The s-Block Elements

• Question 221

## The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrieds decreases in which of the following orders? CsH > RbH > KH > NaH >LiH KH > NaH> LiH > CsH >RbH NaH > LiH >KH> RbH> CsH LiH > NaH > KH >RbH >CsH

Solution

D.

LiH > NaH > KH >RbH >CsH

Small anion forms stable compounds with small cation.
The thermal stability of alkali metal hydrides decreases as:
LiH> NaH > KH > RbH  >CsH
because the size of cation increases as;
Li+ < Na+ < K+ < Rb+ <Cs+

Question 222

## The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is: BeCO3 < MgCO3 < K2CO3 <CaCO3 BeCO3 < MgCO3 < CaCO3 <K2CO3 MgCO3 < BeCO3 < CaCO3 < K2CO3 K2CO3 < MgCO3 < CaCO3 < BeCO3

Solution

B.

BeCO3 < MgCO3 < CaCO3 <K2CO3

Thermal stability of carbonates increases in a group as we move from top to bottom and decreases in a period as we move from left to right. so, the correct order of thermal stability of given carbonates is:
BeCO3 < MgCO3 < CaCO3 < K2CO3
Be, Mg and Ca present in second group and K present in the first group.

Question 223

## In which of the following the hydration energy is higher than the lattice energy?  BaSO4 MgSO4 RaSO4 SrSO4

Solution

B.

MgSO4

Hydration energy of sulphate decreases from top to bottom in an IInd group. Mg2+ is smaller than other ions of that group so Mg2+ is readily hydrated. MgSO4 has higher hydration energy than lattice energy.

Question 224

## The correct order of the mobility of the alkali metal ions in aqueous solution is: Li+>Na+ > K+ > Rb+ Na+>K+ > Rb+  > Li+ K+ > Rb+ >Na+ > Li+ Rb+ > K+ > Na+ > Li+

Solution

D.

Rb+ > K+ > Na+ > Li+

The correct order of the mobility of the alkali metal ions in aqueous solutions
Rb+ > K+ > Na+ > Lidue to following order of hydration energy of these ions Li+ > Na+ > K+ > Rb+ and due to hydration of ion, mobility decreases.