G, the free energy change of a chemical process under non-standard state conditions (concentrations not 1 molar and pressures not 1 atmosphere) can be determined two different ways:
Using a Standard State Free Energy Change and the Reaction Quotient to Determine a Non-standard State Free Energy ChangeFrom the Standard State Free Energy Change and the Reaction Quotient From Cell Potentials
If we know the standard free energy change for a process, G^{o}, and the reaction quotient, Q, for the change we can determine the non-standard state free energy change, G, for the process using the following equation:
In this equation:
Using Cell Potentials to Determine Non-standard State Free Energy Changes
If we know the potential, E, for an electrochemical cell operating at non-standard state conditions (or if we can determine the cell potential) we can calculate the free energy change, G, for the cell reaction using the equation:
In this equation: