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Reversible reactions and equilibrium
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Some reactions can go both ways. In a closed system they reach a balance called dynamic equilibrium.
Reversible reactions
The symbol ⇌ shows a reaction can go forwards and backwards.
ammonium chloride ⇌ ammonia + hydrogen chloride
ammonium chloride ⇌ ammonia + hydrogen chloride
Dynamic equilibrium
In a closed system, the forward and backward reactions happen at the same rate.
The amounts of reactants and products stay constant, although both reactions continue.
The amounts of reactants and products stay constant, although both reactions continue.
The Haber process and changing conditions (Higher)
N2 + 3H2 ⇌ 2NH3 (forward reaction is exothermic). Conditions: 450 °C, 200 atmospheres, iron catalyst.
If conditions change, the equilibrium shifts to oppose the change:
Higher pressure → shifts to the side with fewer gas molecules.
Higher temperature → shifts in the endothermic direction.
If conditions change, the equilibrium shifts to oppose the change:
Higher pressure → shifts to the side with fewer gas molecules.
Higher temperature → shifts in the endothermic direction.
Why does high pressure increase the yield of ammonia?
- N2 + 3H2 has 4 gas molecules; 2NH3 has 2.
- Higher pressure favours the side with fewer molecules.
Answer: It shifts the equilibrium towards ammonia.
⇌ means reversible. At dynamic equilibrium, forward and backward rates are equal and amounts stay constant. Changes shift the equilibrium to oppose them.
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