Electrochemistry

• The oxidising agent is the one that gets reduced. Say it until it is automatic.
• Loss of electrons is a rise in oxidation number. Total rise = total fall. The oxidising agent is the one that gets reduced.

• 6.1.1 I can calculate — calculate oxidation numbers of elements in compounds and ions
• 6.1.2 I can use — use changes in oxidation numbers to help balance chemical equations
• 6.1.3 I can explain — explain and use the terms redox, oxidation, reduction and disproportionation in terms of electron transfer and changes in oxidation number
• 6.1.4 I can explain — explain and use the terms oxidising agent and reducing agent
• 6.1.5 I can use — use a Roman numeral to indicate the magnitude of the oxidation number of an element

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “The oxidising agent is oxidised.” Correct The oxidising agent is the one that gets reduced. Its name says what it does to the other species; it takes their electrons, so its own oxidation number falls. The reducing agent is the one that gets oxidised.
• “Chromium in Cr₂O₇²⁻ is +12.” Correct +12 is the total for two chromium atoms. An oxidation number is per atom: +6.
• “Oxygen is always −2 and hydrogen is always +1.” Correct Oxygen is −1 in peroxides (H₂O₂, Na₂O₂, BaO₂) and +2 in OF₂. Hydrogen is −1 in metal hydrides (NaH, CaH₂).
• “An oxidation number of +8/3 must be a mistake.” Correct A fractional value is an average over atoms of one element that are not all in the same state. Fe₃O₄ holds one Fe at +2 and two at +3; the average is +8/3. Do not round it.
• “Chlorine appears on both sides, so Cl₂ + 2KI → 2KCl + I₂ is disproportionation.” Correct Disproportionation needs the same element, starting in one species, to be both oxidised and reduced. Here chlorine is only reduced and iodine only oxidised.
• “Oxidation numbers are written 2+ and ionic charges +2.” Correct The other way round. An oxidation number is written sign first (+2, −1); an ionic charge is written number first (Fe²⁺, a 2+ ion). In a name the magnitude alone goes in Roman numerals: iron(II).
• “Acidify the manganate(VII) titration with hydrochloric acid.” Correct Use dilute sulfuric acid. Manganate(VII) oxidises chloride ions to chlorine, so extra titrant is used and the titre is too high.
• “Oxygen is always −2.” Repair −1 in peroxides (H₂O₂, Na₂O₂, BaO₂) and +2 in OF₂, because fluorine is more electronegative than oxygen.
• “Hydrogen is always +1.” Repair −1 in metal hydrides (NaH, CaH₂), where the metal is the less electronegative partner.
• “Chlorine is always −1 in compounds.” Repair +1 in ClO⁻, +5 in ClO₃⁻, +7 in ClO₄⁻: bonded to oxygen, chlorine is the less electronegative atom and is positive.
• “Chromium in Cr₂O₇²⁻ is +12.” Repair +12 is the total for two atoms; the oxidation number is per atom, +6.
• “An oxidation number cannot be a fraction, so Fe in Fe₃O₄ must be +3.” Repair +8/3 is the correct average of one Fe at +2 and two at +3. Report the average; do not round it.
• “Nitrogen in NH₄NO₃ is +1.” Repair The salt is two ions; assign each separately: −3 in NH₄⁺ and +5 in NO₃⁻. An average over different ions describes neither nitrogen.
• “The oxidising agent is oxidised.” Repair It oxidises the other species and is itself reduced: it accepts electrons and its oxidation number falls.
• “H₂ + Cl₂ → 2HCl is not redox because no ions form.” Repair Oxidation numbers change (H 0 → +1, Cl 0 → −1), so it is redox; that is why the oxidation-number definition exists.
• “Cl₂ + 2KI → 2KCl + I₂ is disproportionation because chlorine is on both sides.” Repair Disproportionation needs the same element to be both oxidised and reduced; here chlorine is only reduced and iodine only oxidised.
• “MnO₄⁻ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ is balanced.” Repair The oxygen and the charge are not (+9 against +17): add 8H⁺ on the left and 4H₂O on the right.
• Leaving 5e⁻ in the overall equation after adding two half-equations. Repair Scale the half-equations so the electrons lost equal the electrons gained; they then cancel exactly. An overall equation contains no electrons.
• “iron (+III) chloride” or “iron III chloride”. Repair iron(III) chloride — brackets, no sign, no space.
• “sodium(I) chloride” or “sodium sulfate(IV)”. Repair The numeral is used only for elements with more than one oxidation number, so sodium chloride; and the syllabus keeps the traditional name sodium sulfite.
• Acidifying a manganate(VII) titration with hydrochloric acid. Repair Dilute sulfuric acid; MnO₄⁻ oxidises Cl⁻ and the titre is too high.
• “Add phenolphthalein to the manganate(VII) titration.” Repair It is self-indicating; the end point is the first permanent pale pink.
• “The largest source of error is human error in reading the burette.” Repair Never acceptable. Name the cause and its effect: the burette's ±0.05 cm³ per reading (0.43% of a 23.10 cm³ titre), or air oxidation of Fe²⁺, which lowers the titre.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 11 (Group 17) reuses the chlorine disproportionations and the halogens as oxidising agents — re-answer the ladder question there. Topic 12 reuses the oxidation numbers of nitrogen and sulfur. Topic 24 gives each half-equation written here an electrode potential, and Topic 28 explains the variable oxidation numbers of manganese, chromium and iron — reassign MnO₄⁻ and Cr₂O₇²⁻ when you reach each. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 6 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on this topic turns on one decision made correctly: an oxidation number in an unfamiliar ion, which species is the oxidising agent, whether a reaction is disproportionation, or the coefficient a balanced equation needs. A Paper 2 question asks you to define the terms, calculate oxidation numbers, deduce a balanced ionic equation and explain a classification from the numbers.
• Oxidation numbers, including fractional averages; the mole ratio from a balanced redox equation; and the titration calculation that uses that ratio. The only Data-section values needed are relative atomic masses from the Periodic Table. Electrode potentials are not used at all in this topic.
• Topic 6 is the theory behind the redox titrations the syllabus names: potassium manganate(VII) with iron(II), hydrogen peroxide or ethanedioate, and sodium thiosulfate with iodine. Burette readings to 0.05 cm³, concordant titres within 0.10 cm³, a mole ratio from the equation, percentage errors and an improvement.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.