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

8/19/2019 CLS Aipmt 15 16 XI Che Study Package 3 SET 1 Chapter 9

1/18

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’ H2O

2 solution?

Sol. This means that one volume of H2O

2(aq) at STP evolves 20 volumes of O

2(g).

3. What is the formula of calgon?

Sol. Na6P

6O

18

4. How many H2O molecule in CuSO

45H

2O is/are hydrogen bonded?

Sol. One.

5. Name the stabiliser used while storing H2O

2.

Sol. Urea.

6. Does H2O

2 has a planar structure?

Sol. No, H2O2 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. NaAlSiO4

xH2O

Chapter 9

Hydrogen

Solutions


2/18

2 Hydrogen Solution of Assignment (Set-1)


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(HCO3)2

Heating Mg(OH)

2 + 2CO

2

Ca(HCO3)2

Heating CaCO

3 + H

2O + CO

2

12. To which class of hydrides does TiH1.5

, B2H

6 and CH

4 belong?

Sol. TiH1.5

– Metallic hydride

B2H

6 – Covalent hydride (electron-deficient)

CH4 – 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 ( OH2) 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 H2O molecule is transferred to another water molecule leaving behind OH– ion and forming a

H3O+ ion.

H2O(l) + H

2O(l) H

3O+(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

2O

2.


3/18

3Solution of Assignment (Set-1) Hydrogen


18. Explain why boiling point of HCl is less than that of HF.

Sol. Fluorine has greater electronegativity than that of chlorine. Therefore, stronger hydrogen bonds are formed

between hydrogen and fluorine atoms of the two neighbouring molecules as compared to those formed between

hydrogen and chlorine. Therefore, HF has higher boiling point than that of HCl.

19. What is demineralized water? Is it fit for drinking purposes?

Sol. Water which does not contain cations or anions is called demineralized water. No, it is not fit for drinking

because drinking water must contain some useful ions.

20. Why hard water does not form lather with soap?

Sol. Hard water contains soluble calcium and magnesium ions (Ca2+, Mg2+). So, when these ions react with soap

(e.g., sodium stearate), they form corresponding calcium and magnesium salt which precipitate out (scum).

So, until whole of the Ca2+ and Mg2+ ions are precipitated, lather is not formed. Therefore, lot of soap is first

wasted to precipitate out the ions.

2

17 35 17 35 2Sodium stearate Calcium stearate

2C H COONa Ca (C H COO) Ca 2Na

217 35 17 35 2

Sodium stearate Magnesium stearate

2C H COONa Mg (C H COO) Mg 2Na

21. What are interstitial hydrides? Give three examples.

Sol. Interstitial hydrides are the metallic hydrides which are formed by transition metals (d -block elements) of group

3, 4, 5 and f -block elements. Interstitial hydrides are the hydrides in which hydrogen is thought to occupy the

interstices in the metal lattice. All these hydrides are non-stoichiometric i.e., their composition is not fixed.

For example :

LaH2.87

, YbH2.55

, TiH1.5 – 1.8

22. Complete the following reactions

(a) P4O

10(s) + 6H

2O(l)

(b) SiCl4(l) + 2H

2O(l)

(c) N3–(s) + 3H2O(l)

Sol. (a) P4O

10(s) + 6H

2O(l) 4H

3PO

4(aq)

(b) SiCl4(l) + 2H

2O(l) SiO

2(s) + 4HCl(aq)

(c) N3–(s) + 3H2O(l) NH

3(g) + 3OH–(aq)

23. Hydrogen peroxide is used to restore the colour of old oil paintings containing lead oxide. Explain.

Sol. Lead oxide (PbO) contained in old oil paintings gets converted into lead sulphide (PbS) which is black in colour

in the presence of H2S gas present in the atmosphere. So, H

2O

2 is used to restore the original whiteness by

keeping the tarnished oil paintings in hydrogen peroxide solution for sometime which results in the oxidation

of lead sulphide to lead sulphate.

