Concise Physical Chemistry

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

60 THERMOCHEMISTRY

2(286)=572

394

-890

-76

methane

elements

CO 2 + 2H 2 O

FIGURE 4.2 A Thermochemical cycle for determining
fH^298 (methane). Not to scale.

The indirect method shown for methane has been extended to very many hydro-

carbons and other organic compounds. Although many inorganic substances do not

burn, they do react. Inorganic reaction cycles similar to Fig. 4.2 can often be set up

to obtain thermochemical data. A free thermochemical database is maintained by

the National Institutes of Standards and Technology (NIST) of the US government

(.gov). Go to webbook.nist.gov.

4.5 ENTHALPIES OF REACTION

Suppose we know the enthalpies of formation
fH^298 of acetylene CH CH, ethene

CH 2 CH 2 , and ethane CH 3 CH 3 (226.7, 52.5, and−84.7 kJ mol−^1 ) by the combustion

method shown in Fig. 4.1. The enthalpy change
rH^298 of the reaction

CH CH(g)+2H 2 (g)→CH 3 CH 3 (g)

can be found by comparing the level given by
fH^298 (CH CH(g)) with that of


fH^298 (CH 3 CH 3 (g)). (Remember that
fH^298 is zero for elemental hydrogen.)

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

=− 84. 7 − 226. 7 =− 311 .4kJmol−^1

This reaction has been carried out with the result 
rH^298 =− 312. 1 ± 0. 6

kJ mol−^1.

A similar reaction, the partial hydrogenation,

CH CH(g)+H 2 (g)→CH 2 CH 2 (g)

cannot be carried out in the laboratory. Hydrogenation doesn’t stop at CH 2 CH 2 (g),

but goes on to the fully hydrogenated product CH 3 CH 3 (g) or gives a mixed product