Reversible reactions in a sealed container
5 marks · HardExplain what happens to the forward and backward reaction rates when a reversible reaction first starts in a sealed container.
Describe what is meant by equilibrium in this context.
State one reason why equilibrium can only be reached if the container is sealed.
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(a) —
(b) —
(c) —
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that forward reaction rate is initially high and backward reaction rate is zero or low.
M2 (a) · 1 mark · Describes how the forward reaction rate decreases and backward reaction rate increases as products form.
B1 (b) · 1 mark · States that equilibrium is reached when forward and backward reaction rates are equal.
B2 (b) · 1 mark · Explains that concentrations of reactants and products remain constant at equilibrium.
B3 (c) · 1 mark · States that no substances can enter or leave a sealed container, allowing a constant system for equilibrium.
Worked solution
(a) At the start of a reversible reaction in a sealed container, the forward reaction rate is high because only reactants are present, while the backward reaction rate is zero since there are no products yet. As the reaction proceeds, the forward reaction rate decreases because the amount of reactants decreases, and the backward reaction rate increases as more products are formed.
(b) Equilibrium is reached when the rates of the forward and backward reactions are equal. At equilibrium, the concentrations of reactants and products remain constant, as both reactions continue but at the same rate.
(c) In a sealed container, equilibrium can be reached because no substances can enter or leave; this keeps the system closed so nothing is lost or gained.