Explaining what's happening
3 marks · EasyA student observes the reaction between excess sodium carbonate and dilute hydrochloric acid. The table shows the concentration of acid over time.
| Time (min) | Acid concentration (mol/dm³) |
|---|---|
| 0 | 1.00 |
| 1 | 0.85 |
| 2 | 0.70 |
| 3 | 0.60 |
| 4 | 0.59 |
| 5 | 0.59 |
Between which two times was the reaction fastest? Explain your answer.
State why the reaction gradually slows down as shown in the table.
According to collision theory, what needs to happen for acid particles to react with sodium carbonate?
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) Between 0–1 min; the concentration of acid decreases most quickly then.
(b) The acid is being used up, so fewer acid particles are available for collisions so frequency of collisions decreases.
(c) The acid particles must collide with the sodium carbonate particles with enough energy (activation energy).
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly identifies 0–1 min as when reaction is fastest and gives a reason related to fastest decrease in concentration.
B1 (b) · 1 mark · Explains reaction slows as acid is used up so fewer particles left to collide.
B2 (c) · 1 mark · States that acid particles must collide with sodium carbonate particles with enough energy (activation energy) to react.
Worked solution
a) Fastest between 0 and 1 min because the acid concentration drops by 0.15 mol/dm³, the largest decrease in any 1-minute interval.
b) The reaction slows because acid is being used up, leaving fewer acid particles, so collisions become less frequent.
c) According to collision theory, acid particles must collide with sodium carbonate particles with at least the activation energy for a reaction.