Hydrogen
“Hydrogen, the most abundant element in the universe and the third most abundant on the surface of the globe, is being visualised as the major future source of energy.”
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
9.1 POSITION OF HYDROGEN IN THE PERIODIC TABLE
Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. As you know by now that the elements in the periodic table are arranged according to their electronic configurations.
Hydrogen has electronic configuration
Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well. Now the pertinent question arises as where should it be placed in the periodic table? Loss of the electron from hydrogen atom results in nucleus
9.2 DIHYDROGEN,
9.2.1 Occurrence
Dihydrogen is the most abundant element in the universe (
9.2.2 Isotopes of Hydrogen
Hydrogen has three isotopes: protium,
The predominant form is protium. Terrestrial hydrogen contains
Table 9.1 Atomic and Physical Properties of Hydrogen
Property | Hydrogen | Deuterium | Tritium |
---|---|---|---|
Relative abundance (%) | 99.985 | 0.0156 | |
Relative atomic mass |
1.008 | 2.014 | 3.016 |
Melting point / K | 13.96 | 18.73 | 20.62 |
Boiling point/ K | 20.39 | 23.67 | 25.0 |
Density / gL | 0.09 | 0.18 | 0.27 |
Enthalpy of fusion |
0.117 | 0.197 | - |
Enthalpy of vaporization |
0.904 | 1.226 | - |
Enthalpy of bond dissociation |
435.88 | 443.35 | - |
Internuclear distance |
74.14 | 74.14 | - |
Ionization enthalpy |
1312 | - | - |
Electron gain enthalpy |
-73 | - | - |
Covalent radius |
37 | - | |
Ionic radius |
208 |
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 (Table 9.1). However, in physical properties these isotopes differ considerably due to their large mass differences.
9.3 PREPARATION OF DIHYDROGEN,
There are a number of methods for preparing dihydrogen from metals and metal hydrides.
9.3.1 Laboratory Preparation of Dihydrogen
(i) It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.
(ii) It can also be prepared by the reaction of zinc with aqueous alkali.
9.3.2 Commercial Production of Dihydrogen below:
The commonly used processes are outlined below:
(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.
(ii) High purity (>99.95\%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
(iii) It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:
at anode:
at cathode:
The overall reaction is
(iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.
e.g.,
The mixture of
The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.
This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.
Presently
9.4 PROPERTIES OF DIHYDROGEN
9.4.1 Physical Properties
Dihydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water. Its other physical properties alongwith those of deuterium are given in Table 9.1.
9.4.2 Chemical Properties
The chemical behaviour of dihydrogen (and for that matter any molecule) is determined, to a large extent, by bond dissociation enthalpy. The
The chemistry of dihydrogen can be illustrated by the following reactions:
Reaction with halogens: It reacts with halogens,
While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.
Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic.
Reaction with dinitrogen: With dinitrogen it forms ammonia.
This is the method for the manufacture of ammonia by the Haber process.
Reactions with metals: With many metals it combines at a high temperature to yield the corresponding hydrides (section 9.5)
where
Reactions 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.
Reactions with organic compounds: It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example : (i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee)
(ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.
9.4.3 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 polyunsaturated 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 (Section 9.5)
- 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
. - 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.
9.5 HYDRIDES
Dihydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary compounds, called hydrides. If ’
The hydrides are classified into three categories :
(i) Ionic or saline or saltlike hydrides
(ii) Covalent or molecular hydrides
(iii) Metallic or non-stoichiometric hydrides
9.5.1 Ionic or Saline Hydrides
These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character. However, significant covalent character is found in the lighter metal hydrides such as
Saline hydrides react violently with water producing dihydrogen gas.
Lithium hydride is rather unreactive at moderate temperatures with
9.5.2 Covalent or Molecular Hydride
Dihydrogen forms molecular compounds with most of the
Molecular hydrides are further classified according to the relative numbers of electrons and bonds in their Lewis structure into :
(i) electron-deficient, (ii) electron-precise, and (iii) electron-rich hydrides.
An electron-deficient hydride, as the name suggests, has too few electrons for writing its conventional Lewis structure. Diborane
Electron-precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (e.g.,
Electron-rich hydrides have excess electrons which are present as lone pairs. Elements of group 15-17 form such compounds.
9.5.3 Metallic or Non-stoichiometric (or Interstitial) Hydrides
These are formed by many
Earlier it was thought that in these hydrides, hydrogen occupies interstices in the metal lattice producing distortion without any change in its type. Consequently, they were termed as interstitial hydrides. However, recent studies have shown that except for hydrides of Ni, Pd, Ce and Ac, other hydrides of this class have lattice different from that of the parent metal. The property of absorption of hydrogen on transition metals is widely used in catalytic reduction / hydrogenation reactions for the preparation of large number of compounds. Some of the metals (e.g., Pd, Pt) can accommodate a very large volume of hydrogen and, therefore, can be used as its storage media. This property has high potential for hydrogen storage and as a source of energy.
