Class 11 - Chemistry - The S Block Elements

Question 10.1

What are the common physical and chemical features of alkali metals?

Answer.

Physical properties of alkali metals:

  • They have low ionization enthalpies.
  • They are highly electropositive in nature.
  • They exhibit +1 oxidation states in their compounds.
  • They impart characteristic color to the flame.

Chemical properties of alkali metals:

  • Their hydroxides are highly basic in nature.
  • They are highly reactive in nature.
  • They dissolve in liquid ammonia to form blue and conducting solution.

 

 

Question 10.2

Discuss the general characteristics and gradation in properties of alkaline earth metals.

Answer.

Properties of Alkaline Earth Metals:

  • Atomic size goes on increasing down the group.
  • They are harder than alkali metals.
  • Ionization energy goes on decreasing down the group.
  • Electropositive character increases on going down the group.
  • They are less electropositive than alkali metals.

 

 

Question 10.3

Why are the alkali metals not found in nature?

Answer.

Alkali metals are highly reactive nature. This is why, they always exist in combined state in nature.

 

Question 10.4

Find out the oxidation state of sodium in Na2O2.

Answer.

Let x is the oxidation state of Na in Na2O2.

2x + 2 (-1) = 0

2x – 2 = 0

2x = +2

x = +1.

 

 

Question 10.5

Explain why is sodium less reactive than potassium.

Answer.

Sodium is less reactive than potassium because ionization enthalpy of K is 419 kJ mol-1.

Ionization enthalpy of sodium = 496 kJ mol-1.

Hence, ionization enthalpy of potassium is less than that of sodium, potassium is more reactive than sodium.

 

 

Question 10.6

Compare the alkali metals and alkaline earth metals with respect to

(i) ionization enthalpy (ii) basicity of oxides (iii) solubility of hydroxides.

Answer.

  • Ionization enthalpy of alkaline metals is higher because of high nuclear charge and smaller atomic size .
  • Basicity of oxides of alkali metals is higher than that of alkaline earth metals.
  • Solubility of hydroxides of alkali metals is higher due to lower ionization enthalpy.
  • Due to lower ionization enthalpy, alkali metals are more electropositive.

 

 

Question 10.7

In what ways lithium shows similarities to magnesium in its chemical behavior?

Answer.

Similarities between lithium and magnesium:

(i) lithium and  magnesium reacts slow with cold water.

(ii) oxides of lithium and  magnesium are less  soluble in H2O. Also the hydroxides of both decompose at high temperature.

2LiOH →Li2​O+H2​O

Mg(OH)2 →MgO + H2O

(iii)  Nitrides are formed from both the lithium and magnesium when they react with N2.

6Li+N2 →2Li3 N

 3Mg+N2 →Mg3 N2

​(iv) Neither Li nor Mg form superoxides nor peroxides.

(v) Both the carbonates of lithium and magnesium are naturally covalent. They decompose on heating.

Li2CO3  →Li2O+CO2

​MgCO3  →MgO + CO2

​ (vi) They do not form bicarbonates which are solid.

(vii) Both MgCl2 and LiCl are soluble in ethanol because they are naturally covalent.

(viii) Both MgCl2 and LiCl are naturally deliquescent. They crystallize as hydrates from aqueous solutions.

Eg.,

LiCl.2H2O and MgCl2​.8H2O

 

Question 10.8

Explain why can alkali metal dissolves in liquid ammonia the solution can acquire different colours.

Explain the reasons for this type of color change.

Answer.

By using a stronger reducing agent, the oxides of metals gets  reduced by the process called chemical reduction.

Alkali and alkaline earth metals are better reducing agents. There are no better reducing agents than the alkali metals.

This is the reason why, these metals are not obtained by chemical reduction methods.

 

 

Question 10.9

Why are potassium and caesium, rather with lithium used in photoelectric cells?

Answer.

Potassium (K) and caesium (Cs) have lower ionization enthalpy than that of lithium.

These metals easily emit electrons on the exposure of light. Due to which, K and Cs are used in photoelectric cells rather than lithium.

  

Question 10.10

When an alkali metal dissolves in liquid ammonia the solution can acquire different colours.

Explain the reasons for this type of colour change.

Answer.

