Question

Elements A and B do not form solid bicarbonate but reacts with Nz to give nitrides. Which of the following can be A and B ?

(1) Li, Mg
(2) Rb, Na
(3) Ca, Cs
(4) Ca, Na

Answer 1

ANSWER:

• The elements A and B are Li, Mg.

GIVEN:

• Elements A and B do not form solid bicarbonate but reacts with N₂ to give nitrides.

TO FIND:

• The elements A and B.

EXPLANATION:

GIiven Elements:

• Li, Mg
• Rb, Na
• Ca, Cs
• Ca, Na

Take Ca, Na:

• Ca(HCO₃)₂ is the formula of Calcium bicorbonate and is less stable in solid state and so it naturally occurs in liquid state.

• NaHCO₃ is formula of Sodium bicorbonate and occur in powered form. As it is in powder form, Sodium bicorbonate exist as solid in room temperature.

As Calcium bicorbonate exist in liquid form, it can be A or B but Sodium bicorbonate exist in solid form so it cannot be A or B.

Take Ca, Cs:

• Ca(HCO₃)₂ is the formula of Calcium bicorbonate and is less stable in solid state and so it naturally occurs in liquid state.

• CsHCO₃ is the formula Cesium bicorbonate which is used to make Cesium salts and hence is a solid compound.

As Calcium bicorbonate exist in liquid form, it can be A or B but Cesium bicorbonate exist in solid form so it cannot be A or B.

Take Rb, Na:

Here we discuss about reaction with N₂.

• Rb + N₂ is basically not possible because Rubidium cannot react directly with N₂.

• We already discussed that NaHCO₃ is formula of Sodium bicorbonate and occur in powered form and is a solid.

As Rubidium bicorbonate cannot react with N₂ directly, it cannot be A or B also Sodium bicorbonate exist in solid form so it cannot be A or B.

Take Li, Mg:

Physical State:

• Both LiHCO₃(Lithium bicarbonate) and Mg(HCO₃)₂ (Magnesium bicarbonate) are less stable in solid state and naturally occur in liquid(aqueous) state.

Reaction with N₂:

6 Li + 3 N₂ -----> 2 Li₃N

3Mg + N₂ -------> Mg₃N₂

Hence Li, Mg satisfies both the condition as both are in liquid state as well as they directly react with N₂ to form Lithium Nitride(Li₃N) and Magnesium Nitride(Mg₃N₂) respectively.

A and B are Lithium and Magnesium.

Answer 2

ANSWER:- Option (1) Li,Mg.

REASON:-

• Li and Mg are small in size and their bicarbonates will be unstable.

• Hence ,Li & Mg do not form bicarbonates but react with N to give nitrites.

Ex : -

• 6Li + 3N2 $\rightarrow$ Li3N

• 3Mg + N2 $\rightarrow$Mg3N2

• So A is Li & B is Mg.

__________________________

HOPE IT HELPS :)