equilibrium

Question 1

3:18 describe reversible reactions such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride

Answer 1

Heating ammonium chloride:
- On heating, white solid ammonium chloride decomposes forming ammonia and hydrogen chloride gas. On cooling, ammonia and hydrogen chloride react to form a white solid of ammonium chloride:
- ammonium chloride ⇌ ammonia + hydrogen chloride

Dehydration of copper(II) sulfate
- heat to turn blue crystals white (endothermic) or add water to turn white crystals blue (exothermic)
- hydrated copper sulphate ⇌ anhydrous copper sulfate + water

Question 2

3:20 (Triple only) know the characteristics of a reaction at dynamic equilibrium

Answer 2

• the concentrations of reactants and products remain constant
• rate of forward reaction = rate of backward reaction

Question 3

3:21 (Triple only) understand why a catalyst does not affect the position of equilibrium in a reversible reaction

Answer 3

A catalyst is a substance which increases the rate of reaction without being chemically changed at the end of the reaction.

A reversible reaction is one where the forward reaction and the backward reaction happen simultaneously.

In such a reaction a catalyst speeds up both the forward and the backward reactions. Hence, although the system will reach dynamic equilibrium more quickly, the addition of a catalyst will not affect the position of equilibrium.

Question 4

3:22 (Triple only) predict, with reasons, the effect of changing either pressure or temperature on the position of equilibrium in a reversible reaction (references to Le Chatelier’s principle are not required)

Answer 4

If the conditions of an equilibrium reaction are changed, the reaction moves to counteract that change.

If a change in conditions moves equilibrium to the right, the yield of the substances on the right is increased.

temperature:
- All reactions are exothermic in one direction and endothermic in the other way
- CO(g) +2H2(g) ⇌ CH3OH(g) ΔH = –91 kJ/mol
- If temperature is decreased the position of the equilibrium will shift to the right because it is an exothermic reaction.

pressure:
- Reactions may have more molecules of gas on one side than on the other.
- CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) ΔH = +210 kJ mol–1
- If the pressure is increased the position of the equilibrium will shift to the left because there are fewer molecules on the left-hand side.