| Date | November 2010 | Marks available | 1 | Reference code | 10N.1.hl.TZ0.33 |
| Level | HL | Paper | 1 | Time zone | TZ0 |
| Command term | Question number | 33 | Adapted from | N/A |
Question
For the electrolysis of aqueous copper(II) sulfate, which of the following statements is correct?
A. Cu and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 1:1
B. \({{\text{H}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 1:1
C. Cu and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 2:1
D. \({{\text{H}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 2:1
Markscheme
C
Examiners report
There were three G2 comments on this particular question, all of which suggested that the question was ambiguous as the nature of the electrodes was not stated for the electrolysis of aqueous copper(II) sulphate. Also noting the fact that candidates do not have access to a Data Booklet for P1. As regards the first point, it is a valid comment that the nature of electrodes could have been specified in the question itself.
However, on close examination of each of the first choices, it should be obvious to the candidate that inert electrodes were used as \({{\text{O}}_{\text{2}}}\) is given as a product in all four cases. This would not be the case if copper electrodes were used. As regards the second point, it is also a fair comment that in the case of the electrolysis of aqueous copper(II) sulfate, candidates can work out the specific products using the standard electrode potentials from Table 14. of the Data Booklet. However, to do so, this would have made this question a clear Objective 3 type question which is outside the realm of P1. This was discussed at length at GA and it was felt that, realistically, candidates should have seen this electrolysis in the laboratory and, hence, as regards this particular electrolysis process candidates should have been able to determine the correct mol ratio of 2:1 for Cu:\({{\text{O}}_{\text{2}}}\) . Although the question was, in fact, the hardest question on the entire paper, 43% of candidates did get the correct answer, C. Yet again this should emphasise the inherent importance of laboratory work as an integral part of the overall IB Diploma Chemistry programme.
