DH = −46.13 kJ mole
−1 , remarkably close to the currently accepted value
3
for the standard heat of formation of ammonia of −46.2 kJ mol
−1 .
Heat capacity therefore had a powerful in uence on enthalpy change but
through the van’t Hoff equation,
@ log e K=@T
ð
Þ p ¼ DH
h
=RT
and its subsequent integration between temperatures T 1 and T 2 ,
log e ðK p2 =K p1 Þ ¼ ÀDH=R 1=T 2 À 1=T 1
ð
Þ
heat capacity change could also be related to equilibrium constants thereby
fully vindicating Le Chatelier’s concerns.
4
3
Chemistry Data Book, J. G. Stark, H .G. Wallace, M. L. McGlashan, 2nd Edition in SI, John Murray
Publishers, 1982. ISBN 0-7195-3951-X.
4
Where here DH now represents the average enthalpy change over the two temperatures.
Appendix A: Heat Capacity and Enthalpy
517