The Direction of Change in Chemical Reactions Thermodynamic Explanation

7 The Direction of Change in Chemical Reactions Thermodynamic Explanation 

The specific examples in Section 14.5 demonstrate that when 1C 1 the reaction has progressed far toward products, and when K 1 the reaction has remained near reactants. The empirical discussion in Section 14.6 shows how the reaction quotient Q and the principle of Te Chatelier can predict the direction of spontaneous reaction and the response of an equilibrium state to an external perturbation. Here, we use the thermodynamic description of K from Section 14.3 to provide the thermodynamic basis for these results obtained empirically in Sections 14.5 and 14.6. We identify those thermodynamic factors that determine the magnitude of K. We also provide a thermodynamic criterion for predicting the direction in which a reaction proceeds from a given initial condition. 

The expression connecting the standard Gibbs free energy change and the equilibrium constant can be rewritten as 

The direction in which a spontaneous chemical reaction proceeds after it is initiated with a given initial concentration of products and reactants is the direction in which AG 0. If the initial condition is to the left of the equilibrium state, products will be formed at the expense of reactants if the initial condition is to the right of the equilibrium state, products will be converted back to reactants. This criterion can be made quantitative and expressed in terms of the initial concentrations as follows. 

Proceeding as in Section 14.3, we find AG for the general gas-phase reaction 

---