2 2 4 2(Black) (White)PbS 4H O PbSO 4H O


4/18



24. In what way is the crystalline form of water of great ecological significance?

Sol. In winters, ice which is the crystalline form of water being less denser forms on the surface of lakes, which

in turn provides thermal insulation so as to ensure the survival of the aquatic life. In this way it is of great

ecological significance.

25. On what principle does the ion-exchange method works for the softening of hard water?

Sol. Ion-exchange method of softening the hard water works on the principle of exchanging the ions responsible

for the hardness of water with the less damaging ions present in some compounds.

26. Calculate the amount of H2O

2 present in 20 ml of 30 volume H

2O

2 solution.

Sol. 20 ml of 30 volume H2O

2 liberate = 20 × 30

= 600 ml of O2 at STP

2 2 2 268 g 22700 mL at STP

2H O 2H O O

Amount of H2O2 required to liberate 22700 mL of O2 = 68 g

Amount of H2O

2 required to liberate 1 mL of O

2 =

68 g

22700

Amount of H2O

2 required to liberate 600 mL of O

2=

68 600 g

22700

= 1.79 g

27. Hydrogen generally forms covalent compounds. Give reason.

Sol. The electronic configuration of hydrogen is 1s1. So, hydrogen has tendency to either lose or gain or share itsone electron to gain stability. The ionisation energy of hydrogen is very high (

iH = 1312 kJ mol–1), so, it does

not lose the electron easily. The electron affinity of hydrogen is low. Therefore, it prefers to share its one electron

to form covalent compounds.

28. Name one compound in which hydrogen occurs in (a) +1 and (b) –1 oxidation state.

Sol. (a) +1 in HCl

(b) –1 in NaH

29. Complete the following reactions :

(a) AlN + H2O

(b) CaC2 + H

2O

Sol. (a) AlN + 3H2O Al(OH)

3 + NH

3

(b) CaC2 + 2H

2O Ca(OH)

2 + C

2H

2

30. Explain the hydroformylation reaction of olefins.

Sol. Hydrogen is used in the hydroformylation of olefins which yield aldehydes first and then on further reduction

give alcohols.

H2 + CO + RCH = CH

2 RCH

2CH

2CHO

H2 + RCH

2CH

2CHO RCH

2CH

2CH

2OH


5/18



Long Answer Type Questions :

31. Describe Clark’s method and Calgon’s method of removal of hardness of water. What type of hardness is

removed by employing the above two methods?

Sol. Clark’s method : This method is employed to remove the temporary hardness of water. In this method, the

calculated amount of lime is added to the hard water which results in the precipitation of calcium carbonate

and magnesium hydroxide, which later are filtered out.

Ca(HCO3)2 + Ca(OH)

2 2CaCO

3 + 2H

2O

Mg(HCO3)2 + 2Ca(OH)

2 2CaCO

3 + Mg(OH)

2 + 2H

2O

Calgon’s method : This method is employed to remove the permanent hardness of water. In this method,

sodium hexametaphosphate (Na6P

6O

18), which is commercially known as calgon is added to hard water which

forms complex anion keeping the Mg2+ and Ca2+ ions in the solution. In method the Ca2+ and Mg2+ ions present

in hard water are exchanged by the Na+ ions of calgon.

6 6 18Calgon

Na P O 2Na+ + Na4P

6O

18

2–

Ca2+ + Na4P

6O

18

2– [Na2CaP

6O

18]2– + 2Na+

32. Give the methods of preparation of an oxide of hydrogen used as a bleaching agent.

Sol. An oxide of hydrogen used as a bleaching agent is hydrogen peroxide. Hydrogen peroxide is prepared by the

following three main methods.

(i) In laboratory, hydrogen peroxide is prepared by the action of sulphuric acid on hydrated barium peroxide

BaO2

8H2O(s) + H

2SO

4(aq) BaSO

4(s) + H

2O

2(aq) + 8H

2O(l)

The excess water is removed by evaporation under reduced pressure giving hydrogen peroxide.

