Unit 09 Hydrogen

Hydrogen is the first element in the periodic table. Hydrogen has electronic configuration 1s’. The position is anomalous since in some properties it resembles alkali metals and in some it resembles halogens.

There are three isotopes of hydrogen : protium, deuterium and tritium.

Dihydrogen has two nuclear spin isomers called ortho and para-dihydrogen.

Hydrogen combines with a large number of metals as well as non-metals to form compounds which are collectively called hydrides

Resemblance with alkali metals

1. Electronic configuration - H contains one electron in its valence shell Is1 same as n s1 in alkali metals.

2. Electropositive character H can lose one electron to form H+

HH++e

3. Oxidation state H shows +1 oxidation state

H+Cl

Na+Cl

4. Liberation at cathode - When aqueous solution of HCl is electrolyzed, H2 is liberated at cathode in the same way as alkali metals are liberated at cathode during electrolysis of their fused halides

5. Reducing character-Hydrogen acts as strong reducing agent.

Fe3O4( s)+4H2( g)Δ3Fe(s)+4H2O(g)

Resemblance with halogens

1. Electronic configuration - Like halogens, H has one electron less than the nearest inert gas.

2. Electronegative character H can gain 1eto form hydride

H+eH

3. Oxidation state-H shows -1 oxidation state Na+H.

4. Liberation at anode - When fused alkali metal hydrides are subjected to electrolysis, hydrogen is liberated at anode.

 2NaH(I) electrolysis  Cathode 2Na(l)+ anode H2( g)

Isotopes of hydrogen

11H12H13H

protium deuterium tritium

They differ from each other only in the number of neutrons in the nucleus. Since they have same electronic configuration 1s1, they have similar chemical properties.

Dihydrogen is the most abundant element in the universe (70 of the total man of the universe)

Preparation

1. By action of water on active metals

2Na(s)+2H2O(l)2NaOH(aq)+H2( g)Ca(s)+2H2O(l)Ca(OH)2(aq)+H2( g)3Fe(s)+4H2O(g)Fe3O4( s)+4H2( g)

2. By electrolysis of water

2H2O(I) electrolysis 2H2( g)+O2( g)

3. By reaction of metals with alkalies

Zn(s)+2NaOH(aq)ΔNa2ZnO2(aq)+H2( g) Sod. zincate Be(s)+2NaOH(aq)ΔNa2BeO2(aq)+H2( g) Sod. beryllate 

4. By reaction of metals with acids

Zn(s)+H2SO4(aq)ZnSO4(aq)+H2( g)Mg(s)+2HCl(aq)MgCl2(aq)+H2( g)

Coal Gasification - Process of producing syn gas from coke or coal.

C(s)+H2O(g)NiΔCO(g)+H2( g) coke \quad steam \qquad \qquad water gas 

Mixture of CO and H2 is used for synthesis of methane and other hydrocarbons so it is known as syn gas.

Water gas shift reaction - Conversion of water gas into syn gas

CO(g)+H2O(g) Water gas +H2O(g)FeCrO4 673K CO2( g)+2H2( g) syn gas 

Properties of dihydrogen

Stable and relatively inert at room temperature due to high HH bond dissociation enthalpy.

Highly combustible gas burns with a pale blue flame to form water, not a supporter of combustion.

Reactions of dihydrogen

H2( g)+S(I)700 KH2 S( g)C(s)+2H2( g)ΔCH4( g)H2( g)+F2( g)Δ2HF(g)H2( g)+Cl2( g)hvΔ2HCl(g)N2( g)+3H2( g)Fe,Mo673 K,20 atm2NH3( g) (Haber’s process) +ΔHCuO(s)+H2( g)ΔCu(s)+H2O(g) (hydrogen as reducing agent) CH2=CH2( g)+H2( g)ΔNiorPtCH3CH3( g) (hydrogenation) 

Uses:

1. Liquid hydrogen (mixed with liquid oxygen) is used as rocket fuel.

2. Atomic hydrogen and oxy - hydrogen torch for cutting and welding.

3. In synthesis of ammonia

4. In manufacturing of vanaspati fat by hydrogenation of poly unsaturated vegetable oil.

5. In manufacturing of bulk organic chemicals

6. In manufacturing of metal hydrides

Hydrides

1. Ionic hydrides : formed by s block elements LiH, NaH, KH. They are stoichiometric.

2. Metallic or interstitial hydrides : formed by d and f block elements. They are non stoichiometric eg. LaH287Yb255H

3. Molecular / covalent hydrides: formed by p block elements

a) Electron deficient hydrides : formed by group 13 elements. They do not have sufficient number of electrons to form normal covalent bonds.

