Hydrogen
1. Position of Hydrogen in the Periodic Table
- Hydrogen 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. However, in elemental form it exists as a diatomic (\[{{H}_{2}}\]) molecule and is called dihydrogen. It forms more compounds than any other element. In the global concern related to energy can be overcome to a great extent by the use of hydrogen as a source of energy.
- Hydrogen has electronic configuration \[1{{s}^{1}}\]. On one hand, its electronic configuration is similar to the outer electronic configuration (\[n{{s}^{1}}\]) of alkali metals, which belong to the first group of the periodic table. On the other hand, like halogens (with \[n{{s}^{2}}n{{p}^{5}}\] configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (\[1{{s}^{2}}\]).
- Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to form unipositive ions, as well as with halogens, which gain one electron to form uninegative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides. However, unlike alkali metals, it has a very high ionization enthalpy and does not possess metallic characteristics under normal conditions. In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens.
- Like halogens, it forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. However, in terms of reactivity, it is very low as compared to halogens.
- Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well.
- Dihydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water.
- Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee).
2. Dihydrogen (\[{{H}_{2}}\])
- Dihydrogen is the most abundant element in the universe (70 per cent of the total mass of the universe) and is the principal element in the solar atmosphere.
- The giant planets Jupiter and Saturn consist mostly of hydrogen. However, due to its light nature, it is much less abundant (\[0.15\]per cent by mass) in the earth's atmosphere.
- In the combined form it constitutes \[15.4\]per cent of the earth's crust and the oceans.
- In the combined form besides in water, it occurs in plant and animal tissues, carbohydrates, proteins, hydrides including hydrocarbons and many other compounds.
3. Isotopes of Hydrogen
- Hydrogen has three isotopes: protium, \[_{1}{{H}^{1}}\], deuterium, \[_{1}{{H}^{2}}\] or D and tritium, \[_{1}{{H}^{3}}\] or T Can we guess how these isotopes differ from each other? 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.
- In the year 1934, an American scientist, Harold C. Urey, got Nobel Prize for separating hydrogen isotope of mass number 2 by physical methods.
- The predominant form is protium. Terrestrial hydrogen contains \[0.0156\]per cent of deuterium mostly in the form of HD. The tritium concentration is about one atom per \[{{10}^{18}}\] atoms of protium. Of these isotopes, only tritium is radioactive and emits low7 energy (\[{{\beta }^{\,\,-}}\]particles (\[{{t}_{1/2,\,\,\,\,\,}}12.33\] years).
- Since the isotopes have the same electronic configuration, they have almost the same chemical properties. The only difference is in their rates of reactions, mainly due to their different enthalpy of bond dissociation. However, in physical properties these isotopes differ considerably due to their large mass differences.
4. Commercial Production of Dihydrogen
- The mixture of \[CO\]and \[{{H}_{2}}\] is called water gas. As this mixture of \[CO\] and \[{{H}_{2}}\] is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or 'syngas'. Nowadays 'syngas* is produced from sewage, saw-dust, scrap wood, newspapers etc. The process of producing 'syngas' from coal is called 'coal gasification'.
- The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.
- Presently \[\tilde{\ }77\]per cent of the industrial dihydrogen is produced from petro-chemicals, 18 percent from coal, 4 per cent from electrolysis of aqueous solutions and 1 per cent from other sources.
5. Uses of Dihydrogen
- The largest single use of dihydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.
- Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of poly- unsaturated vegetable oils like soyabean, cotton seeds etc. It is used in the manufacture of bulk organic chemicals, particularly methanol. It is widely used for the manufacture of metal hydrides.
- It is used for the preparation of hydrogen chloride, a highly useful chemical. In metallurgical processes, it is used to reduce heavy metal oxides to metals.
- Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. Atomic hydrogen atoms (produced by dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of \[4000\] K. It is used as a rocket fuel in space research.
- Dihydrogen is used in fuel cells for generating electrical energy. It has many advantages over the conventional fossil fuels and electric power. It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and other fuels.
6. Water
- A major part of all living organisms is made up of water. Human body has about 65 per cent and some plants have as much as 95 per cent water. It is a crucial compound for the survival of all life forms. It is a solvent of great importance. The distribution of water over the earth's surface is not uniform.
- Estimated World Water Supply in Oceans \[\left( 97.33 \right),\]Saline lakes and inland seas \[\left( 0.008 \right),\]Polar ice and glaciers \[\left( 2.04 \right),\]Ground water \[\left( 0.61 \right),\]Lakes \[\left( 0.009 \right),\]Soil moisture \[\left( 0.005 \right),\]Atmospheric water vapour \[\left( 0.001 \right),\]Rivers \[\left( 0.0001 \right).\]
- The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence of extensive hydrogen bonding between water molecules. This leads to high freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to \[{{H}_{2}}S\] and \[{{H}_{2}}Se\]. In comparison to other liquids, water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc. These properties allow water to play a key role in the biosphere.
- The high heat of vaporisation and heat capacity are responsible for moderation of the climate and body temperature of living beings. It is an excellent solvent for transportation of ions and molecules required for plant and animal metabolism. Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol and carbohydrates dissolve in water.
- The crystalline form of water is ice. At atmospheric pressure ice crystallises in the hexagonal form, but at very low temperatures it condenses to cubic form. Density of ice is less than that of water. Therefore, an ice cube floats on water. In winter season ice formed on the surface of a lake provides thermal insulation which ensures the survival of the aquatic life. This fact is of great ecological significance.
