Preparation of Dihydrogen

Molecular hydrogen, sometimes called dihydrogen, is a diatomic molecule that is composed of two hydrogen atoms held together by a covalent bond with the chemical formula H_2.

Preparation of Dihydrogen:

Dihydrogen may be obtained from water by reduction either with active metals or by electricity. It can be prepared from three sources:

1. from water

2. from alkalis

3. from acids

1. From Water:

a) By the Action of Water on Active Metals:

· Cold Water: Very active metals i.e. alkali and certain alkaline earth metals like Na, K , Ca react with water at room temperature evolving dihydrogen.

The reaction with alkali metal is so vigorous and exothermic that the hydrogen involved catches fire. To slow down the reaction, amalgams of these metals are generally used.

In amalgams, only a small surface area of the metal comes in contact with water and therefore the reaction is slowed down.

· Boiling water: Less active metals like Zn, Mg, Al decompose boiling water liberating dihydrogen.

Zn + H2O	→	ZnO + H2
Mg + H2O	→	MgO + H2
2Al + 3 H2O	→	Al2O3+ H2

· Steam: Still less active metals like Fe, Sn, Ni decompose steam at high temperature (1023-1073K) evolving dihydrogen.

3Fe + 4H₂O → Fe₃O₄ + H₂

b) By Electrolysis of Water:

Dihydrogen of high purity is usually obtained by the electrolysis of water in the presence of small amount of an acid or a base. During electrolysis, dihydrogen is collected at cathode while dioxygen is liberated at anode.

2H₂O (l) → 2H₂ (g) + O₂ (g)

Pure water is only weakly ionized and hence is a poor conductor of electricity but presence of an acid or a base makes it a better conductor of electricity.

This method is not commercially used since it is quite expensive.

2. From Alkalis:

Metals like Be, Zn, Sn react with boiling alkali solution liberating dihydrogen.

Zn + 2NaOH	→	Na2ZnO2 + H2
Be + 2NaOH	→	Na2BeO2 + H2
Sn + 2NaOH + H2O	→	Na2SnO3 + H2
2Al + 2NaOH + 2 H2O	→	2NaAlO2 + 3H2

3. From Acids:

Metals which are more electropositive than hydrogen (lie above hydrogen in the electrochemical series) such as Zn, Fe, Mg react with dilute mineral acids to liberate dihydrogen gas.

Zn + H2SO4	→	ZnSO4 + H2
Fe +2HCl	→	FeCl2 + H2

Metals like Copper, silver, Mercury which are less electropositive than hydrogen (lie below hydrogen in the electrochemical series) do not liberate hydrogen from acids.

There are two methods in preparation of dihydrogen:

1. Laboratory preparation

2. Commercial preparation

1. Laboratory Preparation:

In laboratory, dihydrogen is prepared by reaction of dil H₂SO₄ on granulated zinc.

Zn + H₂SO₄ → ZnSO₄ + H₂

Granulated pieces of zinc are placed in a Woulfe’s bottle and are covered with water. The bottle is fitted with a thistle funnel and a delivery tube.

Conc.H₂SO₄ is poured slowly through the thistle funnel. As the acid falls in the Woulfe’s bottle,it gets diluted and then reacts with zinc evolving dihydrogen gas. It is collected by downward displacement of water.

Sometimes the bubbles of dihydrogen produced stick to the surface of the zinc metal preventing the further reaction of the acid on the metal. Such a situation can be avoided by adding few crystal of copper sulphate to the reaction mixture.

Preparation of Pure Dihydrogen Gas:

· By the action of pure sulphuric acid on magnesium ribbon

Mg + H₂SO₄ (dil) → MgSO₄ + H₂

· By the electrolysis of a warm solution of barium hydroxide using platinum or Nickel electrodes

· By the action of water on sodium hydride

NaH + H₂O → NaOH + H₂

· By the action of KOH on scrap aluminium

2Al + 2KOH + 2H₂O → 2KAlO₂ + 3H₂

2. Commercial Preparation:

· By the Electrolysis of Water:

A small quantity of acid or alkali is added to water to make it a good conductor and electrolysed in a cell. In this cell, iron sheet is used as a cathode while nickel plated iron sheet act as anode.

The two electrodes are separated from each other by an asbestos diaphragm which prevents mixing of dihydrogen and dioxygen. On passing electric current, dihydrogen gas is collected at cathode while dioxygen at anode.

When 20% NaOH solution is used for electrolysis, the decomposition of water takes place as follows:

At cathode,

H^+ + e‾	→	H
H^+ + H^+	→	H2

At anode,

4OH‾	→	4OH + e‾
4OH‾	→	2H2O + O2

· From Syngas (Bosch process):

When superheated steam is passed over red hot coke or coal at 1270K in the presence of nickel catalyst, a mixture of carbon monoxide and dihydrogen is produced.

C (s) + H₂O (g) → CO + H₂

A mixture 1:1 of CO and H₂ was called water gas. All mixtures of CO and H₂irrespective of their composition are called synthetic gas or syngas.

This process of producing syngas from coke or coal is coal gasification.

Syngas is produced from sewage, sawdust, scrap wood, newspaper.

It is difficult to obtain pure hydrogen from water gas or Syngas, since CO is difficult to remove. To remove CO and increase the production of dihydrogen from syngas, CO of the syngas is oxidised to CO₂ by mixing it with more steam at 673 K in presence of iron chromate as catalyst.

CO (g) + H₂O (g) + H₂O (g) → CO₂ (g) + 2H₂ (g)

CO₂ thus produced is removed either by scrubbing the mixture with sodium arsenide solution or bypassing the mixture through water under 30 atm pressure when carbon dioxide dissolves leaving behind dihydrogen which is collected.

· From Hydrocarbons:

Partial Oxidation of Hydrocarbons: A mixture of hydrocarbons is mixed with steam and passed over heated Nickel catalyst at 1270 K.

C_nH_2n+2 + nH₂O → nCO + (2n+1) H₂

Natural gas may also be used.

CH₄ (g) + H₂ (g) CO (g) + 3H₂ (g)

Whole process of obtaining dihydrogen from natural gas is called steam reforming process.

Thermal Cracking of Natural Gas: Dihydrogen may also be obtained by thermal cracking of natural gas at 1270 K in the presence of a catalyst.

CH₄ → C + 2 H₂

· Lane’s Process:

Dihyrogen can also be manufactured by passing alternate currents of steam and water gas over red hot iron. It consists of two stages:

Oxidation Stage: Super heated steam is passed over iron filling heated to about 1023 – 1073 K when hydrogen is formed and magnetic oxide of iron is left behind.

3Fe + 4 H₂O → Fe₃O₄ + 4H₂

Reduction stage: When the whole of iron has being oxidised, the steam supply is cut off and a steam of water gas is passed to reduce Fe₃O₄ back to iron.

Fe3O4 + 4H2	→	3Fe + 4H2O
Fe3O4 + 4CO	→	3Fe + 4CO2

By passing steam and water gas alternatively over heated iron, dihydrogen gas can be manufactured from a small quantity of iron.

· As a By – Product:

Large quantities of dihydrogen are obtained as a by-product in various industries.

1) From petroleum cracking plants

2) In the manufacture of sodium hydroxide and chlorine by electrolysis of brine solution.

At anode,

2Cl‾ (aq) → Cl₂ (g) + 2e‾

At Cathode,  

2H₂O (l) + 2e‾ → H₂ (g) + OH‾ (aq)

Overall reaction:  

2Cl‾ (aq) +  2H₂O (l) → Cl₂ (g) + H₂ (g) + OH‾ (aq)