Give reason for the following
a. The process `1/2H_(2(g))+e^(ɵ)toH_((g))^(ɵ)` is endothermic, yet ionic hydrides are known.
b. A mixture of hydrazine and `H_(2)O_(2)` with `Cu(II)` catalyst is used as a rocket propellant.
c. It is possible to remove completely the temporary hardness caused due to `Mg(HCO_(3))_(2)` by boiling.

Explanation

a. The process `1/2 H_(2(g))+e^(ɵ)toH_((g))^(ɵ)`, i.e. formation of hydride ion is endothermic, yet group `1` and certain group `2` elements form ionic hydrides. This is due to the fact that the lattice energy released during the formation of ionic hydrides more than compensates the energy required for the formation of hydrides ion.
b. the reaction between hydrazines and `H_(2)O_(2)` using `Cu(II)` catalyst is highly exothermic and is accompanied by a large increases in the volume of the products, hence this mixutres is used as rocket propellant.
`H_(2)N-NH_(2(l))+2H_(2)O_(2(l))overset(Cu(II))toN_(2(g))+4H_(2)O_(g)`
c. Temprory hardness of `H_(2)O` caused due to `Mg(HCO_(3))_(2)` can be completely removed by boiling as soluble `Mg(HCO_(3))_(2)` is converted into insoluble `MgCO_(3)` which can be removed by filtration.
`Mg(HCO_(3))_(2)tooverset("Boil")toMgCO_(3)darr+CO_(3)uarr+H_(2)O`