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Balance the following reactions by oxidation number method : (i) FeS_2+O_2 rarrFe_2O_3+SO_2 (ii) MnO_4^(-) +Fe^(2+) rarr underset(("in acidic medium")) (Mn^(2+) +Fe^(3+)) (iii) Cr(OH)_3+IO_3^(-) rarr I^(-)+underset(("in basic medium"))(CrO_4^(2-)) |
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Answer» Solution :(i) `FeS_2+O_2 rarrFe_2O_3+SO_2` 1. The skeleton equation with oxidation number is : `overset(+2)(Fe)overset(-1)(S_2)+overset(0)O_2rarroverset(+3-2)(Fe_2O_3)+overset(+4-2)(SO_2)` (Note : Oxidation number of S in `FeS_2` is -1 and not -2.) 2. The oxidation number of iron as well as shulphure increases from +2 to +3 and -1 to +4 respectively. Since Fe and S must MAINTAIN their atomic RATIO , the change in oxidation number will be considered together . On the other hand, the oxidation number of oxygen decreases from0 to -2Thus, O.N. increases 1 per Fe and 5 per S atom i.e., `1+2 xx5 =11` per `FeS_2` .  3. To balance increase and decrease of oxidation number , multiply `FeS_2` by 4 and `O_2` by 11. `4FeS_2+11O_2 rarr Fe_2O_3 +SO_2` 4. BALANCING all atoms `4FeS_2+11O_2 rarr 2Fe_2O_3 +8SO_2` (ii) `MnO_4^(-) +Fe^(2+)+H^(+)rarrMn^(2+)+Fe^(3+)+H_2O` 1. The skeleton equation along with oxidation number of each atom is as : `overset(+7)(MN)overset(-2)(" "O_(4)^(-))+overset(+2)(Fe^(2+))+H^+rarroverset(+2)Mn^(2+)+overset(+3)Fe^(3+)+overset(+1-1)(H_2O)`  2. The oxidation number of Mn decreases from +7 to +2 while that of Fe increases from +2 to +3. 3. Multiply `Fe^(2+)` by 5 and `MnO_4^(-)` by 1 to balance increases and decreases in oxidation number. `MnO_4^(-) +5Fe^(2+)+H^(+)rarrMn^(2+)+Fe^(3+)+H_2O` 4. Balance all atoms (except H and O) `MnO_4^(-) +5Fe^(2+)+H^(+)rarrMn^(2+)+5Fe^(3+)+H_2O` This reaction takes place in acidic medium . The O and H atoms are balanced as given below : 5. Balance O - atomsby adding equal number of `H_2O` molecules as `MnO_4^(-) +5Fe^(2+)+H^(+)rarrMn^(2+)+5Fe^(3+)+H_2O+3H_2O` 6. Balance H - atoms by adding `H^+` ions on the side deficient in II - atom. `MnO_4^(-) +5Fe^(2+)+H^(+)+7H^(+)rarrMn^(2+)+5Fe^(3+)+4H_2O` or `MnO_4^(-) +5Fe^(2+)+8H^(+)rarrMn^(2+)+5Fe^(3+)+4H_2O` |
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