(i) $\mathrm{Cr}_2\mathrm{O}_7^{2-}+6\mathrm{Fe}^{2+}+14\mathrm{H}^{+} \rightarrow 6\mathrm{Fe}^{3+}$ $+2\mathrm{Cr}^{3+}+7\mathrm{H}_2\mathrm{O}$ (ii) $2\mathrm{CrO}_4^{2-}+2\mathrm{H}^{+} \rightarrow \mathrm{Cr}_2\mathrm{O}_7^{2-}+\mathrm{H}_2\mathrm{O}$ (iii) $2\mathrm{MnO}_4^{-}+5\mathrm{C}_2\mathrm{O}_4^{2-}+16\mathrm{H}^{+} \rightarrow 2\mathrm{Mn}^{2+}$ $+10\mathrm{CO}_2 +8\mathrm{H}_2\mathrm{O}$ OR (i) Transition metals shows variable oxidation state because they have tendency to loose electron from their penultimate $d$-subshell and exhibit various oxidation state. (ii) $\mathrm{Zr}$ and $\mathrm{Hf}$ have almost same radii because of lanthanoid contraction. Filling up of electrons in $4f$ series shows poor shielding effect. (iii) $4d$ series and other transition metals act as catalyst because they participate in various type of chemical reactions i.e., $\mathrm{N}_2+3\mathrm{H}_2 \underset{\frac{\mathrm{Fe}\,\text{High temp. High pressure}}{\mathrm{Mo}}}{\rightleftharpoons} 2\mathrm{NH}_3$