Reversibility & Equilibrium (AQA GCSE Chemistry): Revision Notes
Reversible reactions
What are reversible reactions?
Most chemical reactions only go in one direction - the reactants turn into products and that's it. But some reactions are special because they can go both ways. These are called reversible reactions.
In a reversible reaction, the products can react together to make the original reactants again. It's like a chemical reaction that can change its mind!
Think of reversible reactions like a busy two-way street where traffic (chemical species) can flow in either direction, unlike regular reactions which are like one-way streets.
How to write equations for reversible reactions
When we write equations, we use different arrows to show what type of reaction it is:
- Normal reactions use a single arrow:
- Reversible reactions use a special split arrow:
The split arrow () tells us that the reaction can go in both directions. The chemicals on the left can make the chemicals on the right, and the chemicals on the right can make the chemicals on the left.
Example of a reversible reaction - ammonium chloride
A great example is what happens when you heat ammonium chloride (a white solid):
Worked Example: Ammonium Chloride Decomposition
When heated:
Here's what happens step by step:
- Heating up: When you heat ammonium chloride, it breaks apart into ammonia gas and hydrogen chloride gas
- Cooling down: When these gases cool down, they react together to form ammonium chloride again
- The complete reaction:
This reaction is very useful for testing. If you see white solid forming when gases cool down, you know ammonia and hydrogen chloride were present.
Energy changes in reversible reactions
Understanding energy changes in reversible reactions is crucial for predicting their behaviour.
Key Energy Rule for Reversible Reactions:
- If the forwards reaction (left to right) gives out energy (exothermic), then the reverse reaction (right to left) takes in energy (endothermic)
- The amount of energy given out in one direction equals the amount of energy taken in when going the other way
For example, with copper sulphate:
- Blue hydrated copper sulphate → white anhydrous copper sulphate + water (takes in energy)
- White anhydrous copper sulphate + water → blue hydrated copper sulphate (gives out energy)
Real-world example - making ammonia
Ammonia is made in industry using a reversible reaction called the Haber process:
Industrial Application: The Haber Process
Temperature affects how much ammonia you get:
- Lower temperatures give more ammonia but the reaction is very slow
- Higher temperatures make the reaction faster but give less ammonia
- Medium temperatures (around 450°C) are used as a compromise
This shows why understanding reversible reactions is essential for making chemicals efficiently in industrial processes.
Key Points to Remember:
- Reversible reactions can go in both directions - products can turn back into reactants
- Use the split arrow () to show a reversible reaction in equations
- Energy changes are opposite - if forwards reaction is exothermic, reverse reaction is endothermic
- Temperature affects which direction is favoured in reversible reactions
- Real examples include ammonium chloride heating/cooling and copper sulphate hydration