They exist in polymeric forms eg. BH3,AlH3

b) Electron precise hydrides - formed by group 14 elements. They can neither act as lewis acid or lewis base eg. CH4,SiH4

c) Electron rich hydrides - formed by group 15, 16, 17 elements. They can act as Lewis base due to presence of lone pair of electrons eg. NH3,HF,H2O.

Water

Properties

It has high boiling point due to presence of intermolecular hydrogen bonding.

Due to its high dielectric constant (78.39) it is a universal solvent.

Amphoteric character - It can act as acid as well as base

H2O(I)+NH3( g)NH4++OH

acid base

H2O(I)+H2 S( g)H3O++HS

base acid

Oxidising agent

2Na(s)+2H2O(l)2NaOH(aq)+H2( g)

Reducing agent

2 F2( g)+2H2O(I)O2( g)+4H++4 F

Water has a higher specific heat, thermal conductivity, surface tension, dipole moment & dielectric constant.

It is an excellent solvent for transportation of ions and molecules required for plant and animal metabolism.

Due to hydrogen bonding with polar molecules, even covalent compounds like alcohols and carbohydrates dissolve in water.

Structure of Water

  • Water is regarded as a universal solvent.
  • Depending upon the behavior of water towards soap, water may be classified as soft or hard water.
  • Degree of hardness is expressed in ppm.

Types of hardness of water

1. Temporary hardness - due to bicarbonates of Ca and Mg.

2. Permanent hardness - due to chlorides and sulphates of Ca and Mg.

Methods of removing hardness

Temporary hardness

1. By boiling

Ca(HCO3)2ΔCaCO3( s)+CO2( g)+H2OMg(HCO3)2ΔMgCO3( s)+CO2( g)+H2O

2. By Clark’s process - calculated amount of quick lime is added

CaO(s)+H2OCa(OH)2( s) quick lime  slaked line Ca(HCO3)2+Ca(OH)22CaCO3( s)+2H2O(I)Mg(HCO3)2+Ca(OH)2CaCO3( s)+MgCO3( s)+2H2O(I)

Permanent hardness

1. Ion exchange method Ca2+ and Mg2+ present in hard water are exchanged by ions present in ion exchangers. (sodium aluminium silicate) 2NaZ+M2+MZ2+2Na+

Cation exchange resin - They can exchange H+ions with cations such as Ca2+ and Mg2+ ions present in hard water.

2RCOOH cation exchange resin +CaCl2 (from hard water) (RCOO)2 (exhausted resin) Ca+2Ca++2Cl

Anion exchange resin - They can exchange OHions with anions such as Cland SO42 present in water.

RN+H3OH+ClRN+H3Cl+OH

Anion exchange resin from hard water

H++OHH2O

Water obtained is free from all cations and anions. Water obtained by this process is called de-iosised water.

2. Calgon Process

2CaCl2+Na2[Na4(PO3)6]Na2[Ca2(PO3)6]+4NaCl

from hard water sod. hexametaphosphate (calgon)

Reactions of heavy water (D2O)

1. 2D2O Electroysis 2D2( g)+O2( g)

2. Ca(s)+2D2OCa(OD)2+D2( g)

3. CaO(s)+D2OCa(OD)2

4. P4O10( s)+6D2O(I)4D3PO4(l) deuterophosphoric acid

5. SO3+D2O(I)D2SO4 deuterosulphuric acid

6. Ca3P2( s)+6D2O3Ca(OD)2+2PD3( g) deuterophosphine

Uses:

1. Tracer compound for studying mechanism of reactions.

2. As a moderator in nuclear reactions to slow down fast moving neutrons.

Hydrogen Peroxide

Preparation

1. From sodium peroxide

Na2O2( s)+H2SO4(aq)Na2SO4(aq)+H2O2

2. From barium peroxide

BaO28H2O+H2SO4BaSO4+H2O2+2H2O

Strength of H2O2 solution

1. Volume strength =5.6x Normality

=5.6×% strength  Eq wt of H2O2(17)=5.6× strength in gL  Eq wt of H2O2(17)

2. Volume strength =11.2× Molarity

=11.2×% strength  Mol. Wt of H2O2(34)=5.6× strength in gL  Eq wt of H2O2(17)

Properties

More viscous than water since molecules are more highly associated through H bonds than H2O molecules.

