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22 Chapter 2
1. Read the question carefully.
2. Identify all the species involved, i.e. the reactants, the products
and their states, i.e. are they in the solid (s), liquid (1) or gaseous
(g) phase? Remember also that hydrocarbons (compounds con-
taining hydrogen and carbon only, such as CH4, C2H6 etc.) burn
in oxygen to form carbon dioxide and water:
e.g. CH4(g) + 202,g) + C02(g) + 2H20(1);
C2H6(g) 3.502(g) + 2co2(g) + 3H20(1).
3. Write a chemical equation for the reaction in question, and
balance it.
4. Identify the data given in the question, and write any corre-
sponding formation reactions of the species from their component
elements.
5. Identify the unknown in the question, e.g. AH:xn, AH:, etc.
6. Examine each of the equations in step 4, and re-arrange them so
that the reactants and products required in step 5 are on the
same side as those in step 3. However, if the direction of a
reaction is changed, the sign of AH also changes.
7. Now add the reactions, and find the sum of their enthalpies, by
applying Hess's Law, i.e. if a reaction is the sum of two or more
reactions, then AH,,, = AH( 1) + AH(2) + . . . etc.
8. Answer any riders to the question.

WORKED EXAMPLE USING THE WORKING METHOD OF
HESS'S LAW
Example: Use Hess's Law to determine the standard enthalpy of
combustion of methane (natural gas), CH4(g), given the following
data: AH,"(CO,(,)) = -393.51 kJ mol-', AH,"(H20(1)) = -285.83
kJ mol-' and AH;(CH4(g,) = -74.81 kJ mol-'.

1. Read the question carefully4ombustion is involved, and all the
data given refer to standard enthalpies of fonnation!
2. Identify the species involved (remember hydrocarbons burn in
oxygen to form carbon dioxide and water-don 't forget this!):
CHqg), 02(g), C02(g) and H20(1).
3. Write a balanced chemical equation for the combustion reaction:
CHq,) + 02(g) C02(,) + H20(1) . . . not yet balanced!
+
CHqg) + 202(,) + C021,) + 2H20(1) . . . balanced!