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Gibbs free energy Calculator

Link between enthalpy, entropy and spontaneity.

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Gibbs Free Energy

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Overview

Gibbs free energy quantifies the maximum amount of non-expansion work extractable from a thermodynamically closed system at constant pressure and temperature. It serves as a critical criterion for spontaneity, where a negative value indicates a reaction will proceed without external energy input by balancing enthalpy and entropy.

Symbols

Variables

H = Enthalpy Change, S = Entropy Change, T = Temperature, G = Gibbs Free Energy

Enthalpy Change
kJ/mol
Entropy Change
kJ/molK
Temperature
Gibbs Free Energy
kJ/mol

Apply it well

When To Use

When to use: Use this equation to predict the spontaneity of chemical reactions or phase changes under conditions of constant temperature and pressure. It is particularly useful when determining the temperature at which a reaction shifts from being non-spontaneous to spontaneous.

Why it matters: This formula is the foundation of chemical energetics, allowing scientists to calculate equilibrium constants and design industrial chemical processes. In biology, it explains how cells couple unfavorable reactions with favorable ones to drive life-sustaining metabolic pathways.

Avoid these traps

Common Mistakes

  • Mixing kJ and J
  • Using Celsius instead of Kelvin.

One free problem

Practice Problem

A reaction has ΔH = -180 kJ/mol and ΔS = -0.15 kJ/(mol·K). Calculate ΔG at T = 500 K. Is the reaction spontaneous at this temperature?

Enthalpy Change-180 kJ/mol
Entropy Change-0.15 kJ/molK
Temperature500 K

Solve for:

Hint: ΔG = ΔH - TΔS. Keep all units in kJ/mol.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Atkins' Physical Chemistry
  2. IUPAC Gold Book: Gibbs energy
  3. Wikipedia: Gibbs free energy
  4. IUPAC Gold Book: Enthalpy
  5. IUPAC Gold Book: Entropy
  6. Callen, Herbert B. Thermodynamics and an Introduction to Thermostatistics
  7. Callen's Thermodynamics and an Introduction to Thermostatistics
  8. IUPAC Gold Book: Gibbs Free Energy