Hydrogen and Its Compounds
Hydrogen (symbol H) has atomic number 1 and mass number 1. It was discovered by Henry Cavendish in 1766 but named by Antoine Lavoisier. It has the simplest atomic structure among all the elements around us in nature. In atomic form, it consists of only one proton and one electron but no neutron. However, in elemental form it exists as a diatomic (H2)molecule and is generally called dihydrogen. It forms more compounds than any other element.
Position of Hydrogen in the Periodic Table
It is the first element in the periodic table. Its electronic configuration is 1s1. It resembles with alkali metals as well as with halogens in some properties. So, it is best placed separately in the periodic table.
Occurrence
Dihydrogen is the most abundant element in the universe (70% of the total mass of universe). It is the principal element in the solar atmosphere. The giant planets Jupiter, Saturn and Stars consist mostly of hydrogen. However, due to its light nature, it is much less abundant in the earth’s atmosphere. It is the ninth most abundant element in the earth’s crust.
Isotopes of Hydrogen
Hydrogeň has three isotopes; protium , deuterium and tritium . (The subscript shows the atomic number and the superscript shows the mass number). These isotopes differ from one another in respect of the presence of neutrons. Ordinary hydrogen (protium) has no neutrons, deuterium (also known as heavy hydrogen) has one and tritium has two neutrons in the nucleus. The predominant form is protium. Tritium is a radioactive isotope (half-life period is 12.4 years). It is a beta emitter.
Deuterium was prepared by Urey, Brickwedde and Murphy in 1931 and is used for studying the mechanism of organic reaction and as a bombarding particle in nuclear reactions.
Special Forms of Hydrogen
Nascent Hydrogen
It is the hydrogen at the moment of its generation ie it is present only in-situ. This is more reactive and powerful reducing agent.
Atomic Hydrogen
It is produced at elevated temperature by the decomposition of molecular hydrogen. Its reducing power is more than that of nascent hydrogen
Ordinary Hydrogen
It is a mixture of two forms; ortho and para. In ortho hydrogen, spins of both the nuclei are in same direction while in para hydrogen, spins of both the nuclei are in opposite directions.
Adsorbed Hydrogen
H2 on bubbling at the surface of Pt, Pd, Ni, etc is adsorbed. These metals adsorbed large amount of H2, at normal and released it at higher temperature. The process of adsorption of H2 over these metals, is called occlusion of hydrogen.
Preparation
Preparation Following methods are used For the preparation of dihydrogen.
- In laboratory, it is prepared by the reaction of granulated zinc with dilute hydrochloric acid.
- Commercially, it is prepared by electrolysis of acidified water using platinum electrodes and by the reaction of steam on hydrocarbons or coke at high temperature in the presence of catalyst.
- It is also prepared by passing water vapours over red hot iron or by treating hydrolith or sodium with water.
Properties of Dihydrogen
The physical and chemical properties of dihydrogen are as follows
Physical Properties
- It is a colourless, odourless, tasteless and combustible gas.
- It is lighter than air and insoluble in water.
- Its melting point is 13.96 K and boiling point is 20.39 K.
- Its density is 0.09 gL-1
Chemical Properties
- It is relatively inert at room temperature due to the high H — H bond energy. It reacts with halogens, X2 , to give hydrogen halides, HX.
( At high temp.) H2(g)+ X2(g) → 2HX(g)
- It reacts with oxygen (or air) at high temperature to form water. The reaction is highly exothermic.
( At Catalyst or heating ) 2H2(g)+ O2(g) → 2H2O(l) + Heat
- It reacts with dinitrogen to form ammonia (Haber’s process).
(At 673k,200atm Fe ,Mo ) 3H2(g)+ N2(g) → 2H3(g) + Heat
- It reacts with metals to form hydrides at high temperature.
H2(g)+ 2M(g) → 2MH(g) + Heat (M = alkali metal )
- It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products.
- When water itself combines chemically with some element or mineral, the reaction is called hydration.
Uses of Dihydrogen
(i) The largest single use of dihydrogen is in the synthesis of ammonia by Haber’s process, which is used in the manufacture of nitric acid and nitrogenous fertilizers.
(ii) It is used in the manufacturing of vanaspati fat by the hydrogenation of poly unsaturated vegetable oils like soyabean, cotton seeds, etc.
(iii) It is used in the manufacturing of bulk organic chemicals particularly methanol.
(iv) It is widely used for the manufacturing of metal hydrides, hydrogen chloride, etc. (v) In metallurgical processes, it is used to reduce heavy metal oxides to metals.
(vi) Atomic hydrogen and oxyhydrogen torches are used for cutting and welding purposes.
