6.4 BETA Verified

Redox

13 learning objectives • 5 core • 8 extended

Topic 6.4: Redox Revision Notes

1. Overview

Redox reactions (Reduction-Oxidation) are fundamental chemical processes where the transfer of oxygen or electrons occurs between substances. These reactions are essential in industrial processes, such as the extraction of metals in a blast furnace and the generation of energy in batteries and fuel cells.

Key Definitions

  • Redox Reaction: A reaction where oxidation and reduction occur simultaneously.
  • Oxidation: The gain of oxygen, the loss of electrons, or an increase in oxidation number.
  • Reduction: The loss of oxygen, the gain of electrons, or a decrease in oxidation number.
  • Oxidising Agent: A substance that oxidises another substance and is itself reduced.
  • Reducing Agent: A substance that reduces another substance and is itself oxidised.
  • Oxidation Number: A number assigned to an atom in a compound that represents the number of electrons lost or gained by that atom.

Core Content

Simultaneous Reactions

In any redox reaction, one substance is always oxidised while another is reduced. They cannot happen in isolation.

Oxidation and Reduction in terms of Oxygen

  • Oxidation is the gain of oxygen.
  • Reduction is the loss of oxygen.

Example: Reaction between Copper(II) Oxide and Hydrogen

  • Word Equation: copper(II) oxide + hydrogen → copper + water
  • Symbol Equation: CuO (s) + H₂ (g) → Cu (s) + H₂O (l)
  • Analysis:
    • Copper(II) oxide loses oxygen to become copper; therefore, CuO is reduced.
    • Hydrogen gains oxygen to become water; therefore, H₂ is oxidised.

Using Roman Numerals

Roman numerals are used in the names of compounds to indicate the oxidation number of an element that can have multiple oxidation states (usually transition metals).

  • Iron(II) chloride: FeCl₂ (Iron has an oxidation state of +2)
  • Iron(III) chloride: FeCl₃ (Iron has an oxidation state of +3)
  • Manganese(IV) oxide: MnO₂ (Manganese has an oxidation state of +4)

Extended Content (Extended Curriculum Only)

Oxidation and Reduction in terms of Electrons

To remember the transfer of electrons, use the mnemonic OIL RIG:

  • Oxidation Is Loss (of electrons)
  • Reduction Is Gain (of electrons)

Example: Magnesium reacting with Oxygen 2Mg (s) + O₂ (g) → 2MgO (s)

  • Magnesium atoms lose electrons: Mg → Mg²⁺ + 2e⁻ (Oxidation)
  • Oxygen atoms gain electrons: O₂ + 4e⁻ → 2O²⁻ (Reduction)

Oxidation Numbers

You can identify redox reactions by tracking changes in oxidation numbers using these rules:

  1. Elements: The oxidation number of an element in its uncombined state is 0 (e.g., Zn, O₂, Cl₂).
  2. Monatomic Ions: The oxidation number is the same as the charge (e.g., Na⁺ is +1, S²⁻ is -2).
  3. Compounds: The sum of oxidation numbers in a neutral compound is 0.
  4. Polyatomic Ions: The sum of oxidation numbers equals the charge on the ion (e.g., in SO₄²⁻, the sum is -2).
  • Oxidation = An increase in oxidation number.
  • Reduction = A decrease in oxidation number.

Identifying Redox using Color Changes

Two specific reagents are used in the lab to test for oxidising or reducing agents:

  1. Acidified Aqueous Potassium Manganate(VII):

    • This is a strong oxidising agent.
    • Colour change: Purple → Colourless.
    • If it turns colourless, it means it has reacted with a reducing agent.

  2. Aqueous Potassium Iodide:

    • This is a reducing agent.
    • Colour change: Colourless → Brown (due to the formation of I₂).
    • If it turns brown, it means it has reacted with an oxidising agent.

Key Equations

Reaction Type Equation Notes
Metal Displacement Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s) Zn is oxidised (0 to +2), Cu is reduced (+2 to 0)
Extraction of Iron Fe₂O₃ (s) + 3CO (g) → 2Fe (l) + 3CO₂ (g) Fe₂O₃ is reduced, CO is the reducing agent
Half-equation (Ox) Mg (s) → Mg²⁺ (aq) + 2e⁻ Shows loss of electrons
Half-equation (Red) Cl₂ (g) + 2e⁻ → 2Cl⁻ (aq) Shows gain of electrons

Common Mistakes to Avoid

  • ❌ Wrong: Saying "The reducing agent is reduced."
  • ✅ Right: The reducing agent is the substance that gets oxidised (it gives electrons away to reduce something else).
  • ❌ Wrong: Forgetting state symbols in redox equations.
  • ✅ Right: Always include (s), (l), (g), or (aq) as they often indicate the type of reaction taking place.
  • ❌ Wrong: Thinking oxidation is only about oxygen.
  • ✅ Right: In the extended curriculum, always check electron transfer or oxidation numbers first.

Exam Tips

  • Command Words:
    • "State": Give a brief answer (e.g., "State the colour change" -> "Purple to colourless").
    • "Explain": You must give a reason (e.g., "Explain why this is redox" -> "Because magnesium loses electrons and copper gains electrons").
    • "Identify": Pick the specific substance from the equation.
  • Typical Contexts: Questions often use the context of vehicle catalytic converters (reducing NOₓ), chemical manufacturing, or thermal decomposition in a blast furnace.
  • Observation Questions: If you see Potassium Manganate(VII) mentioned, the answer almost always involves the word "Purple." If you see Potassium Iodide, the answer almost always involves "Brown."
  • Oxidation Numbers: Remember that Oxygen is almost always -2 (except in peroxides) and Hydrogen is +1 (except in metal hydrides). Use these as anchors to calculate unknown oxidation numbers.