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BITSAT 2013
Exam
BITSAT
Year
2013
Subject
Chemistry
Chapter
s Block Elements

BITSAT 2013 · Chemistry · s Block Elements

The alkali metals form salt-like hydrides by the direct synthesis...

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?

Options

  1. ACsH>RbH>KH>NaH>LiH\mathrm{CsH} > \mathrm{RbH} > \mathrm{KH} > \mathrm{NaH} > \mathrm{LiH}
  2. BKH>NaH>LiH>CsH>RbH\mathrm{KH} > \mathrm{NaH} > \mathrm{LiH} > \mathrm{CsH} > \mathrm{RbH}
  3. CNaH>LiH>KH>RbH>CsH\mathrm{NaH} > \mathrm{LiH} > \mathrm{KH} > \mathrm{RbH} > \mathrm{CsH}
  4. DLiH>NaH>KH>RbH>CsH\mathrm{LiH} > \mathrm{NaH} > \mathrm{KH} > \mathrm{RbH} > \mathrm{CsH}

Correct answer

D. LiH>NaH>KH>RbH>CsH\mathrm{LiH} > \mathrm{NaH} > \mathrm{KH} > \mathrm{RbH} > \mathrm{CsH}

Step-by-step solution

The stability of alkali metal hydrides decreases from Li to Cs. It is due to the fact that M−H\mathrm{M}-\mathrm{H} bonds becomes weaker with increase in size of alkali metals as we move down the group from Li\mathrm{Li} to Cs\mathrm{Cs}. Thus the order of stability of hydrides is LiH>NaH>KH>RbH>CsH\mathrm{LiH} > \mathrm{NaH} > \mathrm{KH} > \mathrm{RbH} > \mathrm{CsH}
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