Chemistry › Redox
Redox
Unit C07 · 3 lessons.
Oxidation is loss of electrons, with or without oxygen, and redox is the bookkeeping of where those electrons go. Oxidation states say where they are, half-equations say how many move, and two half-equations combine into a balanced redox equation. The titrations with manganate(VII) and with thiosulfate then use those equations to find an unknown amount.
A half-equation must balance in charge as well as in atoms, so count the charge on each side before writing the electrons in. A titration answer then depends on the ratio from the full equation: one manganate(VII) ion reacts with five iron(II) ions, so a mole ratio of one to one puts the answer out by a factor of five.
Questions on it can come up on AQA Papers 1 and 3, OCR A Papers 1, 2 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1, 2 and 4; the past-paper pages say what else each of those papers covers.
Specification sections: AQA 3.1.7, 3.2.3.2, 3.2.5.5 · OCR A 2.1.5, 3.1.3, 5.2.3 · Edexcel 3.1 to 3.13, 4.3 to 4.12, 14.1 to 14.19 · CAIE 6.1, 11.4, 24.2, 28.2, 33.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Oxidation states and the rules for assigning them
Oxidation and reduction as loss and gain of electrons and as a rise and fall in oxidation state, the rules for assigning oxidation states in order of priority including peroxides, OF₂ and metal hydrides, oxidation states in polyatomic ions and in compounds such as K₂Cr₂O₇, NH₄NO₃, Fe₃O₄ and Na₂S₂O₃, the metal in complex ions and oxo-ions, Group 2 metals with chlorine, metals with dilute acids, oxidation number as the other boards' name, oxidising and reducing agents, and Roman numerals in names.
- Half-equations and building redox equations
Half-equations for oxidation and reduction, the four-step method for balancing a half-equation in acidic solution, half-equations in alkaline solution such as chromium(III) to chromate(VI), combining two half-equations by matching electrons, balancing from oxidation-number changes, the manganate(VII) with iron(II) and dichromate(VI) with iodide equations, identifying the species oxidised and reduced, spectator ions, and disproportionation of chlorine in water and in cold sodium hydroxide, and in hot concentrated sodium hydroxide to chlorate(V).
- Redox titrations: manganate(VII) and thiosulfate
Manganate(VII) titrations, why they are self-indicating and acidified with sulfuric acid rather than hydrochloric acid, the 1 : 5 ratio with iron(II) and the 2 : 5 ratio with ethanedioate and methanoic acid, autocatalysis, iodine–thiosulfate titrations and when to add starch, chlorate(I) in bleach, standardising thiosulfate with potassium iodate(V), primary standards and dichromate(VI) as a titrant, the calculation chain from titre to moles to mass, the mass of iron in a tablet, copper by iodine liberation, and percentage purity.
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The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
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