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7.THERMOCHEMISTRY eLearn.Punjab

As a result, of above mentioned problems, the chemists had to look for methods of obtaining
standard enthalpies of formation indirectly. The energy cycle shows two routes for converting
graphite and oxygen to CO2, whilst the alternative route goes via CO. It would seem reasonable
that the overall enthalpy change for the conversion of graphite to CO is independent of the route
taken, that is,

DH = DH1+DH2 ....................... (8)

If the enthalpy of combustion for graphite to form CO2 and the enthalpy of combustion of CO
to form CO2 are known, we can determine the enthalpy of formation for CO. To clear the idea look

at the following cycle. The oxidation of carbon (graphite) can be written as follows.

C(graphite) + O2(g) → CO2(g) DH (graphite) =-393.7kJ mol-1

CO(gas) + 1 O2(g) → CO2(g) DH2 (CO)=-283kJ mol-1
2

1 DH1(CO) =?
C(graphite) + 2 O2(g) → CO(g)

Applying equation (8) DH = DH1 + DH2

or DH1 = DH - DH2 C + O2 ΔH → CO2

= -393- (-283) DH1 1 O2 1 O2 DH 2
2 2

= -110kJ mol-1 CO

So, the enthalpy change for the formation of CO(g) is -110.0 kJmol-1.
The method we have just used in obtaining equation (8), is a specific example of Hess’s law of
constant heat summation. This law states that
If a chemical change takes place by several different routes, the overall energy change is the
same, regardless of the route by which the chemical change occurs, provided the initial and final
conditions are the same.

Let A can be converted to D directly in a single step and heat evolved is DH. If the reaction can
have a route from A → B → C as shown below.

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