Question

How would you account for the following?
(i) Transition metals exhibits variable oxidation state.
(ii) `Zr(Z=40)` and `Hf(Z=72)` have almost identical radii.
(iii) Transition metals and their compounds act as catalyst.
OR
Complete the following chemical equation:
(i)`Cr_(2)O_(7)^(2-)+6Fe^(2+)+14H^(+)rarr`
(ii) `CrO_(4)^(2-)+2H^(+)rarr`
(iii) `2MnO_(4)^(2-)+5C_(2)O_(4)^(2-)+16H^(+)rarr`

Answer 1

Correct Answer - (i) The energy of ns-electrons and `(n-1)` d-electrons are nearly same, therefore,ns electrons as well as `(n-1)`d-electrons can take part in bond formation in transition elements.
(ii) `Zr(Z=40):[Kr]4d^(2) 5s^(2);Hf(Z=72):[Xe]4f^(14)5d^(2)6s^(2)`
Due to Lanthanoids contraction, which essentially compensates for the expected increase in atomic size with increasing atomic number. Hence ,the second and the third d-series exhibit identical radii `(e.g,Zr160 pm,Hf 150 pm )` and have similar physical and chemical properties.
OR
(i) `Cr_(2)O_(7)^(2-)+Fe^(2-)+14H^(+) to 2CrO_(4)^(2-)+2H^(+) to Cr_(2O_(7)^(2-)+H_(2)O`
(ii) `2CO_(4)^(2-)+2H^(+) to Cr_(2)O_(7)^(2-)+H_(2)O`
(iii)`2MnO_(4)^(2-)+5C_(2)O_(4)^(2-)+16H^(+) to 2Mn^(2+)+10CO_(2)+8H_(2)O`