Question

(a) Complete the following equations:
(i) ${Cr}_2{O}_7^{2-}+2O{H}^{-}\to$ (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 larger number of complexes.
(iii) The $E^0$ value for the ${Mn}^{3+}/{Mn}^{2+}$ couple is much more positive than that for ${Cr}^{3+}/{Cr}^{2+}$ couple.

Answer 1

(a) The balanced chemical equation :

(i) $C{r}_2{O}_2^{7-}+2O{H}^{-}\to 2Cr{O}_2^{4-}+H_{2}O$

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

(b)(i) Zinc has the stable filled valence shell $d^{10}$ electronic configuration in its ground state as well as in its most common oxidation state of +2. Hence, it is not considered as a transition element.

(ii) Ions formed by transition metals have small sizes and high ionic charges. They possess vacant d-orbitals to accommodate lone pairs of electrons for bond formation. As a result transition metals forms a large number of complexes.

(iii) The $E^o$ value for the $M{n}^{+3}/M{n}^{2+}$ couple is more positive than that for $C{r}^{3+}/C{r}^{2+}$ couple. This is because $M{n}^{2+}$ ion is particularly stable due to extra stability of its half filled valence electronic configuration($d^5$). Thus $M{n}^{+3}$ ion has a very high tendency to gain an electron and form the much more stable $M{n}^{+2}$ ion.