---
title: "Free Energy and Whether a Reaction Is Feasible"
description: "Free energy change combines enthalpy and entropy with temperature. Its sign says whether a reaction is feasible, and setting it to zero gives the temperature at which feasibility flips."
canonical: https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM
source: https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM.md
retrieved: 2026-09-12
---

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# Free Energy and Whether a Reaction Is Feasible

Free energy change combines enthalpy and entropy with temperature. Its sign says whether a reaction is feasible, and setting it to zero gives the temperature at which feasibility flips.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM

## Ready when they can

- Calculates a free energy change from enthalpy and entropy values with the units made consistent first
- Reads the sign to say whether a reaction is feasible at that temperature
- Finds the temperature at which a reaction becomes feasible by setting the free energy change to zero
- Explains why a feasible reaction can still be far too slow to see

## Lesson: Feasible or Not

Gibbs free energy combines enthalpy and entropy into one test. Delta G equals delta H minus T times delta S, with T in kelvin. Negative means the reaction is feasible at that temperature, positive means it is not. For pure ice and water at normal pressure, that is why melting runs on its own just above 0 degrees Celsius while freezing runs on its own just below.

Watch the units before you touch the maths. Enthalpy usually arrives in kilojoules per mole while entropy comes in joules per mole per kelvin. Convert one of them first or the answer lands wrong by a factor of a thousand.

**Example.** Set delta G to zero and solve: T equals delta H divided by delta S. Endothermic changes that gain entropy turn feasible when hot. Exothermic changes that lose entropy stay feasible only when cold.

Feasible never means fast. Diamond turning to graphite is feasible at room temperature yet far too slow to see, because few particles clear the activation barrier. Thermodynamics says if, kinetics says when.

**Recap.** Balance enthalpy against temperature weighted entropy, fix the units first, and never read speed from the sign.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Hess's Law and Enthalpy Cycles](https://lightmysky.com/learn/science/hesss-law-and-enthalpy-cycles-mt_530V9czLOb)
- [Entropy and the Spread of Energy](https://lightmysky.com/learn/science/entropy-and-the-spread-of-energy-mt_a_2feH2gnE)

## Opens up

- [Rate Equations and Orders of Reaction](https://lightmysky.com/learn/science/rate-equations-and-orders-of-reaction-mt_1x3ogDKGxC)
- [Coupled Reactions and Why ATP Is the Cell's Currency](https://lightmysky.com/learn/science/coupled-reactions-and-why-atp-is-the-cells-currency-mt_1yVJRArCow)
- [Standard Electrode Potentials and Predicting Redox Reactions](https://lightmysky.com/learn/science/standard-electrode-potentials-and-predicting-redox-reactions-mt_LDI10CjoAI)
- [Cells That Do Work: Batteries, Fuel Cells and Corrosion](https://lightmysky.com/learn/science/cells-that-do-work-batteries-fuel-cells-and-corrosion-mt_NQQzlecX4v)
- [Chemical Potential and Gibbs Energy That Depends on Composition](https://lightmysky.com/learn/science/chemical-potential-and-gibbs-energy-that-depends-on-composition-mt_Tdl6K-gFO3)
- [Free Energy and the Direction of Spontaneous Change](https://lightmysky.com/learn/science/free-energy-and-the-direction-of-spontaneous-change-mt_ww43eqzupN)