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Chemistry · Unit 8

Redox Reactions

Oxidation and reduction

Redox is the other half of reaction chemistry alongside acid-base, and it covers rusting, respiration, batteries and metal extraction with one framework.

The unit covers oxidation and reduction in terms of electron transfer, oxidation states, half equations, oxidising and reducing agents, and the reactivity series.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps
20
Questions
120
Difficulty
1-3

What this unit covers

  • Oxidation and Reduction
  • Oxidation States
  • Half Equations
  • Oxidising and Reducing Agents
  • The Reactivity Series

Where this fits

Needs Chemical Reactions. Required for Electrolysis and for Metals and Extraction.

Where people slip

The agent is the opposite of what it does. An oxidising agent is itself reduced, because it took the electrons - a wording that trips people up on every paper.

How the unit is structured

Redox Reactions runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Half Equations

  • Build the sentenceLevel 2

    1. Build the rule for combining two half-equations into an overall equation.

    Answer: multiply each half-equation so the electrons cancel

    The electrons lost must equal the electrons gained, so you scale each half-equation until the electrons cancel.

  • Fact or fibLevel 2

    2. In an oxidation half equation, the electrons are written on the product (right-hand) side.

    Answer: True

    Oxidation is loss of electrons, so the electrons appear as a product on the right.

  • Fill the blankLevel 2

    3. In the half equation 2Cl- -> Cl2 + 2e-, the chloride ions are being ____.

    • oxidisedcorrect
    • reduced
    • neutralised
    • dissolved

    The chloride ions lose electrons (shown on the right), so they are oxidised.

  • Guess the numberLevel 2

    4. How many electrons are lost in the half equation Fe -> Fe3+ + ? e- ?

    Answer: 3 electrons

    Iron goes from 0 to +3, so it loses three electrons: Fe -> Fe3+ + 3e-.

  • Multiple choiceLevel 2

    5. Which half equation represents oxidation?

    • Zn -> Zn2+ + 2e-correct
    • Cu2+ + 2e- -> Cu
    • Cl2 + 2e- -> 2Cl-
    • Fe3+ + e- -> Fe2+

    Zn -> Zn2+ + 2e- shows a loss of electrons, which is oxidation.

  • Match the pairsLevel 3

    6. Match each part of standard cell notation to its meaning.

    Answer: Single vertical line | = Boundary between two phases; Double vertical line || = Salt bridge; Species on the far left = The anode, where oxidation occurs; Species on the far right = The cathode, where reduction occurs

    In cell notation a single line marks a phase boundary, a double line marks the salt bridge, oxidation is written on the left and reduction on the right.

Oxidation States

  • Multiple choiceLevel 1

    7. What is the oxidation state of an atom in an uncombined element, such as Fe metal or O2?

    • 0correct
    • +1
    • -1
    • +2

    Atoms in an uncombined element always have an oxidation state of 0.

  • Guess the numberLevel 2

    8. What is the usual oxidation state of oxygen in a compound like H2O or CO2?

    Answer: -2 oxidation state

    Oxygen almost always has an oxidation state of -2 in its compounds.

  • Match the pairsLevel 2

    9. Match each atom (in the given species) to its oxidation state.

    Answer: Fe in Fe metal = 0; Cl in NaCl = -1; Al in Al2O3 = +3; O in MgO = -2

    Uncombined Fe is 0, chloride is -1, aluminium is +3, and oxide is -2.

  • Sort into groupsLevel 2

    10. Sort each change by whether the species is oxidised or reduced.

    Answer: Fe2+ turns into Fe3+ = Oxidised; Cl2 turns into Cl- = Reduced; Zn turns into Zn2+ = Oxidised; Cu2+ turns into Cu = Reduced

    Losing electrons (oxidation number rises) is oxidation; gaining electrons (number falls) is reduction.

  • Fill the blankLevel 3

    11. Oxygen is usually -2, but in oxygen difluoride, OF2, its oxidation number is ____ because fluorine is more electronegative.

    • +2correct
    • -2
    • -1
    • 0

    Fluorine is always -1 and is more electronegative than oxygen, so the two fluorines give -2 and oxygen must be +2.

  • Odd one outLevel 3

    12. In which species does sulfur NOT have an oxidation number of +6?

    • SO2correct
    • SO4^2-
    • H2SO4
    • SO3

    In SO2 the two oxygens give -4, so sulfur is +4; in the other three species sulfur is +6.

Oxidation and Reduction

  • Fact or fibLevel 1

    13. The rusting of iron is an example of an oxidation reaction.

    Answer: True

    Rusting is oxidation - iron reacts with oxygen and water to form hydrated iron(III) oxide.

  • Fill the blankLevel 1

    14. The mnemonic OIL RIG means Oxidation Is Loss, Reduction Is ____ (of electrons).

    • Gaincorrect
    • Loss
    • Oxygen
    • Heat

    OIL RIG stands for Oxidation Is Loss, Reduction Is Gain - of electrons.

  • Multiple choiceLevel 1

    15. In terms of oxygen, what does oxidation mean?

    • A substance gains oxygencorrect
    • A substance loses oxygen
    • A substance gains hydrogen
    • A substance loses mass

    In terms of oxygen, oxidation is defined as the gain of oxygen by a substance.

