12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Heat and Chemical Reactions

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) + ŻO2(g) « H2O(l) + 68 kcal

or in the more conventional way of writing the heat of reaction,

H2(g) + ŻO2(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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