Question

The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following orders?

• A. \( \text{CsH} > \text{RbH} > \text{KH} > \text{LiH} \)
• B. \( \text{KH} > \text{NaH} > \text{LiH} > \text{RbH} \)
• C. \( \text{NaH} > \text{LiH} > \text{RbH} > \text{CsH} \)
• D. \( \text{LiH} > \text{NaH} > \text{RbH} > \text{CsH} \)

Hint:

The thermal stability of hydrides decreases as the size of the alkali metal cation increases.

Answer 1

The Correct Option is A

Step 1: Understand the thermal stability of hydrides.
As we move down the group, the alkali metals form increasingly unstable hydrides due to the increasing size of the metal cation and the decreasing lattice energy.

Step 2: Conclusion.
The thermal stability decreases in the order: \( \text{CsH} > \text{RbH} > \text{KH} > \text{LiH} \).

Final Answer:
\[ \boxed{\text{CsH} > \text{RbH} > \text{KH} > \text{LiH}} \]