CAIE IGCSE Chemistry 6.4.9 Redox Using Oxidation Numbers a Questions

Calculate oxidation numbers using charge rules and formula sums, then use changes in oxidation number to identify redox.

Exam points

apply rules for uncombined elements, monatomic ions, neutral compounds and ions
calculate an unknown oxidation number from a neutral formula or charged ion
use oxidation-number changes to identify oxidation and reduction
check oxidation states in dichromate, permanganate, halogen and metal-ion compounds

[Maximum number: 2]

Hydrogen iodide thermally decomposes into iodine and hydrogen. The reaction is reversible.

\underset{\text { colourless gas }}{2 \mathrm{HI}(\mathrm{~g})} \rightarrow \underset{\substack{\mathrm{I}_{2}(\mathrm{~g}) \\ \text { purple gas }}}{\rightleftharpoons}+\underset{\substack{\mathrm{H}_{2}(\mathrm{~g}) \\ \text { colourless gas }}}{ }

Fig. 4.1 shows a gas syringe containing a mixture of hydrogen iodide, iodine and hydrogen gases. The gas syringe is sealed and the mixture is heated to $300^{\circ} \mathrm{C}$ . The mixture of gases reaches equilibrium and is purple.

Fig. 4.1

Deduce the oxidation number of iodine, I, in:

HI
$\mathrm{I}_{2}$ .

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CAIE IGCSE Chemistry 6.4.9 Redox Using Oxidation Numbers a Questions question 1