  • Build the sentenceLevel 2

    16. Build the electron definition of oxidation.

    Answer: oxidation is the loss of electrons

    In terms of electrons, oxidation is the loss of electrons.

  • Choose all that applyLevel 2

    17. Which of these are examples of oxidation? (Select all that apply.)

    • Iron rustingcorrect
    • Magnesium burning in aircorrect
    • Ice melting into water
    • Sugar dissolving in water

    Rusting and burning both involve gaining oxygen (oxidation); melting and dissolving are physical changes, not oxidation.

  • Odd one outLevel 2

    18. Three of these describe a galvanic cell. Which statement is the odd one out?

    • It needs an external power supply to make the reaction happencorrect
    • It converts chemical energy into electrical energy
    • The reaction inside it is spontaneous
    • It produces a positive EMF

    A galvanic cell runs on a spontaneous reaction and needs no external power; requiring a power supply describes an electrolytic cell.

Oxidising and Reducing Agents

  • Build the sentenceLevel 2

    19. Build the sentence that states when a redox reaction is feasible.

    Answer: a positive cell EMF shows the reaction is feasible

    A positive cell EMF shows the reaction is feasible under standard conditions.

  • Fill the blankLevel 2

    20. The standard EMF of a cell is found using Ecell = E(cathode) - E(____).

    • anodecorrect
    • cathode
    • salt bridge
    • external wire

    The standard cell EMF equals the cathode potential minus the anode potential.

  • Match the pairsLevel 2

    21. Match each oxidising agent to the species it is reduced to.

    Answer: MnO4- = Mn2+; Cr2O7^2- = Cr3+; I2 = I-; Cl2 = Cl-

    Each oxidising agent gains electrons: MnO4- becomes Mn2+, Cr2O7^2- becomes Cr3+, I2 becomes I-, and Cl2 becomes Cl-.

  • Choose all that applyLevel 3

    22. Which conditions are part of the standard hydrogen electrode? (Select all that apply.)

    • Hydrogen gas at a pressure of 100 kPacorrect
    • A temperature of 298 Kcorrect
    • H+ ions at a concentration of 1 mol/dm3correct
    • A copper wire as the electrode

    The standard hydrogen electrode uses hydrogen gas at 100 kPa, a temperature of 298 K and 1 mol/dm3 H+, all at an inert platinised platinum electrode, not a copper one.

  • Fact or fibLevel 3

    23. A more positive standard electrode potential means a species is a stronger oxidising agent.

    Answer: True

    A more positive electrode potential shows the species is more readily reduced, so it takes electrons more easily and is a stronger oxidising agent.

  • Guess the numberLevel 3

    24. A cell is built from a magnesium half-cell (E = -2.37 V) and a copper half-cell (E = +0.34 V). What is the standard cell EMF in volts?

    Answer: 2.71 V

    Ecell = E(cathode) - E(anode) = (+0.34) - (-2.37) = +2.71 V.

The Reactivity Series

  • Multiple choiceLevel 1

    25. Which of these metals is the most reactive?

    • Potassiumcorrect
    • Iron
    • Copper
    • Gold

    Potassium sits near the top of the reactivity series, so it is the most reactive of these.

  • Fill the blankLevel 2

    26. Iron only rusts when it is in contact with both water and ____.

    • oxygencorrect
    • nitrogen
    • carbon dioxide
    • hydrogen

    Rusting is the oxidation of iron, which needs both water and oxygen; hydrated iron(III) oxide is formed.

  • Put in orderLevel 2

    27. Put these metals in order from most reactive to least reactive.

    Answer: Potassium -> Magnesium -> Zinc -> Copper

    The reactivity series places potassium above magnesium, then zinc, then copper.

  • Sequence recallLevel 2

    28. Recall the top of the reactivity series from most to least reactive.

    Answer: Potassium -> Sodium -> Calcium -> Magnesium

    The reactivity series begins potassium, sodium, calcium, magnesium.

  • Tap the pairsLevel 2

    29. Match each metal to its chemical symbol.

    Answer: Potassium = K; Sodium = Na; Iron = Fe; Copper = Cu

    These are the standard symbols: potassium K, sodium Na, iron Fe and copper Cu.

  • Sort into groupsLevel 3

    30. Sort each metal by how it is usually extracted from its ore.

    Answer: Aluminium = Electrolysis; Sodium = Electrolysis; Iron = Carbon reduction; Zinc = Carbon reduction

    Aluminium and sodium are more reactive than carbon and need electrolysis, while iron and zinc are less reactive and can be extracted by reduction with carbon.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

  • Build the sentence
  • Choose all that apply
  • Fact or fib
  • Fill the blank
  • Guess the number
  • Listen and choose
  • Match the pairs
  • Multiple choice
  • Odd one out
  • Picture question
  • Put in order
  • Sequence recall
  • Sort into groups
  • Spell it
  • Tap the pairs
  • True or false
  • Type the answer

Practise Redox Reactions

120 questions across 20 steps. Start with step one and crawl at your own pace.

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Read about Redox Reactions

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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