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

Chapter

9

Hydrogen Solutions 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’ H2O2 solution?

Sol. This means that one volume of H2O2(aq) at STP evolves 20 volumes of O2(g). 3.

What is the formula of calgon?

Sol. Na6P6O18 4.

How many H2O molecule in CuSO45H2O is/are hydrogen bonded?

Sol. One. 5.

Name the stabiliser used while storing H2O2.

Sol. Urea. 6.

Does H2O2 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 Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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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. Heating Mg(HCO3)2   Mg(OH)2  + 2CO2 

Heating Ca(HCO3)2   CaCO3  + H2O + CO2 

12. To which class of hydrides does TiH1.5, B2H6 and CH4 belong? Sol. TiH1.5 – Metallic hydride B2H6 – 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) + H2O(l)

H3O+(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 H2O2. Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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Hydrogen

3

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.

2C17H35 COONa  Ca2   (C17H35 COO)2 Ca  2Na Sodium stearate

Calcium stearate

2C17H35 COONa  Mg2   (C17H35 COO)2 Mg  2Na Sodium stearate

Magnesium stearate

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)

P4O10(s) + 6H2O(l) 

(b)

SiCl4(l) + 2H2O(l) 

(c)

N3–(s) + 3H2O(l) 

Sol. (a)

P4O10(s) + 6H2O(l)  4H3PO4(aq)

(b)

SiCl4(l) + 2H2O(l)  SiO2(s) + 4HCl(aq)

(c)

N3–(s) + 3H2O(l)  NH3(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, H2O2 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.

PbS  4H2O2   PbSO4  4H2O

(Black)

(White)

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Hydrogen

Solution of Assignment (Set-1)

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 H2O2 present in 20 ml of 30 volume H2O2 solution. Sol. 20 ml of 30 volume H2O2 liberate = 20 × 30 = 600 ml of O2 at STP

2H2O2

 

68 g

2H2O



O2 22700 mL at STP

Amount of H2O2 required to liberate 22700 mL of O2 = 68 g Amount of H2O2 required to liberate 1 mL of O2 =

68 g 22700

Amount of H2O2 required to liberate 600 mL of O2 =

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 its one 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 + H2O 

Sol. (a) (b)

AlN + 3H2O  Al(OH)3 + NH3 CaC2 + 2H2O  Ca(OH)2 + C2H2

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 = CH2  RCH2CH2CHO H2 + RCH2CH2CHO  RCH2CH2CH2OH Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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Hydrogen

5

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  2CaCO3  + 2H2O Mg(HCO3)2 + 2Ca(OH)2  2CaCO3  + Mg(OH)2  + 2H2O Calgon’s method : This method is employed to remove the permanent hardness of water. In this method, sodium hexametaphosphate (Na6P6O18), 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. 2– Na6P6 O18  2Na+ + Na4P6O18 Calgon

2–  [Na CaP O ]2– + 2Na+ Ca2+ + Na4P6O18 2 6 18

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) + H2SO4(aq)  BaSO4(s) + H2O2(aq) + 8H2O(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. Electrolysis 2HSO4–(aq)   HO3SOOSO3H(aq)

HO3 SOOSO3H(aq)   2HSO4–(aq) + 2H+(aq) + H2O2(aq) Hydrolysis

Peroxodisulphate

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

2   H2O2  Oxidised product 2-ethylanthraquinol 

H2 /Pd

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Hydrogen

Solution of Assignment (Set-1)

33. Distinguish between (a)

Hard water and soft water

(b)

Temporary hard water and permanent hard water

Sol. (a)

Hard water

Soft water

(i) It does not form lather with soap.

(i) It readily forms lather with soap.

(ii) It contains calcium and magnesium salts in (ii) This water is free from soluble salts of calcium the form of hydrogen carbonate, chloride and and magnesium. sulphate in the water. (iii) Example : sea water, river water. (b)

(iii) Distilled water, demineralised water.

