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Thermodynamics 11 27
6. Substitute the values, including the stoichiometry factors:
AH: = ('12 x (-393.5) + 11 x (-285.8)]) - ([I x (-2219)
+ 12 x (O)]) = - 5646.8 kJ mol- '.
Answer: AH," = -5646.8 kJmol-'

HEAT CAPACITY
The heat capacity of a body, C, is the amount of heat required to raise
the temperature of that body by 1 kelvin. The spec@ heat capacity, c,
is the amount of heat required to raise the temperature of 1 gram of a
body by 1 kelvin, i.e. C = c x molar mass (M). In general,
heat lost or gained = mass x specific heat capacity x change in
temperature of a body, i.e. q = rncAT, from which c = q/(mAT).

Summary: Heat capacity C= q/AT JK-'
Specific heat capacity c = q/(mAT) J g-' K- '

Therefore, Cm,p is the amount of heat required to raise the tempera-
ture of I mole of a substance by 1 K, at constant pressure, and Cm,v is
the amount of heat required to raise the temperature of I mole of a
substance by 1 K, at constant volume. From this, two important
equations can be derived:
I. At constant pressure: AU = qp + wP.
But, the work done by the system on the surroundings (expansion
work) at constant pressure is -PA V, as shown in Chapter 2.
+ AU= qp -pAV+ qp = AU+pAV= AH. But, since qp = nCm,pAT,
this must equal AH, i.e. qp = AH = nCm,pAT.

2. At constant volume and constant pressure:AH= AU+pA V= AU,
since A V= 0.
Hence, qv = nCm,vAT = AU.

Summary: 1. At constant pressure: qp = AH = nCm,,AT
2. At constant volume: qv = AU = nC,,"AT

No. I; When a flask containing 500 g of water is heated,
the temperature of the water increases from 25 "C to 75 "C.
Determine the amount of heat the water absorbs, given that the
specific heat capacity of water is 4.184 J g- ' K-