Endothermic reactions (AQA GCSE Chemistry Combined Science): Revision Notes
Endothermic reactions
What is an endothermic reaction?
An endothermic reaction is a chemical reaction that takes in energy from its surroundings. When these reactions happen, the products end up with more energy than the reactants started with. This extra energy has to come from somewhere - and that somewhere is the surroundings.
The word "endothermic" helps us remember this: "endo" means "in" or "inside", so energy goes into the reaction.
This is the opposite of "exothermic" reactions where energy flows out of the reaction and into the surroundings, making them feel hot.
How can you tell if a reaction is endothermic?
There are clear signs that tell us a reaction is endothermic:
- The temperature drops - the surroundings get colder
- Energy is absorbed from the area around the reaction
- The reaction mixture feels cold to touch
- Heat energy is taken in rather than given out
The most reliable sign of an endothermic reaction is that the surroundings become cooler. If you feel the temperature drop during a chemical reaction, you're witnessing energy being absorbed!
Energy changes in endothermic reactions
In an endothermic reaction, the energy story looks like this:
- The reactants start with a certain amount of energy
- During the reaction, energy is absorbed from the surroundings
- The products end up with more energy than the reactants had
- This means the surroundings lose energy and get cooler
The reaction profile (energy diagram) shows the products sitting higher up on the energy scale than the reactants. The difference between them is the energy that was absorbed.
Think of it like lifting a book onto a higher shelf - you need to put energy in to get it to that higher energy position. The products are like the book on the higher shelf.
Common examples of endothermic reactions
Thermal decomposition
This is when compounds break down when heated. A common example is:
- Calcium carbonate breaking down into calcium oxide and carbon dioxide
- This happens when limestone is heated in a kiln
The chemical equation for this reaction is:
Acid and carbonate reactions
When citric acid reacts with sodium hydrogen carbonate:
- The mixture gets noticeably colder
- This reaction is used in instant cold packs for sports injuries
Real-life application: instant cold packs
Instant cold packs are a brilliant example of endothermic reactions in action. Understanding how they work shows us the practical importance of endothermic processes in everyday life.
Worked Example: How Instant Cold Packs Work
Step 1: Initial setup
- Inside the pack are two separate compartments
- One contains water, the other contains a chemical (often ammonium nitrate)
Step 2: Activation
- When you squeeze the pack, the barrier breaks and the chemicals mix
- The dissolving process is endothermic - it absorbs energy
Step 3: Energy absorption
- This makes the pack feel cold against your skin
- The temperature can drop by 10-15°C almost instantly
- The pack can only be used once because the reaction cannot be reversed easily
Result: Instant cooling without needing a freezer - perfect for treating sports injuries!
This is why sports teams use these packs to treat injuries - they provide instant cooling without needing a freezer!
Key Points to Remember:
- Endothermic reactions absorb energy from their surroundings
- The temperature goes down when these reactions happen
- Products have more energy than the reactants
- Instant cold packs are a perfect everyday example
- Think "endo = energy in" to remember the direction of energy flow