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The s-Block Elements

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The s-Block Elements 1) What are the common physical features of alkali metals? Solution- i) Common physical features of alkali metals: i) Alkali metals are silvery white, soft and light metals. ii) They have low density , which increases from Li to Cs down the group. iii) The general outer electronic configuration of all alkali metals in ns 1 (n= 2 to 7). iv) The atoms of alkali metal have large atomic size in a particular period. The atomic size increases from Li to Cs down the group due to addition of extra shell from element to the next. v) They have low ,melting and boiling points due to weak metallic bonding. vi) They have low ionisation enthalpy due to large atomic size. The ionization enthalpy decreases down the group from Li to Cs as atomic size increases. vii) The hydration enthalpies of alkali metal ions decreases with increases in ionic size. Li + > Na + > K + >Rb + > Cs + ii) Common chemical features of alkali metals: i) Alkali metals are highly reactive due to low ionisation enthalpy and large atomic size. The reactivity decreases down the group as ionization enthalpy decreases. ii) Alkali metal tarnish in dry air due to the formation of oxides, which, in turn, reacts with moisture to form hydroxides. iii) Alkali metals burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide and other form superoxides. iv) They react with water to form metal hydroxides and liberate hydrogen gas. 2M + 2H 2 O 2MOH + H 2 (M= Li, Na, K, Rb, Cs) iv) Alkali metals react vigorously with halogens to form ionic halides of type MX (except for lithium , which forms covalent halides) CONTACT US @ 08889388421 ABHYAAS CLASSES BY ROHIT SIR ABHYAAS CLASSES BY ROHIT SIR www.abhyaasclasses.in ABHYAAS CLASSES BY ROHIT SIR
Transcript

The s-Block Elements

1) What are the common physical features of alkali metals?

Solution-

i) Common physical features of alkali metals:

i) Alkali metals are silvery white, soft and light metals.

ii) They have low density , which increases from Li to Cs down the group.

iii) The general outer electronic configuration of all alkali metals in ns1 (n= 2 to 7).

iv) The atoms of alkali metal have large atomic size in a particular period. The atomic size increases from

Li to Cs down the group due to addition of extra shell from element to the next.

v) They have low ,melting and boiling points due to weak metallic bonding.

vi) They have low ionisation enthalpy due to large atomic size. The ionization enthalpy decreases down the

group from Li to Cs as atomic size increases.

vii) The hydration enthalpies of alkali metal ions decreases with increases in ionic size.

Li+

> Na+

> K+ >Rb

+ > Cs

+

ii) Common chemical features of alkali metals:

i) Alkali metals are highly reactive due to low ionisation enthalpy and large atomic size. The reactivity

decreases down the group as ionization enthalpy decreases.

ii) Alkali metal tarnish in dry air due to the formation of oxides, which, in turn, reacts with moisture to

form hydroxides.

iii) Alkali metals burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms

peroxide and other form superoxides.

iv) They react with water to form metal hydroxides and liberate hydrogen gas.

2M + 2H2O →2MOH + H2

(M= Li, Na, K, Rb, Cs)

iv) Alkali metals react vigorously with halogens to form ionic halides of type MX (except for lithium ,

which forms covalent halides)

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2M + X2 →2MX

(M= Li, Na, K, Rb, Cs)

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

Solution-

General characteristics of alkaline earth metals and gradation in properties:

i) All the alkaline earth metals have a general outer electronic configuration of ns2 (n= 2 to 7).

ii) The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali

metals belonging to the same period due to an increase in the nuclear charge. The atomic and ionic radii

increase from Be to Ra due to the addition of shells from one element to the next, as the atomic number

increases down the group.

iii) The alkaline earth metals have low ionization enthalpies due to to large atomic size of their atoms.

Their ionisation enthalpies decrease down the group due to an increases in atomic size.

iv) The hydration enthalpies of alkaline earth metal ions decrease with an increase in ionic size down the

group.

Be2+

> Mg2+

> Ca2+

>Sr2+

> Ba2+

3) Why are alkali metals not found in nature?