Temporary hard water

Permanent hard water

(i) It is due to the presence of bicarbonates of (i) It is due to the presence of chlorides and calcium and magnesium in water. sulphates of calcium and magnesium. (ii) This type of hardness is easily removed by (ii) This type of hardness cannot be removed on simply boiling water. boiling water. (iii) Also known as carbonate hardness.

(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 :

2R  COOH

(Cation exchange resin)



Ca2

(From hard water)

  (RCOO)2 Ca  2H (Exhausted resin)

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) + H2O(l) RNH3+OH–(s) + X–(aq)

RNH3+OH–(s) RNH3+ X–(s) + OH–(aq)

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Hydrogen

7

For example :

R  NH3 OH  (Anion exchange resin)



Cl

(From hard water)

  R  NH3 Cl  OH (Exhausted resin)

In the reaction OH– ion exchanges for the anions in the water. OH– ions are liberated which neutralise H+ ions set free in the cation exchange resin. H+(aq) + OH–(aq)  H2O(l)  35. (a)

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. Discuss the structure of H2O2

(b)

Discuss the bleaching action of H2O2

(c)

Write important uses of H2O2

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

H 95.0 pm

98.8 pm

147.5 pm

O

O

94.8°

145.8 pm

111.5°

O

101.9°

H

(a) Gas phase

(b)

O

90.2°

H

(b) Solid phase

Bleaching action of H2O2 : H2O2 acts as a bleaching agent due to oxidation. It oxidises the colouring matter to the colourless product. H2O2  H2O + [O] Colouring matter + O  Colourless mass (Bleached)

(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?

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Hydrogen

Solution of Assignment (Set-1)

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, Na2S. Similarly, hydrogen also forms stable oxide (H2O), halides (HF) and sulphide (H2S).

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 + 2D2O  C2D2 + Ca(OD)2 SO3 + D2O  D2SO4 Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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Hydrogen

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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 MgCO3 because Mg(OH)2 is precipitated easily. Whereas Ca(HCO3)2 is changed to insoluble CaCO3 and gets precipitated. These precipitates can be removed by filtration process. So, the filtrate obtained will be soft water. Heating Mg(HCO3)2   Mg(OH)2  + 2CO2  Heating Ca(HCO3)2   CaCO3  + H2O + CO2 

(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(HCO3)2 + Ca(OH)2  2CaCO3  + 2H2O Mg(HCO3)2 + 2Ca(OH)2  2CaCO3  + Mg(OH)2  + 2H2O

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 (BeH2 and MgH2) 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

Na + e

–

Na

–

2H

Anode

–

H2 + 2e

4. These hydrides react violently with water producing dihydrogen gas. NaH(s) + H2O(aq)  NaOH(aq) + H2(g) 5. They burn spontaneously in air on heating on account of their decomposition into hydrogen which is inflammable. CaH2  Ca + H2 Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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Hydrogen

Solution of Assignment (Set-1)

6. Lithium hydride is used in the synthesis of some useful hydrides since it is rather unreactive at moderate temperatures with O2 and Cl2. For example 8LiH + Al2Cl6  2LiAlH4 + 6LiCl 2LiH + B2H6  2LiBH4 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–  H2(g) + 2OH–(aq)

Overall reaction :

2Cl–(aq) + 2H2O(l)  Cl2(g) + H2(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

C  H2 O   CO  H2 Ni  

Coke

Water gas

1270 K CnH2n  2  nH2 O   nCO  (2n  1)H2 Ni

Hydrocarbon

e.g., 1270 K

 CO(g) + 3H2(g) CH4(g) + H2O(g)  Ni 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 673 K CO(g) + H2O(g)   CO2(g) + H2(g) FeCrO 4

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

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Sol. (a)

Hydrogen

11

Reaction with halogens : Hydrogen reacts with halogens to give hydrogen halides with general formula HX. H2(g) + X2(g)  2HX(g) (X = F, Cl, Br, I) In case of fluorine reaction occurs even in dark Dark H2 + F2   2HF

