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The choice of a reducing agent in a particular case depends on thermo -dynamic factor. Explain with two examples. |
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Answer» Solution :The choice of the a reducing agent in a particular CASE depends on thermodynamic factor. This fact can be explained by considering the following EXAPLES. `rarr` out of Cand CO , carbon nonoxide (CO) is a better reducting agent at 673K. `rarr` At 983K (or) CO , carbon nonoxide (CO) is a better reducting agent at 673K . `rarr`The above observations are form Ellingham diagram. Zinc is not extracted from Zinc oxide through reduction using CO. Explanation : `2Zn+O_(2)rarr2ZnO,DeltaG^(@)=-650KJ` `2CO+O_(2)rarr2CO_(2),DeltaG^(@)=-450KJ` `2ZnO+2COrarr2Zn+2CO_(2')DeltaG^(@)=200KJ` `DeltaG^(@)` = Positive indicatis that the reaction is not feasible. The extraction of copper from pyrites is more difficult than that from its oxide ore through reduction. Explanation : Pyrites is `(Cu_(2)S)` cannot be reduced by carbir (or) hydrogent because the standar free energy of formation`(DeltaG^(@))` of`Cu_(2)S` is greater then those of`CS_(2)` and `H_(2)S` `{:(2Cu_(2)S+Crarr4Cu+CS_(2)),(Cu_(2)S+H_(2)rarrCu+H_(2)S):}}" Not fessible"` `rarr` The `DeltaG^(@)` of copper oxide is less than that of`CO_(2)` `therefore` The sulphide ore is firts converted to oxide by roasting and then reduced . `2Cu_(2)S+3O_(2)underset("Heat")overset("Roasting")rarr2Cu_(2)+2SO_(2)` `2Cu_(2)+O+Crarr4Cu+CO_(2)["feasible"].` The following are the conditions that Al might be expected to reduce MgO. The Equations for the formation of two oxides are `(4)/(3)Al_((s))+O_(2(g))rarr(2)/(3)Al_(2)O_(3(s))` `2Mg_((s))+O_(2(g))rarr2MgO_((s))` From the Ellingham diagram the two curves of these oxides formation intersect each otherrat a certain point. The corresponding value of`DeltaG^(@)` becomes zero for the reduction of MgO by aluminium metal, `2MgO_((s))+(4)/(3)hArr2Mg_((s))+(2)/(3)Al_(2)O_(3_(s))` From the above information the reduction of MgO by Al metal cannot occur below this temperature(1665 K) instead ,Mg can reduce`Al_(2)O_(3)` to al below 1665K. `rarr` Al-Metal can reduce MgO to Mg above 1665 K because`DeltaG^(@)` for `Al_(2)O_(3)`is less compared to that of MgO. `3MgO_((s))+2Al_((s))overset(T gt 1665K)rarrAl_(2)O_(3(s))+2Mg_((s))` |
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