Hydrogen

Hydrogen

Hydrogen, H2, is the simplest element. It is the first element in the periodic table, and it is placed in Group I of the periodic table. It has three Isotopes

Deuterium, D2, (which is also known as Heavy Hydrogen) is an isotope of hydrogen. The atomic number of deuterium is 1, its atomic mass is 2.0144, and its boiling point is 23.6 deg K


Discovery

Hydrogen was discovered by Henry Cavendish in 1776AD.


Occurrence

Hydrogen is the lightest element and the most abundant element in the universe.

Hydrogen occurs naturally as a mixture of the three isotopes :


Preparation

The preparation of hydrogen gas is usually from a reduction of a compound containing hydrogen that is in the +1 oxidation state. This reduction is accomplished either electrically or chemically.

WARNING
Before collecting hydrogen great care must be taken to ensure that all the air has been displaced from the apparatus since a mixture of hydrogen with air is highly explosive.
WARNING

Preparation of Hydrogen by Electrolysis

Electrolytic hydrogen is the purest commercially available grade of hydrogen and is made by the electrolysis of water.


		2H2O   ==>   2 H2(g)   +   O2(g)        
Pure hydrogen is best prepared by electrolysis with nickel electrodes of a warm saturated barium hydroxide solution. The gas is passed over hot platinum gauze which oxidises any residual oxygen in the gas, and it is then dried by passing the gas over potassium hydroxide pellets and pure redistilled powdered phosphorus pentoxide. 'Electrolytic Hydrogen' is relatively expensive because of the cost of the electrical energy necessary to make it.

Preparation of Hydrogen by the Action of Metals

The alkali metals, lithium, sodium, and potassium react violently with water at the ordinary temperature, yielding hydrogen.

		2 Li   +   2 H2O   ==>   H2   +   2 LiOH        

Calcium reacts with water more slowly unless the water is hot, when the action is more vigorous.


		Ca   +   2 H2O   ==>   H2   +   Ca(OH)2 

Preparation of Hydrogen by Decomposition of Water

Cold water is decomposed by amalgamated aluminium (i.e. an alloy of aluminium and mercury which is made by rubbing aluminium foil with damp mercuric chloride).

		2 Al   +   6 H2O   ==>   2 Al(OH)3   +   3 H2   
Hot water is decomposed by zinc-Copper couple (i.e. solid granules of zinc covered by a surface layer of copper which made by pouring a solution of copper sulphate over granulated zinc).

		Zn   +   2H2O   ==>   Zn(OH)2   +   H2  
Boiling water is slowly decomposed by magnesium power.

		Mg   +   2H2O   ==>   Mg(OH)2   +   H2  
Steam is decomposed when passed over heated magnesium, zinc, and iron.

		Mg   +   H2O   ==>   MgO   +   H2       
		Zn   +   H2O   ==>   ZnO   +   H2       
	      3 Fe   +   H2O   <==>   Fe3O4   +   4H2 
The last reaction, (i.e. the action of iron on steam) is reversible, depending on the experimental conditions.

Preparation of Hydrogen from Action of Acids

Hydrogen is prepared in the laboratory by the action of acids on metals. Dilute sulphuric acid containing 1 volume of concentrated acid to 5 volumes of water, or dilute hydrochloric acid containing 1 volume of concentrated acid to 4 volumes of water, is added to granulated zinc. Zinc sulphate or zinc chloride is formed in solution and the hydrogen that is evolved is collected over water in a trough.

	    Zn   +   H2SO4   ==>    ZnSO4   +   H2  
	    Zn   +   2 HCl   ==>   ZnCl2   +   H2   
Since hydrogen is very much lighter than air it may also be collected by upward displacement.
WARNING
Before collecting hydrogen great care must be taken to ensure that all the air has been displaced from the apparatus since a mixture of hydrogen with air is highly explosive.
WARNING


Manufacture

Pure hydrogen is manufactured industrially by the steam reforming of natural gas, and by the electrolysis Of water.

The manufacture of hydrogen on an industrial scale involves the reaction between steam and iron. Spongy iron from the reduction of spathic iron ore (ferrous carbonate) is heated to redness and steam passed over it.


		3 Fe  +  4 H2O  ==>   Fe3O4  +  4 H2    
The hot ferrosoferric oxide, Fe3O4, is then reduced with water gas:

		Fe2O4   +   4 H2   ==>   3 Fe  +  4 H2O 

Fe2O4 + 4CO ==> 3 Fe + 4 CO2 Water gas is made by passing steam over red hot carbon and it consists of a mixture of carbon monoxide and hydrogen, with a smaller amount of carbon dioxide:


			   bright-red      
			      heat        
		C   +   H2O   ==>    CO  +  H2  

			     dull-red        
				heat 
		C   +   2 H2O   ==>   CO2  +  2 H2


Properties

Hydrogen is

Combustion of Hydrogen


A mixture of hydrogen with oxygen or air explodes violently when kindled, provided either gas is not present in too large excess.


Reaction with Non-Metals

Hydrogen readily combines with fluorine and chlorine, less readily with bromine, iodine, sulphur, phosphorous, nitrogen, and carbon.

		H2   +   F2   ==>   2 HF        
Hydrogen burns in chlorine gas and a mixture of hydrogen and chlorine explodes violently when kindled or exposed to bright sunlight.

		H2   +   Cl2   ==>   2 Hcl      
Hydrogen combines with nitrogen on sparking or in presence of a catalyst, forming ammonia.

		N2   +   3 H2   ==>   2 NH3     

Hydride Formation

Hydrogen forms hydrides, (e.g. NaH) with a number of metals, including lithium, sodium and calcium.


		H2   +  2 Na   ==>   2 NaH      
These hydrides when pure are white salt-like compounds rapidly decomposed by water.

		NaH   +   H2O   ==>   NaOH   +   H2     
The hydrogen atom in these hydrides behaves to some extent like a halogen or electronegative element. For example, on the electrolysis of fused lithium hydride, the hydrogen is liberated at the positive electrode (i.e. a negatively charged hydrogen ion is discharged), and not the negative electrode as is the case when water is electrolysed.

Hydrogen is also evolved at the anode in the electrolysis of a solution of calcium hydride, in fused mixture of potassium chloride and lithium chloride. This indicates that the ionic structure of the lithium hydride is Li(+)H(-).

Reducing Properties

When hydrogen is passed over many heated metallic oxides (e.g. copper oxide, iron oxide, or lead oxide), they are reduced to the metals.

		CuO   +   H2   ==>   Cu   +   H2O


Uses

Hydrogen is used


Principal Compounds

Hydrogen is widely distributed in industrially important compounds and is present
  1. in a wide range of inorganic compounds, including
    Ammonia
    Hydrogen Sulphide
    Water
  2. in the strong acids, including
    Sulphuric Acid
    Nitric Acid
    Hydrochloric Acid
    Hydrobromic Acid
    Hydrofluoric Acid

    and

  3. in almost all organic compounds.
    Alkanes
    Alkenes
    Alkynes
    Alcohols
    etc.


Start of Hypertext .... Elements .... Compounds .... Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.