SAT Subject Test Chemistry,10 edition

Standard free energy change, ΔGº, is defined as the ΔG of a process occurring under standard
conditions, and for which the concentrations of any solutions involved are 1 M. The standard free
energy of formation of a compound, ΔGºf, is the free-energy change that occurs when 1 mole of a
compound in its standard state is formed from its elements in their standard states. The standard
free energy of formation of any element in its most stable form (and, therefore, its standard state) is
zero. The standard free energy of a reaction, ΔGºrxn, is the free energy change that occurs when that
reaction is carried out under standard state conditions; i.e., when the reactants in their standard
states are converted to the products in their standard states, at standard conditions of T and P.
Bearing in mind what we did for enthalpy changes, we can write:

ΔG°rxn  =   (sum    of  ΔG°f    of  products)   –   (sum    of  ΔG°f    of  reactants).

BASIC CONCEPT

ΔG° positive:   small   K,  reaction    not favored

ΔG° negative:   large   K,  reaction    favored

Relation Between Free Energy and the Equilibrium Constant

The value of the free energy change in general (under nonstandard conditions) is related to the
standard free energy change by the following equation:

ΔG  =   ΔG° +   RT  ln  Q

where R is the gas constant, T is the temperature in Kelvins, ln stands for the natural logarithm
function, and Q is the reaction quotient we mentioned briefly in passing in the last chapter. For the
reaction a A + b B c C + d D, the reaction quotient is