(ii) On commercial scale, hydrogen peroxide is prepared by the electrolytic oxidation of acidified sulphate

solution which at high current density gives peroxodisulphate.

2HSO4

–(aq)Electrolysis HO

3SOOSO

3H(aq)

Hydrolysis3 3Peroxodisulphate

HO SOOSO H(aq) 2HSO4

–(aq) + 2H+(aq) + H2O

2(aq)

(iii) On the industrial scale, hydrogen peroxide is prepared by the auto-oxidation of 2-alkylanthraquinols.

2

2

O (air)2 2H /Pd

2-ethylanthraquinol H O Oxidised product


6/18



33. Distinguish between

(a) Hard water and soft water

(b) Temporary hard water and permanent hard water

Sol. (a)Hard water

(i) It does not form lather with soap.

(ii) It contains calcium and magnesium salts in

the form of hydrogen carbonate, chloride and

sulphate in the water.

(iii) Example : sea water, river water.

Soft water

(i) It readily forms lather with soap.

(ii) This water is free from soluble salts of calcium

and magnesium.

(iii) Distilled water, demineralised water.

(b) Temporary hard water

(i) It is due to the presence of bicarbonates of

calcium and magnesium in water.

(ii) This type of hardness is easily removed bysimply boiling water.

(iii) Also known as carbonate hardness.

Permanent hard water

(i) It is due to the presence of chlorides and

sulphates of calcium and magnesium.

(ii) This type of hardness cannot be removed onboiling water.

(iii) Also known as non-carbonate hardness.

34. Discuss synthetic resins method employed to remove hardness of water.

Sol. Synthetic resins method : This method of softening of water is prevalent nowadays. The method is superior

to zeolite method because they remove all types of cations as well as anions present in water.

Synthetic resins are insoluble polymeric solids having giant hydrocarbon network containing reactive acidic or

basic groups. Synthetic resins are of two types :

Cation exchange resins : Cation exchange resins contain large organic molecule with –SO3H group and

are water insoluble. First ion exchange resin (RSO3H) gets changed to RNa after treating it with NaCl.

Now, RNa resin exchanges Na+ ions with Ca2+ and Mg2+ ions present in hard water to make water soft.

2RNa(s) + M2+(aq) R2M(s) + 2Na+(aq)

In order to obtain pure de-mineralised water free from all soluble mineral salts, then cation exchange (in

the H+ form) is used.

2RH(s) + M2+(aq) MR2(s) + 2H+(aq)

For example :

22

(Cation exchange resin) (From hard water) (Exhausted resin)

2R COO H Ca (RCOO) Ca 2H

In the above process as proton is released, thus it makes the water acidic.

Anion exchange resin : These resins contain giant hydrocarbon framework attached to basic groups

such as –NH2 group or OH– group.

RNH2(s) + H

2O(l) RNH

3

+OH–(s)

RNH3

+OH–(s) + X–(aq) RNH3

+X–(s) + OH–(aq)


7/18



For example :

3 3(From hard(Anion exchange (Exhausted

water)resin) resin)

R NH OH Cl R NH Cl OH

In the reaction OH– ion exchanges for the anions in the water. OH– ions are liberated which neutraliseH+ ions set free in the cation exchange resin.

H+(aq) + OH–(aq) H2O(l)

Regeneration of resin : The cation exchanger is regenerated by the action of dilute acid and the anion

exchanger is regenerated by treating it with dilute alkali.

35. (a) Discuss the structure of H2O

2

(b) Discuss the bleaching action of H2O

2

(c) Write important uses of H2O

2

Sol. (a) Structure of H2O2 : Hydrogen peroxide has a non-planar structure as the two O–H bonds are in different

planes. It has an open-book like structure with dihedral angle 111.5° in the gaseous phase but is reduced

to 90.2° in the crystalline state because of hydrogen bonding.

