Which of the following is incorrect regarding the reaction of alkali metals with hydrogen ?

Options

A. All the alkali metals react with dihydrogen on heating to form colorless crystalline ionic hydrides `M^(+)H^(-)`.
B. The order of reactivity of the alkali metals towards dihydrogen increases as we go down the group.
C. The ionic character of the hydrides decreases from Li to Cs.
D. The stability of these hydrides decreases from LiH to CsH.

Answer

B

Explanation

The reactivity of alkali metals towards dihydrogen decreases as we move down the group due to decrease of lattice enthalpy with increase of the size of metal ion. Since hydride ion `(H^(-))` is quite small, `Li^(+)` ion on account of smaller size is able to provide high lattice enthalpy.
(1) Lithium reacts with dihydrogen at 1073 K while other alkali metals react with dihydrogen at 673 K to form ionic or salt-like hydrides :
`2M+H_(2)overset(Delta)to 2M^(+)H^(-)`
(3) Cation polarizes the anion and leads to covalent character. As the size of the cation increases, the smaller hydride anion can polarize the cation more easily. consequently, the covalent character increases while the ionic character decreases.
(4) Due to the decrease of lattice enthalpy, the stability of the hydrides decreases from LiH to CsH while the reducing power increases.