Tin(IV) oxide can be reduced by heating it with carbon. The equation for this reaction is: SnO₂ + 2C → Sn + 2CO. Explain why tin(IV) oxide is reduced in this reaction and why carbon is oxidised. In your answer, use both the gain/loss of oxygen definition and the gain/loss of electrons definition.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tin has been extracted from tin(IV) oxide by heating with carbon for thousands of years. The equation for this reaction is: SnO₂ + 2C → Sn + 2CO.
Model answer (5 marks)
Tin(IV) oxide is reduced because it loses oxygen: SnO₂ → Sn + 2O. The loss of oxygen means the tin ions (Sn⁴⁺) are reduced to tin metal (Sn⁰). In terms of electrons, the Sn⁴⁺ ions gain 4 electrons to form Sn⁰.
Carbon is oxidised because it gains oxygen: C + 2O → CO₂ (or CO). The gain of oxygen means the carbon atoms lose electrons, changing from 0 to +2 oxidation state. Thus carbon is the reducing agent, providing the electrons that reduce Sn⁴⁺ to Sn⁰.
Therefore, SnO₂ is reduced (loss of O, gain of e⁻) and C is oxidised (gain of O, loss of e⁻).
Carbon is oxidised because it gains oxygen: C + 2O → CO₂ (or CO). The gain of oxygen means the carbon atoms lose electrons, changing from 0 to +2 oxidation state. Thus carbon is the reducing agent, providing the electrons that reduce Sn⁴⁺ to Sn⁰.
Therefore, SnO₂ is reduced (loss of O, gain of e⁻) and C is oxidised (gain of O, loss of e⁻).
Examiner tips
- Use the exact wording ‘loss of oxygen’ and ‘gain of electrons’ for reduction; ‘gain of oxygen’ and ‘loss of electrons’ for oxidation.
- Show both definitions in the same sentence to demonstrate understanding of the dual nature of redox.
Common mistakes
- Confusing the direction of electron flow – writing that tin gains oxygen instead of losing it.
- Failing to mention that carbon is the reducing agent or that it loses electrons.
Mark scheme (5 marks)
- Tin(IV) oxide is reduced because it loses oxygen (from SnO₂ to Sn)
- Carbon is oxidised because it gains oxygen (from C to CO)
- Reduction is the gain of electrons, so tin ions (Sn⁴⁺) gain electrons to form tin metal
- Oxidation is the loss of electrons, so carbon atoms lose electrons during the reaction
- Carbon acts as a reducing agent because it causes the reduction of tin(IV) oxide (by providing electrons or removing oxygen)
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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