Gibbs Free Energy, Chemical Potential, and the Equilibrium Constant

The Gibbs free energy (G,) of a substance / is defined as G, = //, - TS,. Free energy cannot be measured directly. As with enthalpy we consider the difference (AG) between the free energies of substances in a reaction. The AG for a reaction is the maximum energy change for that reaction as useful work, measured at constant temperature and pressure. For a reaction we can write 

Next we will show how AG° is related to the equilibrium constant for a reaction. Consider the general reaction 

For a moles of A, aG = AG + RT InJA] . Now AG, for the general reaction equals the difference in the sum of values for the products minus that for the reactants 

Our earlier general expression for the Gibbs free energy of a reaction can be restated as 

At equilibrium, of course, Q = and G,. = 0. When Q and AG, 0, the reaction tends to go from left to right. Conversely, when Q Kg.q and AG,. 0, the reaction tends to go from right to left. 

---

The concepts of enthalpy (H) and entropy (S) are basic to the definitions of Gibbs free energy, chemical potential, and the equilibrium constant (K ), and allow us to predict the effect of temperature on G and Kg. We will also see that entropy is a measure of the state of order-disorder in solids and solid solutions, and helps explain the stabilities of aqueous complexes. 

Sec. 1.3 Gibbs Free Energy, Chemical Potential, and the Equilibrium Constant 7... 

---