Hydrogen Chapter 9 Class XI notes

HYDROGEN

Chapter - 9

It is placed in first group and first period (first element of the periodic table). It is the lightest element known. It exists as a diatomic molecule i.e. H₂. That is why it is called dihydrogen.

It was discovered by Henry Cavendish in 1766. He prepared it by the action of dilute H₂SO₄ on iron. Its name hydrogen was proposed by Lavoisier because it produces water on burning with oxygen gas. [Hydro = water, gen = producer]

Unique position of Hydrogen:

a) It resembles alkali metals and can be placed in group 1. It can lose one electron [1s¹] and has electropositive character with +1 oxidation state. It combines with electronegative elements or non-metals like H₂O, HCl, H₂S.

b) It resembles halogens and can be placed in group 17. It can gain one electron [1s¹] and has electronegative character with -1 oxidation state. It combines with metals as NaH, CaH₂.

Occurrence: It is the ninth element in order of abundance (0.9% earth’s crust by weight). Isotopes of Hydrogen are ¹H₁ (protium- most abundant 99.985%), ²H₁ (deuterium 0.015%), ³H₁ (tritium 10^-15%). Deutrium was discovered by Urey in 1934.

Preparation of dihydrogen: 

a) Lab preparation – It is prepared by reaction of granulated zinc with dilute HCl.    

        Zn + 2 HCl (dilute) → ZnCl₂ + H₂

         Zn + 2 NaOH (aq) → Na₂ZnO₂ + H₂

b) Commercial preparation – 

(i) By electrolysis of water:   

2 H₂O (l)   →  2H₂(g) + O₂(g)

Electrolyte = water + acid or alkali (to make it a good conductor), cathode = Fe sheet, anode = Nickel plated iron sheet or platinum electrodes.

(ii) By reaction of steam on hydrocarbons at high temperature:

C_nH_2n+2 + nH₂O →    n CO + (2n+1) H₂

E.g. CH₄ (g) + H₂O (g)   →   CO + 3 H₂   [Mixture of CO and H₂ is known as ‘water gas’. It is used for synthesis of methanol and many hydrocarbons, so it is known as synthesis gas or syn gas.]

(iii) Syn gas or water gas is also prepared from sewage, saw-dust, scrap wood, etc. Process of producing syn-gas from coal is known as coal gasification. 

C(s) + H₂O(g) → CO + H₂(g)

(iv) Water-gas shift reaction or Bosch process –

CO + H₂ + H₂O   →    CO₂ + 2H₂ (g)

OR    

C + 2H₂O(g)  →   CO₂ + 2H₂(g)

Properties of H₂ : 

a) Physical properties – Colourless, odourless, tasteless and combustible gas with high calorific value, lighter than air and insoluble in water.

b) Chemical properties –

(i) It is neutral to litmus paper.

(ii) It has high bond dissociation energy (H-H bond).

(iii) N₂ + 3H₂→ 2NH₃ (Haber’s process).

(iv) Acts as good reducing agent: H₂ + Pd²⁺→ 2H⁺ + Pd.

(v) 2C + H₂    →   HCΞCH (g) (ethyne or acetylene)

Uses: Hydrogen is used to prepare ammonia, used in hydrogenation of vegetable oils, used as rocket fuel in liquid form, used in air-ships and balloons for meteorological purposes.

Hydrides – Compound of an element with hydrogen.

Classification of hydrides:

a) Ionic hydrides or saline or salt like hydrides – These are formed by these metals whose electronegativity values are less than hydrogen (or those which are more electropositive than hydrogen) i.e. s-block elements [ group 1 and 2 metals except Be and Mg].

Properties:

(i)            Ionic hydrides are crystalline, non-volatile and conduct electricity in aqueous solution.

(ii)          They have high melting and boiling point.

(iii)         They act as strong bases  LiH + H₂O → LiOH + H₂.

(iv)         They react violently with water to form H₂. NaH + H₂O → NaOH + H₂.                                         

(v)          4LiH + AlCl₃ → LiAlH₄ + 3LiCl  [LiAlH₄ acts as a strong reducing agent]

b) Covalent hydrides – These are formed by p-block elements and Be and Mg as their electronegativity difference with hydrogen is very small. These hydrides usually consist of covalent molecules which are held together by weak Vander Waal’s force of attraction. So, they are known as covalent or molecular hydrides.

