Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 30

Q uest i o n s a n d P ro b lem s

14.22 Using data from Appendix 2, calculate ?S° and
rxn
?Ssurr for each of the reactions in Problem 14.10 and
determine if each reaction is spontaneous at 25°C.
14.23 Using data from Appendix 2, calculate ?S° and
rxn
?Ssurr for each of the reactions in Problem 14.11 and
determine if each reaction is spontaneous at 25°C.
14.24 When a folded protein in solution is heated to a high
enough temperature, its polypeptide chain will unfold
to become the denatured protein—a process known as
“denaturation.” The temperature at which most of the
protein unfolds is called the “melting” temperature. The
melting temperature of a certain protein is found to be
63°C, and the enthalpy of denaturation is 510 kJ/mol.
Estimate the entropy of denaturation, assuming that the
denaturation is a single-step equilibrium process; that
is, folded protein
denatured protein. The single
polypeptide protein chain has 98 amino acids. Calculate
the entropy of denaturation per amino acid.

Section 14.5:? Predicting Spontaneity
Review Questions
14.25 Define free energy. What are its units?
14.26 Why is it more convenient to predict the direction
of a reaction in terms of ?Gsys instead of ?Suniv?
Under what conditions can ?Gsys be used to predict
the spontaneity of a reaction?
14.27 What is the significance of the sign of ?Gsys?
14.28 From the following combinations of ?H and ?S,
predict if a process will be spontaneous at a high or low
temperature: (a) both ?H and ?S are negative, (b) ?H
is negative and ?S is positive, (c) both ?H
and ?S are positive, (d) ?H is positive and ?S is
negative.
Problems
14.29 Assuming that ?H and ?S do not change with
temperature, determine ?G for the denaturation in
Problem 14.24 at 20°C.
14.30 Calculate ?G° for the following reactions at 25°C:
(a) N2(g) + O2(g)
2NO(g)
(b) H2O(l)
H2O(g)
(c) 2C2H2(g) + 5O2(g)
4CO2(g) + 2H2O(l)
(Hint: Look up the standard free energies of formation
of the reactants and products in Appendix 2.)
14.31 Calculate ?G° for the following reactions at 25°C:
(a) 2Mg(s) + O2(g)
2MgO(s)
(b) 2SO2(g) + O2(g)
2SO3(g)
(c) 2C2H6(g) + 7O2(g)
4CO2(g) + 6H2O(l)
(See Appendix 2 for thermodynamic data.)
14.32 From the values of ?H and ?S, predict which of the
following reactions would be spontaneous at 25°C:
reaction A: ?H = 10.5?kJ/mol, ?S = 30 J/K ? mol;
reaction B: ?H = 1.8?kJ/mol, ?S = –113 J/K ? mol.
If either of the reactions is nonspontaneous at 25°C, at
what temperature might it become spontaneous?
14.33 Find the temperatures at which reactions with the
following ?H and ?S values would become spontaneous:
(a) ?H = –126?kJ/mol, ?S = 84 J/K ? mol;
(b) ?H = –11.7 kJ/mol, ?S = –105?J/K ? mol.

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599

14.34 The molar heats of fusion and vaporization of ethanol
are 7.61 and 26.0 kJ/mol, respectively. Calculate the
molar entropy changes for the solid-liquid and liquidvapor transitions for ethanol. At 1 atm pressure, ethanol
melts at –117.3°C and boils at 78.3°C.
14.35 The molar heats of fusion and vaporization of mercury
are 23.4 and 59.0 kJ/mol, respectively. Calculate the
molar entropy changes for the solid-liquid and liquidvapor transitions for mercury. At 1 atm pressure,
mercury melts at –38.9°C and boils at 357°C.
14.36 Use the values listed in Appendix 2 to calculate ?G°
for the following alcohol fermentation:
C6H12O6(s)

2C2H5OH(l) + 2CO2(g)

14.37 Certain bacteria in the soil obtain the necessary energy
for growth by oxidizing nitrites to nitrates:
2NO – + O2
2

2NO–
3

Given that the standard Gibbs free energies of formation
of NO – and NO – are –34.6 and –110.5 kJ/mol,
2
3
respectively, calculate the amount of Gibbs free energy
released when 1 mole of NO – is oxidized to 1 mole
2
of NO –.
3

Section 14.6:?Thermodynamics
in Living Systems
Review Questions
14.38 What is a coupled reaction? What is its importance in
biological reactions?
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