Solution

Alkali metals are not found in a free state in nature due to their high reactivity. They always exist in a

combined state with another element in the form of mineral s and ores in the earth’s crust.

4) Find out the oxidation state of sodium in Na2O2.

Solution

In Na2O2 (sodium peroxide) each oxygen has -1 oxidation state due to peroxy bond.

∴Theoxidationstateofsodiumis�1inNa2O2 .

Let the oxidation state of Na be x . Then

2x -2 = 0

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2x = 2

x= 2/2 = +1

∴OxidationstateofNainNa2O2 is +1.

5) Explain why is sodium less reactive than potassium.

Solution

Sodium is less reactive than potassium, due to its bigger size and low ionisation enthalpy. Potassium has a

greater tendency to lose an electron and get oxidised than sodium.

6) Compare the alkali metals and alkaline earth metals with respect to ionization enthalpy.

Solution

i) Ionization enthalpy:

Alkaline metals Alkaline earth metals

The ionisation enthalpy of

alkali metals is low due to

their large atomic size.

Example :

Li -520 kJ mol-1

Na – 496 kJ mol-1

The ionisation enthalpy of

alkaline earth metals is also

low, but they are higher than

alkaline earth metals

corresponding to the alkali

metals of the same period

due to large nuclear change

and their small size

compared to alkali metals.

Example:

Be- 899 kJ mol-1

Mg- 737 kJ mol-1

ii) Basicity of oxides:

Alkaline metals Alkaline earth metals

The oxides of alkali metals

are strongly basic and form

very strong bases when

dissolved in water.

Example:

Na2O + H2O →2NaOH

K2O + H2O → 2KOH

The oxides of alkaline earth

metals are basic, but not as

strongly basic as alkali metal

oxides. The oxides of

alkaline earth metals are less

basic than alkaline metal

oxides.

Example :

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CaO + H2O →Ca (OH)2

BaO + H2O →Ba (OH)2

iii) Solubility of hydroxides:

Alkaline metals Alkaline earth metals

The hydroxides of alkali

metals are highly soluble in

water:

Example :

NaOH , KOH

The hydroxides of alkaline

earth metals are less or

sparingly soluble in water.

The solubility increases

down the group as the atomic

number of the elements

increases.

Example:

Ca(OH)2 , Mg (OH)2

7) In what ways lithium shows similarities to magnesium in its chemical behaviour?

Solution

i) Li and Mg, both react slowly with water.

ii) The oxides and hydroxides of Li and Mg are less soluble, and the hydroxides decompose on heating.

iii) Li and Mg, both combine with nitrogen directly to form nitrides, Li3N and Mg3N2.

iv) Li and Mg , do not form superoxides on combining with excess oxygen.

v) The carbonates of Li and Mg are less stable and decompose easily on heating to form oxide with the

liberation of CO2 gas.

vi) Li and Mg do not form solid hydrogen carbonates.

vii) LiCl and MgCl2 are deliquescent and crystallise, from their aqueous solution as hydrates, LiCl.2H2O

and MgCl2.8H2O.

8) Explain why can alkali and alkaline earth metals not be obtained by chemical reduction methods?

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Solution

Alkali and alkaline earth metals cannot be obtained by chemical reduction methods as they are strong

reducing agents and other reducing agents cannot reduce them . this is due to the low ionization potential of

these elements and their tendency to lose electrons easily. This is evident from their large negative

reduction potential value.

9) Why are potassium and caesium, rather than lithium used in photoelectric cells?

Solution

The atomic size of lithium is small, because of which the electron requires more energy to be removed

from its valence shell than potassium and caesium. Therefore, potassium and caesium which can lose

valence electrons by absorbing light of less energy, are used in photoelectric cells.

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

the reasons for this type of colour change.

Solution

When an alkali metal dissolves in liquid ammonia, the solution acquires deep blue colour, because the

ammoniated electron absorb energy in the visible region of light.

In a concentrated solution , the blue colour changes to bronze.

