---
title: "Free Energy and the Direction of Spontaneous Change"
description: "A system in contact with surroundings at fixed temperature settles where free energy is least, balancing low energy against high entropy. That is why some processes run even though they absorb heat."
canonical: https://lightmysky.com/learn/science/free-energy-and-the-direction-of-spontaneous-change-mt_ww43eqzupN
source: https://lightmysky.com/learn/science/free-energy-and-the-direction-of-spontaneous-change-mt_ww43eqzupN.md
retrieved: 2026-09-12
---

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# Free Energy and the Direction of Spontaneous Change

A system in contact with surroundings at fixed temperature settles where free energy is least, balancing low energy against high entropy. That is why some processes run even though they absorb heat.

Subject: Science · Area: Thermal & Statistical Physics · Ages 20 to 22
Page: https://lightmysky.com/learn/science/free-energy-and-the-direction-of-spontaneous-change-mt_ww43eqzupN

## Ready when they can

- Writes the free energy as energy minus temperature times entropy and identifies the competing terms
- Predicts the direction of a change from the sign of the free energy difference
- Explains how the balance shifts as temperature rises

## Lesson: Which way will it go: free energy decides

Free energy is nature's tug of war scored in a single number. One team is energy: systems slide toward lower enthalpy, releasing heat as bonds settle. The other team is entropy: systems slide toward more spread-out arrangements. Gibbs free energy combines both as G equals H minus TS, so a change reads delta H minus T times delta S. The sign is the whole verdict: negative can run on its own and even supply work, positive must be driven, zero marks equilibrium.

**Example.** Temperature is the dial that shifts the balance, because only the entropy term carries a T. At low temperature the enthalpy team dominates, favouring heat-releasing processes. Crank the heat and T times delta S takes over, favouring disorder-making processes even when they absorb heat. Ice refuses to melt in a freezer but surrenders in a warm room for exactly this reason. Four sign combos cover every case: heat releasing plus disorder gaining runs at all temperatures, heat absorbing plus disorder losing never runs, and the mixed cases flip at T equals delta H over delta S.

Fix one misunderstanding first: spontaneous says nothing about speed. A spontaneous process can happen without continuous outside help, like a ball rolling downhill, yet be geological or instant. Rusting iron, melting ice, and exploding mixtures are all spontaneous at wildly different rates. Thermodynamics picks the direction and kinetics picks the pace. Diamond turning to graphite is spontaneous but glacial.

**Tip.** Judge any process with this routine. Write delta H minus T times delta S and name the signs of each term. Read the verdict off the total sign. For mixed cases compute the flip temperature, delta H over delta S, and ask which side of it you stand on.

**Recap.** Free energy balances enthalpy against T times entropy, its sign picks the direction, and temperature decides which team wins.

## Practice

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

## Needs first

- [Free Energy and Whether a Reaction Is Feasible](https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM)
- [The Boltzmann Factor and the Partition Function](https://lightmysky.com/learn/science/the-boltzmann-factor-and-the-partition-function-mt_u9MSoTgJkF)

## Opens up

- [The Microcanonical and Canonical Ensembles](https://lightmysky.com/learn/science/the-microcanonical-and-canonical-ensembles-mt_NSCYLK-brt)
- [The Grand Canonical Ensemble and the Chemical Potential](https://lightmysky.com/learn/science/the-grand-canonical-ensemble-and-the-chemical-potential-mt_sdrQrWhBAF)