Download CLS Aipmt 15 16 XI Che Study Package 3 SET 1 Chapter 9 PDF

TitleCLS Aipmt 15 16 XI Che Study Package 3 SET 1 Chapter 9
TagsHydride Properties Of Water Hydrogen Ion Exchange Magnesium
File Size934.9 KB
Total Pages18
Document Text Contents
Page 1

Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

SECTION - A

School/Board Exam. Type Questions

Very Short Answer Type Questions :

1. Is the present position of hydrogen in the periodic table satisfactory?

Sol. No, the position of hydrogen in the periodic table is not satisfactory because, hydrogen resembles both alkali

metals and halogens in its properties yet it is placed on top of alkali metals.

2. What is the significance of the term ‘20 volume’ H
2
O

2
solution?

Sol. This means that one volume of H
2
O

2
(aq) at STP evolves 20 volumes of O

2
(g).

3. What is the formula of calgon?

Sol. Na
6
P

6
O

18

4. How many H
2
O molecule in CuSO

4
5H

2
O is/are hydrogen bonded?

Sol. One.

5. Name the stabiliser used while storing H
2
O

2
.

Sol. Urea.

6. Does H
2
O

2
has a planar structure?

Sol. No, H
2
O

2
has a non-planar structure.

7. Which anions produce permanent hardness in water?

Sol. Permanent hardness in water is due to the presence of chloride and sulphate anions.

8. By what name is hydrogen peroxide solution commercially available for washing wounds?

Sol. Perhydrol.

9. Why is molecular hydrogen very little reactive?

Sol. Molecular hydrogen is very little reactive because the bond dissociation enthalpy is very high.

10. What is the chemical composition of zeolite?

Sol. NaAlSiO
4
xH

2
O

Chapter 9

Hydrogen

Solutions

Page 2

2 Hydrogen Solution of Assignment (Set-1)

Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

Short Answer Type Questions :

11. Explain how boiling removes temporary hardness of water.

Sol. Temporary hardness of water is due to the presence of soluble magnesium and calcium bicarbonates in water.

When hard water is boiled then, this soluble magnesium bicarbonate changes into insoluble magnesium

hydroxide, whereas calcium bicarbonate changes into insoluble calcium carbonate. The precipitates formed can

then be filtered out leaving behind soft water.

Mg(HCO
3
)
2


Heating Mg(OH)
2
 + 2CO

2


Ca(HCO
3
)
2


Heating CaCO
3
 + H

2
O + CO

2


12. To which class of hydrides does TiH
1.5

, B
2
H

6
and CH

4
belong?

Sol. TiH
1.5

– Metallic hydride

B
2
H

6
– Covalent hydride (electron-deficient)

CH
4
– Covalent hydride (electron-precise)

13. Why the ionisation energy of hydrogen is higher than that of lithium?

Sol. Ionisation energy is the energy required to remove an electron from an isolated gaseous atom. Ionisation energy

depends upon the size of the atom. The size of hydrogen is much smaller than that of lithium. Though they

both have one valence electron in their atom, still more energy is required to remove the electron from hydrogen

atom because of its small size.

14. Why does water act as Lewis base?

Sol. Water is an electron rich hydride with two lone pair of electrons ( OH
2
) and that is why it can act as a Lewis

base.

15. Write the two disadvantages of hard water.

Sol. The two disadvantages of hard water are

(i) It is unsuitable for the laundry purpose as insoluble precipitates are formed called scum and no lather

is produced until all the calcium and magnesium ions are precipitated.

(ii) Hard water reduces the efficiency of the boilers as lot of deposition of salts in the form of scales take

place.

16. What is meant by the auto protolysis of water?

Sol. Auto protolysis is a reaction in which two same molecules react to give ions with proton transfer. In water, a

proton from one H
2
O molecule is transferred to another water molecule leaving behind OH


ion and forming a

H
3
O

+
ion.

H
2
O(l) + H

2
O(l) H

3
O

+
(aq) + OH


(aq)

17. Why is hydrogen peroxide stored in wax-lined glass bottles?

Sol. Hydrogen peroxide is stored in wax-lined glass bottles because the presence of metal surfaces or traces of

alkali in glass containers catalyses the decomposition of H
2
O

2
.

Page 9

9Solution of Assignment (Set-1) Hydrogen

Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

39. What is temporary hardness of water? How it is removed?

Sol. Temporary hardness of water is due to the bicarbonates of calcium and magnesium. It is also known as

carbonate hardness.

