Question 19M.3.sl.TZ2.2a
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.3.sl.TZ2.2a |
| Level | sl | Paper | 3 | Time zone | TZ2 |
| Command term | Determine | Question number | a | Adapted from | N/A |
Bromine and methanoic acid react in aqueous solution.
Br2 (aq) + HCOOH (aq) → 2Br− (aq) + 2H+ (aq) + CO2 (g)
The reaction was monitored by measuring the volume of carbon dioxide produced as time progressed.
Determine from the graph the rate of reaction at 20 s, in cm3 s−1, showing your working.
[3]
tangent drawn to curve at t = 20 s [✔]
slope/gradient calculation [✔]
0.35 «cm3 s–1» [✔]
Note: Accept values in the range 0.32–0.42 «cm3 s–1».
This question was challenging for many students. Quite a few candidates did draw a tangent line at 20s for 1 mark, show a slope/gradient calculation of the line for 1 mark, and had a reasonable final value for the final mark. Some candidates only found the average rate by finding the ratio of the value at that data point and received one mark (16/20=0.80 cm3 s-1). Candidates also received one mark if they had a correct answer with no work since the question clearly asked students to show their work.
