Question
Predict conditions under which Al might be expected to reduce $MgO.$
$($Hint: See Intext question $6.4)$
$($Hint: See Intext question $6.4)$
✓
Answer
The equations for the formation of the two oxides are
$4/3Al(s) + O_2(s) → 2/3Al_2O_3(s)$
$2Mg(s) + O_2(s) → 2MgO(s)$
If we look at the plots for the formation of the two oxides of the Ellingham diagram, we find that they intersect at certain point. The corresponding value of $\Delta\text{G}^{\circ}$ becomes zero for the reduction of $MgO$ by $Al$ metal.
$2\text{MgO}(\text{s})+4/3\text{Al(s)}\rightleftharpoons2\text{Mg}(\text{s})+2/3\text{Al}_2\text{O}_3(\text{s})$
This means that the reduction of $MgO$ by Al metal can occur below this temperature. Aluminium (Al) metal can reduce $MgO$ to $Mg$ above this temperature because $\Delta^{\circ}\text{G}$ for $Al_20_3$ is less as compared to that of $MgO.$
$3\text{MgO}(\text{s})+2\text{Al}(\text{s})\xrightarrow[]{(>1665\text{k})}\text{Al}_2\text{O}_3(\text{s})+3\text{Mg}(\text{s})$
$4/3Al(s) + O_2(s) → 2/3Al_2O_3(s)$
$2Mg(s) + O_2(s) → 2MgO(s)$
If we look at the plots for the formation of the two oxides of the Ellingham diagram, we find that they intersect at certain point. The corresponding value of $\Delta\text{G}^{\circ}$ becomes zero for the reduction of $MgO$ by $Al$ metal.
$2\text{MgO}(\text{s})+4/3\text{Al(s)}\rightleftharpoons2\text{Mg}(\text{s})+2/3\text{Al}_2\text{O}_3(\text{s})$
This means that the reduction of $MgO$ by Al metal can occur below this temperature. Aluminium (Al) metal can reduce $MgO$ to $Mg$ above this temperature because $\Delta^{\circ}\text{G}$ for $Al_20_3$ is less as compared to that of $MgO.$
$3\text{MgO}(\text{s})+2\text{Al}(\text{s})\xrightarrow[]{(>1665\text{k})}\text{Al}_2\text{O}_3(\text{s})+3\text{Mg}(\text{s})$
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