H

O O O O

95.0 pm

147.5 pm 111.5°

94.8°H

H

H

98.8 pm

145.8 pm 90.2°

101.9°

(a) Gas phase (b) Solid phase

(b) Bleaching action of H2O2 : H

2O

2 acts as a bleaching agent due to oxidation. It oxidises the colouring

matter to the colourless product.

H2O

2 H

2O + [O]

Colouring matter + O (Bleached)

Colourless mass

(c) Uses of H2O2 :

(i) It acts as a bleaching agent for bleaching fine and delicate materials.

(ii) It is used as an antiseptic for washing wounds and is sold in the market under the name perhydrol.

(iii) It is used in the synthesis of hydroquinone, tartaric acid, certain food products and pharmaceuticals

etc.

36. (a) Define isotopes.

(b) What is heavy hydrogen? Write the number of electrons, protons and neutrons in it.

(c) In what way are isotopes similar and dissimilar?


8/18



Sol. (a) Isotopes : Isotopes are the atoms of the same element which have the same atomic number but different

mass numbers.

(b) Deuterium is also known as heavy hydrogen. Deuterium has 1 electron, 1 proton and 1 neutron in its atom.

(c) Isotopes have the same electronic configuration. Example, all the isotopes of hydrogen have only one

electron. So, because of same electronic configuration they have almost same chemical properties.

Whereas, isotopes differ in their rates of reactions because they have different enthalpy of bond

dissociation.

37. List the points of similarities and differences between hydrogen and Group I elements.

Sol. Group 1 elements are the alkali metals.

Resemblance with Alkali Metals

1. Like alkali metals, hydrogen has only one electron in its outer shell.

2. Alkali metals have a strong tendency to lose one electron from their outermost shell to form unipositive

ion (Li+, Na+, K+, Rb+, Cs+) and attain nearest noble gas configuration. Similarly, hydrogen also loses

electron to form H+ ion.

3. The alkali metals form stable oxides, halides and sulphides, example sodium forms Na2O, NaCl, Na

2S.

Similarly, hydrogen also forms stable oxide (H2O), halides (HF) and sulphide (H

2S).

Difference from Alkali Metals

1. The ionisation energy of hydrogen is very high in comparison with alkali metals. Example : iH of Li is

520 kJ mol–1 and that of hydrogen is 1312 kJ mol–1.

This is because the 1s electron of hydrogen is tightly held by the nucleus because of its small size.

2. The size of H+ ion is very small (10–3 pm) as compared to that of alkali metal ions. Therefore, H+ does

not exist freely and is always associated with other atoms or molecules.

38. Explain the following

(a) Why cannot water extinguish petrol fire?

(b) Why is ice less denser than water? What type of attractive force must be overcome to melt ice?

(c) Write uses of heavy water.

Sol. (a) Petrol is lighter than water, so when water is added to petrol fire to extinguish it, petrol floats over water.

As a result, fire instead of getting extinguished, flares up.

(b) Ice has an open-cage like structure on account of extensive hydrogen bonding with large number of vacant

spaces making ice less denser than water. Hydrogen bonding is the type of attractive force which has

to be overcome by providing energy in the form of heat so as to melt ice.

(c) Heavy water is D2O. It has some important uses as

(i) Heavy water is used as a moderator in nuclear reactions so as to slow down the speed of fast moving

neutrons.

(ii) It is used as a tracer compound to study the reaction mechanism reactions.

(iii) It is used as a starting material for the preparation of number of deuterium compounds

CaC2 + 2D

2O C

2D

2 + Ca(OD)

2

SO3 + D

2O D

2SO

4


9/18



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(HCO3)2 is converted into Mg(OH)

2 instead of MgCO

3 because Mg(OH)

2

is precipitated easily. Whereas Ca(HCO3)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(HCO3)2

Heating Mg(OH)

2 + 2CO

2

Ca(HCO3)2

Heating CaCO

3 + H

2O + CO

2

(ii) Clark’s method : In this process the calculated amount of lime (calcium hydroxide) is added to hardwater containing bicarbonates of calcium and magnesium. Lime precipitates out calcium carbonate and

magnesium hydroxide which are then filtered to obtain soft water.

