Redox in Terms of Oxygen
This topic introduces the foundational definitions of oxidation and reduction based on the transfer of oxygen. Understanding these basic concepts is the first step towards analysing more complex redox reactions.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Oxidation is the chemical process where a substance gains oxygen atoms.
- Reduction is the chemical process where a substance loses oxygen atoms.
- A common example of oxidation is combustion, such as magnesium burning in air to form magnesium oxide (2Mg + O2 → 2MgO).
- A common example of reduction is the extraction of metals from their ores, like iron oxide being reduced to iron in a blast furnace (Fe2O3 + 3CO → 2Fe + 3CO2).
Definitions
- Oxidation
- The gain of oxygen by a substance during a chemical reaction.
- Reduction
- The loss of oxygen from a substance during a chemical reaction.
Worked example
Consider the following reaction, which is used to extract lead from its ore: 2PbO + C → 2Pb + CO2. Identify the substance that undergoes oxidation and the substance that undergoes reduction.
- 1
Analyse the lead oxide (PbO).
In the reactants, lead is bonded to oxygen.
In the products, it exists as pure lead (Pb).
PbO has lost its oxygen.
- 2
The definition of reduction is the loss of oxygen.
Therefore, lead oxide (PbO) is reduced.
- 3
Analyse the carbon (C).
It starts as a pure element in the reactants and becomes carbon dioxide (CO2) in the products.
Carbon has gained oxygen.
- 4
The definition of oxidation is the gain of oxygen.
Therefore, carbon (C) is oxidised.
Answer: Carbon (C) is oxidised, and lead oxide (PbO) is reduced.
Common mistakes
- ×Confusing the two definitions. Remember that 'Oxidation' sounds like 'Oxygen', reinforcing the idea that it involves gaining oxygen.
- ×Incorrectly identifying the species being changed. Focus on the entire substance that loses or gains the oxygen (e.g., PbO is reduced), not just the metallic element within it.
No-calculator tips
- ✓This is a conceptual topic. There are no calculations.
- ✓In a chemical equation, physically trace the 'O' atoms from the reactants side to the products side. The substance that 'gives away' its oxygen is reduced; the substance that 'takes on' oxygen is oxidised.