Classification of molecular hydrides-

(i)     Electron deficient hydrides – B₂H₆, BeH₂ -These are lewis acids (electron acceptors).

(ii)    Electron precise compounds – CH₄ -They have complete octets.

(iii) Electron rich hydrides – NH₃, H₂O, HF -They have lone pairs (lewis bases i.e. electron donors).

c) Metallic hydrides – They are formed by d-block and f-block elements except metals of group 6,7,8 and 9 (but CrH chromium hydride also exists). These hydrides have properties similar to those of parent metals. So, they are metallic hydrides (except Ni, Pd).

Hydrogen atoms being small in size, occupy some space in the metallic lattice (interstitial sites) but not all. So, they are interstitial hydrides. E.g. in Pd, Pt. This property of high potential for hydrogen storage is used as a source of energy.

They conduct electricity. They are non-stoichiometric (are deficient in hydrogen) like LaH_2.87, NiH_0.6-0.7, PdH_0.6-0.8 i.e. they do not hold law of constant proportion.

Water – (human body contains 65% of water). It is a universal solvent as it can dissociate almost all compounds in it and has high dielectric constant.

Structure of water –

(i) In gaseous state, water exists as discrete molecule. So, it is a bent molecule with bond angle of 104.5⁰ and O-H bond length of 95.7pm (a highly polar molecule with sp³ hybridisation.  Magnetic moment = μ = 1.84 D as Oxygen is an electronegative atom.

(ii) In liquid state, water molecules are held by intermolecular H-bonds and are in association. Each water molecule is generally H-bonded with four other water molecules. (Dipole-dipole interactions)

(iii) In solid state, crystalline form of water is in hexagonal form at atmospheric pressure but at low temperature, it condenses to cubic form. In structure, each oxygen atom is surrounded tetrahedrally by four other oxygen atoms distance of 276pm. It contains void spaces in its cage-like structure.

Ø                Water has maximum density at 277K or 4⁰C. Reason- Ice contains a large number       of vacant spaces in it due to which it has large volume and lesser density in its            cage like structure. When ice melts, some of the bonds are broken and cage-like           structure partially breaks up. Therefore, volume decreases and density increases.

As temperature is raised further above 273K, more H-bonds break resulting in more decrease in volume and increase in density. It goes on till 4⁰C and all void spaces are broken up.

As temperature is raised above 277K, there is increase in volume due to expansion of liquid water and decrease in density. Hence, density of water is maximum at 277K.

Ø               

             Ice floats on water because density of ice is less than water as the ice has cage like  structure having many void spaces due to which it has more volume and less               density.

 Physical properties of water:

a) Colourless, odourless and tasteless.                                                                    

b) Unusual properties of water – It has high boiling point, freezing point, heat of fusion and vapourization than H₂S, H₂Se, H₂Te in its group due to the presence of intermolecular H-bonding between water molecules. It has high specific heat, surface tension, thermal conductivity, dipole moment and dielectric constant.                                                                                                                       

c) It is an excellent solvent for the transportation of ions and molecules required for plants and animals body.

Q Why H₂O, NH₃ and HF have high melting and boiling points?                                                                     

 Ans. They consist of N, O and F electronegative atoms which result in intermolecular H-bonding between molecules and leads to association. Order of boiling point: H₂O > HF > NH₃.

As water is associated with four other water molecules by H-bonding which HF is associated with two other HF molecules. F is more electronegativity than N. So, HF has more stronger H-bonding than NH₃.

