(a) (i) Account for the following: (I) Orange colour of Cr_2 O_7^2- ion changes to yellow when treated with an alkali. (II) Zn, Cd and Hg are…

Account for the following :

Orange colour of $\displaystyle \mathrm{Cr}_{2} \mathrm{O}_{7}{ }^{2-}$ ion changes to yellow when treated with an alkali.

Zn, Cd and Hg are non-transition elements.

$\displaystyle \mathrm{E}^{\circ}$ value for $\displaystyle \mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ couple is highly positive $\displaystyle (+1 \cdot 57 \mathrm{~V})$ as compared to $\displaystyle \mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}$.

What happens when :

Manganate ion undergoes disproportionation reaction in acidic medium ?

$\displaystyle \mathrm{KMnO}_{4}$ is heated ?

Answer the following questions :

What is 'Misch metal'? Give its one use.

Write the formula of an oxoanion of chromium in which it shows the oxidation state equal to its group number.

Why does Vanadium pentoxide $\displaystyle \left(\mathrm{V}_{2} \mathrm{O}_{5}\right)$ act as a catalyst ?

Why do transition elements have high enthalpies of atomisation ?

How do you prepare $\displaystyle \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ from $\displaystyle \mathrm{Na}_{2} \mathrm{CrO}_{4}$ ?