(a) Give reasons:
(i) Transition metals and their compounds show catalytic activities.
(ii) Separation of a mixture of Lanthanoid elements is difficult.
(iii) Zn,Cd and Hg are soft and have low melting point.
(b) Write the preparation of the following:
(i) `Na_2Cr_2O_7` from `Na_2CrO_4`
(ii) `K_2MnO_4` from `MnO_2`

Explanation

a) (i) Transition metals and their compounds show catalytic activities because of their ability to adopt multiple oxidation states and to form complexes.
(ii) Identical radii of Lanthanoid elements accounts for their occurrence together and it makes their separation difficult.
(iii) The d-orbitals of Zn, Cd and Hg are completely filled. As a result, they cannot participate in metallic bonding with other atoms of the same element. Therefore, they are soft and have low melting point.
(b) (i) The yellow solution and sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate `Na_2Cr_2O_7.2H_2O` can be crystallised.
`2Na_2CrO_4+2H^+to Na_2Cr_2O_7+2Na^+ +H_2O`
(ii) `K_2MnO_4` from `MnO_2`
`MnO_2` is fused with an alkali metal hydroxide and an oxidising agent like `KNO_3` this produces dark green `K_2MnO_4`
`2MnO_2+4KOH +O_2 to 2K_2MnO_4+2H_2O`