Chemistry Class 11 NCERT Solutions: Chapter 10 the s Block Elements Part 3

Q: 6. Compare the alkali metals and alkaline earth metals with respect to

(i) Ionization enthalpy

(ii) Basicity of oxides and

(iii) Solubility of hydroxides.

Alkali vs Alkaline Metals
Key differences between the alkali and alkaline earth metals

Alkali metals

Alkaline earth metals

(i)

Ionization enthalpy:

These have lowest ionization

Enthalpies in respective periods. This is because of their large atomic sizes. Also, they lose their only valence electron easily as they attain stable noble gas

configuration after losing it.

(i)

Ionization enthalpy:

Alkaline earth metals have smaller atomic size and higher effective nuclear charge as compared to alkali metals. This causes their first ionization enthalpies to be higher than that of alkali metals. However, their second ionization enthalpy is less than the corresponding alkali metals. This is because alkali metals, after losing one electron, acquires noble gas configuration, which is very stable.

(ii)

Basicity of oxides:

The oxides of alkali metals are very basic in nature. This happens due to the highly

electropositive nature of alkali metals, which makes these oxides highly ionic. Hence, they readily

dissociate in water to

give hydroxide ions.

(ii)

Basicity of oxides:

The oxides of alkaline earth metals are quite basic but not as basic as those of alkali metals. This is because alkaline earth metals are less electropositive

than alkali metals.

(iii)

Solubility of hydroxides:

The hydroxides of alkali metals are more soluble than those of alkaline earth metals.

(iii)

Solubility of hydroxides:

The hydroxides of alkaline earth metals are less soluble than those of alkali metals. This is due to the high lattice energies of alkaline earth metals. Their higher charge densities (as compared to alkali metals) account for higher lattice

energies.

Q: 7. In what ways lithium shows similarities to magnesium in its chemical behaviour?

Answer:

Similarities between lithium and magnesium are as follows.

(i) Both Equation react slowly with cold water.

(ii) The oxides of both Equation are much less soluble in water and their hydroxides decompose at high temperature.

Equation

Equation

(iii) Both Equation react with Equation to form nitrides.

Equation

Equation

(iv) Neither Equation nor Equation form peroxides or superoxides.

(v) The carbonates of both are covalent in nature. Also, these decompose on heating.

Equation Equation

Equation

(vi) Equation do not form solid bicarbonates

(vii) Both Equation are soluble in ethanol owing to their covalent nature.

(viii) Both Equation are deliquescent in nature. They crystallize from aqueous solutions as hydrates, for example, Equation and Equation

Q: 8. Explain why alkali and alkaline earth metals cannot be obtained by chemical reduction methods?

Answer:

In the process of chemical reduction, oxides of metals are reduced using a stronger reducing agent. Alkali metals and alkaline earth metals are among the strongest reducing agents and the reducing agents that are stronger than them are not available. Therefore, they cannot be obtained by chemical reduction of their oxides.

Q: 9. Why are potassium and cesium, rather than lithium used in photoelectric cells?

Answer

All the three, lithium, potassium, and cesium, are alkali metals. Still, Equation and Equation are used in the photoelectric cell and no Equation . This is because as compared to Equation and K, Equation is smaller in size and therefore, requires high energy to lose an electron. While on the other hand, Equation and Equation have low ionization energy. Hence, they can easily lose electrons. This property of Equation and Equation is utilized in photoelectric cells.

Q: 10. When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.

Answer:

When an alkali metal is dissolved in liquid ammonia, it results in the formation of a deep blue coloured solution.

Equation

The ammoniated electrons absorb energy corresponding to red region of visible light. Therefore, the transmitted light is blue in colour. At a higher concentration (3 M), clusters of metal ions are formed. This causes the solution to attain a copper-bronze colour and a characteristic metallic lustre.

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