6.4 Redox

2026 Syllabus Objectives

Core:

  1. Use a Roman numeral to indicate the oxidation number of an element in a compound
  2. Define redox reactions as involving simultaneous oxidation and reduction
  3. Define oxidation as gain of oxygen and reduction as loss of oxygen
  4. Identify redox reactions as reactions involving gain and loss of oxygen
  5. Identify oxidation and reduction in redox reactions

Supplement: 6. Define oxidation in terms of: (a) loss of electrons (b) an increase in oxidation number 7. Define reduction in terms of: (a) gain of electrons (b) a decrease in oxidation number 8. Identify redox reactions as reactions involving gain and loss of electrons 9. Identify redox reactions by changes in oxidation number using: (a) the oxidation number of elements in their uncombined state is zero (b) the oxidation number of a monatomic ion is the same as the charge on the ion (c) the sum of the oxidation numbers in a compound is zero (d) the sum of the oxidation numbers in an ion is equal to the charge on the ion 10. Identify redox reactions by the colour changes involved when using acidified aqueous potassium manganate(VII) or aqueous potassium iodide 11. Define an oxidising agent as a substance that oxidises another substance and is itself reduced 12. Define a reducing agent as a substance that reduces another substance and is itself oxidised 13. Identify oxidising agents and reducing agents in redox reactions

Combustion and Redox Reactions

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What is Combustion? 🔥

Combustion is a chemical reaction in which a substance reacts with oxygen. The reaction is exothermic, meaning it releases energy in the form of heat and light.

Examples of combustion reactions:

1. Combustion of Methane (Natural Gas)

Methane is the main component of natural gas and is an important source of energy for homes and industry.

CH4(g)+2O2(g)→CO2(g)+2H2O(g)\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}

In this reaction:

  • Methane gains oxygen
  • Carbon dioxide and water vapour are produced
  • Energy is released

2. Respiration - Combustion of Glucose

Our bodies obtain energy from food through a process called respiration. During digestion, carbohydrates are broken down into glucose. The glucose then reacts with oxygen in our cells to release energy.

C6H12O6+6O2→6CO2+6H2O\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}

This overall reaction of respiration is:

  • Exothermic (releases energy)
  • Essential for everyday activities
  • A combustion reaction

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