Chemistry Class 11 NCERT Solutions: Chapter 10 the s Block Elements Part 7

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Q: 19. Draw the structure of

(i) (vapour)

(ii) (Solid)

Answer:

(A) Structure of (Solid)

exists as a polymer in condensed (solid) phase.

In the vapour state, exists as a monomer with a linear structure.

Q: 20. The hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain.

Answer:

The atomic size of sodium and potassium is larger than that of magnesium and calcium. Thus, the lattice energies of carbonates and hydroxides formed by calcium and magnesium are much more than those of sodium and potassium. Hence, carbonates and hydroxides of sodium and potassium dissolve readily in water whereas those of calcium and magnesium are only sparingly soluble.

Q: 21. Describe the importance of the following:

(i) Limestone

(ii) Cement

(iii) Plaster of Paris

Answer:

(i) Chemically, limestone is

Importance of limestone

(A) It is used in the preparation of lime and cement.

(B) It is used as a flux during the smelting of iron ores

(ii) Chemically, cement is a mixture of calcium silicate and calcium aluminate.

Importance of cement

(A) It is used in plastering and in construction of bridges

(B) It is used in concrete

(iii) Chemically, plaster of Paris is

Importance of plaster of Paris

(A) It is used in surgical bandages

(B) It is also used for making casts and moulds

Q: 22. Why are lithium salts commonly hydrated and those of the other alkali metal ions usually anhydrous?

Answer

Lithium is the smallest in size among the alkali metals. Hence, can polarize water molecules more easily than other alkali metals. As a result, water molecules get attached to lithium salts as water of crystallization. Hence, lithium salts such as trihydrated lithium chloride are commonly hydrated. As the size of the ions increases, their polarizing power decreases. Hence, other alkali metal ions usually form anhydrous salts.

Q: 23. Why is almost insoluble in water whereas soluble not only in water but also in acetone?

Answer:

is insoluble in water. On the contrary, is soluble not only in water, but also in acetone. This is mainly because of the greater ionic character of as compared to . The solubility of a compound in water depends on the balance between lattice energy and hydration energy. Since fluoride ion is much smaller in size than chloride ion, the lattice energy of is greater than that of . Also, there is not much difference between the hydration energies of fluoride ion and chloride ion. Thus, the net energy change during the dissolution of in water is more exothermic than that during the dissolution of in water. Hence, low lattice energy and greater covalent character are the factors making soluble not only in water, but also in acetone.