Heat Changes in Chemical Reactions.
The subject of thermochemistry deals with the heat changes accompanying chemical reactions. As will be seen shortly the laws of thermochemistry are based-largely on the principle of the conservation of energy or the first law of thermodynamics. Different substances have different amounts of internal (chemical) energy, and so the total energy of the products of a reaction is generally different from that of the reactants; hence, the chemical change will be accompanied by the liberation or absorption of energy, which may appear in the form of heat. If heat is liberated in the reaction the process is said to be exothermic, but if heat is absorbed it is described as endothermic. The majority of, although not all, chemical reactions which go to virtual completion at ordinary temperatures are exothermic in character, since they are accompanied by an evolution of heat. If a chemical reaction is associated with a volume change, as is particularly the case for many processes involving the combination of gases, the magnitude of the heat change will depend on whether the reaction is carried out at constant pressure or at constant volume. Since many reactions normally occur at constant (atmospheric) pressure it is the usual practice to record heat changes by quoting the value of qp, the heat absorbed at constant pressure; this may, of course, be identified with ΔH , the increase of heat content under the same conditions. This quantity is often referred to as the heat of reaction; it represents the difference in the heat contents of the reaction products and of the reactants, at constant pressure and at definite temperature, with every substance in a definite physical state. From the value of qp (or ΔH) the value of gv (or ΔE) can be readily determined if the volume change ΔV at the constant pressure P is known as will be seen below.
The heat change accompanying a reaction, for example, that between solid carbon (graphite) and gaseous oxygen to yield carbon dioxide gas, is represented in the form of a thermochemical equation, as follows:
C(s) + 02(g) = C02 (g) ΔH = -94.00 kcal.