With chlorine, it occurs in presence of sunlight Sunlight H2 + Cl2   2HCl

Bromine reacts only upon heating 673 K H2 + Br2   2HBr

With iodine, it requires catalyst for the reaction Pt

 2HI H2 + I2  673 K This shows that F2 is more reactive than Cl2 which is further more reactive than Br2 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 CC or C=C bond, in the presence of catalyst to form saturated hydrocarbons. 473 K

H  C  C  H(g)  H2 (g)   H3 C  CH3 (g) Ni Ethene

Ethane

The above reaction is called catalytic hydrogenation. (c)

Reaction with nitrogen : Dihydrogen is employed in Haber’s process for the formation of ammonia 673 K, 200 atm

 2NH3(g) H = –92.6 kJ mol–1 3H2(g) + N2(g)  Fe 43. (a) (b) Sol. (a)

Give the structure of water in gaseous phase. List some physical properties of water. 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 bond is polar because of high electronegativity of oxygen in comparison to that of hydrogen.

O

2

O

95.7 pm

104.5°

H

H

–



+

O 

H

+

H

(a)

(b)

(c)

Bent structure of water

Water molecule as a dipole

H2O molecule Orbital overlap picture in water molecule

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Hydrogen

(b)

Solution of Assignment (Set-1)

Physical Properties of Water 1.

Pure water is colourless, odourless and tasteless.

2.

Water is present in the liquid state at room temperature.

3.

Water boils at 100°C and changes into the gaseous state whereas it freezes at 0°C to form ice.

4.

Water molecules undergo extensive hydrogen bonding.

5.

The existence of hydrogen bonding is responsible for high freezing point, high boiling point, high value of specific heat, high latent heat of fusion and high latent heat of vaporisation, in comparison to H2S and H2Se molecules.

6.

The high heat of vaporisation and high heat capacity are responsible for moderation of the climate and body temperature of the living beings.

7.

Being an excellent solvent it is essential for the transportation of metabolites in plants.

8.

Alcohols and carbohydrates dissolve in water due to the formation of hydrogen bonding.

44. Explain the ability of water to hydrolyse many ionic and covalent compounds. Sol. Water is an excellent solvent due to its high dielectric constant (81). Water can easily hydrolyse many ionic and covalent compounds. (i)

Water hydrolyses oxides and halides of non-metals forming their respective acids P4O10(s) + 6H2O(l)  4H3PO4(aq) SiCl4(l) + 2H2O(l)  SiO2(s) + 4HCl(aq) SO3(g) + H2O(l)  H2SO4(aq)

(ii)

Water hydrolyses the carbides, hydrides, nitrides and phosphides of some metals with the liberation of acetylene, hydrogen, ammonia, phosphine respectively. CaC2 Calcium carbide

 2H2 O  Ca(OH)2  C2H2

Acetylene

CaH2 (s)  2H2O(l)   Ca(OH)2 (aq)  2H2 (g) Calcium hydride

AlN  3H2 O  Al(OH)3  NH3

Aluminium nitride

Ammonia

Ca3P2  6H2 O  3Ca(OH)2  2PH3 Calcium phosphide

Phosphine

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Solution of Assignment (Set-1)

Hydrogen

13

45. Calculate the normality of 20 volume of hydrogen peroxide. Sol. Mass of H2O2 per litre of the solution : 2H2O2  2H2O + O2 2 × 34 g

22.7 L at STP

= 68 g

22.7 L of O2 at STP is obtained from H2O2 = 68 g 

1 L of O2 at STP is obtained from H2O2 =

20 L of O2 at STP is obtained from H2O2 =

68 g 22.7 68  20 g = 59.91 g = 60 g 22.47

So, the strength of H2O2 in 20 volume H2O2 solution = 60 g/L Normality =

Strength Equivalent mass

Equivalent mass of H2O2 : 2H2O2  2H2O + O2 68 parts by mass

32 parts by mass Equivalent mass of O2 = 8

32 parts by mass of O2 are evolved from H2O2 = 68 parts 8 parts by mass of O2 are evolved from H2O2 =

68  8 = 17 parts 32

Thus, equivalent mass of H2O2 = 17 or 17 g/equiv. Calculation of normality of the solution :

Normality of solution =

Strength of solution 60 g/L = = 3.52 N Equivalent mass 17 g/eq.

