Chemistry Reference
In-Depth Information
15.9.
Thermal Properties
15.9.1.
Specific Heat Capacity and Enthalpy
The specific heat capacity of a material is defined as the quantity of
thermal energy required to raise the temperature of unit mass of the material
by 18C; its units are J kg
1
K
1
. In what follows, 'specific heat capacity'
means specific heat capacity at constant pressure, rather than at constant
volume; the distinction between the two is not of practical importance for
solids, or for liquids outside the critical region, and in most practical situa-
tions isobaric conditions exist.
As the specific heat capacity of fluid milk products is temperature
dependent, the amount of heat energy to be added or removed to effect a
given temperature change is given by the equation
q
¼
Z
2
ð
Jkg
1
Þ
c
ðÞ
d
(51)
1
where q is the quantity of heat energy added or removed per kg,
is tempera-
ture (8C),
2
1
is temperature change (
1
to
2
or
2
to
1
) brought about by
adding or removing q and c(
) is specific heat capacity expressed as some
function of temperature (J kg
1
K
1
).
Under isobaric conditions and in the absence of phase transitions,
chemical reactions, changes in composition and forms of work other than
displacement work, the heat added to or removed from a material is equal to
the change in enthalpy of the material (Smith and Van Ness, 1987; Figura and
Teixeira, 2007). Thus
h
¼
Z
2
ð
Jkg
1
Þ
c
ðÞ
d
(52)
1
where h is the enthalpy change per unit mass (J kg
1
).
Absolute values of enthalpy cannot be determined. However, Equation
52 may be used to calculate enthalpy, h, at any temperature,
2
, relative to an
arbitrarily assumed value of h
¼
0 at temperature
1
(the datum temperature
in this case).
Differentiation of Equation 52 gives
c
ðÞ¼
dh
ð
Jkg
1
K
1
Þ
d
ðÞ
(53)
