---
title: "Entropy and the Spread of Energy"
description: "Entropy measures how many ways a system's energy can be arranged. Changes that make more gas, or more particles, raise it, and that is why some endothermic changes happen at all."
canonical: https://lightmysky.com/learn/science/entropy-and-the-spread-of-energy-mt_a_2feH2gnE
source: https://lightmysky.com/learn/science/entropy-and-the-spread-of-energy-mt_a_2feH2gnE.md
retrieved: 2026-09-12
---

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# Entropy and the Spread of Energy

Entropy measures how many ways a system's energy can be arranged. Changes that make more gas, or more particles, raise it, and that is why some endothermic changes happen at all.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/entropy-and-the-spread-of-energy-mt_a_2feH2gnE

## Ready when they can

- Ranks solid, liquid and gas by entropy and explains the order in terms of arrangements
- Predicts the sign of the entropy change for a reaction from the states and the number of particles
- Calculates an entropy change from standard entropy values
- Explains why ammonium nitrate dissolving cools the water and still happens on its own

## Lesson: More Ways, More Entropy

Entropy, S, measures how spread out energy and particles are. More arrangements mean higher entropy. For one substance, gas beats liquid and liquid beats solid, because particles gain freedom at each step.

To predict the sign, count gas moles first. Making more gas, or turning solids and liquids into gas, raises entropy. Reactions that consume gas to make solids lower it.

**Example.** Dissolving ammonium nitrate cools the water, so it is endothermic, yet it happens alone. The ions spreading through the solution give a big entropy gain that outweighs the unfavourable enthalpy.

**Tip.** To calculate, add the standard entropies of the products and subtract the reactants, scaling each by its moles. Units are joules per mole per kelvin, so divide by 1000 before mixing with enthalpy in kilojoules.

**Recap.** Rank the states, count gas moles for the sign, and let spreading explain why some endothermic changes run alone.

## Practice

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

## Needs first

- [Lattice Enthalpy and Born-Haber Cycles](https://lightmysky.com/learn/science/lattice-enthalpy-and-born-haber-cycles-mt_pUku4iUjYa)
- [Reactions That Release or Absorb Heat](https://lightmysky.com/learn/science/reactions-that-release-or-absorb-heat-mt_UoqUPI_uNz)

## Opens up

- [The Boltzmann Distribution over Molecular Levels](https://lightmysky.com/learn/science/the-boltzmann-distribution-over-molecular-levels-mt_-DHIox6GBm)
- [Entropy as a State Function](https://lightmysky.com/learn/science/entropy-as-a-state-function-mt_bg_K5nU-GZ)
- [Entropy from Counting: The Molecular Meaning of the Third Law](https://lightmysky.com/learn/science/entropy-from-counting-the-molecular-meaning-of-the-third-law-mt_ihl5fEjoLL)
- [Free Energy and Whether a Reaction Is Feasible](https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM)