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



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



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




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+



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.

NCERT Book Store

NCERT Sample Papers

Entrance Exams Preparation