Question 22N.2.hl.TZ0.5d
| Date | November 2022 | Marks available | [Maximum mark: 2] | Reference code | 22N.2.hl.TZ0.5d |
| Level | hl | Paper | 2 | Time zone | TZ0 |
| Command term | Suggest | Question number | d | Adapted from | N/A |
Lignite, a type of coal, contains about 0.40 % sulfur by mass.
Sodium thiosulfate reacts with hydrochloric acid as shown:
Na2S2O3 (aq) + 2HCl (aq) → S (s) + SO2 (aq) + 2NaCl (aq) + H2O (l)
The precipitate of sulfur makes the mixture cloudy, so a mark underneath the reaction mixture becomes invisible with time.
Suggest two variables, other than concentration, that should be controlled when comparing relative rates at different temperatures.
[2]
Any two of:
depth/volume «of solution» ✔
colour/darkness/thickness/size/background of mark ✔
intensity of lighting in the lab ✔
Accept same size flask.
Accept position of observation/person observing.
Accept same equipment/apparatus.
Do not accept catalyst/particle size/pressure/time.
As is often the case with questions referencing practical skills, this was not well answered with the average being just over half marks with the mark achieved being largely unrelated to performance on the rest of the paper. It is unclear why factors that would have no effect and which cannot be controlled anyway (pressure and size of particle precipitated), are non-variables (such as not adding a catalyst), or are one of the actual variables measured (time/temperature) were more attractive to students than more obvious factors, such as the flask being used, the volumes of reagents, the mark being observed and the person observing it. Another shortcoming was that, even though two variables were specified, students frequently gave more and invoking the "list principle" which often resulted in the additional incorrect answers cancelling out marks already gained for correct responses.
