| Date | May 2013 | Marks available | 2 | Reference code | 13M.3.sl.TZ2.E4 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | State | Question number | E4 | Adapted from | N/A |
Question
In order to make waste water acceptable for drinking, it is treated in a series of steps to remove hazardous substances.
Tertiary treatment removes phosphates, nitrates and heavy metal ions from water.
State an ionic equation, including the state symbols, to show how hydrogen sulfide gas, \({{\text{H}}_{\text{2}}}{\text{S(g)}}\), is able to remove mercury(II) ions, \({\text{H}}{{\text{g}}^{2 + }}{\text{(aq)}}\), when it is bubbled through a water sample.
Markscheme
\({\text{H}}{{\text{g}}^{2 + }}{\text{(aq)}} + {{\text{S}}^{2 - }}{\text{(aq)}} \to {\text{HgS(s)}}/{{\text{H}}_2}{\text{S(g)}} + {\text{H}}{{\text{g}}^{2 + }}{\text{(aq)}} \to {\text{HgS(s)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}}/\)
\({{\text{H}}_2}{\text{S(aq)}} + {\text{H}}{{\text{g}}^{2 + }}{\text{(aq)}} \to {\text{HgS(s)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}}\);
correctly balanced equation;
correct state symbols;
Neither mark can be awarded for an incorrect equation.
Examiners report
Part a), relating to the reaction precipitating mercury sulfide, was very challenging and arguably marginal to the syllabus and as a result hardly any students gained any marks; unbalanced equations, especially yielding hydrogen as a product, proliferated. The process of eutrophication was better known, but many students incorrectly attributed the depletion of oxygen to the excessive growth of plants and algae rather than their subsequent death and decay.
