In a reversible reaction the products can turn back into the reactants
Definition
Reversible reaction
A reaction in which the products can react to form the original reactants.
- In some reactions the products can react together to form the original reactants again.
- A reaction like this is a reversible reaction.
- It is written with the reversible arrow ⇌\rightleftharpoons⇌ instead of a single arrow.
- The change from left to right is the forward reaction, and the change from right to left is the reverse reaction.
Key Idea
- A reversible reaction can go both ways, shown by the ⇌\rightleftharpoons⇌ symbol.
- The products of the forward reaction are the reactants of the reverse reaction.
Changing the conditions can change the direction
Definition
Reaction conditions
The physical factors around a reaction, such as temperature, that can be changed to affect how the reaction proceeds.
- Changing the reaction conditions, such as the temperature or how much of a substance is present, can change which direction happens.
- Heating solid ammonium chloride breaks it down into ammonia and hydrogen chloride gas.
- On cooling, the two gases react again to form solid ammonium chloride: NH4Cl(s)⇌NH3(g)+HCl(g)\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}NH4Cl(s)⇌NH3(g)+HCl(g).
Hydrated copper sulfate is a clear reversible change to remember
- Blue hydrated copper(II) sulfate contains water joined to the copper sulfate.
- Heating it drives off the water and leaves white anhydrous copper(II) sulfate.
- Adding water to the white solid turns it blue again: CuSO4⋅5H2O⇌CuSO4+5H2O\text{CuSO}_4{\cdot}5\text{H}_2\text{O} \rightleftharpoons \text{CuSO}_4 + 5\text{H}_2\text{O}CuSO4⋅5H2O⇌CuSO4+5H2O.
- This colour change is used as a test for the presence of water.
Exam technique
- Use the ⇌\rightleftharpoons⇌ symbol whenever a reaction is described as reversible.
- Name the colours both ways for copper sulfate: blue when hydrated, white when anhydrous.
- State the condition that drives each direction, such as heating to remove water.
Self review
- What is a reversible reaction?
- What symbol shows that a reaction is reversible?
- What colour change happens when water is added to anhydrous copper sulfate?
- Give one condition that can change the direction of a reversible reaction.