M + (x+y) NH3 →[M(NH3)x]+ + [e(NH3)y]

-

Ammoniated electron

11) Beryllium and magnesium do not give colour to flame where as other alkaline earth metals do so why?

Solution

The electrons in Be and Mg atoms are strongly bound and cannot be excited by the flame due to their

small size compared to other elements. Therefore , Be and Mg do not give colour to the flame.

12) Discuss the various reactions that occur in the Solvay process.

Solution

The Solvay process is used to prepare sodium carbonate (washing soda).

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The principal reactions that take place in the Solvay ammonia process are:

i) 2NH3 +H2O+CO2→(NH4)2CO3

ii) →(NH4)2CO3+ H2O+CO2→2NH4HCO3

iii) NH4HCO3 +NaCl→ NH4Cl + NaHCO3

iv) 2 NaHCO3→ Na2CO3 + H2O+CO2

13) Potassium carbonate cannot be prepared by Solvay process. Why?

Solution

Potassium carbonate cannot be prepared by the Solvay process, as potassium hydrogen carbonate is soluble

in water , and cannot b precipitated by adding of ammonium hydrogen carbonate to a saturated solution of

potassium chloride.

14) Why is Li2CO3 decomposed at a lesser temperature whereas Na2CO3 at higher temperature.

Solution

Li2CO3 decomposes at a lower temperature due to the small size of the Li+

ion . The small size of Li+

,

makes the lattice of Li2O more stable than that of Li2CO3 . This results in the decomposition of Li2CO3 to

Li2O and CO2 . Therefore, Li2CO3 is not thermally stable. On the other hand, Na2CO3, due to its larger size,

forms a thermally stable lattice.

15) Compare the solubility and thermal stability of the compounds of the alkali metal with those of the

alkaline earth metal nitrates.

Solution

i)

Alkaline metals Alkaline earth metals

Solubility :

The nitrates of alkali metals

are soluble in water. The

water solubility increases

down the group as the lattice

enthalpy decrease rapidly

The nitrates of alkaline earth

metal decompose on heating

to form metal oxide with the

liberation of nitrogen dioxide

and oxygen.

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than the hydration enthalpy.

Thermal stability:

The nitrates of alkali metals,

except lithium, decompose

on heating , to form the

corresponding nitrites with

the evolution of oxygen gas.

However, lithium nitrate, on

heating, decomposes to form

lithium oxide, nitrogen

dioxide and oxygen.

2MNO3 →

2MNO2 + O2

Alkali metal nitrite Alkali

metal nitrate

2M(NO3)2 → 2MO

+ 4NO2 + O2

Alkali metal nitrate Alkali

metal oxide

(M=Be, Mg, Ca, Sr, Ba)

ii) Carbonates:

iii)Sulphates:

Alkaline metals Alkaline earth metals

Solubility :

Alkali metal carbonates are

soluble in water.

Thermal stability:

Alkali metal carbonates,

except lithium carbonate, are

highly stable, do not

decompose on heating , and

the stability increases down

the group.

Lithium carbonate

decomposes to form lithium

oxide and carbon dioxide on

heating.

2Li2CO3 → Li2O

+ O2

Lithium carbonate

Lithium monoxide

Solubility :

Alkaline earth metal

carbonates are insoluble in

water.

Thermal stability:

Alkaline earth metal

carbonates decompose on

heating, to from the

corresponding metal oxide ,

with the liberation of carbon

dioxide. The thermal stability

increases down the group

with an increase in cationic

size.

MCO3 →MO + CO2

(M=Be, Mg, Ca, Sr, Ba)

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16) a)Starting with sodium chloride how would you proceed to prepare

i) Sodium metal ii) Sodium hydroxide

Solution

i) Sodium metal from sodium chloride:

Sodium metal can be obtained by the electrolysis of molten or fused NaCl (Down’s process). Sodium metal

is deposited at the cathode, while chlorine is liberated at the anode.

2NaCl(l) →2Na(s) +Cl2 (g)

ii) Sodium hydroxide from sodium chloride:

Sodium hydroxide is obtained by the electrolysis of aqueous solution of NaCl (brine solution) (Nelson cell

process). Sodium hydroxide is obtained and H2 gas is liberated at the cathode. Cl2 gas is liberated at the

anode.

