Updated By SaraNextGen

On March 11, 2024, 11:35 AM

Question 10.15:

Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c) Sulphates.

Answer:

(i) Nitrates

Thermal stability

Nitrates of alkali metals, except LiNO₃, decompose on strong heating to form nitrites.

LiNO₃, on decomposition, gives oxide.

Similar to lithium nitrate, alkaline earth metal nitrates also decompose to give oxides.

As we move down group 1 and group 2, the thermal stability of nitrate increases.

Solubility

Nitrates of both group 1 and group 2 metals are soluble in water.

(ii) Carbonates

Thermal stability

The carbonates of alkali metals are stable towards heat. However, carbonate of lithium, when heated, decomposes to form lithium oxide. The carbonates of alkaline earth metals also decompose on heating to form oxide and carbon dioxide.

Solubility

Carbonates of alkali metals are soluble in water with the exception of Li₂CO₃. Also, the solubility increases as we move down the group.

Carbonates of alkaline earth metals are insoluble in water.

(iii) Sulphates

Thermal stability

Sulphates of both group 1 and group 2 metals are stable towards heat.

Solubility

Sulphates of alkali metals are soluble in water. However, sulphates of alkaline earth metals show varied trends.

BeSO₄ Fairly soluble

MgSO₄ Soluble

CaSO₄ Sparingly soluble

SrSO₄ Insoluble

BaSO₄ Insoluble

In other words, while moving down the alkaline earth metals, the solubility of their sulphates decreases.

Question 10.16:

Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?

Answer:

(a) Sodium can be extracted from sodium chloride by Downs process.

This process involves the electrolysis of fused NaCl (40%) and CaCl₂ (60 %) at a temperature of 1123 K in Downs cell.

Steel is the cathode and a block of graphite acts as the anode. Metallic Na and Ca are formed at cathode. Molten sodium is taken out of the cell and collected over kerosene.

(ii) Sodium hydroxide can be prepared by the electrolysis of sodium chloride. This is called Castner–Kellner process. In this process, the brine solution is electrolysed using a carbon anode and a mercury cathode.

The sodium metal, which is discharged at cathode, combines with mercury to form an amalgam.

(iii) Sodium peroxide

First, NaCl is electrolysed to result in the formation of Na metal (Downs process).

This sodium metal is then heated on aluminium trays in air (free of CO₂) to form its peroxide.

(iv) Sodium carbonate is prepared by Solvay process. Sodium hydrogen carbonate is precipitated in a reaction of sodium chloride and ammonium hydrogen carbonate.

These sodium hydrogen carbonate crystals are heated to give sodium carbonate.

Question 10.17:

What happens when (i) magnesium is burnt in air (ii) quick lime is heated with silica (iii) chlorine reacts with slaked lime (iv) calcium nitrate is heated ?

Answer:

(i) Magnesium burns in air with a dazzling light to form MgO and Mg₃N₂.

(ii) Quick lime (CaO) combines with silica (SiO₂) to form slag.

(iii) When chloride is added to slaked lime, it gives bleaching powder.

(iv) Calcium nitrate, on heating, decomposes to give calcium oxide.

Question 10.18:

Describe two important uses of each of the following: (i) caustic soda (ii) sodium carbonate (iii) quicklime.

Answer:

(i) Uses of caustic soda

(a) It is used in soap industry.

(b) It is used as a reagent in laboratory.

(ii) Uses of sodium carbonate

(a) It is generally used in glass and soap industry.

(b) It is used as a water softener.

(iii) Uses of quick lime

(a) It is used as a starting material for obtaining slaked lime.

(b) It is used in the manufacture of glass and cement.

Question 10.19:

Draw the structure of (i) BeCl₂ (vapour) (ii) BeCl₂ (solid).

Answer:

(a) Structure of BeCl₂ (solid)

BeCl₂ exists as a polymer in condensed (solid) phase.

