| Date | May 2016 | Marks available | 2 | Reference code | 16M.3.hl.TZ0.8 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | State | Question number | 8 | Adapted from | N/A |
Question
Metal ions may cause unwanted environmental effects.
The presence of iron(III) ions can catalyse the formation of hydroxyl radicals from O2− and H2O2 in the Haber–Weiss reaction. State the equations for this process.
Zinc ions, toxic to aquatic life, may be removed by adding a solution containing hydroxide ions. Determine the concentration of zinc ions in a saturated solution of zinc hydroxide at 298K using information from section 32 of the data booklet.
Markscheme
Fe3+ +•O2- →Fe2+ +O2
Fe2+ +H2O2 →Fe3+ +•OH+OH-
Award [1] for the net equation H2O2 + • O2– → HO•+OH– +O2.
Accept the hydroxyl and superoxide radicals represented without the radical symbols as long as there is consistent use of the radical symbol.
Ksp=[Zn2+][OH-]2
OR
[OH-] = 2[Zn2+]
\(\ll {\rm{[Z}}{{\rm{n}}^{\rm{2}}}{\rm{] = }}\sqrt[3]{{7.5 \times {{10}^{ - 18}}}}{\rm{ = }}\gg {\rm{2.0}} \times {\rm{1}}{{\rm{0}}^{ - {\rm{6}}}}\ll {\rm{mold}}{{\rm{m}}^{ - {\rm{3}}}}\gg \)
Award [2] for correct final answer.
Award [1 max] for 5.5×10–9 OR 6.0×10–9«moldm–3».
