10) is of great importance
18.32 Calculate LGo for the process
in chemistry.
C(diamond) -+ C(graphite)
Problems
Is the reaction spontaneous at 25oC? If so, why is it
18.23 Calculate Kp for the following reaction at 25oC: that diamonds do not become graphite on standing?

H2(g) + I2(g) ? 2HI(,!,) fsGo = 2.60 kJ THERMODYNAMICS IN LMNG SYSTEMS
N8.24 For the autoionization of water at 25oC,
Review Questions
H20(I) ? H"(aq) + OH (aq:)
18.33 What is a coupled reaction? What is its importance
KW is 1.0 X 10 14. What is LGo for the process?
in biological reactions?
18.25 Consider the following reaction at 25oC: 18.34 What is the role of ATP in biological reactions?

Fe(OH);i(s) ? Fe2'(aq) + 20H (aq) Problems

Calculate b9o for the reaction. Ksp for Fe(OH)2 is 18.35 Referring to the metabolic process involving glucose
on p. 747, calculate the maximum number of moles
1.6 X 10 "'. of ATP that can be synthesized from ADP from the

N8.26 Calculate LGo and Kp for the following equilibrium breakdown of one mole of glucose.

reaction at 25oC. 18.36 In the metabolism of glucose, the first step is the con-

QUESTIONS AND PROBLEMS 793

19.17 Which species in each pair is a better oxidizing agent THE EFFE€:T OF CONCENTRATION ON CELL EMF

under standard-state conditions ? (a) Br2 or Au3+, (b) Review Questions
Hz or Ag+, (c) Cd2+ or Cr3+, (d) 02 in acidic me-
19.27 Write the Nernst equation and explain all the terms.
dia or 02 in basic media 19.28 Write the Nernst equation for the following
19,10 Which species in each pair is a better reducing agent
processes at some temperature 7:a
under standard-state conditions ? (a) Na or Li, (b) Hz
(a) Mg(s) + Sn"+(a4) ? Mg2+(aq) + Sn(s)
or I2, (c) Fe2+ or Ag, (d) Br or Co2+
(b) 2Cr(s) + 3Pb"+(aq) ? 2Cr3+(aq) + 3Pb(s)
SPONTANEITY OF REDOX REACTIONS
Problems
fleview Questions
19.29 What is the potential of a cell madle up of 2Zn/Zn"+
19.19 Write the equations relating AGo and K to the stan- and Cu/Cu2' half-cells at 25oC if [(Zznn22++])=, 0.25 M
and [Cu2'] = 0.15 A/?
dard emf of a ceM. Define all the terms.
19,30 Calculate Eo, E, and AG for the following cell re-
19.20 Compare the ease of measuring the equilibrium con-
stant electrochemically with that by chemical means actions.
[see Equation (18.10)].
(a) Mg(s) + Sn"+(aq) -+ Mg"+(aq) + Sn(s)
Problems [Mg2"] = 0.045 M, [Sn2+] = 0.035 M
(b)-3Zn(s) + 2Cr3+(aq) ? 3Zn"+(aq) + 2Cr(s)
19.21 What is the equilibrium constant for the following (cr")4 o.oio rvr, (zn2j = 0.0085 w

reaction at 25oC? 19.31 Calculate the standard potential of the cell consist-

Mg(s) + Zn2+(aq) ? Mg2+(aq) + Zn(s) ing of the Zn/Zn2+ halfHcell and the SHE. What will

19,22 The equilibrium constant for the reaction th; emf of the cell be if tzn2+) = 0.45 M, PH2 =

Sr(s) + Mg2+(aq) ? Sr"+(aq) + Mg(s) 2.0 atm, and [H+] = 1.8 M?
19,32 What is the ernf of a cell consisting of a Pb/Pb2+
is 2.69 X 1@12 at 25oC. Calculate Eo for a cell
half-cell and a Pt/H2/H+ half-cell if [Pb2+] =
made up of Sr/Sr2+ and Mg/Mg2+ half-cells. 0.10 M, [H+] = 0.050 M, and PH2 = 1.0 atm?
19.33 Referring to the arrangement in Figure 19.1, calcu-
19.23 Use the standard reduction potentials to find the late the [Cu"+]/[Zn"+] ratio at which the following
equilibrium constant for each of the following re-
reaction is spontaneous at 25oC:
actions at 25oC:
Cu(s) + Znz+(aq) ? Cu2+(aq) + Zn(s)
(a) Br2(}) + 2I (aq) -? 2Br (aq) + I2(s)
19,34 Calculate the ernf of the following concentration
(b) 2Ce"+(aq) + 2Cl (aq) ?
'- - Cl2(!) + 2Ce3+(aq) cell:

(c) 5Fe2+(aq) + Mn04 (aq) + 8H"(aq) ? Mg(s)l Mg?'+(0.24 M) 11 Mg2+(0.53 M)i Mg(s)
Mn2+(aq) + 4H20 + 5Fe3+(aq)
BATTERIES AND FUEL CELLS
19,24 Calculate LGo and Kc for the following reactions at
Review Questions
25oC:
19.35 Explain the differences between a primary electro-
(a) Mg(s) + Pb"+(aq) ? Mg2+(aq) + Pb(S) chemical cell-one that is not rechargeable-and a
storage cell (for example, the lead storage battery),
(b) Br2(I) + 2I (aq) ? 2Br (aq) + I2(s) which is rechargeable.

(c) 0:?(g) + 4H+(aq) + 4Fe"+(aq) ? 19.36 Discuss the advantages and disadvantages of fuel
cells over conventional power plants in producing
22+eaq electricity.

(d) 2Al(s) + 3I2(s) ? 2Al3+(aq) + bI (aq) Problems

19.25 Under standard-state conditions, what spontaneous 19.37 The hydrogen-oxygen fuel cell is described in
reaction will occur in aqueous solution among the Section 19.6. (a) What volume of H2(g), stored at
ions Ce4+, Ce3+, Fe3+, and Fe"+? Calculate AGo 25oC at a pressure of 155 atm, would be needed to
and Kc for the reaction.

19,26 Given that Eo = 0.52 V for the reduction Cu+(aq)

+ e -? Cu(s), calculate Eo, AGo, and K for the
following reaction at 25oC:

2Cu+(aq) ? Cu"+(aq) + Cu(s).