Concise Physical Chemistry

c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

ENTHALPIES OF REACTION 61

under all known experimental conditions. That doesn’t matter because of Hess’s law.

The enthalpies of formation yield

rH^298 =
fH^298 (CH 2 CH 2 (g))−
fH^298 (CH CH(g))

= 52. 5 − 226. 7 =− 174 .4kJmol−^1

An equivalent way of finding
rH^298 of partial hydrogenation is by measuring

the enthalpy change−136.3 kJ mol−^1 for the reaction

CH 2 CH 2 (g)+H 2 →CH 3 CH 3 (g)

When this is done, one has a first law triangle like that of Fig. 4.1, with only
rH^298

of partial hydrogenation yet to be determined. It is

rH^298 (partial)=− 312. 1 −(− 136 .3)=− 175 .8kJmol−^1

as compared to the previous value of−174.4 kJ mol−^1.

One can also find the enthalpy of hydrogenation of ethene:

rH^298 =
fH^298 (CH 3 CH 3 (g))−
fH^298 (CH 2 CH 2 (g))

=− 84. 7 − 52. 5 =− 137 .2kJmol−^1

This is a complete reaction to a well-defined final state. It has been carried out in the

laboratory. The experimental value is− 136. 3 ± 0 .2kJmol−^1.

Although, for simplicity, we shall use hydrocarbons to illustrate the principles

of thermochemistry, there is no such restriction in practice. For example, Pitzer

et al. (1961) give the standard enthalpies of formation
fH◦(KCl)=− 435 .9 and


fH◦(KClO 3 )=− 391 .2kJmol−^1. (The superscript◦indicates the standard state in

this notation; please do not confuse it with a superscripted zero of temperature 0.)

These values lead to the enthalpy of reaction
rH◦necessary to convert solid KCl(s)

to solid KClO 3 (s):

KCl(s)+^32 O 2 (g)→KClO 3 (s)

rH◦=− 391. 2 −(− 435 .9)= 44 .7kJmol−^1

Including the notation (s) to indicate the solid state is a precaution rather than a

necessity because in the standard states, both KCl and KClO 3 aresolids.

Even from these few simple examples, it should be clear that with a sufficient

database of enthalpies of formation, it is possible to calculate the enthalpies of an

almost limitless array of chemical reactions of industrial, medical, and pharmaceutical

importance. This is the reason why so much effort has gone into populating the