2NaCl (aq) + 2H2O(l) →2NaOH (aq) + H2 +Cl2(g)

b) Starting with sodium chloride how would you proceed to prepare

iii) Sodium peroxide iv) Sodium carbonate.

Solution

iii) Sodium peroxide from sodium chloride:

Alkaline metals Alkaline earth metals

Solubility :

Alkali metal sulphates are

soluble in water.

Thermal stability:

Alkali metal sulphates ,

except lithium sulphate, are

thermally stable and do not

decompose on heating.

Lithium sulphate

decomposes on heating to

form lithium monoxide, with

the liberation of sulphur

dioxide and oxygen.

2LiSO4 → 2Li2O +2SO2 +

O2

Solubility :

BeSO4 and MgSO4 are

soluble in water and the

solubility decreases from

CaSO4 to BaSO.

Thermal stability:

Alkaline earth metal

sulphates decompose on

heating to form the

corresponding metal oxide

with the liberation of sulphur

dioxide and oxygen.

2MSO4 →2MO + 2SO2 +O2

(M= Be, Mg, Ca, Sr, Ba)

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Sodium is obtained by electrolysis of fused NaCl . Sodium, when burnt in oxygen, froms sodium peroxide.

2Na + O2 → Na2 O2

iv) Sodium carbonate from sodium chloride:

Sodium carbonate is obtained by the action of ammonia and CO2 on an aqueous NaCl solution.

2NH3 + H2O +CO2 →(NH4)2CO3

(NH4)2CO3+ H2O +CO2 → 2NH4HCO3

NH4HCO3 +NaCl → NH4Cl + NaHCO3

Sodium bicarbonate on heating forms sodium carbonate.

NaHCO3 → Na2CO3 + CO2 +H2O

17) a) What happens when

i) magnesium is burnt in air ii) quick lime is heated with silica?

Solution

i) When magnesium is burnt in the air, it burns with a dazzling light with emission U.V

radiations and forms magnesium oxide and magnesium nitride.

5Mg(s) + O2(g) +N2(g) →2MgO (s) + Mg3N2 (s)

ii) When quick lime is heated with silica , it forms calcium silicate.

CaO (s) + SiO2 (s) →CaSiO3 (s)

b) What happens when

i) chlorine reacts with slaked lime iv) calcium nitrate is heated

iii) When chlorine reacts with slaked lime, it forms bleaching powder.

Ca(OH)2 + Cl2 → CaOCl2 +H2O

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iv) When calcium nitrate is heated, it undergoes decomposition with the evolution of

nitrogen dioxide gas.

2Ca(NO3)2(s) →2 CaO(s) + 4NO2 (g) + O2(g)

18) Describe two uses of the following:

i) Caustic soda ii) Sodium carbonate iii) Quick lime

Solution

i) Two uses of caustic soda:

a) It is used in the manufacture of soap.

b) It is used in the extraction of aluminium from bauxite, in the refining of bauxite.

ii) Two uses of sodium carbonate:

a) It is used in water softening and as a cleaning agent.

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

iii) Two uses of quick lime:

a) It is used in the manufacture of cement.

b) It is used in the purification of sugar.

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

Solution

i) BeCl2 (vapour):

BeCl2 has a linear structure with 180° bond angle in vapour state.

ii) BeCl2 (solid)

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Solid state BeCl2 has a polymeric chain structure with chlorine bridges.

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.

Solution

The solubility of an ionic compound in water depends on its lattice energy and hydration

energy. The lattice energy of the hydroxides and carbonates , of sodium and potassium is

less than their hydration energy, due to the large size of their cations. Therefore, the

hydroxides and carbonates of sodium and potassium are soluble in water. On the other

hand, the lattice energy of the hydroxides and carbonates, of magnesium and calcium is

higher than the hydration energy, due to the small size of their cations and relativley more

magnitude of the charge. Therefore, the hydroxides and carbonates , of magnesium and

calcium are sparingly soluble in water.