Behaves as weak acid

H2O2(I)H++HO2(hydroperoxide iorn)

H2O2(I)2H++O22 (peroxide iron)

Oxidising character

In acidic medium

H2O2(I)+2H++2e2H2O(I)

2Fe2+(aq)+H2O2(I)+2H+2Fe3+(aq)+2H2O(l)

2I(aq)+H2O2(I)+2H+I2+2H2O(I)

PbS(s)+4H2O2(I)PbSO4( s)+H2O(l)

black white

Restores the white colour of lead paintings which have blackened due to formation of lead sulphide.

In alkaline medium

H2O2(aq)+2e2OH

Cr3+(aq)+3H2O2(aq)+10OH2CrO42(aq)+8H2O(l)

Mn2+(aq)+H2O2(aq)+2OHMnO2( s)+2H2O(l)

Reducing character

In acidic medium

H2O2(aq)2H++O2( g)+2e

Cr2O72(aq)+8H++3H2O2(aq)2Cr3+(aq)+7H2O(I)+3O2( g)

2MnO4(aq)+6H++5H2O2(aq)2Mn2+(aq)+8H2O+5O2( g)

In alkaline medium

H2O2(aq)+2OH2H2O+O2( g)+2e

I2+H2O2(aq)+2OH2I+2H2O+O2

2Fe3+(aq)+H2O2(aq)+2OH2Fe2+(aq)+O2( g)+2H2O

Bleaching action-due to liberation of nascent oxygen produced on decomposilion.

H2O2(aq)H2O+[0]

colouring matter +[0] colourless matter

It liberates I2 with KI which gives blue colour with starch.

Structure of H2O2

Structure of H2O2(a ) in the gas phase, dihedral angle =111.5 and (b) in the solid phase at 100 K, dihedral angle =90.2.

Hydrogen peroxide acts as an oxidizing agent, reducing agent and bleaching agent.

Strength of H2O is expressed in terms of volume of O2 liberated on heating its unit volume.

The search for an alternative source of energy has given rise to the hydrogen economy.

Liquid hydrogen is an important fuel.

Hydrogen as a Fuel

Dihydrogen releases large quantities of heat on combustion.

Pollutants in combustion of dihydrogen will be less than petrol. The only pollutant will be the oxide of dinitrogen which is present as an impurity. Thus hydrogen economy is an alternative the bane principle is the transportation and storage of energy in the form of liquid or gaseous dihydrogen. Advantage is that energy is transmitted in the form of dihydrogen and not as electric power.

Solved Problems

1. HClO4H2O is

a) A covalent solid containing hydrogen bonded H2O

b) An ionic solid containing H+ions

c) An ionic solid containing ClO4ions

d) Aliquid

Show Answer Solution: HClO4H2O is [H3O+][ClO4]in the same way as NH4Cl is [NH4+][Cl] after the proton shift. Therefore answer is c.

2. ZnH2 is a/an

a) ionichydride

b) covalent hydride

c) interstitial hydride

d) intermediate hydride

Show Answer Solution : Like ZnH2,CuH2,CdH2,MgH2 and HgH2 have properties intermediate between ionic and covalent hydrides. Therefore answer is d.

3. The bond dissociation energy of H2,D2 and T2 follow the order (D-Deuterium, T-Tritium)

a) H2>D2>T2

b) T2>D2>H2

c) D2>T2>H2

d) T2>H2>D2

Show Answer

Solution: Bond dissociation energy of

T2 is 446.9 kJ mol1

D2 is 443.4 kJ mol1 and

H2 is 435.9 kJ mol1 Therefore the answer is b.

