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Preparation of DiHydrogen

There are different methods to prepare dihydrogen from various sources.

In Laboratory, Dihydrogen is normally prepared by action of Conc. H2SO4 on Zinc Granules.

The experimental set up is as shown in the diagram below

Objective: To prepare dihydrogen gas in laboratory by the action of acid on metal
Apparatus required: Woulfe's bottle, thistle funnel, delivery tube, cylindrical jar, beaker, stoppers
Chemicals, Distilled H2O, Zn metal fillings, Conc. H2SO4
Principle: H2SO4 oxidizes Zn to form ZnSO4 and in the process H2 gas is liberated as Zn lies below H in the electrochemical series. The gas, molecular hydrogen or dihydrogen, is produced according to the reaction: Zn(s) + dil. H2SO4 ZnSO4(aq) + H2(g)

Experimental Procedure
  1. Take granulated pieces of Zn in a Woulfe's bottle and immerse them in Distilled H2O
  2. Place a cylindrical jar in inverted fashion in a beaker filled with distilled water
  3. Fit the bottle with a thistle funnel and a delivery tube as shown in the diagram
  4. Pour Conc. H2SO4 in drops through thistle funnel.
  5. As the acid enters the Woulfe's bottle, it gets diluted, and then reacts with the metal.
  6. The evolution of H2 gas is observed through the formation of air bubbles.
  7. The produced H2 is collected by the downward displacement of water as H2 is insoluble in water.
To obtain very pure dihydrogen gas, the following methods are employed
  1. Treat a ribbon of Mg with Dilute H2SO4
    The chemistry involved being: Mg(s) + dil. H2SO4 MgSO4(aq) + H2(g)
  2. Pure dihydrogen can also be produced by electrolyzing a warm solution of Ba(OH)2 using inert electrodes such as Pt electrodes.
  3. Oxidation of NaH by water, according to the reaction
    NaH + H2O  NaOH +H2
  4. Preparation of pure dihydrogen for military purpose. This method is known as Uyeno's method. In this method, aluminium scrap is treated with aqueous KOH to give KAlO2 and H2 according to the equation
    2 Al + 2 KOH + 2 H2O  2 KAlO2 + 3 H2

Commercial Preparation of H2

Method-1: By Electrolysis of Water.
Objective: To produce pure sample of H2 gas from inexpensive and abundantly available source such as water.
Apparatus: Electrolysis cell, Asbestos Diaphragm, Metal Electrodes, Iron sheet as Cathode and Ni plated Fe sheet as anode.
Required: Electricity in good supply
Chemicals: Water, NaOH
Theory: Water on electrolysis undergoes self oxidation and reduction and anode and cathode respectively according to the following equations
Water ionizes as - H2O H+ + OH-
At Anode: 4 OH- 4 OH + 4 e-
4 OH 2 H2O + O2
At Cathode: H+ + e- H
H + H H2

Experimental Procedure
  1. Take water to be electrolysed in an electrolytic cell fixed with Fe sheet as cathode and Ni plated Fe sheet as anode.
  2. Separate the two chambers by using an Asbestos Diaphragm to prevent the mixing of O2 and H2 air bubbles.
  3. Add about 15-25 % of NaOH solution to increase the conductivity of water and make the electrolysis faster.
  4. Pass electricity after setting up the experiment as shown in the diagram.
  5. H2 is collected at cathode while O2 is collected at anode.

Observation: Decomposition of water usually occurs once 20% of NaOH solution is electrolysed.
Disadvantage: The major disadvantage of this method for the preparation of H2 is that in India (Our Country) electricity is quite expensive and cannot be afforded to be used for preparation of H2 on a commercial scale.
Thus, other methods are employed.

Syngas - Bosch process: A method for the preparation of H2 gas on a commercial scale with high purity.
Chemistry Involved: H2 gas of very high purity is obtained on a commercial scale according to the equation
C(s,coke) + H2O(g,steam) CO(g) + H2(g)
The mixture of H2 and CO is termed as "Synthesis gas". This gas is used for the synthesis of CH3OH and a number of hydrocarbons, on a commercial scale. Previously, a 1:1 mixture of CO and H2 was called as "Water Gas".

This reaction is observed when super heated steam is passed over red hot coke at 1270 K using Ni as catalyst.

This process is also called as "Coal Gasification"

Major drawback of this process is that, it is difficult to produce H2 in a very pure state through this method. This can be overcome by oxidizing CO to CO2 by mixing CO with more steam in the presence of FeCrO4 as catalyst at 673 K.

This process is known as water gas shift reaction.
CO(g) + H2O(g) CO2(g) + H2(g)

From Hydrocarbons: This is relatively a novel method for the commercial preparation of dihydrogen of very high purity.
This can be attained by either of the two processes
  1. Partial oxidation of Hydrocarbons - Here, the crude hydrocarbon CnH2n+2 is mixed with steam. The mixture is passed through a finely divided Ni catalyst heated at 1270 K
    CnH2n+2 + n H2O(steam) n CO + (2n+1) H2
    In place of Hydrocarbon, natural gas, CH4 can also be employed in accordance with the equation
    CH4(g) + H2O(g,steam) CO(g) + 3 H2(g)
    Thus, syngas is again formed by this method and H2 of very high purity can be prepared as described in previous section.
    The whole process of obtaining H2 from hydrocarbons is known as Steam reforming process.
  2. Thermal cracking of Natural Gas
    On heating natural gas (CH4) to a temperature above 1270 K in the presence of finely divided Ni as catalyst, H2 gas is produced according to the reaction,
    CH4 C(s) + 2 H2(g)
Lane's Process: This process involves the passage currents of hot steam and water gas alternatively, over a red hot iron rod. The process involves two steps:
  1. Oxidation: Production of dihydrogen gas, leaving behind magnetic oxide of iron, Fe3O4, when super heated steam is passé over hot iron fillings at 1050 K and above, according to the equation
    3 Fe + 4 H2O(steam) Fe3O4 + 4 H2
  2. The next stage consists of reduction of Fe3O4 to Fe by water gas, according to the equation:
    Fe3O4 + 4 H2 3 Fe + 4 H2O
    Fe3O4 + 4 CO  3 Fe + 4 CO2
Thus, by alternatively passing steam and water gas, a good amount of H2 can be generated from a small quantity of Fe fillings.

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