Reversible reactions and equilibrium
Reversible reactions
In a , the products can react to form the reactants again. The symbol ⇌ shows this:
A + B ⇌ C + D
The goes from left to right (reactants → products). The goes from right to left. Changing the conditions can change the direction.
Example: heating ammonium chloride breaks it down into ammonia and hydrogen chloride. On cooling, they react to form ammonium chloride again.
ammonium chloride ⇌ ammonia + hydrogen chloride
Key terms
Energy in reversible reactions
If a reversible reaction is in one direction, it is in the other. The same amount of energy is transferred each way.
Example: heating blue hydrated copper sulfate drives off its water, leaving white anhydrous copper sulfate. This direction is endothermic.
CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O
Adding water to white anhydrous copper sulfate turns it blue again and releases energy — this direction is exothermic. This colour change is used as a test for water.
Key terms
Dynamic equilibrium
When a reversible reaction happens in a (nothing can get in or out), it reaches . At equilibrium:
- the forward and reverse reactions happen at exactly the same rate
- the of all the reactants and products stay the same — although they are not necessarily equal
It is called dynamic equilibrium because both reactions are still happening. They just cancel each other out, so nothing seems to change.