21) i) Describe the importance of the Limestone.

Solution

i) Limestone is calcium carbonate(CaCO3).

Importance:

a) It is used as building material in the form of marble.

b) It is used in the manufacture of quicklime .

c) It is used in flux in the extraction of metals in metallurgy.

d) It is used as an antacid, and as a mild abrasive in tooth paste.

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e) It is a constituent of chewing gum, and is used as filler in cosmetics.

ii) Describe the importance of the Cement.

Solution

Cement:

Cement is an important building material. It is also called as Portland cement.

Composition of cement:

CaO(50-60%),

SiO2(2-25%),

Al2O3(5-10%),

MgO (2-3%)

H2O3(1-2%)

and SO3(1-2%)

Importance:

Cement is a very important commodity of material necessity for any country after iron and

steel.

a) It is used to make concrete and reinforced concrete.

b) It is used in plastering and in the construction of bridges, dams and buildings.

iii) Describe the importance of the Plaster of Paris.

Solution

Plaster of Paris

Hemihydrate of calcium sulphate is plaster of Paris. Its chemical formula is CaSO4.1/2H2O

Importance:

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a) It is used as building material.

b) It is used in the plastering of walls and for interior decoration.

c) It is used as plaster for joining fractured ones and also used in dentistry.

d) It is used for making casts of statues and busts.

22) Why are lithium salts commonly hydrated and those of the other alkali ions usually

anhydrous?

Solution

Because of its very small size, a lithium atom has the tendency to get hydrated . Therefore,

lithium salts are hydrated, when they come in contact with moisture. On the other hand, the

cations of other alkali metal ions are very large and cannot be hydrated easily. Therefore ,

other alkali metal salts are anhydrous.

23) Why are LiF almost insoluble in water , whereas LiCl soluble not only in water but also

in acetone?

Solution

LiF is insoluble in water due its high lattice enthalpy, which is due to the very small size of

the F- ion. On the other hand, LiCl is soluble in water due to the relatively low lattice

enthalpy, ehich is due to the large size of Cl- ions. It is soluble in acetone because of dipolar

attractions created by the polar nature of acetone.

24) Explain the significance of sodium, potassium , magnesium and calcium in biological

fluid.

Solution

Biological importance o sodium and potassium:

Sodium and potassium are considered as microelements, that are present in their ionic form

in body fluids.

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a) Sodium and potassium ions transmit nerve signals.

b) They regulate the flow of water across cell membranes.

c) They also transport sugars and amino acids into the cells.

d) They activate the enzymes that carry out regular metabolic activities, like oxidation of

glucose to produce ATP.

e) Sodium and potassium ions are present in high concentration on either side of the cell

membrane. These ions are continuously exchanged between the cell membrane by a

discriminating mechanism called sodium-potassium pump.

This consumes one-third of the ATP of a resting animal.

Biological importance of magnesium and calcium:

A healthy adult body contains about 25g of Mg and 1200 g of Ca.

Importance of magnesium:

a) Magnesium is the cofactor for all enzymes that utilise ATP in phosphate transfer.

b) The green pigment is plants, chlorophyll , has magnesium as the central metal ion.

Chlorophyll absorbs sunlight and produces food from CO2 and H2O by a process called

photosynthesis.

Importance of calcium :

a) Nearly 99% of the calcium in the body is in the bones and teeth, which support the

muscular mass of the body.

b) It plays a major role in neuromuscular function and inter neuronal transmission.

c) It also plays an important role in cell membrane integrity and blood coagulation.

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?

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Solution

i) When sodium metal is dropped in water, sodium hydroxide is formed.

The reaction takes place with a hissing sound and is an explosive reaction. The metal

catches fire because the hydrogen that evolved during the reaction is highly combustible

and catches fire.

2Na + 2H2O → 2NaOH + H2

ii) When sodium metal is heated in free supply of air sodium peroxide is formed.

2Na + O2 → Na2 O2

iii) When sodium peroxide dissolves in water, oxygen gas is evolved with the formation of

sodium hydroxide.

