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Suggesta conditionunders whichmagnesiumcouldreduce alumina. |
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Answer» Solution :The tworedox REACTIONSARE : ` ""(4)/(3) Al+O_ 2to(2)/(3)Al_2 O _ 3 , Delta _f G ^(@)""_("("Al, Al_2O_3")") ""` ... (i) ` "" 2Mg +O _ 2to2Mg O,Delta _ f G^(@) ""_("("Mg, MgO ")")"" `...(ii) THEMG, MgOand Al, `Al_2O_ 3`curveintersectatpointA (1623K) where`Delta _ rG^(@) = 0 `. The ` Delta _fG^(@) `valueof` Al_2 O _ 3 `, attemperatures below1623 K , is LESS- vethan thatof MgO. Therefore, whenEq.(i) issubtrated fromEq. (ii),` Delta _r G^(@) ` of thetwocombinedredox reactions, i.e.,Eq. (iii) is -ve. ` 2 Mg +(2)/(3) Al_2O _ 3 to2 MgO+(4)/(3)Al , Delta _ rG^(@) = - ve"" `... (iii) Therefore, below 1623 K, Mg can reduce` Al_2O_ 3`to`Al`. However,above1623 K, the` Delta _f G^(@) `value for` Al_ 2O_3`is more-vethanthatofMgO . Therefore,when Eq. (ii)is subtractedfrom Eq.(i),` Delta _r G^(@) `of thetwocombinedredoxreactions , i.e.,Eq. (iv)is-ve. ` (4)/(3) Al +2 MgOto(2)/(3) Al_2O _ 3+2Mg , Delta_ rG^(@) =- ve"" `... (iv) Therefore,above 1623 K, AlcanreduceMgOtoMg. |
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