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:

From 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:

• R = 8.314 J mol^-1 K^-1 or 0.008314 kJ/mol^-1 K^-1.
• T is the temperature on the Kelvin scale.
• ln Q is the natural logarithm of the reaction quotient.

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:

• n is the number of moles of electrons exchanged in the cell reaction.
• F is 96.485 kJ volt^-1 mol^-1 (the "Faraday").
• E is the cell potential under non-standard state conditions (concentrations not 1 molar, pressures not 1 atmosphere).