Chemistry Class 11 NCERT Solutions: Chapter 10 the s Block Elements Part 9
Q: 27. State as to why
(A) A solution of is alkaline?
(B) Alkali metals are prepared by electrolysis of their fused chlorides?
(C) Sodium is found to be more useful than potassium?
(A) When sodium carbonate is added to water, it hydrolyses to give sodium bicarbonate they themselves are very strong reducing agents. They cannot be prepared by displacement reaction either (wherein one element is displaced by another ). This is because these elements are highly electropositive. Neither can electrolysis of aqueous solutions be used to extract these elements. This is because the liberated metals react with water.
(B) It is not possible to prepare alkali metals by the chemical reduction of their oxides as they themselves are very strong reducing agents. They cannot be prepared by displacement reactions either (wherein one element is displaced by another). This is because these elements are highly electropositive. Neither can electrolysis of aqueous solutions be used to extract these elements. This is because the liberated metals react with water.
Hence, to overcome these difficulties, alkali metals are usually prepared by the electrolysis of fused chlorides.
(C) Blood plasma and the interstitial fluids surrounding the cells are the regions where sodium ions are primarily found. Potassium ions are located within the cell fluids. Sodium ions are involved in the transmission of nerve signals, in regulating the flow of water across the cell membranes, and in transporting sugars and amino acids into the cells. Hence, sodium is found to be more useful than potassium.
Q: 28. Write balanced equations for reactions between
(A) and water
(B) and water
(A) The balanced chemical equation for the reaction between and water is:
(B) The balanced chemical equation for the reaction between and water is:
(C) The balanced chemical equation for the reaction between is:
Q: 29. How would you explain the following observations?
(i) is almost insoluble but in soluble in water,
(ii) is soluble but is insoluble in water,
(iii) is more soluble than KI in ethanol
(i) is almost insoluble in water and is soluble in water. is a small cation with a high polarizing power and is a small anion. The size compatibility of and is high. Therefore, the lattice energy released during their formation is also very high. When is dissolved in water, the hydration energy of its ions is not sufficient to overcome the high lattice energy. Therefore, is insoluble in water. On the other hand, ion is a large anion. Hence, can easily polarize ions, making unstable. Thus, the lattice energy of is not very high and so it is soluble in water
(ii) is soluble in water, but is not. is a large cation and is a small anion. The size compatibility of and is not high. As a result, is unstable. The lattice energy released during its formation is also not very large. It can easily be overcome by the hydration energy of the ions. Therefore, is soluble in water. In are both large-sized. The lattice energy released is high. Hence, it is not soluble in water.
(iii) is more soluble than in ethanol. As a result of its small size, the lithium ion has a higher polarizing power than the potassium ion. It polarizes the electron cloud of the iodide ion to a much greater extent than the potassium ion. This causes a greater covalent character in than in KI. Hence, than in Hence, is more soluble in ethanol.
Q: 30. Which of the alkali metal is having least melting point?
Atomic size increases as we move down the alkali group. As a result, the binding energies of their atoms in the crystal lattice decrease. Also, the strength of metallic bonds decreases on moving down a group in the periodic table. This causes a decrease in the melting point. Among the given metals, is the largest and has the least melting point.
Q: 31. Which one of the following alkali metals gives hydrated salts?
Answer: Smaller the size of an ion, the more highly is it hydrated. Among the given alkali metals, is the smallest in size. Also, it has the highest charge density and highest polarizing power. Hence, it attracts water molecules more strongly than the other alkali metals. As a result, it forms hydrated salts such as . The other alkali metals are larger than Li and have weaker charge densities. Hence, they usually do not form hydrated salts.
Q: 32. Which one of the alkaline earth metal carbonates is thermally the most stable?
Thermal stability increases with the increase in the size of the cation present in the carbonate. The increasing order of the cationic size of the given alkaline earth metal carbonates is
Hence, the increasing order of the thermal stability of the given alkaline earth metal carbonates is