Iron rusting is an example of a redox reaction. Explain what is meant by a redox reaction and describe what happens to iron and oxygen in terms of electrons and oxidation states during rusting.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron reacts with oxygen and water to form iron(III) oxide, commonly known as rust. The overall process involves the transfer of electrons between species.
Model answer (5 marks)
A redox reaction is one in which oxidation and reduction occur at the same time.
During rusting iron is oxidised – it loses electrons. The oxidation state of iron rises from 0 in the metal to +3 in Fe₂O₃.
Oxygen is reduced – it gains electrons. Its oxidation state falls from 0 in O₂ to –2 in the oxide.
Thus iron loses electrons and its oxidation number increases, while oxygen gains electrons and its oxidation number decreases.
During rusting iron is oxidised – it loses electrons. The oxidation state of iron rises from 0 in the metal to +3 in Fe₂O₃.
Oxygen is reduced – it gains electrons. Its oxidation state falls from 0 in O₂ to –2 in the oxide.
Thus iron loses electrons and its oxidation number increases, while oxygen gains electrons and its oxidation number decreases.
Examiner tips
- Use the phrase ‘oxidation and reduction occur simultaneously’ to show you understand redox. Mention the change in oxidation states (+3 for Fe, –2 for O) to hit the specific marks. Keep the answer concise – no extra explanation beyond the required points.
- common_mistakes
- :
- Writing that iron gains electrons or that oxygen loses electrons. Forgetting to state the change in oxidation numbers. Using the wrong oxidation state for iron (e.g. +2 instead of +3).
Mark scheme (5 marks)
- A redox reaction is one in which both oxidation and reduction occur simultaneously
- Iron loses electrons during rusting (iron is oxidised)
- Oxygen gains electrons during rusting (oxygen is reduced)
- The oxidation state of iron increases (e.g. from 0 to +3) during rusting
- The oxidation state of oxygen decreases (e.g. from 0 to −2) during rusting
Key terms in this question
redox reaction · oxidation · oxidation state
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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