Physical Chemistry
7 marks · EasyThis question is about catalysts and dynamic equilibrium.
State what is meant by a catalyst.
Describe how you could show that manganese(IV) oxide is a catalyst in the decomposition of hydrogen peroxide.
Define dynamic equilibrium in a reversible reaction.
Explain why the concentrations of reactants and products remain constant at dynamic equilibrium, although both reactions are still occurring.
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(a) A catalyst is a substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction. · must show not used up, speeds up reaction
(b) Measure and compare the rate of oxygen production with and without manganese(IV) oxide. Filter and dry the manganese(IV) oxide after the reaction and show its mass is unchanged. · must show compare rate with/without catalyst, recover and weigh catalyst to show unchanged
(c) Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction in a closed system. · must show rate of forward = rate of reverse, closed system
(d) At dynamic equilibrium, both the forward and reverse reactions continue to occur at the same rate. As a result, the concentrations of reactants and products remain constant over time, even though reactions are still taking place. · must show reactions continue, equal rates cause constant concentrations
Common mistakes: Saying catalysts are used up in reactions (wrong); Saying concentration of reactants equals concentration of products at equilibrium (wrong)
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Marking points
B1 (a) · 1 mark · Correctly states that a catalyst increases rate but is chemically unchanged after the reaction.
B2 (b) · 1 mark · Describes measuring and comparing the rate of oxygen evolution with and without the catalyst.
B3 (b) · 1 mark · Explains recovery and weighing of catalyst before and after to show it is unchanged.
B4 (c) · 1 mark · States at equilibrium, rate of forward reaction equals rate of reverse reaction.
B5 (c) · 1 mark · Mentions that equilibrium occurs in a closed/sealed system.
B6 (d) · 1 mark · Explains both reactions continue at same rate at equilibrium.
B7 (d) · 1 mark · Explains that concentrations remain constant because forward and reverse reactions are balanced.
Worked solution
a) A catalyst is a substance that increases the rate of a chemical reaction but is not used up and remains chemically unchanged at the end.
b) To show manganese(IV) oxide is a catalyst, compare the rate of the hydrogen peroxide decomposition reaction with and without it. The reaction is much faster with the catalyst. Afterwards, filter, dry and weigh the manganese(IV) oxide—its mass will be the same as before, showing it was not used up.
c) Dynamic equilibrium refers to the state in a closed system when the rate of the forward reaction equals the rate of the reverse reaction.
d) Even though both reactions are happening at the same time, the rates are equal. This means that the concentrations of all reactants and products stay constant—the amount of each does not change over time, even though particles continue to react.