The Foundations of Chemistry

644 CHAPTER 15: Chemical Thermodynamics

*112.(a) Is the reaction C(diamond) nC(graphite) sponta-

neous at 25°C and 1 atm? (b) Now are you worried about

your diamonds turning to graphite? Why or why not?

(c) Is there a temperature at which diamond and graphite

are in equilibrium? If so, what is this temperature?

(d) How do you account for the formation of diamonds

in nature? (Hint:Diamond has a higher density than

graphite.)

Mixed Exercises

*113.An ice calorimeter, shown here, can be used to measure

the amount of heat released or absorbed by a reaction

that is carried out at a constant temperature of 0°C. If

heat is transferred from the system to the bath, some of

the ice melts. A given mass of liquid water has a smaller

volume than the same mass of ice, so the total volume of

the ice and water mixture decreases. Measuring the vol-

ume decrease using the scale at the left indicates the

amount of heat released by the reacting system. As long

as some ice remains in the bath, the temperature remains

at 0°C. In Example 15-2 we saw that the reaction

CuSO 4 (aq)2NaOH(aq)88n

Cu(OH) 2 (s)Na 2 SO 4 (aq)

releases 846 J of heat at constant temperature and

pressure when 50.0 mL of 0.400 MCuSO 4 solution and

50.0 mL of 0.600 MNaOH solution are allowed to react.

(Because no gases are involved in the reaction, the vol-

ume change of the reaction mixture is negligible.)

Calculate the change in volume of the ice and water mix-

ture that would be observed if we carried out the same

experiment in an ice calorimeter. The density of H 2 O(
)

at 0°C is 0.99987 g/mL and that of ice is 0.917 g/mL.

The heat of fusion of ice at 0°C is 334 J/g.

it is easier to measure the standard enthalpy of combus-

tion by reaction of the compound with excess O 2 (g) to

form CO 2 (g) and H 2 O(
). From the following standard

enthalpies of combustion at 25°C, determine 
Hf^0 for the

compound. (a) cyclohexane, C 6 H 12 (
), a useful organic

solvent: 
H^0 combustion3920 kJ/mol; (b) phenol,

C 6 H 5 OH(s), used as a disinfectant and in the production

of thermo-setting plastics: 
H^0 combustion3053 kJ/mol.

*115.Standard entropy changes cannot be measured directly in

the laboratory. They are calculated from experimentally

obtained values of 
G^0 and 
H^0. From the data given

here, calculate 
S^0 at 298 K for each of the following

reactions.

(a) OF 2 (g)H 2 O(g)88nO 2 (g)2HF(g)

oxygen difluoride

H^0 323.2 kJ/mol 
G^0 358.4 kJ/mol

(b) CaC 2 (s)2H 2 O(
)88nCa(OH) 2 (s)C 2 H 2 (g)

calcium carbide acetylene

H^0 125.4 kJ/mol 
G^0 145.4 kJ/mol

(c) CaO(s)H 2 O(
)88nCa(OH) 2 (aq)

H^0 81.5 kJ/mol 
G^0 26.20 kJ/mol

*116.Calculate q, w,and 
Efor the vaporization of 10.0 g of

liquid ethanol (C 2 H 5 OH) at 1.00 atm at 78.0°C, to form

gaseous ethanol at 1.00 atm at 78.0°C. Make the follow-

ing simplifying assumptions: (a) the density of liquid

ethanol at 78.0°C is 0.789 g/mL, and (b) gaseous ethanol

is adequately described by the ideal gas equation. The

heat of vaporization of ethanol is 855 J/g.

*117.We add 0.100 g of CaO(s) to 125 g H 2 O at 23.6°C in a

coffee-cup calorimeter. The following reaction occurs.

What will be the final temperature of the solution?

CaO(s)H 2 O(
)88nCa(OH) 2 (aq)


H^0 81.5 kJ/mol rxn

*118.(a) The accurately known molar heat of combustion of

naphthalene, C 10 H 8 (s), 
H5156.8 kJ/mol C 10 H 8 , is

used to calibrate calorimeters. The complete combustion

of 0.01520 g of C 10 H 8 at constant pressure raises the tem-

perature of a calorimeter by 0.212°C. Find the heat

capacity of the calorimeter. (b) The initial temperature of

the calorimeter (part a) is 22.102°C; 0.1040 g of C 8 H 18 (
),

octane (molar heat of combustion 
H5451.4 kJ/mol

C 8 H 18 ), is completely burned in the calorimeter. Find the

final temperature of the calorimeter.

CONCEPTUAL EXERCISES

*119.When a gas expands suddenly, it may not have time to

absorb a significant amount of heat: q0. Assume that

1.00 mol N 2 expands suddenly, doing 3000 J of work.

*114.It is difficult to prepare many compounds directly from

their elements, so 
Hf^0 values for these compounds can-

not be measured directly. For many organic compounds,

Ice in

water

Insulation

System