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chemistry

Gibbs Free Energy Calculator

Calculate the Gibbs free energy change (ΔG) from enthalpy, entropy, and temperature to predict reaction spontaneity.

By Christopher FloiedPublished Updated

Enthalpy change of the reaction.

Entropy change in J/(mol·K).

Results

ΔG

-155.3 kJ/mol

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Gibbs Free Energy Calculator. Follow the units and input format printed next to each field. Where a unit selector is available, choose the units that match your data.

2

Review your inputs

Double-check that all values are correct and match the units or format shown for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Gibbs Free Energy Calculator computes the output in your browser and displays the results, with units clearly labeled, as soon as they are ready — no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

When to Use This Calculator

  • •Use the Gibbs Free Energy Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
  • •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
  • •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
  • •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.

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About Gibbs Free Energy Calculator

The Gibbs free energy calculator determines whether a chemical reaction or physical process is thermodynamically spontaneous by computing ΔG = ΔH − TΔS. A negative ΔG indicates a spontaneous (favorable) process under the given conditions, a positive ΔG indicates a non-spontaneous process, and ΔG = 0 indicates equilibrium. This relationship, derived by Josiah Willard Gibbs in the 1870s, unifies the effects of enthalpy (heat exchange) and entropy (disorder) into a single criterion for spontaneity at constant temperature and pressure. It is one of the most important equations in chemistry and is used extensively in thermodynamics, biochemistry, materials science, and chemical engineering.

The Math Behind It

The Gibbs free energy function G = H − TS combines the first and second laws of thermodynamics for processes at constant temperature and pressure. The change ΔG = ΔH − TΔS tells us the maximum useful work obtainable from a process (at constant T and P). When ΔG < 0, the process releases free energy and can proceed spontaneously. When ΔG > 0, the process requires energy input. At equilibrium, ΔG = 0, and the system has no net driving force in either direction. The interplay between ΔH and TΔS creates four scenarios: (1) ΔH < 0 and ΔS > 0: spontaneous at all temperatures; (2) ΔH > 0 and ΔS < 0: non-spontaneous at all temperatures; (3) ΔH < 0 and ΔS < 0: spontaneous at low temperatures; (4) ΔH > 0 and ΔS > 0: spontaneous at high temperatures. The crossover temperature where ΔG = 0 is T = ΔH/ΔS. Note that ΔS is often given in J/(mol·K) while ΔH is in kJ/mol, so you must divide ΔS by 1000 (or multiply by 0.001) to ensure consistent units before multiplying by T.

Formula Reference

Gibbs Free Energy

ΔG = ΔH − TΔS

Variables: ΔG = Gibbs free energy change (kJ/mol); ΔH = enthalpy change (kJ/mol); T = temperature (K); ΔS = entropy change (J/(mol·K), converted to kJ)

Worked Examples

Example 1: Combustion reaction

ΔH = −200 kJ/mol, ΔS = −150 J/(mol·K), T = 298 K.

Step 1:Convert ΔS: −150 J/(mol·K) = −0.150 kJ/(mol·K).
Step 2:TΔS = 298 × (−0.150) = −44.70 kJ/mol.
Step 3:ΔG = −200 − (−44.70) = −155.30 kJ/mol.

ΔG = −155.30 kJ/mol; the reaction is spontaneous at 298 K.

Common Mistakes & Tips

  • !Mixing units: ΔH in kJ but ΔS in J — always convert to the same energy unit.
  • !Using Celsius instead of Kelvin for temperature.
  • !Interpreting ΔG as reaction rate — spontaneity does not imply speed.

Related Concepts

Activation Energy

While ΔG tells if a reaction is favorable, activation energy determines how fast it proceeds.

Nernst Equation

Gibbs free energy is related to cell potential: ΔG = −nFE.

Used in These Calculators

Calculators that build on or apply the concepts from this page:

Activation Energy Calculator

Nernst Equation Calculator

Frequently Asked Questions

Does a negative ΔG mean the reaction happens instantly?

No. ΔG indicates thermodynamic favorability, not kinetics. A reaction can be thermodynamically spontaneous but kinetically slow (e.g., diamond converting to graphite). The activation energy determines the rate.

What is standard Gibbs free energy (ΔG°)?

ΔG° is the Gibbs free energy change under standard conditions (1 atm, 1 M concentration, 298 K). The actual ΔG under non-standard conditions is ΔG = ΔG° + RT ln(Q), where Q is the reaction quotient.

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