1

### AIPMT 2008

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

## Explanation

The stability of alkali metal hydrides decreases from Li to Cs. It is due to the fact that M–H bonds 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

LiH > NaH > KH > RbH > CsH
2

### AIPMT 2008

Equimolar solutions of the following were prepared in water separately. Which one of the solutions will record the highest pH?
A
MgCl2
B
CaCl2
C
SrCl2
D
BaCl2

## Explanation

Equimolar solutions of the given chlorides when prepared in water forms their respective hydroxides.

SrCl2 + 2H2O $\to$ Sr(OH)2 + 2HCl

BaCl2 + 2H2O $\to$ Ba(OH)2 + 2HCl

MgCl2 + 2H2O $\to$ Mg(OH)2 + 2HCl

CaCl2 + 2H2O $\to$ Ca(OH)2 + 2HCl

Be(OH)2 is amphoteric, but the hydroxides of other alkaline earth metals are basic. The basic strength increases down the group. Hence higher the basic character higher will be the pH.
3

### AIPMT 2007

In which of the following the hydration energy is higher than the lattice energy?
A
MgSO4
B
RaSO4
C
SrSO4
D
BaSO4

## Explanation

Hydration energy of sulphate decreases on moving down the group-II. Mg2+ is smaller than other ions of group II, so Mg2+ is readily hydrated and hence MgSO4 has higher hydration energy than the lattice energy.
4

### AIPMT 2007

The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is
A
BeCO3 < MgCO3 < CaCO3 < K2CO3
B
MgCO3 < BeCO3 < CaCO3 < K2CO3
C
K2CO3 < MgCO3 < CaCO3 < BeCO3
D
BeCO3 < MgCO3 < K2CO3 < CaCO3

## Explanation

As the basicity of metal hydroxides increases down the group from Be to Ba, the thermal stability of their carbonates also increases in the same order. Further group 1 compounds are more thermally stable than group 2 because their hydroxide are much basic than group 2 hydroxides therefore, the order of thermal stability is

BeCO3 < MgCO3 < CaCO3 < K2CO3