Explaining what's happening
4 marks · NormalThe graph below shows the concentration of hydrochloric acid over time during its reaction with excess calcium carbonate.
Describe how the rate of reaction changes during the experiment.
Explain the shape of the curve using 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) — · must show Rate is fastest at the start (steepest part of curve), Rate slows down as curve flattens
(b) — · must show At start, high concentration so more frequent collisions, As acid decreases, less frequent collisions, reaction slows
Common mistakes: Saying 'the rate stays the same' or 'curve is straight down'
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Marking points
B1 (a) · 1 mark · States that the reaction is fastest at the start (steepest part of curve)
B2 (a) · 1 mark · States that the rate slows down as the curve flattens / reaction slows as time passes
M1 (b) · 1 mark · Explains that a higher concentration at the start causes more frequent collisions
A1 (b) · 1 mark · States that as concentration falls, fewer collisions occur and the reaction slows (using collision theory language)
Worked solution
a) The rate of reaction is fastest at the start where the graph is steepest, then slows down as the graph flattens until the reaction stops.
b) At the beginning, there is a high concentration of acid so more frequent collisions between reactant particles. As the reaction proceeds, the concentration of acid decreases, meaning there are fewer acid particles and fewer collisions, which slows the reaction.