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.
3H1 : 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
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 WO3 + 3D2
(C) By the electrolysis of an alkaline solution of heavy water.
2D2O 2D2 + O2
(at cathode) (at anode)
Preparation of T2 –
1. By bombarding deuterium compounds (e.g.- D3PO4, ND4CL etc.) with fast moving deuterons
2. By bombarding beryllium with fast deuterons
3. By bombarding Lithium with slow neutrons
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 2DF Deuterium fluoride
D2 + Cl2 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 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