PREPARATION, PROPERTIES AND USES OF HYDROGEN
Hydrogen (H) is the most abundant element found in the universe. The sun and the stars are composed mainly of hydrogen. Scientists have estimated that 90% of the atoms in the universe are hydrogen atoms. Hydrogen is a predominant component found in most of the compounds, than any other element. Water is the most copious compound, found on earth which is made up of hydrogen. Hydrogen is an important constituent of organic compounds like hydrocarbons, many minerals, cellulose and starch, sugar, fats, oils, alcohols, acids, and thousands of other substances.
PREPARATION OF DIHYDROGEN
A. Laboratory Preparation of Dihydrogen
- Reacting Zinc with Acid
A common way to prepare dihydrogen gas in the laboratory is by a reaction of Zinc with a diluted acid (H2SO4, HCl etc.). This reaction results in the formation of Zinc Sulphate (or zinc chloride) and release of hydrogen in a gaseous form.
Zn + H2SO4 → ZnSO4 + H2
Zn +HCl → ZnCl2 + H2
- Reacting Zinc with Alkali
Zinc can also react with an aqueous alkali such as Sodium Hydroxide, the sodium and zinc will combine and give dihydrogen as a product. The reaction is as follows:
Zn + 2NaOH → Na2ZnO2 + H2
B. Commercial Preparation of Dihydrogen
There is a huge demand for hydrogen, as it is required for the production of fertilizers and oil refining process. Therefore, the large-scale production of dihydrogen is required. The commercial production of dihydrogen is by the following ways:
- Electrolysis of Water
This method helps in the production of a very pure form of hydrogen from water. In this method, electricity is passed through the water in the presence of a small amount of acid or base. The water molecule breaks up, as a result, the hydrogen collects at the cathode while oxygen collects at the anode.
2H2O → 2H2 + O2
- 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
Net reaction isCO + H2O → CO2 + H2
- From Natural Gas
It is the cheapest way to produce hydrogen on a commercial scale. The natural gas is heated to high temperatures (of about 1100°C) with steam and Nickel as a catalyst. As a result, the methane molecules break into Carbon Monoxide and Hydrogen.
CH4 + H2O→ CO + 3H2
Thermal Cracking of natural gas at a temperature of 1270K and in the presence of finely powdered nickel as a catalyst, dihydrogen is produced according to the following reaction:
CH4 → C(s) + H2(g)
- Bosch’s Process
The maximum quantity of commercial dihydrogen is prepared by Bosch’s method. It involves the following steps:
- 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 steal reforming of hydrocarbons.
(b) 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 CO2dissolves 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 Hydrocarbons
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.
CnH2n+2 + nH2O(steam) → nCO + (2n+1) H2
PROPERTIES OF DIHYDROGEN
- Physical Properties of Dihydrogen
- At room temperature, dihydrogen molecule is found in a gaseous state.
- It is a colourless, odourless and tasteless gas which makes it very hard to detect.
- It is the lightest element of the periodic table. Has the molecular weight of 1.00794
- iv. It has the lowest density of all elements
- v. It is highly combustible or inflammable. On combustion it produces water. Also, it gives a blue flame on burning.
- vi. Dihydrogen liquefies at a very low temperature of about -250°C.
- Chemical Properties of Dihydrogen
- Dihydrogen is a non-metal.
- Dihydrogen is a stable molecule.
- It has high enthalpy as it has a stable H-H bond.
- It is an oxidizing agent when it reacts with metals, and forms metal hydrides. It reacts with metals like Na, Ca, Li, etc, to form their hydrides. Under such reaction conditions, it acquires the oxidation state of -1 and acts as an oxidizing agent. In case of metals like Pt, Pd, Ni, etc, hydrogen combines to form interstitial hydrides in which hydrogen atoms occupy the interstitial space in the metallic crystals. This property is referred to as occlusion. Occluded hydrogen can be liberated from the metals by strong heating.
- It forms compounds with a large number of elements under appropriate experimental conditions.
- During the reaction with metal oxide, such as iron and metal less active than iron, hydrogen acts as a reducing agent.
- Dihydrogen reacts with carbon monoxide at 700 K in the presence of a catalyst Zn/Cr2O3 to produce methanol.
- The Unsaturated hydrocarbons like ethene(CH2=CH2) and ethyne(CH≡CH) react with dihydrogen to form saturated hydrocarbons.
- When dihydrogen (under pressure) is passed through edible vegetable oils such as groundnut or cottonseed oil at about 473 K in presence of finely nickel as a catalyst, they undergo hardening and leads to the formation of edible fats known as Vanaspati Ghee. This process is known as hydrogenation or hardening of oils.
Uses of Dihydrogen
- Dihydrogen is used in the preparation of ammonia which is a starting material for the manufacture of various fertilizers such as urea, ammonium sulphate, calcium ammonium nitrate, etc.
- It is used in the hydrogenation of vegetable oils.
- It is needed for the manufacture of synthetic petrol.
- Dihydrogen is used in the oxy-hydrogen torch for welding, if the temperature is around 2500°C, and in the atomic hydrogen torch for welding, if the temperature is around 4000°C.
- Liquid hydrogen, when mixed with liquid oxygen, is used as rocket fuel in space programmes.
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