Exothermic & Endothermic Reactions (AQA GCSE Chemistry): Revision Notes
Endothermic reactions
What are endothermic reactions?
An endothermic reaction is a chemical reaction that takes in energy from its surroundings. This happens when the products formed have more energy stored in them than the original reactants.
When an endothermic reaction occurs:
- Energy is absorbed from the surrounding area
- The temperature of the surroundings goes down
- The products end up with more energy than what you started with
Think of it like this: the reaction needs to "borrow" energy from around it to make the new products.
Energy profile for endothermic reactions
The energy profile shows us what happens to energy during the reaction:
- Reactants start at a lower energy level
- Products end up at a higher energy level
- The difference shows how much energy was absorbed during the reaction
- There's an energy barrier (activation energy) that must be overcome first
The graph goes upward from reactants to products, showing that energy has been taken in.
Common examples of endothermic reactions
Thermal decomposition
This is when heat breaks down a compound into simpler substances.
Chemical Equation Example: Calcium Carbonate Decomposition
Calcium carbonate breaks down when heated:
You need to keep adding heat energy for this to happen.
Cold packs (sports injury packs)
When you squeeze the pack, water mixes with a solid chemical. The dissolving process absorbs energy from your skin, which makes the pack feel cold against your injury.
The cold pack gets cold because it's taking heat energy away from you - this is a perfect example of an endothermic process in action!
Citric acid and sodium hydrogen carbonate reaction
This reaction absorbs energy from the surroundings, so you might feel the container getting cooler during the reaction.
Key features to remember
Understanding the key characteristics of endothermic reactions helps you identify them in both theory and practice.
Key Features of Endothermic Reactions:
Energy changes:
- Products have higher energy than reactants
- Energy is absorbed from the surroundings
- The surroundings get cooler
How to spot them:
- The reaction mixture or container feels cold
- You need to keep adding heat to keep some reactions going
- The temperature drops during the reaction
Identifying endothermic reactions in practice
When you're working in the laboratory, there are several clear indicators that can help you recognise when an endothermic reaction is taking place.
Signs of Endothermic Reactions in Experiments:
If you're doing an experiment and notice:
- The test tube or beaker feels cold to touch
- A thermometer reading decreases
- You need to keep heating something to make it continue reacting
These are all signs you're dealing with an endothermic reaction!
Key Points to Remember:
- Endothermic reactions absorb energy from their surroundings
- Products have more energy than the reactants you started with
- Temperature goes down in the area around the reaction
- Common examples include thermal decomposition and cold packs
- Easy way to remember: ENdothermic = ENergy goes IN to the reaction