Section 1 of 4What Makes a Reaction Reversible
Products react back
- In a reversible reaction the products can react with each other or decompose to reform the original reactants.
Symbol
- The symbol ⇌ shows that the forward and reverse reactions happen at the same time, with one half-arrow pointing each way.
Closed systems
- A reversible reaction reaches a balance only in a closed system, where substances cannot escape or enter the container.
Going to completion
- Some reactions go to completion, when the reactants are all used up and the products do not react back.
Changing direction
- Changing conditions such as temperature can favour the forward or the reverse reaction.
Section 2 of 4Ammonium Chloride
1
Heating solid ammonium chloride decomposes it into ammonia and hydrogen chloride gases: NH₄Cl(s) ⇌ NH₃(g) + HCl(g).
2
The forward reaction is endothermic, because energy is taken in to break the bonds in the solid.
3
As the hot gases cool, they recombine to form solid ammonium chloride on the cooler parts of the apparatus.
4
The reverse reaction is exothermic, releasing heat energy to the surroundings as new bonds form.
Section 3 of 4Hydrated Copper(II) Sulfate
Hydrated Copper(II) Sulfate
Water of crystallisation
- Water of crystallisation is water that is held inside the crystal structure of some salts.
Hydrated salt
- Hydrated copper(II) sulfate, CuSO₄·5H₂O, forms blue crystals containing five water molecules per formula unit.
Anhydrous salt
- Anhydrous copper(II) sulfate is a white powder, formed when the blue crystals are heated to drive off the water.
Simple test
- The colour change from white to blue on adding water is used as a simple test for the presence of water.
| Direction | Reaction | Energy change | Colour change |
|---|---|---|---|
| Forward | CuSO₄·5H₂O → CuSO₄ + 5H₂O | Endothermic | Blue to white |
| Reverse | CuSO₄ + 5H₂O → CuSO₄·5H₂O | Exothermic | White to blue |
Section 4 of 4Exam Checklist & Tips
3.3.1 – Key Points for the Exam
- Reversible reaction A reaction that can proceed in both the forward and reverse directions.
- Reversible symbol (⇌) A pair of opposing half-arrows used to show that a reaction is reversible.
- Water of crystallisation Water molecules that are part of the crystal structure of a salt.
- Hydrated Containing water of crystallisation.
- Anhydrous Containing no water.
Common Mistakes
- Using a single forward arrow for a reversible reaction, rather than the double arrow ⇌.
- Stating that a reversible reaction reaches balance in an open container, where gases can escape.
- Confusing the coloursanhydrous copper(II) sulfate is white, and the hydrated salt is blue.
- Describing the ammonium chloride reaction as going only one way, without its reverse.