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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
NaH > LiH > KH > RbH > CsH
B
LiH > NaH > KH > RbH > CsH
C
CsH > RbH > KH > NaH > LiH
D
KH > NaH > LiH > CsH > RbH
Detailed Solution
Alkali metals form ionic (salt-like) hydrides $M^+H^-$ on heating with hydrogen: $2M + H_2 \rightarrow 2MH$
The size of the metal cation increases from $Li^+$ to $Cs^+$.
The small $H^-$ ion is held most strongly by the small $Li^+$ ion; as the cation becomes larger, the lattice energy of the hydride decreases.
So the M-H bond becomes weaker from LiH to CsH and the thermal stability of these hydrides decreases as follows:
LiH > NaH > KH > RbH > CsH
LiH is stable up to about 1000 K, while the others decompose at lower temperatures.
The size of the metal cation increases from $Li^+$ to $Cs^+$.
The small $H^-$ ion is held most strongly by the small $Li^+$ ion; as the cation becomes larger, the lattice energy of the hydride decreases.
So the M-H bond becomes weaker from LiH to CsH and the thermal stability of these hydrides decreases as follows:
LiH > NaH > KH > RbH > CsH
LiH is stable up to about 1000 K, while the others decompose at lower temperatures.