Lithiumis the smallest in size among the alkali metals. Hence, Li⁺ ion can polarizewater molecules more easily than other alkali metals. As a result, watermolecules 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 - 23 : -
Whyis LiF almost insoluble in water whereas LiCl soluble not only in water butalso in acetone?

Answer - 23 : -

LiFis insoluble in water. On the contrary, LiCl is soluble not only in water, butalso in acetone. This is mainly because of the greater ionic character of LiFas compared to LiCl. The solubility of a compound in water depends on thebalance between lattice energy and hydration energy. Since fluoride ion is muchsmaller in size than chloride ion, the lattice energy of LiF is greater thanthat of LiCl. Also, there is not much difference between the hydration energiesof fluoride ion and chloride ion. Thus, the net energy change during thedissolution of LiCl in water is more exothermic than that during thedissolution of LiF in water. Hence, low lattice energy and greater covalentcharacter are the factors making LiCl soluble not only in water, but also in acetone.

Question - 24 : -
Explainthe significance of sodium, potassium, magnesium and calcium in biologicalfluids.

Answer - 24 : -

Importanceof sodium, potassium, magnesium, and calcium in biological fluids:

(i) Sodium(Na):

Sodiumions are found primarily in the blood plasma. They are also found in theinterstitial 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):

Potassiumions 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):

Magnesiumand calcium are referred to as macro-minerals. This term indicates their higherabundance 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 - 25 : -
Whathappens 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 - 25 : -

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

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

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

Question - 26 : -
Commenton each of the following observations:
(a) The mobilities of the alkali metal ionsin 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 - 26 : -

(a) On moving down the alkaligroup, the ionic and atomic sizes of the metals increase. The given alkalimetal 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 highlyis it hydrated. Since Li⁺ is the smallest, it gets heavilyhydrated in an aqueous solution. On the other hand, Cs⁺is the largest and so itis the least hydrated. The given alkali metal ions can be arranged in thedecreasing order of their hydrations as:

Li⁺ > Na⁺ > K⁺> Rb⁺ > Cs⁺

Greater the mass of a hydrated ion, thelower is its ionic mobility. Therefore, hydrated Li⁺ is the least mobileand hydrated Cs⁺ is the most mobile. Thus, the givenalkali metal ions can be arranged in the increasing order of their mobilitiesas:

Li⁺ < Na⁺ < K⁺< Rb⁺ < Cs⁺

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

(i)Ionisation enthalpy

(ii)Enthalpy of hydration

(iii)Enthalpy of vaporisation

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

Question - 27 : -
Stateas 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 - 27 : -

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

(b) It is not possible toprepare alkali metals by the chemical reduction of their oxides as theythemselves are very strong reducing agents. They cannot be prepared bydisplacement reactions either (wherein one element is displaced by another).This is because these elements are highly electropositive. Neither canelectrolysis of aqueous solutions be used to extract these elements. This isbecause the liberated metals react with water.

Hence,to overcome these difficulties, alkali metals are usually prepared by theelectrolysis of their fused chlorides.

(c) Blood plasma and the interstitial fluidssurrounding the cells are the regions where sodium ions are primarily found.Potassium ions are located within the cell fluids. Sodium ions are involved inthe transmission of nerve signals, in regulating the flow of water across thecell membranes, and in transporting sugars and amino acids into the cells.Hence, sodium is found to be more useful than potassium.

Question - 28 : -
Writebalanced equations for reactions between
(a) Na₂O₂and water
(b) KO₂ and water
(c) Na₂O and CO₂

Answer - 28 : -

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

(b) The balancedchemical equation for the reaction between KO₂and water is:

Question - 29 : -
Howwould you explain the following observations?
(i) BeO is almost insoluble but BeSO₄ in soluble in water,
(ii) BaO is soluble but BaSO₄ is insoluble inwater,
(iii)LiI is more soluble than KI in ethanol.

Answer - 29 : - (i) BeOis almost insoluble in water and BeSO₄ issoluble in water. Be²⁺ isa small cation with a high polarising power and O^2– is a small anion. The size compatibilityof Be²⁺ and O^2– ishigh. Therefore, the lattice energy released during their formation is alsovery high. When BeO is dissolved in water, the hydration energy of its ions isnot sufficient to overcome the high lattice energy. Therefore, BeO is insolublein water. On the other hand, ion is a large anion. Hence, Be²⁺ can easily polarise ions, making BeSO₄ unstable. Thus, thelattice energy of BeSO₄ is not very high and so it is solublein water.

(ii) BaO is soluble in water, but BaSO₄is not. Ba²⁺ isa large cation and O^2– isa small anion. The size compatibility of Ba²⁺ andO^2– is not high. As a result, BaO is unstable.The lattice energy released during its formation is also not very large. It caneasily be overcome by the hydration energy of the ions. Therefore, BaO issoluble in water. In BaSO₄,Ba²⁺ and

are both large-sized. The lattice energy releasedis high. Hence, it is not soluble in water.

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

Question - 30 : -
Whichof the alkali metal is having least melting point?
(a)Na (b) K (c) Rb (d) Cs

Answer - 30 : -

Atomicsize increases as we move down the alkali group. As a result, the bindingenergies of their atoms in the crystal lattice decrease. Also, the strength ofmetallic bonds decreases on moving down a group in the periodic table. Thiscauses a decrease in the melting point. Among the given metals, Cs is thelargest and has the least melting point.

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