Rates of Reaction
7 marks · HardA laboratory chemist is investigating two gas-phase reactions: the catalytic decomposition of hydrogen peroxide, and the reaction of nitrogen and hydrogen to make ammonia. She explores how changes in pressure and the use of catalysts can affect the rate of these reactions.
Explain how increasing the pressure affects the rate of both these gas-phase reactions. Give a reason for your answer.
Describe the role of a catalyst in the decomposition of hydrogen peroxide. Name a catalyst commonly used for this reaction.
The chemist doubles the pressure in the reaction vessel for the decomposition of hydrogen peroxide (in the gas phase) while also adding a catalyst. Predict and explain how the combined changes will affect the rate of reaction, compared to the original conditions.
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(a) Increasing the pressure increases the rate of gas-phase reactions because the gas particles are closer together and collide more frequently. · must show States that increased pressure increases rate, Explains that it causes more frequent collisions
(b) A catalyst speeds up the decomposition of hydrogen peroxide by providing an alternative route with lower activation energy. Manganese(IV) oxide (MnO₂) is a common catalyst for this reaction. · must show Defines catalyst as substance that speeds up the reaction and is unchanged, Mentions lower activation energy or alternative pathway, Names MnO₂ or manganese(IV) oxide
(c) Both changes will increase the rate. Increasing pressure increases collision frequency, and the catalyst increases the proportion of successful collisions. Together, they cause a much faster reaction. · must show States that both pressure and catalyst increase rate, Explains effect of each (collision frequency, activation energy), States or implies combined effect is greater than either alone
Common mistakes: Saying pressure does not affect gases; Saying catalyst is used up; Confusing effect on yield with effect on rate
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Marking points
M1 (a) · 1 mark · States that increasing pressure increases reaction rate for gases.
M2 (a) · 1 mark · Explains increase is due to more frequent collisions.
M3 (b) · 1 mark · Describes role of catalyst: speeds up reaction, not used up, unchanged.
M4 (b) · 1 mark · Names a correct catalyst for hydrogen peroxide decomposition (e.g. manganese(IV) oxide / MnO₂).
M5 (c) · 1 mark · States both changes increase rate.
M6 (c) · 1 mark · Explains effect of pressure (collision frequency) and/or catalyst (lower activation energy/increases successful collisions).
A1 (c) · 1 mark · States or implies the combined effect is greater than either change alone.
Worked solution
a) Increasing the pressure in reactions involving gases increases the rate because it forces the gas particles closer together, so they collide more frequently in a given volume.
b) A catalyst is a substance that increases the rate of a chemical reaction but is not used up or changed permanently. In the decomposition of hydrogen peroxide, manganese(IV) oxide (MnO₂) is commonly used. The catalyst provides an alternative reaction pathway with a lower activation energy.
c) Doubling the pressure increases the frequency of particle collisions, so the reaction is faster. Adding a catalyst also increases the rate by making more collisions successful. Both changes together cause the reaction to proceed much faster than under original conditions, as the effects combine.