| Date | November 2009 | Marks available | 1 | Reference code | 09N.3.sl.TZ0.C4 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Identify | Question number | C4 | Adapted from | N/A |
Question
Rechargeable nickel-cadmium batteries are used in portable electrical equipment and emergency lighting.
The discharge process can be summarized by the equation below.
\[{\text{2NiO(OH)(s)}} + {\text{Cd(s)}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O(l)}} \rightleftharpoons {\text{2Ni(OH}}{{\text{)}}_{\text{2}}}{\text{(s)}} + {\text{Cd(OH}}{{\text{)}}_{\text{2}}}{\text{(s)}}\]
State the change in oxidation number of the cadmium and deduce if it is acting as the positive or negative electrode during the discharge process.
Identify a physical property of Cd(OH)2 which allows this process to be reversed and the battery recharged.
Markscheme
\(0 \to + 2\) / increase by 2;
negative;
If decrease by 2, positive, award [1]. If decrease by 2, negative, award [0].
insoluble (\({\text{C}}{{\text{d}}^{2 + }}\) ions do not escape into solution);
Do not accept solid.
Examiners report
Most candidates were able to identify the change of oxidation number of cadmium but very few identified the insolubility of cadmium hydroxide as the physical property which allows the process to be reversed.
Most candidates were able to identify the change of oxidation number of cadmium but very few identified the insolubility of cadmium hydroxide as the physical property which allows the process to be reversed.
