Question

(a) Complete the following equations :
(i) ${C{r}_2{O}_7}^{2-}+2O{H}^{-}\to$
(ii) ${Mn{O}_4}^{-}+4H^{+}+3e^{-}\to$
(b) Account for the following:
(i) $Zn$ is not considered as a transition element.
(ii) Transition metals form a large number of complexes.
(iii) The $E^0$ value for the $M{n}^{3+}/M{n}^{2+}$ couple is much more positive than that for $C{r}^{3+}/c{r}^{2+}$ couple.

Answer 1

a)i) $C{r}_2{O}_7^{2-}+$ $2O{H}^{-}\to 2Cr{O}_4^{2-}+H_{2}O$

ii)$Mn{O}_4^{-}+4H^{+}+3e^{-}\to Mn{O}_2+2H_{2}O$

b)i)The transition metals are compounds that form at least 1 stable ion where the compound has an incomplete d subshell. Zn is not a transition metal because it forms only $Z{n}^{2+}$ ions with all the 3d electrons present.

ii)Transition metal shows a variable oxidation state, so they form a large number of complexes.

iii)$M{n}^{2+}$ has the outer electronic configuration of $3d^4$ and $M{n}^{2+}$ has the outer electronic configuration of $3d^5$. Thus, the conversion is highly favorable and $E^o$ value much greater than a conversion of $C{r}^{3+}$ to $C{r}^{2+}$.