ExamGOAL
Books
44
MCQ (Single Correct Answer)

Assertion (A) The carbonate of lithium decomposes easily on heating to form lithium oxide and $\mathrm{CO}_2$.

Reason (R) Lithium being very small in size polarises large carbonate ion leading to the formation of more stable $\mathrm{Li}_2 \mathrm{O}$ and $\mathrm{CO}_2$.

A
Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$
B
Both $A$ and $R$ are correct but $R$ is not the correct explanation of $A$
C
Both A and R are not correct
D
A is not correct but R is correct
45
MCQ (Single Correct Answer)

Assertion (A) Beryllium carbonate is kept in the atmosphere of carbon dioxide.

Reason (R) Beryllium carbonate is unstable and decomposes to give beryllium oxide and carbon dioxide.

A
Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$
B
Both $A$ and $R$ are correct but $R$ is not the correct explanation of $A$
C
Both A and R are not correct
D
A is not correct but R is correct
46
Subjective

The $s$-block elements are characterised by their larger atomic sizes, lower ionisation enthalpies, invariable +1 oxidation state and solubilities of their oxosalts. In the light of these features describe the nature of their oxides, halides and oxosalts.

Explanation

Due to low ionisation energy and large atomic size, alkali metals form cation readily and so their compounds are ionic.

Oxides

Due to +1 oxidation state, alkali metals form normal oxides of general formula $\mathrm{M}_2 \mathrm{O}$.

Only Li forms normal oxide $\mathrm{Li}_2 \mathrm{O}$ when heated in air. Other form peroxide and superoxide.

Oxides of alkali metals are strongly basic and are soluble in water.

The basic character of oxide increases gradually from $\mathrm{Li}_2 \mathrm{O}$ to $\mathrm{Cs}_2 \mathrm{O}$ due to increased ionic character.

Halides

Except lithium halides all other alkali metal halides are ionic. Due to high polarising power of $\mathrm{Li}^{+}$. Lithium halide is covalent in nature. Due to +1 oxidation states alkali metal halides have general formula MX. Low ionisation enthalpy allows formation of ionic halides.

Oxo salts

All alkali metals form solid carbonates of general formula $\mathrm{M}_2 \mathrm{CO}_3$. Carbonates are stable except $\mathrm{Li}_2 \mathrm{CO}_3$ due to high polarising capacity of $\mathrm{Li}^{+}$ which is unstable and decomposes. All the alkali metals (except Li ) form solid bicarbonates $\mathrm{MHCO}_3$. All alkali metals form nitrates having formula $\mathrm{MNO}_3$. They are colourless, water soluble, electrovalent compounds.

47
Subjective

Present a comparative account of the alkali and alkaline earth metals with respect to the following characteristics.

(a) Tendency to form ionic/covalent compounds

(b) Nature of oxides and their solubility in water

(c) Formation of oxosalts

(d) Solubility of oxosalts

(e) Thermal stability of oxosalts

Explanation

(a) Alkaline earth metals form compounds which are predominantly ionic but less ionic than the corresponding compounds of alkali metals due to increased nuclear charge and small size.

(b) Oxides of alkaline earth metals are less basic than corresponding oxides of alkali metals. The oxides dissolve in water to form basic hydroxides and evolve a large amount of heat. The alkaline earth metal hydroxides, are however less basic and less stable than alkali metal hydroxides.

(c) Alkaline earth metals form oxoacids as alkali metals. The formation of alkali metal oxoacids is much more faster and stronger than their corresponding alkaline earth metals due to increased nuclear charge and small size.

(d) Solubility of alkaline oxoacids is more than alkali oxoacids because alkaline earth metals have small size of cation and higher hydration energy. Salts like $\mathrm{CaCO}_3$ are insoluble in water.

(e) Oxosalts of alkali metals are thermally more stable than those of alkaline earth metals. As the electropositive character increases down the group, the stability of carbonate and hydrogen carbonates of alkali metal increases.

Whereas for alkaline earth metals, carbonate decomposes on heating to give carbon dioxide and oxygen.

48
Subjective

When a metal of group 1 was dissolved in liquid ammonia, the following observations were obtained

(a) Blue solution was obtained initially.

(b) 0 n concentrating the solution, blue colour changed to bronze colour.

How do you account for the blue colour of the solution? Give the name of the product formed on keeping the solution for some time.

Explanation

(a) The reaction that takes place when alkali metal is dissolved in liquid ammonia is

$$M+(x+y) \mathrm{NH}_3 \longrightarrow\left[M\left(\mathrm{NH}_3\right)_x\right]^{+}+\left[\left(\mathrm{NH}_{3) y}\right]^{-} e\right.$$

The blue colour of the solution is due to the presence of ammoniated electron which absorb energy in the visible region of light and thus, impart blue colour to the solution.

(b) In concentrated solution, the blue colour changes to bronze colour due to the formation of metal ion clusters. The blue solution on keeping for some time liberate hydrogen slowly with the formation of amide.

$$\underset{\text { Ammoniacal }}{\mathrm{M}^{+}+\mathrm{e}^{-}}+\mathrm{NH}_3 \longrightarrow \underset{\text { Amide }}{\mathrm{MNH}_2}+\frac{1}{2} \mathrm{H}_2 $$