Gibbs Free Energy Calculator for A-Level Chemistry

Last updated: June 2026

Use this Gibbs free energy calculator to calculate ΔG, ΔH, ΔS or temperature from the Gibbs equation. It is designed for A-Level and Singapore H2 Chemistry students studying thermodynamic feasibility.

Key takeaway: The Gibbs equation is ΔG = ΔH − TΔS. A negative ΔG means that the reaction is thermodynamically feasible under the stated conditions, but it does not mean that the reaction must be fast.

Table of Contents

What is Gibbs free energy?

Gibbs free energy change combines enthalpy change and entropy change into one quantity that predicts whether a process is thermodynamically feasible at a particular temperature.

ΔG = ΔH − TΔS

In this equation, ΔG and ΔH are commonly expressed in kJ mol⁻¹, T must be an absolute temperature in kelvin, and ΔS must use units compatible with ΔH.

How to use this calculator

Enter exactly three of the four quantities.
Leave the quantity you want to calculate completely blank.
Select the unit used for entropy and temperature.
Press Calculate missing value.
Follow the numbered working to reproduce the method in an examination.

Rearranging the Gibbs equation

The same relationship can be rearranged to calculate any missing quantity:

ΔH = ΔG + TΔS

ΔS = (ΔH − ΔG) / T

T = (ΔH − ΔG) / ΔS

Important unit trap: Entropy is often given in J K⁻¹ mol⁻¹, while enthalpy and Gibbs free energy are given in kJ mol⁻¹. Divide entropy by 1000 before using it with values measured in kJ mol⁻¹.

Interpreting ΔG

Value of ΔG	Interpretation
ΔG < 0	The reaction is thermodynamically feasible under the stated conditions.
ΔG = 0	The system is at equilibrium under the stated conditions.
ΔG > 0	The reaction is not thermodynamically feasible as written.

Thermodynamic feasibility and reaction rate are different ideas. A reaction with a negative ΔG may still be extremely slow when its activation energy is high.

Temperature and feasibility

Temperature affects the size of the TΔS term. At the boundary between feasible and non-feasible conditions, ΔG = 0, so the threshold temperature is:

T = ΔH / ΔS

This special form applies only when ΔG is zero. The signs of ΔH and ΔS determine whether increasing temperature makes a reaction more or less favourable.

Worked example

Suppose ΔH = +180 kJ mol⁻¹, ΔS = +400 J K⁻¹ mol⁻¹ and T = 500 K.

First convert entropy: 400 J K⁻¹ mol⁻¹ = 0.400 kJ K⁻¹ mol⁻¹ .

ΔG = 180 − 500(0.400) = −20 kJ mol⁻¹

Since ΔG is negative, the reaction is thermodynamically feasible at 500 K.

Common examination traps

Trap 1: Do not use temperature in degrees Celsius in the equation. Convert it to kelvin first.

Trap 2: Keep the units of ΔH, ΔG and TΔS consistent.

Trap 3: If ΔS is negative, subtracting TΔS adds a positive contribution to ΔG.

Trap 4: A negative ΔG describes thermodynamic feasibility, not the speed of reaction.

Frequently asked questions

What should I leave blank?

Leave only the quantity you want the calculator to find blank. Exactly three boxes must contain values.

Can the calculator accept entropy in joules?

Yes. Keep the entropy unit set to J K⁻¹ mol⁻¹ and the calculator will convert it internally to kJ K⁻¹ mol⁻¹.

Why must temperature be in kelvin?

The Gibbs equation uses absolute temperature. Celsius does not begin at absolute zero, so it cannot be substituted directly.

Does a negative ΔG guarantee that a reaction occurs quickly?

No. A reaction may be thermodynamically feasible but kinetically slow because it has a large activation energy.