DP Chemistry (last assessment 2024)
Question 21N.2.sl.TZ0.6c(iv)
| Date | November 2021 | Marks available | [Maximum mark: 1] | Reference code | 21N.2.sl.TZ0.6c(iv) |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Deduce | Question number | c(iv) | Adapted from | N/A |
c(iv).
[Maximum mark: 1]
21N.2.sl.TZ0.6c(iv)
Biochemical oxygen demand (BOD) can be determined by the Winkler Method.
A 25.00 cm3 sample of water was treated according to the Winkler Method.
Step I: 2Mn2+ (aq) + O2 (g) + 4OH− (aq) → 2MnO2 (s) + 2H2O (l)
Step II: MnO2 (s) + 2I− (aq) + 4H+ (aq) → Mn2+ (aq) + I2 (aq) + 2H2O (l)
Step III: 2S2O32− (aq) + I2 (aq) → 2I− (aq) + S4O62− (aq)
The iodine produced was titrated with 37.50 cm3 of 5.000 × 10−4 mol dm−3 Na2S2O3.
(c(iv))
The three steps of the Winkler Method are redox reactions.
Deduce the reduction half-equation for step II.
[1]
Markscheme
MnO2 (s) + 2e− + 4H+ (aq) → Mn2+ (aq) + 2H2O (l) ✔
