Question

Balance the following equation by oxidation number method
i) $K_2{Cr}_2{O}_7+KI+H_2{SO}_4\to K_2{SO}_4+{Cr}_2{\left({SO}_4\right)}_3+I_2+H_{2}O$
ii) $KMn{O}_4+{Na}_2{SO}_3\to Mn{O}_2+{Na}_2{SO}_4+KOH$
iii) $Cu+H{NO}_3\to Cu{\left({NO}_3\right)}_2+{NO}_2+H_{2}O$
iv) $KM{nO}_4+H_2{C}_2{O}_4+H_2{SO}_4\to K_2{SO}_4+{MnSO}_4+{CO}_2+H_{2}O$

Answer 1

(i) $K_{2}C{r}_2{O}_7+KI+H_{2}S{O}_4\to K_{2}S{O}_4+C{r}_2(S{O}_4{)}_3+I_2+H_{2}O$

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Equate increase and decrease of oxidation number

$K_{2}C{r}_2{O}_7+6KI+H_{2}S{O}_4\to K_{2}S{O}_4+C{r}_2(S{O}_4{)}_3+3I_2+H_{2}O$

Equate number of potassium and then $S{O}_4^{2-}$ and then hydropen

$K_{2}C{r}_2{O}_7+6KI+7H_{2}S{O}_4\to 4K_{2}S{O}_4+C{r}_2(S{O}_4{)}_3+3I_2+7H_{2}O$

(ii) $KMn{O}_4+N{a}_{2}S{O}_3\to Mn{O}_2+N{a}_{2}S{O}_4+OH$

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Equate increase and decrease $3\times 2.12\times 3$

$2KMn{O}_4+3N{a}_{2}S{O}_3\to 2Mn{O}_2+3N{a}_{2}S{O}_4+KOH$

Equate number of K and then balance number of hydrogen

$2KMn{O}_4+3N{a}_{2}S{O}_3+H_{2}O\to 2Mn{O}_2+3N{a}_{2}S{O}_4+2KOH$

(iii) $Cu+HN{O}_3\to Cu(N{O}_3{)}_2+N{O}_2+H_{2}O$

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equate increase and decrease and also equate nitrate

$Cu+4HN{O}_3\to Cu(N{O}_3{)}_2+2N{O}_2+2H_{2}O$

Equate hydrogen

(iv) $KMn{O}_4+H_2{C}_2{O}_4+H_{2}S{O}_4\to K_{2}S{O}_4+MnS{O}_4+C{O}_2+H_{2}O$

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Equate increase and decrease oxidation number

$KMn{O}_4+5H_2{C}_2{O}_4+H_{2}S{O}_4\to K_{2}S{O}_4+MnS{O}_4+10C{O}_2+H_{2}O$

Equate K and $S{O}_4^{2-}$ and then hydrogen

$2KMn{O}_4+5H_2{C}_2{O}_4+3H_{2}S{O}_4\to K_{2}S{O}_4+2MnS{O}_4+10C{O}_2+8H_{2}O$