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Exercise · Q10

Q.Balance the following ionic equation in acidic medium by the ion-electron method, showing both half-reactions separately: Cr2O72−+I−→Cr3++I2\text{Cr}_2\text{O}_7^{2-} + \text{I}^{-} \to \text{Cr}^{3+} + \text{I}_2.

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Reduction half: Cr2O72−→2Cr3+\text{Cr}_2\text{O}_7^{2-}\to2\text{Cr}^{3+} (Cr already balanced); balance O with 7H2O7\text{H}_2\text{O}, H with 14H+14\text{H}^{+}: Cr2O72−+14H+→2Cr3++7H2O\text{Cr}_2\text{O}_7^{2-}+14\text{H}^{+}\to2\text{Cr}^{3+}+7\text{H}_2\text{O}; balance charge (+12+12 left vs +6+6 right) with 6e−6e^{-} on the left: Cr2O72−+14H++6e−→2Cr3++7H2O\text{Cr}_2\text{O}_7^{2-}+14\text{H}^{+}+6e^{-}\to2\text{Cr}^{3+}+7\text{H}_2\text{O}.\n\nOxidation half: 2I−→I22\text{I}^{-}\to\text{I}_2 (I balanced 2=2); balance charge (−2-2 left vs 00 right) with 2e−2e^{-} on the right: 2I−→I2+2e−2\text{I}^{-}\to\text{I}_2+2e^{-}.\n\nCombine: LCM of 6 and 2 is 6; the reduction half already has 6 electrons, multiply the oxidation half by 3: 6I−→3I2+6e−6\text{I}^{-}\to3\text{I}_2+6e^{-}. Add and cancel the 6e−6e^-: $\text{Cr}_2\text{O}_7^{2-} …

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