The shape of the lesson
By the end of the lesson, learners can
- Oxygen in, oxygen out: The simplest definitions use oxygen.
- Electrons and oxidation numbers: Extended.
- Oxidising and reducing agents: Extended.
- Oxygen: gain is oxidation, loss is reduction.
- Electrons (Extended): loss is oxidation, gain is reduction.
- Oxidation number: up is oxidation, down is reduction.
- Oxidising agent is reduced; reducing agent is oxidised.
One picture every idea returns to
Oxidation is gain of oxygen and reduction is loss of oxygen; a reaction in which both happen is a redox reaction. A Roman numeral in a name gives the oxidation number of the element, as in iron(III) oxide. Extended: oxidation is also loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number. An oxidising agent oxidises another substance and is itself reduced; a reducing agent reduces another substance and is itself oxidised.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One loses, the other gains.. Oxidation and reduction always happen together: if one substance gains oxygen, another loses it; if one loses electrons, another picks them up. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Oxygen in, oxygen out | The simplest definitions use oxygen. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Electrons and oxidation numbers | Extended. Project tab 2 of the lab (Electrochemistry lab, electron transfer) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Oxidising and reducing agents | Extended. Project tab 4 of the lab (Electrochemistry lab, oxidised or reduced?) and let a student drive it while the class predicts. | Jot Thoughts |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Oxidising and reducing agents, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The oxidising agent is the substance that is reduced, not the one that is oxidised. | It sounds backwards, so reason it out: it causes oxidation in something else by taking that thing's electrons, and taking electrons is reduction. |
Redox: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
In the blast furnace, Fe2O3 + 3CO → 2Fe + 3CO2. Identify the substance oxidised and the substance reduced, explaining in terms of oxygen.
Give the formulae of (a) copper(II) oxide and (b) iron(II) chloride, and state what the Roman numerals show.
(Extended) Magnesium reacts with copper(II) sulfate solution: Mg + Cu2+ → Mg2+ + Cu. In terms of electrons, state which species is oxidised and which is reduced, and name the reducing agent.
(Extended) Use oxidation numbers to show that 2Fe2+ + Cl2 → 2Fe3+ + 2Cl− is a redox reaction.
(Extended) Define oxidising agent and reducing agent.
(Extended) Describe a test to show that a solution contains an oxidising agent, giving the result.
Answer key and marking notes
- Carbon monoxide is oxidised: it gains oxygen to form carbon dioxide.
- Iron(III) oxide is reduced: it loses oxygen to form iron.
- (a) CuO; (b) FeCl2.
- The Roman numeral gives the oxidation number (the charge on the metal ion): +2 in both.
- Magnesium is oxidised: it loses two electrons (0 → +2).
- Cu2+ is reduced: it gains two electrons (+2 → 0).
- Magnesium is the reducing agent.
- Iron: +2 → +3, an increase, so Fe2+ is oxidised.
- Chlorine: 0 → −1, a decrease, so chlorine is reduced. Both happen, so it is redox.
- Oxidising agent: a substance that oxidises another substance and is itself reduced (it gains electrons).
- Reducing agent: a substance that reduces another substance and is itself oxidised (it loses electrons).
- Add aqueous potassium iodide.
- It turns from colourless to brown (iodine forms), showing an oxidising agent is present.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Lead is extracted by heating lead(II) oxide with carbon: 2PbO + C → 2Pb + CO2. Which substance is oxidised?
- Acarbon
- Blead(II) oxide
- Clead
- Dcarbon dioxide
What is the formula of iron(III) chloride?
- AFeCl3
- BFeCl2
- CFe3Cl
- DFe2Cl3
Which of these reactions is a redox reaction?
- A2Cu + O2 → 2CuO
- BNaOH + HCl → NaCl + H2O
- CCaCO3 → CaO + CO2
- DAgNO3 + NaCl → AgCl + NaNO3
What is the oxidation number of sulfur in sulfur dioxide, SO2?
- A+4
- B+2
- C−2
- D+6
Which describes the change Fe2+ → Fe3+ + e−?
- Aoxidation, because an electron is lost
- Breduction, because an electron is lost
- Creduction, because the charge on the ion increases
- Doxidation, because an electron is gained
In the reaction Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g), which species is the oxidising agent?
- AH+
- BMg
- CMg2+
- DH2
A solution Z turns aqueous potassium iodide from colourless to brown, but it does not change the colour of acidified potassium manganate(VII). What does this suggest about Z?
- AZ is an oxidising agent
- BZ is a reducing agent
- CZ is both an oxidising agent and a reducing agent
- DZ is neither an oxidising agent nor a reducing agent
Bromine water reacts with aqueous potassium iodide: Br2 + 2I− → 2Br− + I2. Which statement explains why this is a redox reaction?
- AThe oxidation number of bromine decreases from 0 to −1 and that of iodine increases from −1 to 0
- BIt cannot be redox, because no oxygen is gained or lost
- CBromine is oxidised from 0 to −1 and iodide ions are reduced
- DIodide ions gain electrons and bromine loses electrons
Quiz key
Carbon gains oxygen to become carbon dioxide, so carbon is oxidised. Lead(II) oxide loses its oxygen, so it is reduced, not oxidised. Lead and carbon dioxide are products; the question of what is oxidised is about which reactant gains oxygen.
Iron(III) means Fe3+, and each chloride ion carries a charge of 1−, so three chloride ions balance one iron ion: FeCl3. FeCl2 is iron(II) chloride. Fe3Cl wrongly reads the numeral as the number of iron atoms, and Fe2Cl3 does not balance the charges.
Copper gains oxygen to form copper(II) oxide, so copper is oxidised and oxygen is reduced: a redox reaction. Neutralisation, the thermal decomposition of calcium carbonate and the precipitation of silver chloride only swap or split up ions and molecules; no element gains or loses oxygen or changes its oxidation number, so none of them is redox.
Two oxygens contribute 2 × (−2) = −4, so sulfur must be +4 for the total to be zero. +2 counts only one oxygen, −2 is sulfur's oxidation number in a sulfide ion, and +6 is its oxidation number in sulfur trioxide, SO3.
The electron appears as a product, so the iron(II) ion loses an electron, and loss of electrons is oxidation; the oxidation number rises from +2 to +3. Loss of electrons is never reduction, an increase in charge (oxidation number) means oxidation rather than reduction, and no electron is gained here.
Each hydrogen ion gains an electron (+1 → 0) to form hydrogen gas, so H+ is reduced and it is the oxidising agent that oxidises the magnesium. Magnesium loses electrons, so it is the reducing agent. Mg2+ and H2 are products, and the agents are always named among the reactants.
Turning iodide ions into brown iodine is an oxidation, so Z is an oxidising agent. A reducing agent would decolourise purple manganate(VII), and Z does not, so Z is not a reducing agent. That rules out both being true, and the positive iodide result rules out neither.
Bromine goes from 0 in Br2 to −1 in Br− (a decrease, so reduction), while iodine goes from −1 in I− to 0 in I2 (an increase, so oxidation). Both happen, so it is redox even though no oxygen is involved. A decrease in oxidation number is reduction, not oxidation, and it is the iodide ions that lose electrons and bromine that gains them.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.