Dihydrogen: Structure, Properties, Preparation & Uses | Chemistry Class 11 - NEET PDF Download

What is Dihydrogen?

Dihydrogen is the homonuclear diatomic molecule formed from two hydrogen atoms. This molecule features a covalent bond between two hydrogen atoms, satisfying each of their required duet configurations. Since hydrogen is the lightest element on the modern periodic table, dihydrogen is the lightest known molecule. Since the standard atomic weight of the hydrogen atom is 1.008u, the molecular weight of the dihydrogen molecule is 2.016 atomic mass units.

Under standard conditions for temperature and pressure (often abbreviated to STP), dihydrogen exists as a colourless, tasteless, and odourless gas which is highly combustible in nature. The molecular formula of dihydrogen is H₂.

Structure of Dihydrogen

The dihydrogen molecule features a single covalent bond between the two hydrogen atoms that constitute it. This molecule has a linear shape and is nonpolar in nature. Each hydrogen atom contributes one electron towards the covalent bond. Therefore, the duet configuration requirements of both the hydrogen atoms that constitute the dihydrogen molecule are satisfied.

Applications of Dihydrogen

In the chemical industry and the petroleum industry, large quantities of H2 are required on a regular basis. Dihydrogen’s main use is in the refining of certain fossil fuels and also in ammonia manufacturing. The petrochemical industry is one of the main consumers of H2. This industry utilizes dihydrogen for processes like hydrodesulfurization, hydrodealkylation, and also hydrocracking. Dihydrogen has many other important applications. For example, this compound can be used as a hydrogenating agent, especially in order to increase the saturation level of unsaturated fats and oils (which are usually found in products like margarine). This compound is also used in methanol development. Similarly, dihydrogen serves as the source of hydrogen in hydrochloric acid production. H2 also acts as a reducing agent and can, therefore, be used to treat metallic ores.

PREPARATION OF DIHYDROGEN (H₂)

1. Lab methods:

• Zn + H₂SO₄ → ZnSO₄ + H₂
  Metals that have a reduction potential lesser than H can liberate H₂ from acids.
  Pure zinc is not used because it reacts slowly. The presence of some impurities increases the rate of reaction due to the formation of electrochemical couples. Conc. sulphuric acid is also not used because it oxidises, H₂ formed into H₂O.
  Zn + 2H₂SO₄(conc.) → ZnSO₄ + SO₂ + 2H₂O
• It can also be prepared by the reaction of zinc with aqueous alkali.
  Zn + 2NaOH → Na₂ZnO₂ + H₂

2. Commercial production of dihydrogen:

• By the electrolysis of acidified water using platinum electrodes.

• From water gas (Bosch process):
  The maximum quantity of commercial dihydrogen is prepared by Bosch’s method. It involves the following steps:
  (i) Preparation of water gas:
  It is prepared by the action of steam with red hot coke as:
  C(coke) + H₂O (steam) → CO(g) + H₂(g)
  Water gas can also be obtained by the action of steam on hydrocarbons in the presence of a catalyst. This process is also called steam reforming of hydrocarbons.
  (ii) Separation of Hydrogen:
  In order to obtain dihydrogen in the pure form, the water gas formed by any of the above methods is mixed with steam and is passed over heated Fe2O3 and Cr2O3 at 770K when CO is oxidized to CO2. A gaseous mixture of CO2 and H2 is then bubbled into cold water under pressure when CO2 dissolves while leaving behind dihydrogen gas which escapes out. This reaction is called the water-gas-shift reaction.
   CO(g) + H₂O(g) → CO₂(g) + H₂(g)
• From steam (Lane’s process):
  In this process, Water Gas (a mixture of carbon monoxide and hydrogen) and steam are alternatively passed over iron at very high temperature up to 800°C. The iron is first oxidized, releasing hydrogen and is replaced with fresh metal. The iron then reduces the water gas back to metal. The reactions are as follows
  3Fe + 4H₂O → Fe₃O₄ + 4H₂
  Fe₃O₄ + 4CO → 3Fe + 4CO₂
  The net reaction is CO + H₂O → CO₂ + H₂
• Highly pure (> 99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
• From hydrocarbons by partial oxidation:
  During the partial oxidation of hydrocarbons, the crude hydrocarbon, CnH2n+2 is mixed with steam. The mixture is passed through the finely divided nickel catalyst heated at 1270K.
  C_nH_2n+2 + nH₂O(steam) → nCO + (2n+1) H₂
• 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:
  Anode: 2Cl^-(aq) → Cl₂(g) + 2e^-
  Cathode: 2H₂O(l) + 2e^- → H₂(g) + OH^-(aq)
  Overall reaction: 2Cl^-(aq) + 2H₂O (l) → Cl₂(g) + H₂(g) + OH^-(aq)

Properties of Dihydrogen

• At STP, dihydrogen exists in the gaseous state.
• The melting point of H₂ is 13.99 Kelvin. Converting this value to the celsius scale, the melting point of dihydrogen can be represented as -259.16 degrees celsius.
• The boiling point associated with dihydrogen corresponds to 20.271 on the Kelvins scale. Converting this value into the celsius scale, the boiling point of H₂ can be expressed as -252.879 degrees celsius.
• The latent heat of fusion associated with the H₂ molecule corresponds to 0.117 kilojoules per mole.
• The latent heat of vaporization (also known as the enthalpy of vaporization) of dihydrogen is equal to 0.904 kilojoules per mole.
• The molar heat capacity of H₂ is roughly equal to 28.83 joules per mole Kelvin.
• The speed of sound when it propagates through gaseous H₂ is 1310 meters per second.

Physical Properties of Dihydrogen

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

Chemical Properties of Dihydrogen:

(ii) Action with non-metals:

(iii) Reaction with metals: Here H₂ acts as oxidizing agent.

(iv) Reducing action of dihydrogen:

(v) Reactions with metal ions and metal oxides:

(vi) Reaction with organic compounds:

Uses of Dihydrogen

1. It is used in the manufacture of CH₃OH.
   
2. It produces a temperature of 2850°C and oxy-atomic hydrogen flame produces a temperature of 4000°C, so it is used in oxy-hydrogen flame.
3. The largest single use of H₂ is in the synthesis of NH₃ which is used in the manufacture of HNO₃ and fertilizers.
4. Liquid hydrogen (H₂) is used as rocket fuel.
5. H₂ is used as a reducing agent in extraction of metals.
6. H₂ is used in fuel cell for generating electrical energy.
7. 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.)