Ca(HCO3)2 + Ca(OH)

2 2CaCO

3 + 2H

2O

Mg(HCO3)2 + 2Ca(OH)

2 2CaCO

3 + Mg(OH)

2 + 2H

2O

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 ionichydrides. 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 (Ca2+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 + 2e2

–

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

NaH(s) + H2O(aq) NaOH(aq) + H

2(g)

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

inflammable.

CaH2 Ca + H

2


10/18



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

temperatures with O2 and Cl

2. For example

8LiH + Al2Cl

6 2LiAlH

4 + 6LiCl

2LiH + B2H

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 Cl2 and NaOH.

During electrolysis, the following reactions take place :

At anode : 2Cl–(aq) Cl2(g) + 2e–

At cathode : 2H2O(l) + 2e– H

2(g) + 2OH–(aq)

Overall reaction : 2Cl–(aq) + 2H2O(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 K2 2Ni

CokeWater gas

C H O CO H

1270 Kn 2n 2 2 2Ni

Hydrocarbon

C H nH O nCO (2n 1)H

e.g.,

CH4(g) + H

2O(g)

1270 K

Ni CO(g) + 3H

2(g)

The mixture of CO and H2 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 H2 (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) + H2O(g)

4

673 K

FeCrO CO

2(g) + H

2(g)

The above reaction is called water-gas shift reaction. The CO2 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


11/18



Sol. (a) Reaction with halogens : Hydrogen reacts with halogens to give hydrogen halides with general formula

HX.

H2(g) + X

2(g) 2HX(g) (X = F, Cl, Br, I)

In case of fluorine reaction occurs even in dark

H2

+ F2

Dark

2HF

With chlorine, it occurs in presence of sunlight

H2 + Cl

2

Sunlight 2HCl

Bromine reacts only upon heating

H2 + Br

2

673 K 2HBr

With iodine, it requires catalyst for the reaction

H2 + I

2

Pt

673 K 2HI

This shows that F2 is more reactive than Cl

2 which is further more reactive than Br

2 and least reactive

among halogens is I2 which requires a catalyst for the reaction.

(b) With unsaturated hydrocarbons : Vapours of dihydrogen are passed through unsaturated hydrocarbons

with CC or C=C bond, in the presence of catalyst to form saturated hydrocarbons.

473 K

2 3 3NiEthene

Ethane

H C C H(g) H (g) H C CH (g)

The above reaction is called catalytic hydrogenation.

(c) Reaction with nitrogen : Dihydrogen is employed in Haber’s process for the formation of ammonia

3H2(g) + N

2(g)

673 K, 200 atm

Fe 2NH

3(g) H = –92.6 kJ mol–1

43. (a) Give the structure of water in gaseous phase.

(b) List some physical properties of water.

Sol. (a) Structure of Water

Gas Phase : In gas phase, each water molecule in the form of water vapour exists as an independent

molecule. Due to the presence of two lone pairs, the geometry of the water gets distorted and acquires

a bent geometry with a bond angle of 104.5° and O–H bond length of 95.7 pm. In water, each O–H bondis polar because of high electronegativity of oxygen in comparison to that of hydrogen.

O95.7 pm

104.5°H H

(a)

O

H H

(b)

+

+

2–

O

(c)

H O molecule

Orbital overlap picturein water molecule

2Bent structure

of water

Water molecule

as a dipole


12/18


13/18



45. Calculate the normality of 20 volume of hydrogen peroxide.

Sol. Mass of H2O2 per litre of the solution :

