The Sblock Elements Ques 47
47. 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?
[2008]
(a) $\mathrm{CsH}>\mathrm{RbH}>\mathrm{KH}>\mathrm{NaH}>\mathrm{LiH}$
(b) $\mathrm{KH}>\mathrm{NaH}>\mathrm{LiH}>\mathrm{CsH}>\mathrm{RbH}$
(c) $\mathrm{NaH}>\mathrm{LiH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}$
(d) $\mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}$
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Answer:
Correct Answer: 47.(d)
Solution: (d) The stability of alkali metal hydrides decreases from $\mathrm{Li}$ to $Cs$. It is due to the fact that $M-\mathrm{H}$ honds becomes weaker with increase in size of alkali metals as we move down the group from $Li$ to $Cs$. Thus the order of stability of hydrides is
$ \mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH} $
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