The Gibbs energy for the decomposition of Al₂O₃ at 500°C is as follows
$2 over 3 space Al subscript 2 straight O subscript 3 space rightwards arrow space 4 over 3 space Al space plus space straight O subscript 2 comma space increment subscript straight r space straight G space equals space plus 966 space kJ space mol to the power of negative 1 end exponent$
The potential difference needed for electrolytic reduction of Al₂O₃ at 500°C is at least

4.5 V

3.0 V

2.5 V

5.0 V

Solution

2.5 V

2 over 3 space Al subscript 2 straight O subscript 3 space rightwards arrow space 4 over 3 space Al space plus space straight O subscript 2 increment straight G space equals space plus space 966 space kJ space mol to the power of negative 1 end exponent space equals 966 space space straight x space 10 cubed space straight J space mol to the power of negative 1 end exponent increment straight G space equals space minus nFE subscript cell 966 space straight x space 10 cubed space equals space minus space 4 space straight x space 96500 space straight x space straight E subscript cell straight E subscript cell space equals space 2.5 space straight V space

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