Question

An element reacts with hydrogen to form a compound A which on treatment with water liberates hydrogen gas. The element can be
A. Nitrogen
B. Chlorine
C. Selenium
D. Calcium

Answer 1

Hint: Hydrides of metals are basic in nature and hydrides of non-metals are acidic in nature, metal hydrides produce H^- ions in the solution and non-metal hydrides produce H⁺ ions in the solutions.

Complete Step by Step Answer:
When elements react with hydrogen it results in the formation of hydride of that element. These hydrides can be basic or acidic in nature based on the nature of the element. If the element is more electropositive than hydrogen it will form a basic hydride as it will produce H^- ions and if the element is more electronegative than hydrogen then it will form an acidic hydride as it will produce H⁺ ions.

Nitrogen produces ammonium hydroxide (NH₄OH) when it reacts with water. When HCl reacts with water it produces H₃O⁺ & Cl^- ions. Selenium hydrides also do not produce H₂ gas when it reacts with water. But Calcium Hydride reacts with water and forms H₂ gas.
$Ca+H_2\rightarrow \ CaH_2$
$CaH_2+H_2O\rightarrow Ca{(OH)}_{2(aq)}+H_{2(g)} \uparrow$

Calcium is an alkaline earth metal that belongs to group II, so its electron affinity is much more than hydrogen cation. Calcium Hydride liberates H^- ions in the solution and it’s basic in nature, when it reacts with water it forms Calcium Hydroxide an alkali and it vigorously liberates Hydrogen gas with some amount of energy in the form of heat.
Thus, Option (D) is correct.

Note: The structure of CaH₂ is salt-like and it can be crystallised into cotunnite like structure. CaH₂ is a desiccant that means it is used as a drying agent. All Hydrides of alkaline earth metals are basic in nature.