Edexcel IAL Chemistry 8.2 Oxidation numbers in compounds and ions

Assign the fixed values first, include every formula subscript and set the weighted sum equal to the species charge before comparing initial and final states.

Exam points

Calculate oxidation numbers in ions, peroxides, metal hydrides and complexes.
Order species by one element's oxidation number or identify equal oxidation states.
Determine oxidation-number changes for each named element across a reaction.

[Maximum number: 1]

Chlorine dioxide, $\mathrm{ClO}_{2}$ , can be used to treat water to make it safe for drinking. The molecule is a free radical. One suggested structure is shown.

Figure for Question 8.2—The oxidation number of elements in compounds and ions question 1 — Edexcel A-Level Chemistry AS

Chlorine dioxide can be made from potassium chlorate(V), $\mathrm{KClO}_{3}$ , in a two-step process.

Step 1 Potassium chlorate(V) reacts with concentrated sulfuric acid to form compound A.

Step 2 Compound A then undergoes a disproportionation reaction to form chlorine dioxide and hydrogen chlorate(VII), $\mathrm{HClO}_{4}$ .

State the oxidation number of chlorine in chlorine dioxide.

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8.2—The oxidation number of elements in compounds and ions question 1