IB Chemistry HL 3.2.2 Half-equations Question Bank

Practise IB Chemistry HL 3.2.2 by applying half-equations to exam-style questions.

[Maximum number: 4]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

Aqueous sodium chlorate(I), NaOCl, the most common active ingredient in chlorine based bleaches, oxidizes coloured materials to colourless products while being reduced to the chloride ion. It will also oxidize sulfur dioxide to the sulfate ion.

Deduce a balanced equation for the reaction between the chlorate(I) ion and sulfur dioxide from the appropriate half-equations.

$\mathrm{ClO}^{-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{Cl}^{-}(\mathrm{aq})$ ;
$\mathrm{SO}_{4}{ }^{2-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{SO}_{2}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) ;$

Marking guidance:

Accept $\mathrm{SO}_{4}{ }^{2-}(a q)+4 \mathrm{H}^{+}(a q)+2 e^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)$ .
For final equation:
$\mathrm{ClO}^{-}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{SO}_{4}{ }^{2-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$
Accept $\mathrm{ClO}^{-}(a q)+\mathrm{H}_{2} \mathrm{SO}_{3}(a q) \rightleftharpoons \mathrm{SO}_{4}{ }^{2-}(a q)+2 \mathrm{H}^{+}(a q)+\mathrm{Cl}^{-}(a q)$ .
correct reactants and products;
balancing and cancelling $\mathrm{e}^{-}, \mathrm{H}^{+}$ and $\mathrm{H}_{2} \mathrm{O}$ ;

Apply ECF if incorrect half-equations written.
Ignore state symbols and absence of equilibrium arrow for all equations and accept inclusion of $\mathrm{Na}^{+}$ in any equation.
(ii) Award [2] for all correct, [1] for 2 or 3 correct.

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3.2.2—Half-equations question 1