2H2O

2 2H

2O + O

2

2 × 34 g 22.7 L at STP

= 68 g

22.7 L of O2 at STP is obtained from H

2O

2 = 68 g

1 L of O2 at STP is obtained from H

2O

2 =

68g

22.7

20 L of O2 at STP is obtained from H

2O

2 =

68 20 g

22.47

= 59.91 g = 60 g

So, the strength of H2O

2 in 20 volume H

2O

2 solution = 60 g/L

StrengthNormality =

Equivalent mass

Equivalent mass of H2O2 :

2H2O

2 2H

2O + O

2

68 parts 32 parts

by mass by mass

Equivalent mass

of O2 = 8

32 parts by mass of O2 are evolved from H

2O

2 = 68 parts

8 parts by mass of O2 are evolved from H

2O

2 =

68 8

32

= 17 parts

Thus, equivalent mass of H2O

2 = 17 or 17 g/equiv.

Calculation of normality of the solution :

Normality of solution =Strength of solution

Equivalent mass=

60 g/L

17 g/eq. = 3.52 N

SECTION - B

Model Test Paper

Very Short Answer Type Questions :

1. What are the constituents of water gas?

Sol. CO and H2.

2. Name the isotope of hydrogen which has equal number of protons, neutrons and electrons.

Sol. Deuterium.


14/18



3. How many hydrogen bonded water molecules are there in hydrated CuSO4?

Sol. One.

4. Write the chemical formula of washing soda.

Sol. Na2CO

3

5. What is the dihedral angle of H2O

2 in solid phase?

Sol. 90.2

6. Why is dihydrogen gas not preferred in balloons?

Sol. Dihydrogen gas is combustible in nature. It may therefore react violently with oxygen. Thus, it is not used in

balloons.

7. What is the trade name of hydrogen peroxide used as an antiseptic?

Sol. Perhydrol.

8. Write the reaction when heavy water is added to aluminium carbide.

Sol. Al4C

3 + 12D

2O 3CD

4 + 4Al(OD)

3


9. Explain how temporary hardness of water is removed by adding lime.

Sol. Temporary hardness of water is removed by adding calculated amount of lime to hard water. Lime precipitates

out calcium carbonate and magnesium hydroxide which are then filtered to obtain soft water.

Ca(HCO3)2 + Ca(OH)

2 2CaCO

3 + 2H

2O

Mg(HCO3)2 + 2Ca(OH)

2 2CaCO

3 + Mg(OH)

2 + 2H

2O

10. Give some physical properties of H2O

2.

Sol. (i) Pure hydrogen peroxide is a syrupy liquid.

(ii) It is colourless but gives a bluish tinge in thick layers.

11. What is non-carbonate hardness of water?

Sol. Non-carbonate hardness of water is the permanent hardness. Permanent hardness of water is due to the

presence of chlorides and sulphates of calcium and magnesium in water. As this type of hardness cannot be

removed by boiling and not due to the presence of bicarbonates, therefore, it is also known as permanent as

well as non-carbonate hardness.

12. Give two points of difference of hydrogen and halogens.

Sol. (i) Hydrogen is less reactive as compared to halogens.

(ii) The electron affinity of hydrogen is less than that of halogens.


15/18



13. Give one reaction each to show that water can act both as oxidising as well as reducing agent.

Sol. Water acts as oxidising agent :

2Oxidisingagent

2H O(l) + 2Na(s) 2NaOH(aq) + H2(g)

In the above reaction, H2O gets reduced to dihydrogen by highly electropositive metal.

Water as reducing agent : Water acts as a reducing agent when it itself gets oxidised.

2Reducingagent

2H O(l) + 2F2(g) 4H+(aq) + 4F–(aq) + O

2(g)

In the above reaction water gets oxidised to O2.

14. Compare soft and hard water.

Sol. Soft water is the one which is free from the soluble salts of calcium and magnesium, whereas hard water

contains soluble salts of calcium and magnesium.

Soft water forms lather easily with soap, whereas hard water does not form lather with soap.

15. How is heavy water prepared?

Sol. Heavy water is prepared by the following methods :

(i) It is prepared by the exhaustive electrolysis of water. Ordinary water contains very small proportion of D2O.

