PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 95

For an isothermal process, equation (3.74) becomes

H(T, p) =H(T, p 1 ) +

∫p

p 1



V−T

(

∂V

∂T

)

p



dp , [T]. (3.77)

3.5.3.2 Ideal gas.

For an ideal gas, the partial derivative of enthalpy with respect to pressure is zero, as follows
from (3.57). The enthalpy of an ideal gas is thus a function of temperature only (isothermal
processes in an ideal gas are identical with processes at constant enthalpy). Equation (3.76)
for an ideal gas is

H◦(T) =H◦(T 1 ) +

∫T

T 1

Cp◦(T) dT. (3.78)

3.5.3.3 Changes at phase transitions

Enthalpy changes at phase transitions are usually known experimentally.

Example

Derive the pressure dependence of enthalpy at constant temperature for a gas obeying the pressure

virial expansion with the second virial coefficient [see equation (2.20)], i.e. the equation of state

z= 1 +

B

RT

p ,

whereB=f(T)is the second virial coefficient.