4. When NaH reacts with water, the gas liberated is

a) oxygen

b) hydrogen

c) steam

d) ozone

Show Answer Solution: NaH+H2ONaOH+H2 Therefore the answer is b.

5. H2O2 on standing decomposes to produce

a) H2

b) H2 and O2

c) H2O and O2

d) OH

Show Answer Solution: 2H2O22H2O+O2 Therefore the answer is c.

6. Dipole moment of H2O2 is

a) Equal to that of water

b) Greater than that of water

c) Less than that of water

d) Unpredictable

Show Answer

Solution: Due to presence of two polar 0H bonds and non-planar structure, dipole moment of H2O2 becomes quite high.

Therefore the answer is b.

7. A hair dye available in market generally contains two bottles, one containing dye and other contains hydrogen peroxide. Two bottles are mixed before applying the dye. Function of H2O2 is to

a) Reduce the dye

b) Oxidise the dye

c) Dilute the dye

d) Acidify the dye solution

Show Answer

Solution : Bleaching action of H2O2 is due to its oxidative property.

Therefore the answer is b.

8. What percentage strength of H2O2 will be marked as ’ 20 vol'?

a) 3%

b) 6%

c) 9%

d) 12%

Hint: 2H2O22H2O+O2

Show Answer

Solution: 2H2O22H2O+O2

22400 cm3=68 g (Since, mass of 2H2O2=68 g )

1 cm3=6822400 g

20 cm3=68 g22400 cm3×20 cm3

Therefore, percentage strength

68×2022400×1000

=6.06%6%

Therefore answer is b.

Practice Questions

Question 1. In which of the following reactions, H2O2 is acting as a reducing agent?

a) SO2+H2O2H2SO4

b) 2KI+H2O22KOH+I2

c) PbS+4H2O2PbSO4+4H2O

d) Ag2O+H2O22Ag+H2O+O2

Show Answer Answer: (d)

Question 2. 10 mL of H2O2 solution is treated with Kl and titration of liberated I2, required 10 mL of 1 N hypo. Thus H2O2 is

a) 1 N

b) 5-6 volume

c) 17gL1

d) all are correct

Show Answer Answer: (d)

Question 3. 30 volumes H2O2 means

a) 30%H2O2

b) 30 cm3 of the solution contains 1 g of H2O2

c) 1 cm3 of the solution liberates 30 cm3 of O2 at STP

d) 30 cm3 of the solution contains one mole of H2O2

Show Answer Answer: (c)

Question 4. In alkaline medium, H2O2 reacts with Fe3+ and Mn2+ respectively to give

a) Fe4+ and Mn4+

b) Fe2+ and Mn2+

c) Fe2+ and Mn4+

d) Fe4+ and Mn2+

Show Answer Answer: (c)

Question 5. In transforming 0.01 mole of PbS to PbSO4, the volume of ’ 10 volume’ H2O2 required will be

a) 11.2 mL

b) 22.4 mL

c) 33.6 mL

d) 44.8 mL

Show Answer Answer: (d)

Question 6. Blackened oil painting can be restored into original form by the action of

a) Chlorine

b) BaO2

c) H2O2

d) MnO2

Show Answer Answer: (c)

Question 7. In the calgon process of softening of water, which of the following is used?

a) Sodium hexametaphosphate

b) Hydrated sodium aluminium silicate

c) Cation exchange resins

d) Anion exchange resins

Show Answer Answer: (a)

Question 8. The reagent commonly used to determine hardness of water quantitatively is

a) Oxalic acid

b) Disodium salt of EDTA

c) Sodium citrate

d) Sodium thiosulphate

Show Answer Answer: (b)

Question 9. The hardness of water sample containing 0.002 mole of magnesium sulphate dissolved in a litre of water is expressed as:

a) 20ppm

b) 240ppm

c) 2000ppm

d) 120ppm

Show Answer Answer: (b)

Question 10. In the following reaction using isotope 18O2H2O2

2MnO4+3H2O2182MnO2+3O2+2H2O+2OH

Isotopic oxygen goes:

a) both in O2

b) both in MnO2

c) both in OH

d) one in O2 and one in MnO2

Show Answer Answer: (a)