Magnesium ribbon is added to dilute hydrochloric acid. Explain the changes in oxidation state that occur during this reaction, identifying which species is oxidised and which is reduced. Include in your answer what is meant by the terms oxidation and reduction in terms of electron transfer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When magnesium ribbon is placed into dilute hydrochloric acid, the ribbon dissolves and bubbles of gas are produced. The overall equation for the reaction is: Mg + 2HCl → MgCl₂ + H₂
Model answer (5 marks)
Oxidation is the loss of electrons by an atom or ion.
Reduction is the gain of electrons by an atom or ion.
In the reaction Mg + 2HCl → MgCl₂ + H₂, magnesium goes from an oxidation state of 0 to +2, so it loses two electrons and is oxidised.
The hydrogen ions from HCl go from an oxidation state of +1 to 0 in H₂, so they gain two electrons and are reduced.
Because oxidation and reduction occur simultaneously, this is a redox reaction.
Reduction is the gain of electrons by an atom or ion.
In the reaction Mg + 2HCl → MgCl₂ + H₂, magnesium goes from an oxidation state of 0 to +2, so it loses two electrons and is oxidised.
The hydrogen ions from HCl go from an oxidation state of +1 to 0 in H₂, so they gain two electrons and are reduced.
Because oxidation and reduction occur simultaneously, this is a redox reaction.
Examiner tips
- Use the exact definition of oxidation and reduction; include electron transfer. Show the change in oxidation state for Mg and H⁺. State which species is oxidised and which is reduced. Mention that both processes occur together, making it a redox reaction.
Common mistakes
- Confusing the oxidation state of H⁺ with that of H₂. Failing to state that Mg is oxidised and H⁺ is reduced. Using vague terms like ‘loses/gains electrons’ without linking to oxidation states.
Mark scheme (5 marks)
- Oxidation is the loss of electrons (by an atom or ion)
- Reduction is the gain of electrons (by an atom or ion)
- Magnesium is oxidised because it loses electrons / goes from oxidation state 0 to +2
- Hydrogen ions (H⁺) are reduced because they gain electrons / go from oxidation state +1 to 0
- This reaction is a redox reaction because oxidation and reduction occur simultaneously
Key terms in this question
oxidation · reduction · oxidation state · electron transfer
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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