CBSE Class 11 Chemistry The S Block Element Notes Set C

S BLOCK ELEMENTS REMEDIAL ACTION PLAN

1. Groups present in the s –block and their names

Alkali metals and alkaline earth metals

2. Electronic configurations

3. General physical properties of alkali metals and alkaline earth metals

4. General chemical properties of alkali metals and alkaline earth metals

5. Diagonal relation ships Li- Mg, Be- B.

6. Applications of alkali metals and alkaline earth metals

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

Ans. Li resembles Mg mainly due to similarity in sizes of their atoms and ions. The main points of similarity are: Both are quite hard. 1 Both LiOH and Mg(OH)2 are weak bases. 2 Carbonates of both on heating decompose to produce oxides and carbondioxide. 3 Both react with nitrogen to give ionic nitrides.

8. various reactions that occur in the Solvay process.

Ans. In Solvay ammonia process. When carbon dioxide is passed through a concentrated solution of brine saturated with NH3, NaHCO3 gets precipitated. NaHCO3 on subsequent heating gives Na2CO3.

NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl₂

NaHCO₃ → Na₂CO₃ +CO₂ + H₂O

CO2 needed for the reaction is prepared by heating calcium carbonate and the quick lime, CaO is dissolved in water to form slaked lime, Ca(OH)₂

CaCO₃ → CaO + CO₂

CaO + H₂O → Ca(OH)₂

NH₃ needed for the purpose is prepared by heating NH₄Cl and Ca(OH)²

2 NH₄Cl + Ca(OH)₂ → 2 NH₃ + CaCl₂ + H₂O

9.What happens when:

a) Sodium metal is dropped in water?

b) Sodium metal is heated in free supply of air?

c) Sodium peroxide dissolves in water?

Ans. a) Sodium metal catches fire and hydrogen gas is evolved 2Na + 2H₂O What happens when:

a) Sodium metal is dropped in water?

b) Sodium metal is heated in free supply of air?

c) Sodium peroxide dissolves in water?

Ans. a) Sodium metal catches fire and hydrogen gas is evolved

2Na + 2H₂O 2NaOH + H₂ + Heat

b) Sodium peroxide is formed

2Na + O₂ Na₂O₂

c) (i) Sodium peroxide reacts with water at ordinary temperature to liberate oxygen gas

Na₂O₂ + 2H₂O 4NaOH + O₂

ii) With ice cold water, H₂O₂ is formed

Na₂O₂ + 2H₂O2 NaOH +H₂ O₂

10. Why are potassium and cesium, rather than lithium used in photoelectric cells?

Ans. The ionization enthalpy of lithium is quite high. The photons of light are not in a position to eject electrons from the surface of lithium metal. Therefore photoelectric effect is not noticed. However, both potassium and cesium have comparatively low ionization enthalpies. This means that the electrons can quite easily be ejected from the surface of these metals when photons of certain minimum frequency (threshold frequency) strike against their surface.

11. Why is Li₂CO₃ decomposed at a lower temperature whereas Na₂CO₃ at higher temperature?

Ans. Li₊ ion is very small in size. It is stabilized more by smaller anions such as oxide ion rather than large anions such as carbonate. Therefore Li₂CO₃ decomposes into Li₂O on mild heating. On the other hand, Na₊ ion is larger in size. It is stabilized more by carbonate ion than oxide ion. Hence, Na₂CO₃ does not undergo thermal decomposition easily.

12. Compare the alkali metals and alkaline earth metals with respect to (i) ionization enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides..
Ans.
(i) Ionization enthalpy (I E): I E of alkaline earth metals are higher than those of alkali metals of group 1. This is because the atoms of alkaline earth metals have smaller size (due to higher nuclear charge) as compared to the alkali metals.
(ii) Basicity of oxides: The oxides of alkali and alkaline earth metals dissolve in water to form their respective hydroxides. These hydroxides are strong bases. The hydroxides of alkaline earth metals are less basic than of alkali metals of the corresponding periods. This is due to their (i) high ionization enthalpy (ii) small ionic size and (iii) dipositive charge on the ions.
As a result M-O bond in these hydroxides is relatively stronger than that of corresponding alkali metals and therefore does not break.
(iii) Solubility of hydroxides: Because of smaller size and higher ionic charge, the lattice enthalpies of alkaline earth metals are much higher than those of alkali metals and hence the solubility of alkali metal hydroxides is much higher than that of alkaline earth metals. However the solubility of the hydroxides of both alkali and alkaline earth metals increase down the group due to large decrease in their lattice enthalpies as compared to their hydration enthalpies.

13. Explain the significance of sodium, potassium, magnesium and calcium in biological fluids.
Ans. Significance of sodium and potassium:
(i) Sodium ions participate in the transmission of nerve signals.
(ii) Sodium ions also regulate flow of water across the cell membranes and in transport of sugars and amino acids into the cells.
(iii) Potassium ions are the most abundant cations within cell fluids, where they activate many enzymes, participate in oxidation of glucose to produce ATP.
(iv) Potassium ions in combination with sodium ions are responsible for transmission of nerve signals.
(v) The functional features of nerve cells depend upon the sodium potassium ion gradient that is established in the cell.

Significance of Magnesium and Calcium:
1. Magnesium ions are concentrated in animal cells and Calcium ions are concentrated in body fluids, outside the cell.
2. All enzymes that utilize ATP in phosphate transfer require magnesium ion as cofactor.
3. In green plants magnesium is present in chlorophyll.
4. Calcium and magnesium ions are also essential for the transmission of impulses along nerve fibres.
5. Calcium ions are important in blood clotting and are required to trigger the contraction of muscles.
6. Calcium ions also regulate the beating of the heart.

14.Explain why can alkali and alkaline earth metals not be obtained by chemical reduction methods?
Ans. The metals belonging to both these families are very strong reducing agents. It is therefore not possible to reduce their oxides by reacting with common reducing agents like carbon (coke), zinc etc. These are normally isolated by carrying out the electrolysis of the salts of these metals in the molten state.

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