Physical Chemistry Third Edition

2.8 Calculation of Energy Changes of Chemical Reactions 101

b. Compute the standard-state enthalpy change for this

reaction at 298.15 K and at 373.15 K, using enthalpy

changes of formation and heat capacity data and

compare with your result from part a.

c.Estimate the enthalpy change of the following reaction,

using average bond energies:

2 CO(g)+O 2 (g)−→2CO 2 (g)

2.60 Estimate the enthalpy changes of the following reactions,
using average bond energies. Compare with the value
obtained from enthalpy changes of formation if possible.
a.N 2 O 4 (g)−→2NO 2 (g)
b. 2SO 2 (g)+O 2 (g)−→2SO 3 (g)
c.SO 3 (g)+H 2 O(g)−→H 2 SO 4 (g)

2.61 Estimate the enthalpy changes of the following reactions,
using average bond energies. Compare with the value
obtained from enthalpy changes of formation if possible.
The P–Cl bond energy is not included in Table A.9, but you
can assume a value of 160 kJ mol−^1.
a.PCl 5 (g)−→PCl 3 (g)+Cl 2 (g)
b. H 2 (g)+F 2 (g)−→2 HF(g)

2.62 a.Find∆H◦for the complete combustion of 1.000 mol of
gaseous acetylene (ethyne) in gaseous oxygen to form
gaseous carbon dioxide and liquid water at 298.15 K.
b.Find∆H◦for the complete combustion of 1.000 mol of
gaseous acetylene (ethyne) in gaseous oxygen to form
gaseous carbon dioxide and gaseous water at 373.15 K.

c.Estimate∆Hfor the reaction using average bond

energies, assuming gaseous water as one of the

products.

2.63 a.Calculate∆H◦for the reaction at 298.15 K:

PCl 5 (g)−→PCl 3 (g)+Cl 2 (g)

b.Calculate∆Ufor the reaction at 298.15 K.

c.Use the result from part b to estimate the average bond

energy of a P–Cl bond, using the value of 244 kJ mol−^1

for the CI–CI bond.

2.64 a.Find∆H◦for the reaction at 298.15 K:

CH 4 (g)+2O 2 (g)−→CO 2 (g)+2H 2 O(g)

b.Find∆U◦for the same reaction at 298.15 K.

c.Find∆H◦and∆U◦for the same reaction at 373.15 K.

d.Estimate∆U◦for the same reaction using average

bond energies. Comment on the comparison of this

value with the correct values.

2.65 For the reaction C 2 H 2 (g)+2H 2 (g)−→C 2 H 6 (g)

a. Calculate∆H◦at 298.15 K.

b. Find∆U◦for the same reaction at 298.15 K.

c.Estimate∆Uusing average bond energies.

d.Calculate∆H◦at 373.15 K. State any assumptions and

approximations.

Summary of the Chapter

The first law of thermodynamics is a form of the law of conservation of energy. This

law definesU, the internal energy of a system, through

∆Uq+w

whereqis the heat added to the system andwis the work done on the system. The

internal energy is a state function of the macroscopic state of the system. Techniques

were introduced to carry out several kinds of useful calculations, including work done

and heat transferred in various processes, including isothermal and adiabatic processes

in ideal gases and real gases.

The enthalpy,H, was defined by

HU+PV