Dihydrogen

Preparation of Dihydrogen, H₂

Laboratory Preparation of Dihydrogen

• It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.

• It can also be prepared by the reaction of zinc with aqueous alkali.

Commercial Production of Dihydrogen

• By the electrolysis of acidified water

• Form water gas (Bosch process)

• From steam (Lane’s process) super-heated steam is passed over iron filings heated to about 1023-1073 k when hydrogen is formed.
• Highly pure (> 99.95 %) dihydrogen is obtained by electrolyzing warm aqueous barium hydroxide solution between nickel electrodes.
• From hydrocarbons by partial oxidation

• It is also obtained as a by-product in the manufacture of NaOH and chlorine by the electrolysis of brine solution.

During electrolysis, the reactions that take place are

Properties of Dihydrogen

Physical Properties

• Dihydrogen is a colourless, odourless and tasteless gas.
• It is a combustible gas.
• It is insoluble in water.
• It is lighter than air.

Chemical Properties of Dihydrogen

• Reaction with halogens: It reacts with halogens, X₂ to give hydrogen halides. HX.

(F can react with hydrogen in dark also, iodine requires a catalyst)

• Reaction with dioxygen:

The reaction is highly exothermic.

• Reaction with dinitrogen: With dinitrogen to form ammonia

• Reaction with metals: Here H₂ acts as oxidising agent.

Where (M = alkali metal)

• Reaction with metal ions and metal oxides: It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals

• Reaction with organic compounds: It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance.
• Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and Vanaspati ghee).
• Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

Use of Dihydrogen

• It is used in the manufacture of CH₃OH.
• It produces temperature of 2850^oC and oxy-atomic hydrogen flame produces a temperature of 4000^oC, so it is used in oxy-hydrogen flame.
• The largest single use of H₂ is in the synthesis of NH₃ which is used in the manufacture of HNO₃ and fertilizers.
• Liquid hydrogen (LH₂) is used as rocket fuel.
• H₂ is used as a reducing agent in extraction of metals.
• H₂ is used in fuel cell for generating electrical energy.
• Hydrogen is used in the manufacture of synthetic petrol.

(By heating H₂ with coal and heavy oils under very high pressure in the presence of catalyst).