Question

a. Why $3$d-series of elements acts as good catalyst?
b. Given reason: $T{i}^{4+}$ salts are colourless where as $C{r}^{3+}$ salts are coloured.

Answer 1

a. Transition metals and their compounds exhibit catalytic activity due to following reasons.
i. They have ability to adopt multiple oxidation states.
ii. Transition metals can form complexes with reactant molecules using vacant d-orbitals.
iii. In finely divided state, transition metals have large surface area .
b.
$T{i}^{+4}$ is colourless due to $d^o$-configuration in the valence shell. d-d transition is not possible as there are no d electrons. $C{r}^{+3}$ has $d^3$ configuration in the valence shell which facilitates d-d transition. Hence, $C{r}^{3+}$ salts are violet coloured.
Note: Ions with no unpaired electrons are colourless and ions with unpaired electrons are coloured.