Gibbs Free Energy Calculator
Combine enthalpy, entropy, and temperature to calculate a Gibbs free-energy change.
01 Set up your calculation
- Enter enthalpy and entropy changes for the same reaction basis and conditions.
- Enter the temperature at which those values apply.
- Read ΔG and its sign; use the crossover estimate only if ΔH and ΔS can be treated as constant.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Activation Energy and Arrhenius Equation.
Follow the numbers
At 298.15 K, ΔH = −40 kJ/mol and ΔS = −100 J/(mol·K) give ΔG = −40 − 298.15 × (−0.100) = −10.185 kJ/mol.
03 Keep the reaction basis consistent
Enthalpy and entropy changes must describe the same process, amounts, phases, and temperature. Multiplying a balanced reaction by two doubles both changes and therefore doubles ΔG, while leaving its thermodynamic direction unchanged.
A standard free-energy result requires standard-state inputs. Mixing standard enthalpy with an entropy change for a different composition does not produce a consistent standard ΔG.
04 Spontaneity and speed answer different questions
At constant temperature and pressure, the sign of the actual Gibbs change indicates the favored thermodynamic direction. It does not state how rapidly the process proceeds. A favorable process can remain slow because of an activation barrier.
The optional crossover temperature solves ΔH − TΔS = 0 while holding both input changes constant. Over broad temperature ranges heat-capacity differences and phase changes may matter, so treat that value as an estimate under the stated approximation.
Questions from the notebook
Why are entropy and enthalpy in different energy units?
The default fields reflect common tables. The calculation converts joules to kilojoules before subtracting.
Does negative ΔG mean the reaction is fast?
No. Kinetics must be evaluated separately.
Is ΔG° = 0 the same as every mixture being at equilibrium?
No. Standard free energy and the actual mixture’s free energy are different quantities. Composition enters the relationship ΔG = ΔG° + RT ln Q.
References & reading
- OpenStax, Rice University — Free Energy ↗
Derives ΔG = ΔH − TΔS and explains its sign at constant temperature and pressure.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.