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InkFlaskCambridge IGCSE

Lesson overview

Redox: Who loses electrons, who gains them?

Stand a zinc strip in blue copper(II) sulfate solution and a copper strip in colourless zinc sulfate, predict which strip will change, and time 10 minutes. Find out what oxidation and reduction are, by oxygen and by electrons, how oxidation numbers show them, which substance is the oxidising or the reducing agent, and two colour tests that identify one.

Suggested sequence

  • Lesson 1 · Oxygen in, oxygen out. Oxidation and reduction as gain and loss of oxygen, both at once in a redox reaction, and the Roman numeral in a name as an oxidation number. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
  • Lesson 2 · Electrons on the move. Supplement: oxidation and reduction as loss and gain of electrons and as changes in oxidation number; oxidising and reducing agents. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
  • Lesson 3 · Two colour tests. Supplement: two colour tests, with acidified potassium manganate(VII) and with potassium iodide, that identify a reducing or an oxidising agent. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
  • End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
  • One to two weeks later · return questions. 3 questions, 15 min, lesson closed.

Learning goals

Part 1: Oxygen in, oxygen out

  • Describe oxidation as gain of oxygen and reduction as loss of oxygen, happening together in a redox reaction; identify the substance oxidised and the substance reduced from an equation; read the Roman numeral in a name as the oxidation number of the element (6.4.1–6.4.5)

Part 2: Electrons on the move

  • Supplement: describe oxidation as loss of electrons and an increase in oxidation number, and reduction as gain of electrons and a decrease in oxidation number; work out oxidation numbers with the rules; identify redox reactions, oxidising agents and reducing agents (6.4.6–6.4.9, 6.4.11–6.4.13)

Part 3: Two colour tests

  • Supplement: describe the colour change of acidified aqueous potassium manganate(VII) with a reducing agent and of aqueous potassium iodide with an oxidising agent, and use the two tests to identify reducing and oxidising agents (6.4.10–6.4.13)

Misconceptions to listen for

  • “Row A.” — That swaps them. Tin(IV) oxide loses oxygen, so it is reduced; carbon gains oxygen, so it is oxidised.
  • “Row B.” — Carbon dioxide and tin are the products. The substances oxidised and reduced are the reactants that gain and lose the oxygen.
  • “Row D.” — The oxygen that tin(IV) oxide loses goes to the carbon, so tin(IV) oxide is reduced at the same time as carbon is oxidised.
  • “Water is oxidised, because it contains oxygen.” — What matters is the change: water loses its oxygen to the magnesium, so water is reduced.
  • “Hydrogen is reduced, because it is set free.” — Hydrogen is a product. The reactant that lost the oxygen is the water, so water is reduced.
  • “The Roman numeral counts atoms, so copper(II) oxide has two copper atoms.” — The numeral gives the oxidation number of the element, here the charge on the metal ion: copper(II) means Cu2+, and copper(II) oxide, CuO, has one copper for each oxygen.
  • “The (IV) counts the oxygen atoms.” — MnO2 has two oxygen atoms, not four. The numeral gives the oxidation number of manganese.
  • “Manganese has four electrons in its outer shell.” — The numeral in a name is about the element in that compound, not about its atom: manganese is +4 in MnO2.
  • “PbO2 is lead(II) oxide, because there are two oxygen atoms.” — The numeral is the oxidation number of lead, not a count of oxygen atoms. Two O2− ions give −4, so lead is +4: lead(IV) oxide.
  • “MgCl2 is magnesium(II) chloride.” — Magnesium only forms Mg2+ ions, so its compounds need no numeral, like those of sodium.

Scope

Redox (6.4 Core 1 to 5; Supplement 6 to 13): oxidation numbers from Roman numerals, oxidation and reduction as gain and loss of oxygen, of electrons and as changes in oxidation number, oxidising and reducing agents, and the colour tests with acidified potassium manganate(VII) and potassium iodide. Half-equations are in ELYS01, the reactivity series and displacement in REACT01, and the extraction of metals in EXTR01.

Open the first part as pupils see it