Exothermic reactions (AQA GCSE Chemistry): Revision Notes
Exothermic reactions
What are exothermic reactions?
An exothermic reaction is a chemical reaction that releases energy to the surroundings. This happens when the products have less energy than the reactants that started the reaction.
The key point is that energy is not created or destroyed - it just moves around. In exothermic reactions, energy transfers from the chemicals to the surroundings.
Key features of exothermic reactions
When an exothermic reaction happens:
- Energy is transferred to the surroundings
- Temperature of the surroundings increases
- Products have lower energy than reactants
- Heat is given out
Understanding reaction profiles
A reaction profile is a graph that shows how energy changes during a reaction. For exothermic reactions:
- The reactants start at a higher energy level
- The products end up at a lower energy level
- The difference in energy is released as heat
- Energy is given out during the reaction
This creates a "downhill" pattern on the energy graph, which is the characteristic shape that helps you identify exothermic reactions on energy diagrams.
Common examples of exothermic reactions
There are three main types you need to know:
1. Combustion
- Burning reactions like wood or fuel
- Always give out heat and light
2. Oxidation reactions
- When substances react with oxygen
- Like rusting or hand warmers
3. Neutralisation
- When acids and alkalis react together
- Always produces heat
Hand warmers - a real example
Worked Example: How Hand Warmers Work
Hand warmers work using an exothermic oxidation reaction:
Iron + Oxygen → Iron oxide
When you activate a hand warmer:
- Iron powder reacts with oxygen from the air
- The oxidation reaction is exothermic
- Heat is released to warm your hands
This is why hand warmers get hot when you open them!
Self-heating drinks
Worked Example: Self-Heating Beverages
Some drinks can heat themselves using exothermic reactions:
- Calcium oxide reacts with water
- This reaction is exothermic
- Heat is released which warms up the drink
The calcium oxide and water are kept separate until you want to heat the drink.
How to spot exothermic reactions
Look for these key signs:
- Temperature goes up
- Heat is given out
- Surroundings get warmer
- Products have less energy than reactants
Key Points to Remember:
- Exothermic reactions release energy to the surroundings
- Temperature increases when exothermic reactions happen
- Common examples include combustion, oxidation and neutralisation
- Products always have lower energy than reactants
- Energy is transferred OUT to the surroundings