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Question

The Gibbs energy for the decomposition of Al2O3 at 500 oC is as follows:
2/3Al2O34/3Al+O2;ΔrG=+966 kJ/mol.
The potential difference needed for electrolytic reduction of Al2O3 at 500oC is at least:

A
5.0V
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B
4.5V
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C
3.0V
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D
2.5V
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Solution

The correct option is D 2.5V
The ionic reactions are,

23×(2Al3+)+4e43Al

23×(3O2)O2+4e

Thus the number of electrons transferred, n=4

ΔF=nFe=4×96500×E

966×103=4×96500×E

E=966×1034×96500=2.5V

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