Exothermic reactions (AQA GCSE Chemistry Combined Science): Revision Notes
Exothermic reactions
What are exothermic reactions?
An exothermic reaction is a chemical reaction that gives out energy to the surroundings. This happens when the products have less energy than the reactants that started the reaction.
The term "exothermic" comes from Greek words meaning "outside heat" - literally heat going outside or outward from the reaction.
During an exothermic reaction:
- Energy is released from the reaction
- The energy goes into the surroundings
- The temperature of the surroundings increases
Think of it like this: the reaction is giving away extra energy, making everything around it warmer.
Energy changes in exothermic reactions
In any chemical reaction, energy cannot be created or destroyed. When an exothermic reaction happens:
- The reactants start with a certain amount of energy
- The products end up with less energy than the reactants
- The leftover energy is transferred to the surroundings as heat
- This makes the temperature go up
Energy Conservation Rule: The total amount of energy before and after a reaction is always the same - it's just moved from the chemicals to the surroundings in exothermic reactions.
Reaction profile for exothermic reactions
A reaction profile is a graph that shows how energy changes during a reaction. Understanding these graphs helps us visualise what happens during chemical reactions.
For exothermic reactions:
- The reactants start at a higher energy level
- Energy increases at first (activation energy needed to start the reaction)
- The products finish at a lower energy level than the reactants
- The difference between start and end energy is given out as heat
The activation energy is like the "energy hill" that reactants must climb over before the reaction can happen, even though the overall reaction releases energy.
Common examples of exothermic reactions
There are three main types of exothermic reactions you need to know:
1. Combustion
- Burning fuels like wood, coal, or petrol
- These reactions release lots of heat and light
2. Some oxidation reactions
- When metals react with oxygen
- Like iron rusting (but this is very slow)
3. Neutralisation
- When acids react with alkalis
- Like hydrochloric acid reacting with sodium hydroxide
Real-world examples
These everyday applications show how exothermic reactions are used in practical situations to provide heat when and where we need it.
Worked Example: Hand Warmers
Hand warmers use an exothermic reaction to produce heat:
Step 1: Identify the reactants
- They contain iron powder
- When activated, oxygen from the air enters
Step 2: The chemical reaction The chemical equation is:
Step 3: Energy release
- This oxidation reaction releases heat
- The heat warms your hands for several hours
Worked Example: Self-heating Drinks
Some drinks can heat themselves up using exothermic reactions:
Step 1: The setup
- The can contains calcium oxide (CaO) and water in separate compartments
- When you activate the can, these chemicals mix
Step 2: The chemical reaction The reaction is:
Step 3: Heat transfer
- This reaction gives out heat
- The heat warms up the drink inside the can
Key Points to Remember:
- Exothermic reactions release energy to the surroundings
- Temperature increases during exothermic reactions
- Products have less energy than the reactants
- Common examples include combustion, some oxidation, and neutralisation
- Hand warmers and self-heating drinks use exothermic reactions in everyday life