Alkali metals dissolve in liquid ammonia and give deep blue solutions which are conducting in nature.

This is because ammoniated electrons absorb energy in the visible region of light and impart blue color.

 

M + (x+y)NH3 --> [M(NH3)x]+ + e-3(NH3)y

                                             Ammoniated electrons

 

Question 10.11

Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?

Answer.

Ionization enthalpy of Be and Mg are much higher than those of the other alkaline earth metals due to its small size.

A large amount of energy is needed to excite their valence electrons. Hence, they do not impart color to the flame.

 

Question 10.12

Discuss the various reactions that occur in the Solvay process.

Answer.

Reactions involved in Solvay process:

Class_11_Chemistry_S-Block_Elements_Solvay_Process 

 

Question 10.13

Potassium carbonate cannot be prepared by Solvay process. Why?

Answer.

Potassium carbonate being more soluble than sodium bicarbonate does not get precipitated when CO2

is passed through a concentrated solution of KCl saturated with ammonia.

 

Question 10.14

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

Answer.

Li2CO3 is a covalent compound whereas Na2CO3 is an ionic compound.

Lattice energy of Na2CO3 is higher than that of Li2CO3.

It is decomposed at a lower temperature.

 

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.

  • Nitrates – Group 1 and 2 elements are soluble in water because hydration energy is more than the lattice energy.
  • Nitrides of both group 1 and group 2 elements are thermally unstable but they decompose differently except LiCO3.

Class_11_Chemistry_S-Block_Elements_Equation1

  • Carbonates – of group 1 elements are soluble in water except Li2CO3. They are also thermally stable except Li2CO3.

Class_11_Chemistry_S-Block_Elements_Equation2

Group 2 carbonates are insoluble in water because their lattice energy are higher than hydration energy.

Thermal stability of group 2 carbonates increase down the group because lattice energy keeps on increasing

due to increase in ionic character.

  • Sulphates – of group 1 are soluble in water except Li2SO4. They are thermally stable.

Solubility of group 2 sulphates decreases down the group because lattice energy dominates over hydration energy.

They are thermally stable which increases down the group due to increase in lattice energy.

 

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.

  • Sodium metal is manufactured by electrolysis of a fused mass of NaCl 40% and CaCl2 60% in
  • Down’s cell at 873K using iron as cathode and graphite as anode. Sodium is liberated at cathode.

At cathode

Na + + e-  --> Na(l)

At anode

2Cl-(melt) ---> Cl2 (g) + 2e-

  • Sodium hydroxide is manufactured by electrolysis of an aqueous solution of NaCl in Castner-Kellner cell.

At cathode

Class_11_Chemistry_S-Block_Elements_Equation_2

At anode

Cl- - e- -->Cl2

Cl + Cl  --> Cl2

  • Sodium peroxide
  • 4Na + O2   --> 2 Na2O + O2
  • Sodium carbonate is obtained by Solvay ammonia process.

 

 Class_11_Chemistry_S-Block_Elements_Solvay_Ammonia_Process

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.

 Class_11_Chemistry_S-Block_Elements_Equation3

 

Question 10.18

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

Answer.

  • Caustic soda – is used in manufacturing soap paper, artificial silk. It is also used in textile industries.
  • Sodium carbonate – is used in softening of water, for laundry and cleaning purposes.
  • It is also used in glass manufacturing.
  • Quicklime – It is used in the preparation of bleaching powder.
  • Also, used in the purification of sugar and manufacturing cement.

 

 

Question 10.19

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

Answer.

Class_11_Chemistry_S-Block_Elements_Equation6

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.

Answer.

Group 1 hydroxides and carbonates are large in size.

Due to which they have higher hydration energy than the lattice energy.

So they are easily soluble in water.

In magnesium and calcium, size in small and lattice energy dominates over hydration energy.

Hence, they are sparingly soluble in water.

 

 

Question 10.21

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

Answer.

Limestone

  • It is extensively used in the manufacturing of high quality paper.
  • It is used in cosmetics.
  • It is used as an antacid.
  • It is used as mild abrasive in toothpaste.
  • Cement
  • It is used in concrete and reinforced cement.
  • It is an important building material.
  • Plaster of Paris
  • It is used in dentistry for making statues.
  • It is used in plasters for correcting the fractured bones.