- Ice has a highly ordered three dimensional hydrogen bonded structure.
- Water reacts with a large number of substances. Water has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance.
- Water can be easily reduced to dihydrogen by highly electropositive metals. Water is oxidised to 0^ during photosynthesis.
- Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water. From aqueous solutions many salts can be crystallised as hydrated salts.
7. Hard and Soft Water
- Rain water is almost pure (may contain some dissolved gases from the atmosphere).
- Being a good solvent, when it flows on the surface of the earth, it dissolves many salts. Presence of calcium and magnesium salts in the form of hydrogencarbonate, chloride and sulphate in water makes water 'hard’. Hard water does not give lather with soap.
- Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.
- Hard water forms scum/precipitate with soap. Soap containing sodium stearate (\[{{C}_{17}}{{H}_{35}}COONa\]) reacts with hard water to precipitate out \[Ca/Mg\]stearate.
- It is, therefore, unsuitable for laundry. It is harmful for boilers as well, because of deposition of salts in the form of scale. This reduces the efficiency of the boiler. The hardness of water is of two types: temporary hardness and permanent hardness.
- Temporary hardness is due to the presence of magnesium and calcium hydrogencarbonates. It can be removed by :
- During boiling, the soluble \[Mg{{(HC{{O}_{3}})}_{2}}\] is converted into insoluble \[Mg{{(OH)}_{2}}\] and \[Ca{{(HC{{O}_{3}})}_{2}}\] is changed to insoluble \[CaC{{o}_{3}}\]. It is because of high solubility product of \[Mg{{(OH)}_{2}}\] as compared to that of \[Mg{{(CO)}_{3}}\], that \[Mg{{(OH)}_{2}}\]is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.
- In Clark's 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 is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness is not removed by boiling. It can be removed by the following methods :
- Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
- Calgon’s method: Sodium hexametaphosphate (\[N{{a}_{6}}{{P}_{6}}{{O}_{18}}\]), commercially called 'calgon'.
- Ion-exchange method: This method is also called zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit. For the sake of simplicity, sodium aluminium silicate (\[NaAISi{{O}_{4}}\]) can be written as\[NaZ\]. When this is added in hard water, exchange reactions take place. Permutit/zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution.
- Synthetic resins method: Nowadays hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process. Cation exchange resins contain large organic molecule with - \[S{{O}_{3}}H\]group and are water insoluble. Ion exchange resin (\[RS{{O}_{3}}H\]) is changed to RNa by treating it with NaCl. The resin exchanges Na+ ions with \[C{{a}^{2+}}\] and \[M{{g}^{2+}}\] ions present in hard water to make the water soft. Here R is resin anion. The resin can be regenerated by adding aqueous NaCl solution.
8. Hydrogen Peroxide (\[{{H}_{2}}{{O}_{2}}\])
- Hydrogen peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents. In the pure state \[{{H}_{2}}{{O}_{2}}\] is an almost colourless (very pale blue) liquid. Hydrogen peroxide has a non-planar structure.
- It acts as an oxidising as well as reducing agent in both acidic and alkaline media.
- \[{{H}_{2}}{{O}_{2}}\] decomposes slowly on exposure to light. \[2{{H}_{2}}{{O}_{2}}(1)\to 2{{H}_{2}}O(1)+{{O}_{2}}(g).\] In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalysed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser. It is kept away from dust because dust can induce explosive decomposition of the compound.
- In daily life it is used as a hair bleach and as a mild disinfectant. As an antiseptic it is sold in the market as perhydrol. It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.
- It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc. It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils, fats, etc.
- Nowadays it is also used in Environmental (Green) Chemistry. For example, in pollution control treatment of domestic and industrial effluents, oxidation of cyanides, restoration of aerobic conditions to sewage wastes, etc.
9. Dihydrogen as a Fuel
- Dihydrogen releases large quantities of heat on combustion.
- The data on energy released by combustion of fuels like dihydrogen, methane, LPG etc. are compared in terms of the same amounts in mole, mass and volume. On a mass for mass basis dihydrogen can release more energy than petrol (about three times). Moreover, pollutants in combustion of dihydrogen will be less than petrol. The only pollutants will be the oxides of dinitrogen (due to the presence of dinitrogen as impurity with dihydrogen).
- This, of course, can be minimised by injecting a small amount of water into the cylinder to lower the temperature so that the reaction between dinitrogen and dioxygen may not take place. However, the mass of the containers in which dihydrogen will be kept must be taken into consideration.
- A cylinder of compressed dihydrogen weighs about 30 times as much as a tank of petrol containing the same amount of energy. Also, dihydrogen gas is converted into liquid state by cooling to 20 K. This would require expensive insulated tanks. Tanks of metal alloy like \[NaN{{i}_{5,}}Ti-Ti{{H}_{2,}}Mg-Mg{{H}_{2}}\] etc. are in use for storage of dihydrogen in small quantities. These limitations have prompted researchers to search for alternative techniques to use dihydrogen in an efficient way.
- In this view Hydrogen Economy is an alternative. The basic principle of hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous dihydrogen. Advantage of hydrogen economy is that energy is transmitted in the form of dihydrogen and not as electric power. It is for the first time in the history of India that a pilot project using dihydrogen as fuel was launched in October, 2005 for running automobiles. Initially 5 per cent dihydrogen has been mixed in CNG for use in four-wheeler vehicles. The percentage of dihydrogen would be gradually increased to reach the optimum level.
Note - Heavy Water (\[{{D}_{2}}O\]) is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.
