Physical Chemistry Third Edition

2.6 Calculation of Enthalpy Changes of Processes without Chemical Reactions 85

273.15

Step 2: Reversible process

Irreversible process

Liquid

T/K

Solid

Process 1: Heating of supercooled liquid

Process 3: Cooling of solid

258.15

Figure 2.9 Irreversible and Reversible Paths for Example 2.28.

The total enthalpy change is

∆H∆H 1 +∆H 2 +∆H 3 −10870 J− 10 .87 kJ

Because the process is at constant pressure,q∆H.

PROBLEMS

Section 2.6: Calculation of Enthalpy Changes of Processes
without Chemical Reactions

2.42 a.Calculate∆Hand∆Ufor heating 1.00 mol of argon
from 100 K to 300 K at a constant pressure of 1.00 atm.
State any assumptions.

b.Calculate∆Hand∆Ufor heating 1.00 mol of argon

from 100 K to 300 K at a constant volume of 30.6 L.

c.Explain the differences between the results of parts a

and b.

2.43 a.Findq,w,∆U, and∆Hfor heating 1.000 mol of
neon gas from 273.15 K to 373.15 K at a constant
pressure of 1.000 atm. State any approximations and
assumptions.

b.Findq,w,∆U, and∆Hfor heating 1.000 mol of

neon gas from 273.15 K to 373.15 K at a constant

volume of 22.4 L. State any approximations and

assumptions.

2.44 Supercooled steam is condensed irreversibly but at a

constant pressure of 1.000 atm and a constant temperature

of 96.5◦C. Find the molar enthalpy change. State any

assumptions and approximations.

2.45 The enthalpy change of fusion of mercury is

2331 J mol−^1. Find∆Hfor converting 100.0 g of solid

mercury at−75.0◦C to liquid mercury at 25.0◦Cata

constant pressure of 1.000 atm. Assume that the heat

capacities are constant and equal to their values in

Table A.6 of the appendix.

2.46 Find∆Hif 100.0 g of supercooled liquid mercury at

− 50. 0 ◦C freezes irreversibly at constant temperature and a

constant pressure of 1.000 atm. The enthalpy change of

fusion at the normal melting temperature is 2331 J mol−^1.