QUESTION 134 (4 marks)
A steel object is connected separately to either magnesium metal or copper metal in an electrolyte. Use the reduction potentials Mg²⁺/Mg = −2.37 V, Fe²⁺/Fe = −0.44 V and Cu²⁺/Cu = +0.34 V. Treat the steel as iron and compare which metal is more readily oxidised.
(a) Identify the metal that can act as a sacrificial anode for iron. Explain using the potentials. [2 marks]
(b) Write the oxidation half-equation for the sacrificial metal. [1 marks]
(c) Explain why attaching copper would not provide sacrificial protection. [1 marks]
Practice marking scheme
Answer
Magnesium is the sacrificial anode; . Copper is less readily oxidised than iron, so it does not sacrifice itself to protect the iron.
Working
A more negative reduction potential indicates a stronger reducing metal. Magnesium oxidises preferentially and supplies electrons, keeping iron cathodic. Copper’s reduction potential is more positive than iron’s, so copper is not preferentially oxidised; the iron can instead become the anode in their galvanic coupling.
Marking criteria
- Identifies magnesium as the sacrificial anode. [1 mark]
- Uses its more negative reduction potential to explain preferential oxidation and iron protection. [1 mark]
- Writes the balanced magnesium oxidation half-equation. [1 mark]
- Explains that copper is less readily oxidised, so iron is not sacrificially protected. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.