CBSE Class 12-science Answered
Coupling reactions:
Many reactions which are non-spontaneous (?G is +ve) can be made to occur spontaneously if these are coupled with reactions having larger negative free energy.
By coupling means both non-spontaneous and spontaneous reactions can be carried out simultaneously.
For example:
Decomposition of Fe2O3 into iron is a non-spontaneous reaction. However this decomposition can take place simultaneously if carbon monoxide is simultaneously burnt in oxygen.
2Fe2O3 ? 4Fe + 3O2 ----- (1) ?G0 = +1487kJmol-1
2CO + O2 ? 2CO2 ----- (2) ?G0 = - 514kJmol-1
Multiplying equation (2) by 3 and then adding to equation (1) we get
6CO + 3O2 ? 6CO2 ?G0 = - 1543kJmol-1
2Fe2O3 ? 4Fe + 3O2 ?G0 = +1487kJmol-1
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2Fe2O3 + 6CO ? 4Fe + 6CO2 ?G0 = -56.2Kjmol-1