Chemical properties of water:

a) H₂O is very stable at room temperature due to highly negative heat of formation but it decomposes at very high temperature. 2H₂O  →   2H₂ (g) + O₂(g)    

b) It has amphoteric nature. Water is weak electrolyte as it shows self-ionization or auto-protolysis. H₂O+H₂O → H₃O⁺ + OH^-                                                                                                                                        

It acts as both acid and base:   (i) H₂O + HCl → H₃O⁺ + Cl^-                

                                                  (ii) H₂O + NH₃ → NH₄⁺ + OH^- 

c) It undergoes redox reactions: (i) 2Na + H₂O → 2NaOH + H₂ (as oxidizing agent)          

(ii) 2F₂+2H₂O→4HF+O₂ (as reducing agent)                                                                                                                 

d) Hydrolytic reactions – It can hydrolyse many oxides, phosphides, nitrides, etc. due to its high dielectric constant.

CaO + H₂O → Ca(OH)₂ , 

SO₂ + H₂O → H₂SO₃ (sulphurous acid),                                          

SiCl₄ + 2H₂O → SiO₂ + 4HCl,  

P₄O₁₀ + 6H₂O → 4 H₃PO₄ (phosphoric acid),                                                        

Mg₃N₂ + 6H₂O → 2NH₃ + 3Mg(OH)₂       

e) Hydrates formation –Water can be associated with ionic compounds as hydrates. This water in coordination with ionic salts is known as water of crystallisation.

Different types of hydrates:                       

 i) Coordinated water: as a constituent of complex [Cr(H₂O)₆]³⁺.3Cl^-

ii) Interstitial water: water is present in the voids of lattice. E.g. BaCl₂.2H₂O

iii) Hydrogen-bonded water: E.g. CuSO₄.5H₂O as [Cu(H₂O)₄]²⁺SO₄^2-.H₂O

Hard and soft water-

Soft water – Water that produces lather with soap readily being free from soluble salts of calcium and magnesium is called soft water. E.g. rain water, distilled water, demineralised water.

Hard water – Water which does not produce lather with soap readily is called hard water due to the presence of bicarbonates, chlorides and sulphates of Ca and Mg ions in it. Hard water forms curdy white precipitate with soaps. Soap is sodium or potassium salt of higher fatty acids like steric acid, palmitic acid or oleic acid.

2 C₁₇H₃₅COONa + CaCl₂→ (C₁₇H₃₅COO)₂Ca (white ppt or scum) + 2NaCl

So, it is unsuitable for laundry and harmful for boilers as it reduces the efficiency of boilers as lot of soap gets wasted in precipitating out Ca²⁺ and Mg²⁺ ions.

Hardness of water –

Types of hardness:

a) Temporary hardness – due to the presence of Ca and Mg ions in the form of bicarbonates. It can be removed by-

(i) By boiling and filtering –

Mg(HCO₃)₂   →  Mg(OH)₂ (insoluble) + 2CO₂ ;

Ca(HCO₃)₂   →   CaCO₃(insoluble) + CO₂ + H₂O

(ii) By Clark’s process – by adding known amount of lime to hard water.

CaO + H₂O → Ca(OH)­₂ ;                

Ca(HCO₃)₂ + Ca(OH)­₂ → 2CaCO₃ (insoluble) + 2H₂O;

Mg(HCO₃)₂ + Ca(OH)­₂ → CaCO₃ (insoluble) + 2H₂O + Mg(OH)₂.

If excess of lime is added then water would again become hard due to the absorption of CO₂.   Ca(OH)­₂ + CO₂ → Ca(HCO₃)₂.

b) Permanent hardness – due to the presence of soluble chlorides and sulphates of calcium and magnesium ions. It is known as non-carbonate hardness. It can be removed by –

(i) Washing-soda process – by adding known amount of Na₂CO₃ and filtering it.                                          CaCl₂ + Na₂CO₃→CaCO₃ + 2NaCl;     

CaSO₄ + Na₂CO₃→ CaCO₃ + Na₂SO₄.

(ii) Ion-exchange resin method – Ca²⁺ and Mg²⁺ ions are exchanged by the ions present in a complex salt called as zeolites or permutit. E.g. Hydrated sodium aluminium silicates Na₂Al₂Si₂O₈.xH₂O (Na₂Z) (naturally occurring).   

Na₂Z + CaCl₂→CaZ + 2NaCl.

Permutit can be regenerated by treating it with aqueous NaCl.           

CaZ + 2NaCl→Na₂Z + CaCl₂.  