2Na2 O2 + 2H2O → 4NaOH + O2

26) Comment of 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 nitride directly.

c) E- for M

2+ (aq) + 2e

− → M(s) (where M= Ca, Sr or Ba) is nearly constant.

Solution

a) The mobility of alkali meta ions in their aqueous solutions depends on the size of the

cation and hydration. As smaller cation is hydrated to a large extent, and hydrated Li+

is

large. Therefore, its mobility is the least. As the size of the cation increases, its hydrated ion

size decreases and the mobility increases.

Li+

< Na+

< K+

<Rb+

< Cs+.

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b) Because of its very strong reducing nature, lithium can easily react with nitrogen to form

nitride. However, due to their low reducing nature, other alkali metals do not combine with

nitrogen.

3Li + N2 →Li3N2

c) The value of E° depends on sublimation enthalpy, ionisation enthalpy and hydration

enthalpy. For the given metals, the overall magnitude of E° remains the same. Therefore,

metals have nearly the same reducing strength.

27) State as to why:

a) A solution of Na2CO3 is alkaline?

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

c) Sodium is found to be more useful then potassium?

Solution

a) Na2CO3 is a salt of a weak acid(H2CO3) and a strong base (NaOH) . The aqueous solution

of Na2CO3 is alkaline due to anionic hydrolysis.

Na2CO3 + 2H2O → 2NaOH + H2CO3

b) Alkali metals are very strong reducing agents. No reducing agents are available to reduce

them. Therefore, they cannot be obtained by normal chemical reduction methods. Hence,

alkali metals are prepared by the electrolysis of their fused chlorides.

c) Sodium is more abundant and also less reactive than potassium. The reactions of sodium

with other substances can be controlled . Therefore, sodium is more useful than potassium.

28) Write balanced equations for reactions between:

a) Na2O2 and water b) Na2O and CO2 c) KO2 and water

Solution

a) 2Na2O2 + 2H2O → 4NaOH + O2

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b) 2KO2 +2H2O → 2KOH + H2O2 + O2

c) Na2O + CO2 → Na2CO3

29) How would you explain the following observation?

i) BeO is almost insoluble but BeSO4 is soluble in water BaO.

ii) BaO is soluble but BaSO4 is insoluble in water.

iii) LiI is more soluble than KI is ethanol.

Solution

i) BeO is insoluble because its lattice enthalpy is high as compared to BeSO4.

This is due to the small size of the O2-

ion as compared to BeSO4 . The high lattice energy

opposes the solubility of a substance in water. Therefore, BeO is insoluble in water, while

BeSO4 due to its low lattice enthalpy , is soluble in water.

ii) BaO is soluble, because of its lattice enthalpy is less than the hydration enthalpy due to

the large size of the Ba2+

ion. In the case of BaSO4 , due to the large size of SO42-

ion, Ba2+

ion is masked and is not hydrated. In BaSO4 , the lattice enthalpy is more than the hydration

enthalpy. Therefore, BaSO4 is insoluble in water.

iii) KI is ionic in nature, while LiI is covalent. Due to its small size, the Li+ ion, polarises

the electron cloud on the I-ion to a greater extent. Due to its covalent nature, LiI, is soluble

in ethanol, while due to its ionic nature, KI, is insoluble in ethanol.

30) Which of the alkali metal is having least melting point?

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

Solution

d) Cs(Caesium)

As the atomic size increases down a group, the strength of the metallic bond decreases, and

the melting point of the elements also decreases. Therefore, Cs, being the largest , ahs the

lowest melting point.

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31) Which of the following alkali metals gives hydrated salts?

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

Solution

a) Li(Lithium)

Li+ ion can attract water molecules easily than any other alkali metal ion, because of its

small size. Therefore, lithium forms hydrated salts.

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

a) MgCO3 b) CaCO3 c) SrCO3 d) BaCO3 e) BeCO3

Solution

d) BaCO3

BaCO3 is thermally more stable, as both Ba2+

ion and CO32-

ions are large leading to the

formation of a stable lattice.

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OHIT SIR


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