HYDROGEN : Introduction

HYDROGEN : Introduction

Hydrogen has the simplest atomic structure of all the elements, and consists of a nucleus containing one proton and one orbital electron. The electronic structure may be written as 1s1.

Isotopes of Hydrogen – There are three isotopes of hydrogen.

1H1 :  Hydrogen or Protium (H), it is stable and 99.986% abundant in total hydrogen available.

2H1 : Deuterium (D), it is stable and nearly 0.0146% abundant in total hydrogen available.

3H : Tritium (T), it is unstable and radioactive with half life time 12.26 years. Tritium is 1 part in 1017 parts hydrogen (7 x 10-16 %)  It decays by emitting out a beta particle

_{ 1 }^{ 3 }{ T }\rightarrow _{ 2 }^{ 3 }{ He+_{ -1 }^{ \quad 0 }e }                

 

Physical constants

H2 D2 T2
Mass of atom (amu) 1.0078 2.0141 3.0160
Freezing point (0C) -259 -254 -252.4
Boiling point (0C) -252.6 -249.3 -248.0
Bond length (A0) 0.741 0.741 0.741
Heat of dissociation (KJ/mole) 435.9 443.4 446.9
Latent heat of fusion (KJ/mole) 0.117 0.197 0.250
Latent heat of vaporization (KJ/mole) 0.904 1.226 1.393

 

Preparation of Hydrogen : –

1. By the action of acids on metals

         Zn + H2SO4 → ZnSO4 + H2

         Fe + 2HCl → FeCl2 + H2

2. Action of alkali on amphoteric metals

         Zn + 2NaOH → Na2ZnO2 + H2

         2Al + 2NaOH + 2H2O → 2NaAlO2  + 3H2

3. Action of water on active metals

         Ca + 2H2O → Ca(OH)2 + H2

         2Na + H2O → 2NaOH + H2

4. From hot water

         Mg + H2O → MgO + H2

5. From steam with iron : Magnetite Fe3O4 (oxide with magnetic properties along with H2 is produced).

         3Fe + 4H2O → Fe3O4 + 4H2

                       magnetite

Manufacture of Hydrogen :

1. Bosch process :

         C + H2O → CO + H2  ; 

         H2 + CO + H2O → 2H2 + CO2       ;         CO2 + H2O → H2CO3

2. Lane process :    3Fe + 4H2O → Fe3O4 + 4H2

 

Preparation of D2

By decomposing heavy water

(A) With sodium:

                   2D2O + 2Na → 2NaOD + D2

                                      Sodium deuteroxide

(B) Heavy water is heated with tungsten

3D2O  + W \underrightarrow { \quad 10{ 0 }^{ 0 }\quad }    WO3 + 3D2

(C) By the electrolysis of an alkaline solution of heavy water.

2D2O \underrightarrow { \quad Electrolysis\quad } 2D2 + O2

(at cathode)     (at anode)

 

Preparation of T2

1. By bombarding deuterium compounds (e.g.- D3PO4, ND4CL etc.) with fast moving deuterons

 _{ 1 }^{ 2 }D+ _{ 1 }^{ 2 }D\rightarrow _{ 1 }^{ 3 }T+_{ 1 }^{ 1 }D

2. By bombarding beryllium with fast deuterons

 _{ 4 }^{ 9 }Be+_{ 1 }^{ 2 }D\rightarrow _{ 1 }^{ 3 }T+_{ 4 }^{ 8 }Be

3. By bombarding Lithium with slow neutrons

 _{ 3 }^{ 6 }Li+_{ 0 }^{ 1 }ns\rightarrow _{ 1 }^{ 3 }T+_{ 2 }^{ 4 }He

Properties of Hydrogen – Colourless, odorless and tasteless gas, Insolvable in water, bad conductor of heat and electricity. Pd metal can adsorb H2 gas.  It exists in ortho and para forms.

Ortho and Para hydrogen : In the H2 molecule, the two nuclei may be spinning in either the same direction (ortho form), or in opposite directions (para form).  This type of spinning isomerism is also found in other symmetrical molecules whose nuclei has spinning momenta (e.g. D2 , N2 , F2 Cl­2).

At room temperature, the equilibrium mixture of H2 contains 66.67%  ortho and 33.33% para forms. As the temperature increases, the proportion of ortho-form increases. The para form has the lower energy, at absolute zero the gas contains 100% of the para form.

 

 

 

 

(i)      Hydrogen is not a reactive gas, because its bond energy is very high (104 Kcal mol–1). It is inflammable gas and burns in air with a pale blue flame forming water.

         2H2 + O2 →  2H2O

(ii)     With non-metals

         N2 + 3H2 →  2NH3 + 22.4 kCals

         C + 2H2 → CH4

         H2 + Cl2 → 2HCl

(iii)    With metals

         2Na + H2 → 2NaH

         Ca + H2 → CaH2

(iv)    Reducing nature

         ZnO + H2 → Zn + H2O

         CuO + H2 → Cu + H2O

(v)     With unsaturated compounds

         R – CH = CH2 + H2  → RCH2 – CH3 (Sabetier & Sanderens reaction)

(vi)    Hydrogenation – oils are the mixture of triglycerides of unsaturated fatty acids. When hydrogen gas is passed through a vegetable oil at 180ºC under pressure in the presence of nickel as a catalyst. It is converted into a triglycerides having higher melting point. ‘Vegetable ghee’ is also known as hardening of oils.

 

Properties of D2 : – In chemical properties, it is similar to hydrogen but slow and less complete in reaction.

1. It burns in oxygen forming deuterium oxide

                   2D2 + O2 → 2D2O     

2. Addition of halogens

          D2 + F2 \underrightarrow { \quad Dark\quad }   2DF Deuterium fluoride

          D2 + Cl2  \xrightarrow [ \quad Sunligh\quad ]{ reduced }   Deuterium Chloride

3. At 700 0C, it reduces AgCl into Ag : 2AgCl + D2 → 2Ag + 2DCI

4. It combines with nitrogen its presence of catalyst forming deuterium ammonia

                   N2 + 3D2 → 2 ND3

5. With unsaturated hydrocarbon

6. It combines with alkali metals at 3600C : 2Na + D2 → 2NaD

7. Exchange reaction

                   C6­H6 + 3D2  \underrightarrow { \quad 20{ 0 }^{ 0 }C\quad }   C6D6   (Deuterobenzene) + 3H2          

                     CH4 + 2D2  →     CD4 + 2H2

 

Uses of D2  –

1. In the production of deuterons, which are used as projectiles in the formation of isotopes of elements.

2. As a tracer in the study of mechanism of chemical reactions