Removal of temporary hardness :

Temporary hardness can be removed by the following methods :

(i) Boiling : The temporary hardness of water can be easily removed by boiling the water in large boilers.

During boiling the soluble Mg(HCO
3
)
2
is converted into Mg(OH)

2
instead of MgCO

3
because Mg(OH)

2

is precipitated easily. Whereas Ca(HCO
3
)
2
is changed to insoluble CaCO

3
and gets precipitated. These

precipitates can be removed by filtration process. So, the filtrate obtained will be soft water.

Mg(HCO
3
)
2


Heating Mg(OH)
2
 + 2CO

2


Ca(HCO
3
)
2


Heating CaCO
3
 + H

2
O + CO

2


(ii) Clark’s method : In this process the calculated amount of lime (calcium hydroxide) is added to hard

water containing bicarbonates of calcium and magnesium. Lime precipitates out calcium carbonate and

magnesium hydroxide which are then filtered to obtain soft water.

Ca(HCO
3
)
2
+ Ca(OH)

2
 2CaCO

3
 + 2H

2
O

Mg(HCO
3
)
2
+ 2Ca(OH)

2
 2CaCO

3
 + Mg(OH)

2
 + 2H

2
O

40. What are saline hydrides? List some of their properties.

Sol. Ionic or saline hydrides

The ionic hydrides are stoichiometric which are formed when hydrogen combines with elements of s-block which

are highly electropositive in nature. There are some elements in s-block like Li, Be, Mg which do not form ionic

hydrides. The hydrides formed by Be and Mg (BeH
2
and MgH

2
) are polymeric in structure. Ionic hydrides are

formed by transfer of electrons from metals to hydrogen atoms and contain hydrogen as H

ion e.g., sodium

hydride (Na
+
H


), calcium hydride (Ca

2+
2H


). Some of the properties of saline hydrides are as follows :

1. These are white crystalline solids, have ionic lattices and behave like salts.

2. They have high melting and boiling points.

3. These are non-volatile and non-conducting in solid state, however they conduct electricity in fused state

liberating hydrogen at anode which confirms the existence of H

ion.

NaH Na + H
+ –

Cathode Anode

Na + e
+ –

Na 2H


H + 2e
2



4. These hydrides react violently with water producing dihydrogen gas.

NaH(s) + H
2
O(aq)  NaOH(aq) + H

2
(g)

5. They burn spontaneously in air on heating on account of their decomposition into hydrogen which is

inflammable.

CaH
2
 Ca + H

2

Page 10

10 Hydrogen Solution of Assignment (Set-1)

Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

6. Lithium hydride is used in the synthesis of some useful hydrides since it is rather unreactive at moderate

temperatures with O
2
and Cl

2
. For example

8LiH + Al
2
Cl

6
 2LiAlH

4
+ 6LiCl

2LiH + B
2
H

6
 2LiBH

4

41. Explain the preparation of dihydrogen by electrolysis of brine and by the action of steam on coke.

Sol. Electrolysis of brine : It is produced as a by-product of the brine (aq. NaCl) electrolysis process for the

manufacture of Cl
2
and NaOH.

During electrolysis, the following reactions take place :

At anode : 2Cl

(aq)  Cl

2
(g) + 2e



At cathode : 2H
2
O(l) + 2e


 H

2
(g) + 2OH


(aq)

Overall reaction : 2Cl

(aq) + 2H

2
O(l)  Cl

2
(g) + H

2
(g) + 2OH


(aq)

By the action of steam on coke : Dihydrogen is prepared by passing steam over coke or hydrocarbons at

high temperature (1270 K) in the presence of Nickel catalyst.

1270 K
2 2Ni

Coke
Water gas

C H O CO H  
�����

1270 K
n 2n 2 2 2Ni

Hydrocarbon

C H nH O nCO (2n 1)H    

e.g.,

CH
4
(g) + H

2
O(g)

1270 K

Ni
 CO(g) + 3H

2
(g)

The mixture of CO and H
2
is called water gas. It is also known as synthesis gas or simply ‘syn gas’ because

it is used in the synthesis of methanol and many other hydrocarbons.

