Gibbs free energy: the equation and the condition for feasibility
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
The Gibbs free energy change is given by delta G = delta H minus T delta S. A reaction is feasible when delta G is less than or equal to zero.
Words that must appear
- Delta G less than or equal to zero
- T
The sign
A negative delta G means the reaction is feasible. To find the temperature at which it just becomes feasible, set delta G to zero, so T equals delta H divided by delta S.
Do not confuse with
Enthalpy change alone. Gibbs free energy includes the entropy term and the temperature.
Common questions
What is the Gibbs free energy equation?
Delta G equals delta H minus T delta S.
When is a reaction feasible?
When delta G is less than or equal to zero.
Why might a feasible reaction not happen?
Feasibility says nothing about rate. A high activation energy can make it immeasurably slow.
Trayna does this automatically. You enter your subjects, your exam dates and how long you have each day, and it builds the route from today to those exams, then adapts it as you go. Maths follows Edexcel; Chemistry, Physics and Biology follow AQA.
Get Trayna on the App Store · on Google Play
Free for 14 days, then from £9.99 a month. Cancel any time.