Explaining what's happening
3 marks · NormalA student reacts marble chips (calcium carbonate) with dilute hydrochloric acid in a beaker. At the start, there are 60 acid particles per cm³ in the solution. Every second, about 18 of these particles collide with the marble chips. Only 5 of these collisions have enough energy to result in a reaction and are called successful collisions.
Calculate the percentage of collisions that are successful each second at the start of the reaction.
Explain, using collision theory, what happens to the rate of reaction as the number of acid particles falls during the experiment.
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Show answer
(a) 28% · must show correct formula for (successful/total collisions) × 100
(b) The rate decreases because fewer acid particles means fewer collisions per second, so fewer successful collisions occur. · must show reference to collision frequency and successful collisions
Common mistakes: Calculating the percentage using total acid particles instead of total collisions
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Selects and applies correct formula: (number of successful collisions ÷ total collisions) × 100
A1 (a) · 1 mark · Calculates percentage as 28% (accept answers in the range 27–29%) (follow-through)
B1 (b) · 1 mark · Explains the fall in rate using collision theory: fewer acid particles ➔ fewer collisions per second ➔ fewer successful collisions
Worked solution
(a) There are 18 collisions each second, and 5 are successful.
(b) As the number of acid particles falls during the reaction, there are fewer particles available to collide with the marble chips each second. This leads to fewer collisions per second and therefore fewer successful collisions, which causes the reaction rate to decrease as the experiment continues.