Gibbs free energy formulas and interpretation
Gibbs free energy combines enthalpy and entropy to assess thermodynamic favorability at constant temperature and pressure.
Convert entropy from joules to kilojoules when enthalpy is entered in kilojoules.
How to use the Gibbs free energy calculator
- Choose a model: Select the relationship matching the problem.
- Choose the unknown: Select the quantity to calculate.
- Enter values: Enter all known values with matching units and signs.
- Calculate: Review the result, formula, units, and direction.
Formula and variables
Free-energy change equals enthalpy change minus absolute temperature times entropy change.
ΔG = ΔH − TΔS- ΔG — Gibbs free energy change
- Thermodynamic driving-force measure (kJ/mol)
- ΔH — Enthalpy change
- Reaction enthalpy (kJ/mol)
- T — Temperature
- Absolute temperature (K)
- ΔS — Entropy change
- Reaction entropy change (J/(mol·K))
Spontaneity example
Use ΔH = −40 kJ/mol, ΔS = −50 J/(mol·K), and T = 298 K.
- ΔH
- −40 kJ/mol
- ΔS
- −50 J/(mol·K)
- T
- 298 K
- ΔG = −40 − 298(−50/1000)
Result: ΔG = −25.1 kJ/mol.
A negative value indicates thermodynamic favorability under the specified conditions.
Understanding your results
Interpreting the result
A negative ΔG indicates thermodynamic favorability, but does not describe reaction speed.
Assumptions
- The selected equation represents the physical system.
- Inputs use a consistent reference direction.
- Values are converted through coherent SI units.
Limitations
- Vector components must be resolved along a common axis.
- External forces or energy losses are not added automatically.
- Results depend on the accuracy of entered measurements.
Common mistakes
- Mixing incompatible units.
- Dropping negative signs that represent direction.
- Using weight where mass is required.
- Entering a zero divisor.
Practical use cases
Physics problems
Check classroom, laboratory, and mechanics calculations.
Practical estimates
Estimate motion, forces, and energy for real systems.
Frequently asked questions
Can a result be negative?
Yes. For directional quantities, the sign indicates direction relative to the chosen positive axis.
Should I use SI units?
The interface can convert supported units, while the formulas are evaluated through coherent SI units.
Sources and review
- SI Brochure, 9th edition — BIPM. Accessed 2026-07-11.
- Special Publication 811 — NIST. Accessed 2026-07-11.
Reviewed 2026-07-11.