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Redox

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Notes de leçon

Oxidation & Reduction (Oxygen Definition)

  • Oxidation is the gain of oxygen by an element or compound.
  • Reduction is the loss of oxygen from an element or compound.
  • A redox reaction is one where oxidation and reduction occur simultaneously.
  • Example: ZnO + C → Zn + CO – zinc oxide is reduced (loses O), carbon is oxidised (gains O).

Reduction and oxidation shown for zinc oxide + carbon

Reduction and oxidation shown for zinc oxide + carbon

Oxidation & Reduction (Electron Definition – Extended)

  • Oxidation is the loss of electrons; reduction is the gain of electrons (OIL RIG).
  • Oxidation increases the oxidation number; reduction decreases it.
  • Half-equations show electron transfer: e.g., Ag → Ag⁺ + e⁻ (oxidation); O₂ + 4e⁻ → 2O²⁻ (reduction).
  • Example: Fe + Cu²⁺ → Fe²⁺ + Cu – Fe loses electrons (oxidised), Cu²⁺ gains electrons (reduced).

Redox reaction between Fe and Cu2+ shown by electron transfer

Redox reaction between Fe and Cu2+ shown by electron transfer

Oxidation Numbers & Roman Numerals

  • Oxidation number (state) shows the charge an atom would have if electrons were completely transferred.
  • Roman numerals in names indicate oxidation number: e.g., iron(II) = Fe²⁺, iron(III) = Fe³⁺.
  • Rules: uncombined elements = 0; monatomic ion = its charge; sum in compound = 0; sum in ion = charge.
  • Group 1 always +1, Group 2 always +2, F always –1, H usually +1, O usually –2.

Bohr model of iron atom showing electron shells. Iron can lose 2 electrons (Fe²⁺) or 3 electrons (Fe³⁺), corresponding to oxidation numbers +2 and +3.

Fe — Bohr model (2,8,14,2)26p30n

Identifying Redox Reactions by Colour Changes

  • Potassium manganate(VII) (KMnO₄) is a purple oxidising agent; with a reducing agent it turns colourless.
  • Potassium iodide (KI) is a colourless reducing agent; with an oxidising agent it turns red-brown (I₂ formed).
  • These colour tests are used to detect oxidising or reducing agents.

Colour-change tests for oxidising and reducing agents

Colour-change tests for oxidising and reducing agents

Oxidising & Reducing Agents (Extended)

  • An oxidising agent oxidises another substance and is itself reduced (gains electrons).
  • A reducing agent reduces another substance and is itself oxidised (loses electrons).
  • Common oxidising agents: O₂, Cl₂, KMnO₄, H₂O₂. Common reducing agents: C, H₂, KI.
  • Example: CuO + H₂ → Cu + H₂O – CuO is oxidising agent (reduced), H₂ is reducing agent (oxidised).

Displacement Reactions & Redox

  • In displacement reactions, the more reactive metal loses electrons (oxidised) and the less reactive metal ion gains electrons (reduced).
  • Example: Fe + CuSO₄ → FeSO₄ + Cu – Fe is oxidised, Cu²⁺ is reduced.
  • Ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu.

Half-Equations & Balancing (Extended)

  • Half-equations show either oxidation or reduction separately with electrons.
  • Oxidation half: atoms/ions lose electrons (e.g., Ni → Ni²⁺ + 2e⁻).
  • Reduction half: atoms/ions gain electrons (e.g., Ni³⁺ + e⁻ → Ni²⁺).
  • Balance atoms and charge: e.g., O₂ + 4e⁻ → 2O²⁻.

Extraction of Metals & Redox

  • Carbon is used to reduce metal oxides (e.g., ZnO + C → Zn + CO).
  • The metal oxide is reduced (loses oxygen), carbon is oxidised (gains oxygen).
  • Metals below carbon in the reactivity series can be extracted by reduction with carbon.

Diapos

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  1. 1.In terms of oxygen, what is oxidation?

    Easy
    • AGain of oxygen
    • BLoss of oxygen
    • CLoss of electrons
    • DGain of electrons

  2. 2.In terms of oxygen, what is reduction?

    Easy
    • ALoss of oxygen
    • BGain of oxygen
    • CLoss of electrons
    • DGain of electrons

  3. 3.What is a redox reaction?

    Easy
    • AA reaction where oxidation and reduction occur simultaneously
    • BA reaction where only oxidation occurs
    • CA reaction where only reduction occurs
    • DA reaction where no electron transfer occurs

  4. 4.In the reaction ZnO + C → Zn + CO, which substance is reduced?

    Easy
    • AZinc oxide (ZnO)
    • BCarbon (C)
    • CZinc (Zn)
    • DCarbon monoxide (CO)

  5. 5.In terms of electron transfer, oxidation is the:

    Easy
    • ALoss of electrons
    • BGain of electrons
    • CGain of oxygen
    • DLoss of oxygen

  6. 6.What is the oxidation number of iron in iron(III) oxide (Fe₂O₃)?

    Easy
    • A+3
    • B+2
    • C-3
    • D0

  7. 7.Which change in oxidation number indicates that the species has been reduced?

    Easy
    • ADecrease from +3 to +2
    • BIncrease from 0 to +2
    • CIncrease from -1 to 0
    • DNo change

  8. 8.In the reaction Cl₂ + 2KI → 2KCl + I₂, which species is reduced?

    Medium
    • AChlorine (Cl₂)
    • BPotassium iodide (KI)
    • CIodine (I₂)
    • DPotassium chloride (KCl)

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