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Question:

The Gibbs energy for the decomposition of Al₂O₃ at 500°C is as follows: 2/3Al₂O₃ → 4/3Al + O₂, ΔrG = +966 kJmol⁻¹. The potential difference needed for electrolytic reduction of Al₂O₃ at 500°C is at least:

5.0 V

2.5 V

4.5 V

3.0 V

Solution:

The correct option is C
2/3Al₂O₃ → 4/3Al + O₂
ΔG = -nFE
E = -ΔG/nF
= -(966 × 10³)/(4 × 96500) = 2.5V
The potential difference needed for the reduction = 2.5V.