It is difficult to obtain dihydrogen from a mixture of CO and H
2
(i.e., water gas). Therefore, the production of

dihydrogen is increased by reacting carbon monoxide of syn gas mixtures with steam in the presence of

ironchromate as catalyst

CO(g) + H
2
O(g)

4

673 K

FeCrO
 CO

2
(g) + H

2
(g)

The above reaction is called water-gas shift reaction. The CO
2
formed in the above reaction is removed by

scrubbing with sodium arsenite solution.

42. Explain the reaction of hydrogen with

(a) Halogens

(b) Unsaturated hydrocarbons

(c) Nitrogen

Page 17

17Solution of Assignment (Set-1) Hydrogen

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Protium
1

1
( H) : It is also known as ordinary hydrogen. It is the most predominant form of hydrogen. It has

one proton, one electron and no neutron in its atom.

Deuterium
2

1
( H or D) : It is also known as heavy hydrogen. Terrestrial hydrogen contains 0.0156 % of

deuterium mostly in the form of HD. It has one proton, one electron and one neutron in its atom.

Tritium
3

1
( H or T) : This isotope of hydrogen is radioactive. Tritium is just about one atom per 1018 atoms of

protium. It has one proton, one electron and two neutrons in its atom.

Long Answer Type Questions :

22. What are hydrides? Explain covalent hydrides.

Sol. Hydrogen combines with a large number of other elements including metals and non-metals except noble gases

to form binary compounds called hydrides. If ‘E’ is the symbol of the element then hydrides are represented

as EH
x
(e.g., BeH

2
) or E

m
H

n
(e.g., B

2
H

6
).

Based on their physical and chemical properties, the hydrides have been classified into three main categories.

(i) Ionic hydrides

(ii) Covalent hydrides

(iii) Metallic hydrides

Covalent or Molecular Hydrides

Covalent or molecular hydrides are the compounds of hydrogen with p-block elements. The most common

hydrides are CH
4
, H

2
O, NH

3
etc. Covalent hydrides are volatile compounds.

Molecular hydrides are further classified according to their relative numbers of electrons and bonds in their Lewis

structures.

1. Electron deficient molecular hydrides : These hydrides have lesser number of electrons than that

required for writing the conventional Lewis structure. All the hydrides of group 13 form electron deficient

compounds like BH
3
which polymerise to form B

2
H

6
. As these are electron deficient, so they have a

tendency to accept the electrons, therefore, act as Lewis acids.

2. Electron-precise hydrides : Such compounds have required number of electrons to write their

conventional Lewis structures. The elements of group 14 form such hydrides. The hydrides have tetrahedral

geometry. e.g., CH
4
.

3. Electron-rich hydrides : Electron-rich hydrides have excess electrons in the form of one or more lone

pairs of electrons around the central more electronegative element. Therefore, these hydrides due to the

presence of lone pair of electrons form electron-rich hydrides. Infact the presence of lone pair of electrons

on electronegative element results in the association of molecules due to hydrogen bonding. These types

of hydrides are formed by elements of group 15-17 (NH
3
has 1 lone pair, H

2
O has 2 lone pairs, HF has

3 lone pairs).

104.5°

O

H H

Page 18

18 Hydrogen Solution of Assignment (Set-1)

Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

23. (a) Why density of ice is maximum at 4°C

(b) Explain the amphoteric nature of water.

Sol. (a) Density of water is maximum at 4°C : The density of water is maximum at 4°C. The density of water

is affected by two factors.

(i) The breaking of hydrogen bonds resulting into closer packed structure.

(ii) The thermal expansion : With the rise in temperature, ice melts, the hydrogen bonds break and

molecules come closer, decreasing the volume and increasing the density until 4°C. With further

increase in temperature the kinetic energy of the molecules increases progressively and they start

moving away from each other increasing the volume and thus decreasing the density.

(b) Water can act both as an acid as well as a base and is thus said to possess amphoteric nature.

According to Lowry-Bronsted concept, it can act as an acid by losing a proton and as a base by accepting

a proton.

Water as base : Water acts as a base towards acids stronger than it

2 2 3

Base Acid

H O(l) H S(aq) H O (aq) HS (aq)
  ��� ���� �

2 3
AcidBase

H O(l) HCl(aq) H O (aq) Cl (aq)
  ���⇀↽���

Water as acid : Water acts as an acid towards bases stronger than it

2 3 4

Acid Base

H O(l) NH (aq) OH (aq) NH (aq)
  ���⇀↽���

� � �

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