Rates of Reaction
7 marks · HardSome students investigate the effect of pressure on the rate of the reaction between two liquids. They then study the catalytic decomposition of a liquid, hydrogen peroxide.
Explain, with a reason, why increasing the pressure has little effect on the rate of reaction between only liquids or only solids.
State what is meant by the term 'catalyst'.
In an experiment, a student adds manganese(IV) oxide to hydrogen peroxide solution and quickly fits a bung and gas syringe. The apparatus is shown below.
Describe how the student could show that manganese(IV) oxide is a true catalyst in this reaction.
Explain why increasing the pressure would not speed up the same hydrogen peroxide decomposition experiment if all the reactants were liquid.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Increasing pressure has little effect on rate because particles in solids and liquids are already close together, so there is little change in collision frequency. · must show States little effect on rate, Explains that particles are close together/packed
(b) A catalyst is a substance that speeds up a chemical reaction but is chemically unchanged at the end. · must show Not used up/chemically unchanged, Speeds up reaction
(c) Weigh the manganese(IV) oxide before and after the reaction or recover it by filtration, dry it, and show its mass is unchanged, demonstrating it is not used up. · must show Weigh/filter and dry the catalyst before and after, Show mass or appearance is unchanged
(d) Because in liquids, the particles are already very close together, so increasing pressure does not increase collision frequency and does not speed up the reaction. · must show Particles already close in liquids, Pressure doesn't change rate in liquids
Common mistakes: Saying pressure affects liquids/solids as much as gases; Not mentioning catalyst is unchanged; Not mentioning weighing or filtering catalyst for proof
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States increasing pressure has little/no effect on rate in solids/liquids.
M2 (a) · 1 mark · Explains that particles are already close together/packed; little change in collision frequency.
M3 (b) · 1 mark · Correctly defines catalyst (speeds up reaction and not used up/chemically unchanged).
M4 (c) · 1 mark · Describes weighing/filtering catalyst before and after reaction/drying and showing mass or appearance unchanged.
M5 (c) · 1 mark · States that mass or appearance is unchanged—proves it is a catalyst.
M6 (d) · 1 mark · States that in liquids, particles are already close together.
M7 (d) · 1 mark · Explains why increased pressure does not increase collision frequency or rate for liquid reactions.
Worked solution
a) The particles in solids and liquids are packed together closely, so compressing them by increasing pressure does not significantly increase their collision frequency. Thus, pressure has little effect on their reaction rate.
b) A catalyst is a substance that increases the rate of a reaction but is not used up (it is chemically unchanged) at the end of the reaction.
c) After the reaction, filter and dry the manganese(IV) oxide. Weigh it before and after. If the mass has not changed and the appearance is the same, then it has not been used up and is a catalyst.
d) In a reaction where all reactants are liquids, the molecules are already very close together, so increasing pressure does not significantly change how often they collide. This means the rate is not affected by pressure.