Oxidation and Reduction (Redox)
Get to grips with Oxidation and Reduction (Redox) for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. In terms of electrons, remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.
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Topic overview
Oxidation and reduction can be defined in two ways. In terms of oxygen, oxidation is gaining oxygen and reduction is losing it. In terms of electrons, oxidation is the loss of electrons and reduction is the gain.
The mnemonic OIL RIG helps: Oxidation Is Loss, Reduction Is Gain — referring to electrons. Both definitions describe the same reactions from different angles.
A redox reaction is one in which both oxidation and reduction occur together. They must occur together, because the electrons lost by one substance are the electrons gained by another. In displacement reactions, the more reactive metal is oxidised as it loses electrons, and the metal ions in solution are reduced.
Revision notes
Two definitions
In terms of oxygen: oxidation is the gain of oxygen, reduction is the loss.
In terms of electrons: oxidation is the loss of electrons, reduction is the gain. OIL RIG — Oxidation Is Loss, Reduction Is Gain.
Why they occur together
The electrons lost by one substance must be gained by another.
Oxidation and reduction therefore always happen together, which is why such reactions are called redox reactions.
Displacement as redox
When magnesium displaces copper from copper sulfate, magnesium atoms lose electrons to form Mg²⁺ ions, so magnesium is oxidised.
Copper ions gain electrons to form copper atoms, so they are reduced. Identifying which substance loses and which gains electrons is how these questions are answered.
Key points
- Oxidation is the gain of oxygen or loss of electrons.
- Reduction is the loss of oxygen or gain of electrons.
- OIL RIG refers to electrons.
- Oxidation and reduction occur together.
- A redox reaction involves both.
- In displacement, the more reactive metal is oxidised.
Worked examples
Example 1
State what happens to electrons during oxidation. [1 mark]
Model answer
Electrons are lost (1 mark).
Example 2
Magnesium reacts with copper sulfate. Explain which substance is oxidised and which is reduced. [4 marks]
Model answer
Magnesium atoms lose electrons to form Mg²⁺ ions (1 mark), so magnesium is oxidised (1 mark). Copper ions gain electrons to form copper atoms (1 mark), so the copper ions are reduced (1 mark).
Example 3
Explain why oxidation and reduction must occur together. [2 marks]
Model answer
The electrons lost by one substance during oxidation must be gained by another substance (1 mark), so reduction always occurs at the same time (1 mark).
Common mistakes
Reversing OIL RIG.
Oxidation Is Loss of electrons; Reduction Is Gain.
Mixing the oxygen and electron definitions.
Gaining oxygen is oxidation, but LOSING electrons is oxidation. They are not contradictory but must not be confused.
Saying only one process occurs.
Both always happen together in a redox reaction.
Not identifying which substance does what.
Say explicitly which is oxidised and which is reduced.
Exam tips
- Use OIL RIG but remember it refers to electrons.
- Say explicitly which substance is oxidised and which reduced.
- Explain that the two processes must occur together.
- Write ion charges correctly, such as Mg²⁺.
Key terms
- Redox
- A reaction involving both oxidation and reduction.
- Oxidation
- Loss of electrons, or gain of oxygen.
- Reduction
- Gain of electrons, or loss of oxygen.
- OIL RIG
- A mnemonic for oxidation and reduction in terms of electrons.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.