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

14 3.

Hydrogen

Solution of Assignment (Set-1)

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

Sol. One. 4.

Write the chemical formula of washing soda.

Sol. Na2CO3 5.

What is the dihedral angle of H2O2 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. Al4C3 + 12D2O  3CD4 + 4Al(OD)3 Short Answer Type Questions : 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  2CaCO3  + 2H2O Mg(HCO3)2 + 2Ca(OH)2  2CaCO3  + Mg(OH)2 + 2H2O 10. Give some physical properties of H2O2. Sol. (i) (ii)

Pure hydrogen peroxide is a syrupy liquid. 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) (ii)

Hydrogen is less reactive as compared to halogens. The electron affinity of hydrogen is less than that of halogens.

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Solution of Assignment (Set-1)

Hydrogen

15

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 : 2H2O(l) + 2Na(s)  2NaOH(aq) + H2(g) Oxidising agent

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. 2H2O(l) + 2F2(g)  4H+(aq) + 4F–(aq) + O2(g) Reducing agent

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 D2 and remaining water, becomes enriched in heavy water.

(ii)

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

Short Answer Type Questions : 16. Why is hard water unsuitable for laundary? Sol. Soaps are sodium and magnesium salts of long chain fatty acids. Soap containing sodium stearate (C17H35COONa) reacts with hard water to form insoluble precipitates called scum M2

(From hard water)



2C17H35 COONa   (C17H35 COO)2 M  2Na  Sodium stearate (Soap)

Metal stearate (ppt)

Where M2+ = Ca2+ or Mg2+ Therefore, no lather is produced until all the calcium and magnesium ions are precipitated. This also results in wastage of soap. 17. Give any three uses of dihydrogen. Sol. (i) (ii)

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. Manufacture of methyl alcohol : Water gas enriched with hydrogen gas in the presence of cobalt catalyst gives methanol Cobalt

CO(g) + 2H2(g)   CH3OH(l) Catalyst (iii) Rocket fuel : It is used as a rocket fuel for space research in the form of liquid hydrogen. Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

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Hydrogen

Solution of Assignment (Set-1)

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 : BaCl2 2H2O

(iii) Hydrogen-bonded water : Example : [Cu(H2O)4]2+ SO42– H2O in CuSO4 5H2O 19. What is the strength (in g/L) of 25 volume solution of H2O2? Sol. 25 volume solution of H2O2 means that 1 L of this H2O2 solution will give 25 L of oxygen at STP 2H2O2 (l)   O2 (g)  H2 O(l) 2  34 g 68 g

22.7 L at STP

On the basis of above equation, 22.7 L of O2 is produced at STP from H2O2 = 68 g 25 L of O2 is produced at STP from H2O2 =

68  25 g = 74.8 g 22.7

Strength of H2O2 in 25 volume H2O2 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. 1270 K

 CO(g) + 3H2(g) CH4(g) + H2O(g)  Ni 1270 K

 CO(g) + H2(g) C(s) + H2O(g)  Ni 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. 673 K

CO(g) + H2O(g)   CO2(g) + H2(g) Catalyst 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. Aakash Educational Services Pvt. Ltd. Regd. Office : Aakash Tower, Plot No.-4, Sector-11, Dwarka, New Delhi-75 Ph.011-47623456

Solution of Assignment (Set-1)

Hydrogen

17

Protium (11H) : 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 (12 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 (13 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., BeH2) or EmHn (e.g., B2H6). 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 B2H6. 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, H2O has 2 lone pairs, HF has 3 lone pairs).

O 104.5°

H

H

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18

Hydrogen

Solution of Assignment (Set-1)

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     H2O(l)  H2S(aq)   H3 O (aq)  HS (aq) Base

Acid

    H2O(l)  HCl(aq)   H3 O (aq)  Cl (aq) Base

Acid

Water as acid : Water acts as an acid towards bases stronger than it     H2O(l)  NH3 (aq)   OH (aq)  NH4 (aq) Acid

Base

  

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