Question

What happens when ${{H}_{2}}$ is passed over lithium at 1073K?
A. Covalent lithium hydride is formed
B. Coloured complex is formed
C. Ionic lithium hydride is formed
D. No reaction takes place

Answer 1

Hint : Think about all the elements in the group 1A or the alkali metals, since lithium is an alkali metal in the second group. Consider the differences in electronegativity and how it may attach the ionic or covalent nature of the bond formed.

Complete step by step solution :
- All alkali metals react with hydrogen at 673K (lithium at 1073K) to form colourless crystalline ionic hydrides of the general formula MH where M stands for alkali metals.
- Lithium and hydrogen are bonded together by ionic bonds as lithium loses an electron to become $L{{i}^{+}}$ and hydrogen accept that electron and became ${{H}^{-}}$.
- We know that to form an ionic bond, there must be more electronegative difference between two atoms and here electronegativity of Li=0.98 and electronegativity of H=2.2. So, due to more electronegative differences between them they cannot form covalent bonds.
- Electronegativity is the tendency of an atom to gain electrons from other elements. A difference in electronegativity will lead the bonds formed to be ionic rather than covalent.
- According to Fajan’s rule, smaller the cation and bigger the anion form a covalent bond. But here, H is not a bigger anion. So, lithium forms ionic lithium hydride in reaction with hydrogen gas. The reaction is as follows:
$2Li+{{H}_{2}}\to 2LiH$

Hence, the correct option is ‘C. Ionic lithium hydride is formed’

Note : Remember that lithium hydride is used as a source of hydrogen for military purposes and for filing meteorological balloons since it has a low molecular weight and on reacting with water, it evolves the highest percentage of hydrogen by weight.