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Whenever
we talk about heats of reaction from now on, we shall be using enthalpies
and not strictly molecular energies. You may think of H as standing
for "heat" in reactions carried out at constant external
pressure.
As an example, when one mole of hydrogen gas and one half mole of
oxygen gas react to produce one mole, or 18 grams, of liquid water,
68 kilocalories of heat are given off (see diagram). When this energy
escapes as heat, the total molecular enthalpy decreases by 68 kcal:
H2(g) + �O2(g)
� H2O(l)
+ 68 kcal
or in the more conventional way of writing the heat of reaction,
H2(g) + �O2(g)
� H2O(l)
DH = -68 kcal mole-1
H2O
DH is negative because the enthalpy
of the chemicals taking part in the reaction decreases. Because
heat is given off, this reaction is described as exothermic. A reaction
that absorbs heat is endothermic. If we regard hydrogen gas, with
a molecular weight of 2 grams per mole, as a fuel, then the energy
yield of this reaction is 68/2 = 34 kcal per gram of fuel.
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