Rates of Reaction
5 marks · EasyA student investigates the rate of reaction between excess marble chips (calcium carbonate) and dilute hydrochloric acid in a flask. The CO produced escapes, causing the total mass to decrease. The student records the following results:
| Time (min) | Mass lost (g) |
|---|---|
| 0 | 0.00 |
| 1 | 0.52 |
| 2 | 0.81 |
| 3 | 0.95 |
Calculate the average rate of reaction in g/min during the first 2 minutes. Show your working.
Use collision theory to explain why the rate of reaction changes after the first 2 minutes. Refer to the results from part (a) in your answer.
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) 0.405 g/min · must show use of mass lost over 2 minutes, appropriate calculation
(b) — · must show reference to fewer acid particles, decreased collision frequency, linked to rate becoming slower (from results in a)
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct subtraction and identifying time interval (mass lost over 2 minutes).
A1 (a) · 1 mark · Correct calculation of rate with suitable units:
B1 (b) · 1 mark · States (or implies) that rate of reaction decreases after first 2 minutes (uses result from a).
M2 (b) · 1 mark · Explains decrease in rate using particle/collision theory (e.g. fewer acid particles/collisions).
M3 (b) · 1 mark · Links decrease in collision frequency or successful collisions to slower rate of reaction.
Worked solution
a) The average rate of reaction = total mass lost / time interval.
So, the average rate during the first 2 minutes is .
b) After the first 2 minutes, the rate of reaction becomes slower. This is because, as the reaction progresses, there are fewer acid particles left to react with the marble chips. According to collision theory, the rate depends on how often the particles collide. Since there are fewer acid particles, collisions are less frequent, so the rate of reaction decreases compared to the start (as shown by the lower mass lost per minute after the first 2 minutes).