Exothermic and Endothermic Reactions
Simple Explanation
An exothermic reaction releases heat to the surroundings (ΞH is negative, and the surroundings get warmer). An endothermic reaction absorbs heat from the surroundings (ΞH is positive, and the surroundings get colder).
Why Do We Need It?
This is the most basic and useful classification of a chemical reaction's energy behaviour β it predicts whether a reaction will feel hot or cold, and whether a product's bonds are stronger or weaker than the reactants' bonds.
Formula
Enthalpy Change of Reaction
ΞH = H(products) β H(reactants)
The heat energy released or absorbed by a reaction at constant pressure.
- ΞH
- β enthalpy change of the reaction, in kilojoules per mole (kJ/mol)
- H(products)
- β total enthalpy (heat content) of the products
- H(reactants)
- β total enthalpy (heat content) of the reactants
When to use it: Whenever you need to classify a reaction as exothermic (ΞH negative) or endothermic (ΞH positive), or state how much heat energy a reaction releases or absorbs.
Worked Example
Classify a reaction from its enthalpy change
The reactants of a reaction have a total enthalpy of 500 kJ/mol; the products have a total enthalpy of 320 kJ/mol. Find ΞH and classify the reaction.
Why Does This Work?
If the products end up with lower total enthalpy than the reactants, the "missing" energy has to have gone somewhere β it is released to the surroundings as heat, making ΞH negative. If the products end up with higher enthalpy, that extra energy had to come from somewhere β it is absorbed from the surroundings, making ΞH positive.
Real-Life Example
Instant hot packs and cold packs
Some first-aid hot packs get warm when activated; instant cold packs get cold.
A hot pack contains an exothermic reaction (like the oxidation of iron), releasing heat to warm your skin; a cold pack contains an endothermic process (like ammonium nitrate dissolving in water), absorbing heat from your skin and its surroundings.
Practice
Photosynthesis absorbs energy from sunlight to build glucose from carbon dioxide and water. Is this reaction exothermic or endothermic?
EasyCommon mistake
Assuming exothermic reactions always happen quickly and endothermic reactions always happen slowly β the sign of ΞH tells you nothing about reaction speed, only about the direction of heat flow; that's a separate topic (reaction kinetics).
Quick Review
- Exothermic: releases heat, ΞH negative, surroundings get warmer.
- Endothermic: absorbs heat, ΞH positive, surroundings get colder.
- ΞH = H(products) β H(reactants).