The formula for Gibbs free energy is:

Ο ΔG = ΔS + TΔΗ

Ο ΔG - ΔΗ - TΔS

Ο ΔG - ΔS - TΔΗ

Ο ΔG = ΔΗ + TAS

D. ΔG° = ΔH° - TΔS°

Explanation:

Standard Gibbs free energy change, ΔG°

Calculating ΔG°

This is how standard Gibbs free energy change is calculated:

ΔG° = ΔH° - TΔS°

That's all you need to know. Learn it!

If you know (or can work out) the enthalpy change for the reaction, and you know (or can work out) the entropy change, and you know the temperature (in kelvin), then it would seem to be really easy to work out ΔG°.

There is an easy mistake to be made though! You have to remember that the entropy change is calculated in energy units of joules, but ΔG° and ΔH° are both measured in kJ.

Because you are calculating ΔG, your answer will be in kJ mol-1. You must remember to change the entropy change value into kJ before you start, otherwise you will get the calculation completely wrong.

On an earlier page in this section, we calculated the entropy change for the reaction

ΔS° worked out as - 242.2 J K-1mol-1.

Before you do anything else, convert this to kJ by dividing by 1000.

ΔS° = - 0.2422 kJ K-1mol-1.

This reaction is actually the combustion of methane, and so we can just take a value of this from a Data Book. But you should, of course, know how to calculate this from enthalpy changes of formation.

ΔH° = - 890.4 kJ mol-1.

So if you had to calculate the Gibbs free energy change at, say, 298 K, you can just slot the numbers in:

ΔG° = ΔH° - TΔS°

ΔG° = - 890.4 - 298 (-0.2442) = - 817.6 kJ mol-1

It is easy as long as you remember to convert the entropy change value into kJ.