Advantage: This method is efficient, cheap and can be used to remove both temporary and permanent hardness.

(iii) Calgon’s method – Calgon( sodiumhexametaphosphate ) is added to hard water.                                        Na₂P₆O₁₈→ 2Na⁺ + Na₄P₆O₁₈^2-;       

Ca²⁺ + Na₄P₆O₁₈^2-→ [Na₂CaP₆O₁₈]^2- + 2Na⁺

(iv) Synthetic resin method – This method is more efficient than ion-exchange. It involves synthesis of cation-exchangers. Synthetic organic exchangers are called as ion-exchange resins. They remove hardness in water and resulting water is known as demineralised water or de-ionized water.            

Cation-exchange resins contain a large hydrocarbon framework attached to acidic groups such as –COOH and –SO₃H group and they can exchange Na⁺ ions with Ca²⁺ and Mg²⁺.                                                           

RSO₃H + Na⁺→RNa + H⁺ (makes water acidic);       

2RNa + Ca²⁺→ R₂Ca + 2Na⁺.    

Resin can be regenerated back by adding aqueous HCl or aqueous NaCl.                                                        

Anion-exchange reins contain a large hydrocarbon framework attached to basic groups such as OH^- as in the form of substituted NH₄OH ( R⁺-NH₃OH^- )

R⁺-NH₃OH^- + Cl^-→R⁺-NH₃Cl^- + OH^- ;      

R⁺-NH₃OH^- + SO₄^2-→ (R⁺-NH₃)₂SO₄^2- + OH^- (makes water basic)

Resin can be regenerated by adding NaOH.

H₂O₂ = Hydrogen peroxide:

Preparation: (i) Lab preparation – BaO₂.8H₂O + H₂SO₄ → BaSO₄ + H₂O₂ + 8H₂O

(ii) Industrial preparation – by electrolysis of 50% sulphuric acid with platinum anode and graphite cathode. Reactions:

2H₂SO₄→ 2H⁺ + 2HSO₄^-;  

at cathode: 2H⁺ + 2e →H₂ ;            

at anode: 2HSO₄→ H₂S₂O₈ + 2e

H₂S₂O₈ + 2H₂O → 2H₂SO₄ + H₂O₂.

This method can be used to prepare D₂O₂.   

K₂S₂O₈ + 2D₂O → 2KDSO₄ + D₂O₂.

Physical properties of H₂O₂–

(i) It is a thick syrupy liquid with pale blue colour.

(ii) It is bitter in taste and more viscous than H₂O due to highly associated with H-bonds.

(iii) It dissolves in water, alcohol and ether as H₂O₂.H₂O . 

(iv) It has dipole moment  of 2.14D [more than water (1.84D)]

Chemical properties of H₂O₂ :

(a) It is an unstable liquid (auto-oxidation and auto-reduction).

2 H₂O₂  →   2H₂O + O₂

(b) Oxidising character:

(i) acidic medium-  2Fe²⁺ + 2H⁺ + H₂O₂ → 2Fe³⁺ + 2H₂O

(ii) in basic medium-   2Fe²⁺ + H₂O₂ → 2Fe³⁺ + 2OH^-

(c) Reducing character:

(i) basic medium- 2MnO₄^- + 3H₂O₂ → 2MnO₂ + 3O₂ + 2H₂O + 2OH^-

(ii) acidic medium-  Mn²⁺ + H₂O₂ → Mn⁴⁺ + 2OH^-

Structure of H₂O₂:  Non-planar molecule. 

2 Oxygen atoms are linked by single covalent bond and each oxygen is linked to hydrogen atom by single bond.

Storage of H₂O₂: As H₂O₂ easily decomposes so it is stored in plastic vessels or wax-lined glass in dark and urea can be added as stabiliser.     

2 H₂O₂   →    2H₂O + O₂

It is kept away from dust because dust can induce explosive decomposition of H₂O₂.

Uses-

1) H₂O₂ is used for bleaching silk, hair, textile, oils, fats, leather (oxidising agent). 

2) It is used as an antichlor (to remove excess Cl₂) in bleaching in textile industry.  

3) It is used as an antiseptic.

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