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6.5 Reversible reactions

What reversible reactions are and the ⇌ symbol, how changing conditions changes the direction of a reaction (ammonium chloride, hydrated copper sulfate), and why the energy transferred is the same in each direction but exothermic one way and endothermic the other.

AQA Edexcel OCR 1 min read

Reactions that go both ways

Key terms

  • Reversible reaction: a reaction in which the products can react to make the original reactants again.
  • Forward reaction: the reaction from left to right in the equation.
  • Reverse (backward) reaction: the reaction from right to left.

In some chemical reactions, the products can react to produce the original reactants. These are reversible reactions, shown with the symbol ⇌:

A + B ⇌ C + D

The direction of a reversible reaction can be changed by changing the conditions, such as temperature.

Examples

Ammonium chloride: when heated, solid ammonium chloride decomposes into two gases. At the cool top of the test tube, they recombine to form white solid again.

ammonium chloride ⇌ ammonia + hydrogen chloride

Hydrated copper sulfate:

Heating blue hydrated copper sulfate gives white anhydrous copper sulfate and water (endothermic); adding water turns it blue again and gives out the same energy (exothermic)
Heating blue hydrated copper sulfate gives white anhydrous copper sulfate and water (endothermic); adding water turns it blue again and gives out the same energy (exothermic)
hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water

Heating removes the water, and the blue crystals turn white. Adding water turns the white powder blue again. This colour change is used as a test for water.

Energy changes

If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case.

  • Heating hydrated copper sulfate is endothermic: energy is taken in.
  • Adding water to anhydrous copper sulfate is exothermic: the same amount of energy is given out, and the mixture gets hot.

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