Reversible reactions
In some chemical reactions, the of the reaction can react together to produce the original . These reactions are called . They can be represented in the following way:
A + B ⇌ C + D
The symbol ⇌ has two half arrowheads, one pointing in each direction. It is used in equations that show reversible reactions:
- the forward reaction is the one that goes to the right
- the backward reaction is the one that goes to the left
The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.
Example
Ammonium chloride
Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again.
This reversible reaction can be modelled as:
ammonium chloride ⇌ ammonia + hydrogen chloride
NH4Cl(s) ⇌ NH3(g) + HCl(g)
Energy changes in reversible reactions
If a reaction is in one direction, it will be in the other direction. The same amount of energy is transferred in both the forwards and reverse reaction.
Copper sulfate
Blue copper sulfate is described as . The copper in its structure are surrounded by water . This water is driven off when blue hydrated copper sulfate is heated, leaving white copper sulfate. This reaction is reversible:
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(l)
The forward reaction is endothermic and the reverse reaction is exothermic.
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Dynamic equilibrium
When a reversible reaction happens in a closed container, it reaches a dynamic . At equilibrium:
- the forward and backward reactions are still happening
- the forward and backward reactions have the same rate of reaction
- the of all the reacting substances remain constant
Question
Nitrogen reacts with hydrogen to produce ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g)
Write the balanced equation for the backward reaction.
