Free Energy


Gibb’s free energy, ΔG, is used to predict the direction of a reaction at constant temperature and constant pressure. If the value for ΔG is negative, the reaction is spontaneous, meaning this is the direction that the reaction proceeds in. For an equibrium process such as a change of state ΔG is zero.

A spontaneous reaction occurs without extra energy being supplied to the system at that temperature, and a feasible reaction is a thermodynamically possible reaction (but this may have a high activation energy making it non-spontaneous).

Gibb’s free energy is equal to enthalpy minus temperature times entropy:


This is measured in kJmol-1.


The correct way to write Gibb’s free energy is:


We could also use the first law of thermodynamics:


to write this more fully:


(U is internal energy, p is pressure, V is volume, T is temperature and S is entropy)



We can differentiate to get:


But since temperature is constant we are left with the A level definition.

For an ideal gas at constant temperature:


Gibb’s free energy is useful in calculations about equilibria.

There are also other kinds of free energy, such as the Helmholtz free energy.

Back to Contents: Chemistry: Thermodynamics

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