Chemistry, Third edition

238 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

Use of bond enthalpies: fuels – where does the heat come

from?

The burning of methane in air follows the equation:

CH 4 (g)2O 2 (g) CO 2 (g)2H 2 O(g) (13.18)

We know that heat is given out during this reaction. But where exactly does the heat

energy come from? We will now calculate the enthalpy change for this reaction using

mean bond enthalpies. To do this we carry out the following operations:

1.We start by writing out the molecular structures for the reactants and products in

full (Fig. 13.9). We see that going from left to right in the equation, the reaction

involves the breaking of four C–H bonds and two O=O bonds and the making of two

C=O bonds and of four O–H bonds.

Fig. 13.9The burning of methane in air.

2.We imagine that the oxygen and methane molecules are completely broken up

into atomsduring the reaction, and that the resulting isolated atoms are then

completely assembledinto the product water and carbon dioxide molecules. (It does

not matter whether or not the reaction really follows this mechanism because,

according to Hess’s law, the results of ‘our energy count’ are independent of how the

molecules actually react or whether or not the products are made in several inter-

mediate steps.)

3.The overall enthalpy change for reaction (13.18) is obtained by adding the

enthalpy changes involved in the breaking up of the reactant molecules and in the sub-

sequent formation of the product molecules. The breaking up of molecules involves

positive enthalpy changes because energy needs to be absorbed in order to break apart

atoms forming a covalent bond. The forming of bonds involves negative enthalpy

changes because of the extra stability achieved by the atoms after combination.

Table 13.5 shows the arithmetic involved in using bond enthalpies to work out

whether, overall, heat is taken in or given out in reaction (13.18). In this calculation,

Table 13.5Estimating the enthalpy change for the burning of methane

Bonds Energy change Totals*/kJ mol^1

Four C–H bonds broken 4  412 1648 kJ taken in  1648

Two O=O bonds broken 2 497 kJ taken in  994

Two C=O bonds made 2 803 kJ given out  1606

Four O–H bonds made 4 463 kJ given out  1852

Estimated enthalpy changeH^ —f = 816

* Positive value indicates heat taken in, negative value indicates heat given out.