In the vapour state, BeCl₂ exists as a monomer with a linear structure.

Question 10.20:

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.

Question 10.21:

Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris.

Answer:

(i) Chemically, limestone is CaCO₃.

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 2CaSO₄.H₂O.

Importance of plaster of Paris

(a) It is used in surgical bandages.

(b) It is also used for making casts and moulds.

Question 10.22:

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, Li⁺ ion 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 (LiCl.3H₂O) are commonly hydrated. As the size of the ions increases, their polarizing power decreases. Hence, other alkali metal ions usually form anhydrous salts.

Question 10.23:

Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone?

Answer:

LiF is insoluble in water. On the contrary, LiCl is soluble not only in water, but also in acetone. This is mainly because of the greater ionic character of LiF as compared to LiCl. 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 LiF is greater than that of LiCl. 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 LiCl in water is more exothermic than that during the dissolution of LiF in water. Hence, low lattice energy and greater covalent character are the factors making LiCl soluble not only in water, but also in acetone.

Question 10.24:

Explain the significance of sodium, potassium, magnesium and calcium in biological fluids.

Answer:

Importance of sodium, potassium, magnesium, and calcium in biological fluids:

(i) Sodium (Na):

Sodium ions are found primarily in the blood plasma. They are also found in the interstitial fluids surrounding the cells.

(a) Sodium ions help in the transmission of nerve signals.

(b) They help in regulating the flow of water across the cell membranes.

(c) They also help in transporting sugars and amino acids into the cells.

(ii) Potassium (K):

Potassium ions are found in the highest quantity within the cell fluids.

(a) K ions help in activating many enzymes.

(b) They also participate in oxidising glucose to produce ATP.

(c) They also help in transmitting nerve signals.

(iii) Magnesium (Mg) and calcium (Ca):

Magnesium and calcium are referred to as macro-minerals. This term indicates their higher abundance in the human body system.

(a) Mg helps in relaxing nerves and muscles.

(b) Mg helps in building and strengthening bones.

(c) Mg maintains normal blood circulation in the human body system.

(d) Ca helps in the coagulation of blood

(e) Ca also helps in maintaining homeostasis.

Question 10.25:

What happens when

(i) sodium metal is dropped in water ?

(ii) sodium metal is heated in free supply of air ?

(iii) sodium peroxide dissolves in water ?

Answer:

(i) When Na metal is dropped in water, it reacts violently to form sodium hydroxide and hydrogen gas. The chemical equation involved in the reaction is:

(ii) On being heated in air, sodium reacts vigorously with oxygen to form sodium peroxide. The chemical equation involved in the reaction is:

(iii) When sodium peroxide is dissolved in water, it is readily hydrolysed to form sodium hydroxide and water. The chemical equation involved in the reaction is:

Question 10.26:

Comment on each of the following observations:

(a) The mobilities of the alkali metal ions in aqueous solution are Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

(b) Lithium is the only alkali metal to form a nitride directly.

(c)   (where M = Ca, Sr or Ba) is nearly constant.

Answer:

(a) On moving down the alkali group, the ionic and atomic sizes of the metals increase. The given alkali metal ions can be arranged in the increasing order of their ionic sizes as:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

Smaller the size of an ion, the more highly is it hydrated. Since Li⁺ is the smallest, it gets heavily hydrated in an aqueous solution. On the other hand, Cs⁺ is the largest and so it is the least hydrated. The given alkali metal ions can be arranged in the decreasing order of their hydrations as:

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Greater the mass of a hydrated ion, the lower is its ionic mobility. Therefore, hydrated Li⁺ is the least mobile and hydrated Cs⁺ is the most mobile. Thus, the given alkali metal ions can be arranged in the increasing order of their mobilities as:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

(b) Unlike the other elements of group 1, Li reacts directly with nitrogen to form lithium nitride. This is because Li⁺ is very small in size and so its size is the most compatible with the N^3– ion. Hence, the lattice energy released is very high. This energy also overcomes the high amount of energy required for the formation of the N^3– ion.