 

 

Question 10.22

Why are lithium salts commonly hydrated and those of the other alkali ions usually anhydrous?

Answer.

Due to a very small size, Li+ can polarize water molecules easily than the other alkali metal ions.

  

Question 10.23

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

Answer.

LiF is almost insoluble in water due to the high lattice energy.

LiCl is soluble in water because its hydration energy is higher than its lattice energy.

 

 

Question 10.24

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

Answer.

Sodium - Sodium ions participate in the transmission of nerve signals and in the transport of sugars and amino acids into the cell.

Potassium – These activate many enzymes and participate in the oxidation of glucose to produce ATP.

Magnesium – All enzymes that utilize ATP in phosphate transfer require magnesium as a cofactor.

It is the main pigment for the absorption of light in plants.

Calcium – These are present in bones and plays important role in neuromuscular functions.

Question 10.25

What happens when

  • Sodium metal is dropped in water?
  • Sodium metal is heated in free supply in air?
  • Sodium peroxide dissolves in water?

Answer.

 Class_11_Chemistry_S-Block_Elements_Equation5

 

Question 10.26

Comment on each of the following observations:

  • The mobilities of the alkli metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ < Cs+.
  • Lithium is the only alkali metal to form a nitride directly.
  • E- for M2+ (aq) + 2e- --> M(s) (where M = Ca, Sr or Ba) is nearly constant.

Answer.

  • Smaller the size of the ion, more highly it is hydrated and greater is the mass of the hydrated ion. Ionic mobility also becomes lesser.
  • Extent of hydration decreases in the order given. So the mobility of Cs+ will be the highest.
  • Lithium is the only alkali metal that form nitride directly because of its smaller size.
  • It is due to the reduction potential. Reduction potential depends on the sublimation energy, ionization energy and hydration energy.
  • Their resultant is almost constant for these ions.

 

 

Question 10.27

State as to why

  • A solution of Na2CO3 is alkaline?
  • Alkali metals are prepared by electrolysis of their fused chlorides?
  • Sodium is found to be more useful than potassium?

Answer.

  • Na2CO3 is a salt of weak acid (carbonic acid and a strong base NaOH).
  • It undergoes hydrolysis to produce strong base NaOH and its aqueous solution is alkaline in nature.
  • Alkali metals are prepared by electrolysis of fused chlorides because the discharge potential of alkali metals is much higher than that of hydrogen.
  • So, when the aqueous solution of an alkali metal chloride is subjected to electrolysis, H2 is produced at the cathode.
  • Potassium is more reactive than sodium and is found in nature to a less extent than sodium. Hence, sodium is more useful.

 

 

Question 10.28

Write balanced equations for reaction between

  • Na2O2 and water
  • KO2 and water
  • Na2O and CO2

Answer.

Class_11_Chemistry_S-Block_Elements_Equation4

Question 10.29

How would you explain the following observations?

  • BeO is almost insoluble but BeSO4 is soluble in water.
  • BaO is soluble but BaSO4 is insoluble in water.
  • LiL is more soluble than KI in ethanol.

Answer.

  • Lattice energy of BeO is comparatively higher than hydration energy. It is almost insoluble in water.
  • BeSO4 is ionic in nature and hydration energy dominates the lattice energy.
  • Both BaO and BaSO4 are ionic compounds but BaO has hydration energy higher than its lattice energy.
  • Hence, it is soluble in water.
  • Size of Li+ ion is very small in comparison to K+ It polarizes electron cloud of I- ion to a greater extent.
  • Hence, LiI dissolves in ethanol more easily than KI.

 

 

Question 10.30

Which of the alkali metal is having least melting point?

  • Na (b) K (c) Rb (d) Cs

Answer.

Correct option is (d)

Size of Cs is the biggest. Thus, its melting point is the lowest.

 

 

Question 10.31

Which one of the following alkali metals gives hydrated salts?

  • Li (b) Na (c) K (d) Cs

 

Answer.

Correct option is (a)

Li is the smallest so it has highest charge density and hence attracts water molecules more strongly.

 

 

Question 10.32

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

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

Answer.

Correct option is (d)

BaCO3

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