12 - Chemical Reactions

Abstract

Chemical reactions entail making or breaking of bonds, so energy is conserved for an isolated system. Reactions at constant volume or pressure exchange heat with the environment by change of internal energy or enthalpy, respectively. Reaction extent is measured by a progress variable; reactions progress until equilibrium is reached or some component is depleted. We define standard states of components and heats of formation of compounds. Affinity is defined as the decrease of Gibbs free energy per unit progress variable; its sign determines the direction of the reaction such that entropy is produced. Change of enthalpy per unit progress variable determines whether the reaction is endothermic or exothermic. At equilibrium the affinity is zero. Equilibrium conditions are expressed by equating a function of temperature and pressure called the “equilibrium constant” to a reaction product that depends on activities and fugacities of chemical components. Special cases include reaction products that can be approximated in terms of partial pressures of ideal gases.