(c) Electrode potential (E°) of any M²⁺/M electrode depends upon three factors:

(i) Ionisation enthalpy

(ii) Enthalpy of hydration

(iii) Enthalpy of vaporisation

The combined effect of these factors is approximately the same for Ca, Sr, and Ba. Hence, their electrode potentials are nearly constant.

Question 10.27:

State as to why

(a) a solution of Na₂CO₃ is alkaline ?

(b) alkali metals are prepared by electrolysis of their fused chlorides ?

(c) sodium is found to be more useful than potassium ?

Answer:

(a) When sodium carbonate is added to water, it hydrolyses to give sodium bicarbonate and sodium hydroxide (a strong base). As a result, the solution becomes alkaline.

(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 their 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.

Question 10.28:

Write balanced equations for reactions between

(a) Na₂O₂and water

(b) KO₂ and water

(c) Na₂O and CO₂

Answer:

(a) The balanced chemical equation for the reaction between Na₂O₂ and water is:

(b) The balanced chemical equation for the reaction between KO₂ and water is:

(c) The balanced chemical equation for the reaction between Na₂O and CO₂ is:

Question 10.29:

How would you explain the following observations?

(i) BeO is almost insoluble but BeSO₄ in soluble in water,

(ii) BaO is soluble but BaSO₄ is insoluble in water,

(iii) LiI is more soluble than KI in ethanol.

Answer:

(i) BeO is almost insoluble in water and BeSO₄ is soluble in water. Be²⁺ is a small cation with a high polarising power and O^2– is a small anion. The size compatibility of Be²⁺ and O^2– is high. Therefore, the lattice energy released during their formation is also very high. When BeO is dissolved in water, the hydration energy of its ions is not sufficient to overcome the high lattice energy. Therefore, BeO is insoluble in water. On the other hand,  ion is a large anion. Hence, Be²⁺ can easily polarise  ions, making BeSO₄ unstable. Thus, the lattice energy of BeSO₄ is not very high and so it is soluble in water.

(ii) BaO is soluble in water, but BaSO₄ is not. Ba²⁺ is a large cation and O^2– is a small anion. The size compatibility of Ba²⁺ and O^2– is not high. As a result, BaO 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, BaO is soluble in water. In BaSO₄, Ba²⁺ and  are both large-sized. The lattice energy released is high. Hence, it is not soluble in water.

(iii) LiI is more soluble than KI in ethanol. As a result of its small size, the lithium ion has a higher polarising power than the potassium ion. It polarises the electron cloud of the iodide ion to a much greater extent than the potassium ion. This causes a greater covalent character in LiI than in KI. Hence, LiI is more soluble in ethanol.

Question 10.30:

Which of the alkali metal is having least melting point?

(a) Na (b) K (c) Rb (d) Cs

Answer:

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, Cs is the largest and has the least melting point.

Question 10.31:

Which one of the following alkali metals gives hydrated salts?

(a) Li (b) Na (c) K (d) Cs

Answer:

Smaller the size of an ion, the more highly is it hydrated. Among the given alkali metals, Li is the smallest in size. Also, it has the highest charge density and highest polarising power. Hence, it attracts water molecules more strongly than the other alkali metals. As a result, it forms hydrated salts such as LiCl.2 H₂O. The other alkali metals are larger than Li and have weaker charge densities. Hence, they usually do not form hydrated salts.

Question 10.32:

Which one of the alkaline earth metal carbonates is thermally the most stable?

(a) MgCO₃ (b) CaCO₃ (c) SrCO₃ (d) BaCO₃

Answer:

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 metals is

Mg < Ca < Sr < Ba

Hence, the increasing order of the thermal stability of the given alkaline earth metal carbonates is

MgCO₃ < CaCO₃ < SrCO₃ < BaCO