6.3 Reversible reactions and equilibrium

1. Features of equilibrium

When the reversible reaction achieves the state of equilibrium in a closed system,

a) the rate of the forward reaction is equal to the rate of the reverse reaction;

b) the concentrations of reactants and products are no longer changing.

Hydrated salt and anhydrous can establish a reversible reaction when heat or addition of water:

3. Factors affect the position of equilibrium

Concentration

With increasing concentration, equilibrium will shift to the side which should consume it. With decreasing concentration, equilibrium will shift the side which should replace it. Exercises: State and explain how to increase the yield of ester (CH₃COOC₂H₅) by changing concentration. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Addition of CH₃COOH or C₂H₅OH. Equilibrium will shift to right side to consume the additional reactants.

Removal of water. Equilibrium will shift to right side to replace the loss of water.

Pressure

Increasing pressure favours to the side which has less gaseous particles.

Decreasing pressure favours to the side which has more gaseous particles.

Exercises: State and explain how increasing pressure affects the yield in the following reaction.

a) 2NO(g) + O₂(g) ⇌ 2NO₂(g)

Increase. Less gaseous particles in the right side.

b) H₂(g) + Cl₂(g) ⇌ 2HCl(g)

No effect. Same amount of gaseous particles on both side.

Temperature

Increasing temperature favours to endothermic process.

Decreasing temperature favours to exothermic process.

Catalyst

Catalyst cannot affect the position of equilibrium because catalyst can speed up both forward and reverse reactions equally by lowering activation energy for both forward and reverse reactions (see the following diagram).

Summary

A(g) + 2B(g) ⇌ AB₂(g) ΔH<0

Change in		Position of equilibrium	Explain
Concentration	Add some A	shifts to right side	to consume the additional A
	Remove some B	shifts to left side	to replace the loss of B
Pressure	compress volume to increase pressure	shifts to right side	less gaseous particles in the right side
	decrease pressure	shifts to left side	more gaseous particles in left side
Temperature	increase temperature	shifts to left side	reverse reaction is endothermic
	decrease temperature	shifts to right side	forward reaction is exothermic
catalyst	addition of catalyst	no effect	catalyst speeds up both forward and reverse reactions equally