Rates of Reaction
5 marks · EasyA student carries out an experiment to measure how quickly hydrogen peroxide decomposes using a catalyst. The volume of oxygen gas collected every 30 seconds is shown on a graph: the curve is steep at first, then becomes less steep and finally flat.
State one piece of apparatus the student could use to collect the oxygen gas.
Describe how using a different catalyst could help the student measure the effect of catalysts on the rate.
State what happens to the rate of reaction as shown by the graph.
Explain why the rate of reaction changes in this way, using ideas from collision theory.
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.
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(a) Gas syringe or measuring cylinder over water
(b) Repeat the experiment using a different catalyst and compare the rates (speed of oxygen production)
(c) The rate decreases as the reaction goes on
(d) At the start, there are more reactant particles, so more frequent collisions per second. As the reactants are used up, there are fewer particles left, resulting in fewer successful collisions and the reaction slows down. · must show More collisions at start, Slower as reactants used
Common mistakes: Suggesting irrelevant apparatus; Not linking rate change to concentration; Mixing up role of catalyst
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Names suitable apparatus for gas collection.
B2 (b) · 1 mark · Describes use of different catalyst in repeated experiment.
B3 (c) · 1 mark · Correctly states that reaction rate decreases over time.
E1 (d) · 1 mark · Explains more collisions at start, rate slows as reactants decrease.
E2 (d) · 1 mark · Specifically mentions collision frequency or successful collisions using collision theory.
Worked solution
a) A gas syringe or an inverted measuring cylinder over water can be used to collect the oxygen gas.
b) Use the same conditions each time but change to a different catalyst, then measure and compare the volume of oxygen collected over time to compare rates.
c) The reaction is fastest at the start and the rate decreases as the reaction progresses.
d) There are more reactant particles at the beginning, so there are more frequent collisions and more successful reactions per second. As reactants are used up, collisions become less frequent and the rate slows.