
74 Fuel Cells
3.9 Specific Heat
Specic heat is dened as the energy needed to raise the temperature of a
unit mass by a unit degree temperature. For constant volume process δw =
pdv = 0 and the rst law reduces to δQ = dU. The constant volume specic heat
is expressed as
C
m
Q
T
u
T
v
v
v
=
=
∂
∂
1 δ
δ
. (3.45)
For a constant pressure process, δw = pdv and the rst law reduces to δQ =
dU + PdV = dH. The constant pressure specic heat is expressed as
C
m
Q
T
h
T
p
p
p
=
=
∂
∂
1 δ
δ
. (3.46)
3.10 Estimation of Change in Enthalpy, Entropy,
and Gibbs Function for Ideal Gases
For ideal gases, enthalpy and internal energy are a function of temperature
only. Hence