(vii) It is used as a rocket fuel in space research.
(viii) It is used in fuel cells for generating electricity and is also called fuel of future.
(ix) A mixture of helium and hydrogen (He= 85% and H= 15%) is used in filling the balloons.
Water ( H2O )
A major part of all living organisms is made up of water. Human body has 65% (about) and some plants have as much as 95% water. 97% part of the entire water is assumed to be confined in oceans while rest is present in pure form. Rain water is the purest form of water.
Properties of Water
- It is colourless and tasteless liquid. In pure form, it is neutral. Its pH is 7.
- Pure water is a bad conductor of electricity, however its conductivity can be increased by adding some strong electrolyte.
- At 4°C, water has maximum density and minimum volume.
- At O°C, water solidifies and converts into its solid form, called ice.
- Due to the presence of extensive H-bonding between water molecules, it has high freezing point, high boiling point, high heat of vaporisation, high heat of fusion in comparison to H2S and H2Se and exists in liquid state.
- In comparison to other liquids, water has a higher specific heat, thermal conductivity surface tension,dipole moment and dielectric constant, etc. Due to its high specific heat, it is a good coolant and is used to cool the engines of cars, buses, trucks, etc
- Because of its high dielectric constant, it is an excellent solvent for the transportation of ions and molecules required for animal metabolism.
- Due to H-bonding with polar molecules, even covalent compounds like alcohol and carbohydrates (glucose, sugar) dissolve in water.
Structure of Water Molecule
In the gas phase, water is a bent molecule with bond angle of 104.5° and O H bond length of 95.7 pm. In ice, each oxygen atom is surrounded tetrahedrally by four other oxygen atoms. H-bonding gives ice, a rather open type structure with wide holes. That’s why density of ice is lesser than that of liquid water although ice is a solid and water is a liquid,
Hard Water
Presence of calcium and magnesium salts (mainly) and iron salt in the form of hydrogen carbonate, chloride and sulphate in water makes water hard. Hard water does not give lather with soap. Hard water forms scum/precipitate with soap. It is, therefore, unsuitable for laundry. It is harmful for boilers as well because of deposition of salts in the form of scale.
Soft Water
Rain water is almost pure. The water free from soluble salts of calcium and magnesium, is called soft water. It gives lather with soap.
Temporary Hardness
Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates in water. It can be removed by
Boiling
During boiling, the soluble magnesium hydrogen carbonate is converted into insoluble magnesium hydroxide and calcium hydrogen carbonate is converted into an insoluble calcium carbonate. These insoluble precipitates can be removed by filtration.
Clark’s Method
In this method, calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.
Permanent Hardness
It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.
Permanent hardness can be removed by the following methods
Treatment with Washing Soda Na2CO3 or Sodium Carbonates) Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
Calgon’s Method Sodium hexametaphosphate (Na6P6O18), commercially called ‘calgon’, when added to hard water, complex anions are formed. The complex anion keeps the Mg2+ and Ca 2+ ions in solution.
Ion Exchange Method This method is also called zeolite/permutit process. Hydrated sodium aluminium silicate is called zeolite. When zeolite is added to hard water. exchange reactions take place which results in softening of water.
Synthetic Resins Method Now-a-days hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process.
Heavy Water( D2O)
It was discovered by Urey and Washburn in 1932 and so called because it is an oxide of heavy hydrogen or deuterium. It is also called deuterated water. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries. In 5000 parts of ordinary water, only one part of heavy water exists. It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It is used for the preparation of other deuterium compounds such as CD4, D2SO4, etc. Its density is more than that of ordinary water.
Hydrogen peroxide (H2O2)
it was discovered by Thenard in 1818. În purē state, it is an odourless and almost colourless (very pale blue) liquid. It is miscible with water in all proportions. It is also called oxygenated water. It can be prepared by acidifying barium peroxide with sulphuric acid or by exposing ultraviolet rays on the oxygen in the presence of water vapour. It generally exists in liquid form due to excessive hydrogen bonding. Hydrogen peroxide acts as oxidising as well as reducing agent.
Uses of Hydrogen peroxide
(i) in daily life, it is used as hair bleach and as a mild disinfectant. Its bleaching property is because of its oxidising nature.
(ii) As an antiseptic, it is sold in the market as perhydrol.
(iii) It is used to manufacture chemicals like sodium perborate and percarbonate which are used in high quality detergents.
(iv) It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils, fats, etc. (v) It is used for examining the milk, wine, etc.
(vi) It is used in glazing the old oil paintings and in exposing its original colour.
(vii) It is used in the,synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin), etc.
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