So, on prolonged electrolysis of water, H2 is liberated much faster than D

2 and remaining water, becomes

enriched in heavy water.

(ii) It is also obtained as a by-product in some fertilizer industries.


16. Why is hard water unsuitable for laundary?

Sol. Soaps are sodium and magnesium salts of long chain fatty acids. Soap containing sodium stearate

(C17

H35

COONa) reacts with hard water to form insoluble precipitates called scum

217 35 17 35 2

(From hard Sodium stearate Metal stearatewater) (Soap) (ppt)

M 2C H COONa (C H COO) M 2Na

Where M2+ = Ca2+ or Mg2+

Therefore, no lather is produced until all the calcium and magnesium ions are precipitated. This also resultsin wastage of soap.

17. Give any three uses of dihydrogen.

Sol. (i) Hydrogenation of oil : Oils are polyunsaturated hydrocarbons. Dihydrogen is added to oils like soyabean

oil, cotton seed oil for the manufacture of vanaspati fat which are saturated hydrocarbons.

(ii) Manufacture of methyl alcohol : Water gas enriched with hydrogen gas in the presence of cobalt

catalyst gives methanol

CO(g) + 2H2(g)

Cobalt

Catalyst CH

3OH(l)

(iii) Rocket fuel : It is used as a rocket fuel for space research in the form of liquid hydrogen.


16/18



18. Give the three types of association of water with many salts along with example.

Sol. (i) Co-ordinated water :

Example : [Cr(H2O)

6]3+ 3Cl–

(ii) Interstitial water :

Example : BaCl22H2O

(iii) Hydrogen-bonded water :

Example : [Cu(H2O)

4]2+ SO

4

2–H2O in CuSO

45H

2O

19. What is the strength (in g/L) of 25 volume solution of H2O

2?

Sol. 25 volume solution of H2O

2 means that 1 L of this H

2O

2 solution will give 25 L of oxygen at STP

2 2 2 22 34 g 22.7 L

68 g at STP

2H O (l) O (g) H O(l)

On the basis of above equation,

22.7 L of O2 is produced at STP from H

2O

2= 68 g

25 L of O2 is produced at STP from H

2O

2 =

6825 g

22.7 = 74.8 g

Strength of H2O

2 in 25 volume H

2O

2 solution = 74.8 g/L

20. Explain water-gas shift reaction.

Sol. Water-gas shift reaction is employed to increase the production of dihydrogen when steam reacts with coke

or hydrocarbons at high temperature in the presence of catalyst.

CH4(g) + H

2O(g)

1270 K

Ni CO(g) + 3H

2(g)

C(s) + H2O(g)

1270 K

Ni CO(g) + H

2(g)

In the above two reactions water-gas or syngas is produced which is a mixture of CO and H2. So, the

production of dihydrogen from the above reactions is increased by reacting carbon monoxide of syngas mixtures

with steam in the presence of iron chromate as catalyst.

CO(g) + H2O(g)

673 K

Catalyst CO

2(g) + H

2(g)

The above reaction is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium

arsenite solution.

21. Briefly discuss the isotopes of hydrogen.

Sol. Hydrogen has three isotopes namely protium, deuterium and tritium. The isotopes of an element are the atoms

having same atomic number but different mass numbers.


17/18



Protium 11

( 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 21

( 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 31

( 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 EHx (e.g., BeH

2) or E

mH

n (e.g., B

2H

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 CH4, H2O, NH3 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 BH3 which polymerise to form B

2H

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., CH4.

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 (NH3 has 1 lone pair, H

2O has 2 lone pairs, HF has

3 lone pairs).

104.5°

O

H H


18/18


AakashEducationalServicesPvt Ltd Regd Office :Aakash Tower PlotNo 4 Sector 11 Dwarka NewDelhi 75Ph 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)

Date post:	08-Jul-2018
Category:	Documents
Upload:	sairaj
View:	232 times
Download:	6 times

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

Documents