Question

Account for the following:
(I)Orange colour of \(\text{Cr}_2\text{O}_7^{2-}\) ion changes to yellow when treated with an alkali.
(II) Zn, Cd and Hg are non-transition elements.
(III) \(E^0\) value for \(\text{Mn}^{3+}/\text{Mn}^{2+}\) couple is highly positive (+1.57 V) as compared to \(\text{Cr}^{3+}/\text{Cr}^{2+}\).

Hint:

The change in colour of \(\text{Cr}_2\text{O}_7^{2-}\) to \(\text{CrO}_4^{2-}\) is a classic example of the impact of pH on the species' structure and charge.

Answer 1

(I) The orange colour of \(\text{Cr}_2\text{O}_7^{2-}\) changes to yellow due to the formation of \(\text{CrO}_4^{2-}\) when treated with an alkali. The alkali provides hydroxide ions that convert dichromate \(\text{Cr}_2\text{O}_7^{2-}\) into chromate \(\text{CrO}_4^{2-}\), which has a yellow colour.
(II) Zinc (Zn), Cadmium (Cd), and Mercury (Hg) are non-transition elements because they do not have partially filled d-orbitals in their stable oxidation states, which is a characteristic feature of transition metals. Zn, Cd, and Hg have full d-orbitals in their stable states.
(III) The high \(E^0\) value for the \(\text{Mn}^{3+}/\text{Mn}^{2+}\) couple indicates that Mn\(^{3+}\) is a strong oxidizing agent. The relatively high positive potential compared to Cr\(^3+\)/Cr\(^2+\) can be attributed to the more effective stabilization of Mn\(^{2+}\) compared to Cr\(^2+\), which makes the Mn\(^{3+}\)/Mn\(^{2+}\) couple more easily reduced.