Thermodynamics of Chemical Reactions

Heats of Reaction Heat of reaction is the energy absorbed or given out by the system when the products of the reaction are restored to the same temperature and pressure as the reactants. If the pressure and temperature are the same for both the reactants and the products, the heat of reaction is the enthalpy change ? H _R. Exothermic reactions are those in which heat is evolved. In endothermic reactions heat is absorbed.

The standard heat of reaction is defined as the change in enthalpy resulting from a reaction which takes place at 1 atm pressure and 25 C. In this case both the reactants and the products are at 1 atm and 25 C.

The heats of reactions can be calculated from the heats of formation and heats of combustion. These are given in standard handbooks. The heat of reaction is dependent on the temperature as follows:

If the heat of reaction is known at a base or reference temperature (usually 25 C = 298 K), the heat of reaction at any other temperature may be calculated by

where ? H _{T 0} is the heat of reaction at T ₀ and ? C _p is the difference in the sum of the molal heat capacities of the products and the sum of the molal heat capacities of the reactants, each multiplied by the stoichiometric coefficient n _i or

where C _pi's, the molal heat capacities, are known...