Arrange the following (i) `CaH_(2), BeH_(2) and TiH_(2)` in order of increasing electrical conductance.
(ii) LiH, NaH and CsH in order of increasing ionic character.
(iii) H–H, D–D and F–F in order of increasing bond dissociation enthalpy.
(iv) `NaH, MgH_(2) and H_(2)O` in order of increasing reducing property.

Explanation

(i) `BeH_(2)` is a covalent hydride, therefore, it does not conduct electricity at all. `CaH_(2)` conducts electricity in the fused statewhile `TiH_(2)` conducts electricity at room temperature. Thus, the order of increasing electrical conductance is : `BeH_(2) lt CaH_(2) lt TiH_(2)`
(ii) Electronegativity decreases down the group from Li to Cs, therefore, the ionic character of their hydrides increases in the same order, i.e., `LiH lt NaH lt CsH`
(iii) Due to greater nuclear mass, the bond pair in D-D bond is attracted more strongly than in H-H bond. Therefore, bond dissociation energy of D-D bond is higher than that of H-H bond.
However, due to repulsions between lone pairs of F and the bond pair, F-F bond dissociation enthalpy is the minimum . Thus the bond dissociation enthalpy increases in the order : ` F - F lt H -H lt D -D`
(iv) Ionic hydrides are powerful reducing agents. Both `MgH_(2)` and `H_(2)O` are covalent hydrides but he bond dissociation energy of `H_(2)O` is much higher than that of `MgH_(2)` . Therefore, the reducing character increases in the order : `H_(2) O lt MgH_(2) lt NaH`