9.6 WATER
A major part of all living organisms is made up of water. Human body has about
Table 9.2 Estimated World Water Supply
Source | % of Total |
---|---|
Oceans | 97.33 |
Saline lakes and inland seas | 0.008 |
Polar ice and glaciers | 2.04 |
Ground water | 0.61 |
Lakes | 0.009 |
Soil moisture | 0.005 |
Atmospheric water vapour | 0.001 |
Rivers | 0.0001 |
9.6.1 Physical Properties of Water
It is a colourless and tasteless liquid. Its physical properties are given in Table 9.3 along with the physical properties of heavy water.
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
Table 9.3 Physical Properties of
Property | ||
---|---|---|
Molecular mass |
18.0151 | 20.0276 |
Melting point/K | 273.0 | 276.8 |
Boiling point/K | 373.0 | 374.4 |
Enthalpy of formation |
-285.9 | -294.6 |
Enthalpy of vaporisation |
40.66 | 41.61 |
Enthalpy of fusion |
6.01 | - |
Temp of max. density/K | 276.98 | 284.2 |
Density |
1.0000 | 1.1059 |
Viscosity/centipoise | 0.8903 | 1.107 |
Dielectric constant/C |
78.39 | 78.06 |
Electrical conductivity |
- |
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.
9.6.2 Structure of Water
In the gas phase water is a bent molecule with a bond angle of
polar molecule, (Fig 9.1(b)). Its orbital overlap picture is shown in Fig. 9.1(c). In the liquid phase water molecules are associated together by hydrogen bonds.
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.
9.6.3 Structure of Ice
Ice has a highly ordered three dimensional hydrogen bonded structure as shown in Fig. 9.2. Examination of ice crystals with
X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of
Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.
9.6.4 Chemical Properties of Water
Water reacts with a large number of substances. Some of the important reactions are given below.
(1) Amphoteric Nature: It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the Brönsted sense it acts as an acid with
The auto-protolysis (self-ionization) of water takes place as follows :
(2) Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive metals.
Thus, it is a great source of dihydrogen.
Water is oxidised to
With fluorine also it is oxidised to
(3) Hydrolysis Reaction: 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.
(4) Hydrates Formation: From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz.,
(i) coordinated water e.g.,
(ii) interstitial water e.g.,
(iii) hydrogen-bonded water e.g.,
9.6.5 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
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: (i) temporary hardness, and (ii) permanent hardness.
9.6.6 Temporary Hardness
Temporary hardness is due to the presence of magnesium and calcium hydrogencarbonates. It can be removed by :
(i) Boiling: During boiling, the soluble
(ii) 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.
9.6.7 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 is not removed by boiling. It can be removed by the following methods:
(i) Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
(ii) Calgon’s method: Sodium hexametaphosphate
The complex anion keeps the
(iii) 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
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.
(iv) 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 -
The resin can be regenerated by adding aqueous
Pure de-mineralised (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the
In this cation exchange process,
The exhausted cation and anion exchange resin beds are regenerated by treatment with dilute acid and alkali solutions respectively.
9.7 HYDROGEN PEROXIDE
Hydrogen peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents.
9.7.1 Preparation
It can be prepared by the following methods.
(i) Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.
(ii) Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.
This method is now used for the laboratory preparation of
(iii) Industrially it is prepared by the autooxidation of 2-alklylanthraquinols.
In this case
9.7.2 Physical Properties
In the pure state
Table 9.4 Physical Properties of Hydrogen Peroxide
Melting point |
272.4 | Density (liquid at |
1.44 |
---|---|---|---|
Boiling point(exrapolated) |
423 | Viscosity |
1.25 |
Vapour pressure |
1.9 | Dielectric constant |
70.7 |
Density (solid at |
1.64 | Electrical conductivity |
9.7.3 Structure
Hydrogen peroxide has a non-planar structure. The molecular dimensions in the gas phase and solid phase are shown in Fig 9.3
9.7.4 Chemical Properties
It acts as an oxidising as well as reducing agent in both acidic and alkaline media. Simple reactions are described below.
(i) Oxidising action in acidic medium
(ii) Reducing action in acidic medium
(iii) Oxidising action in basic medium
(iv) Reducing action in basic medium
9.7.5 Storage
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.
9.7.6 Uses
Its wide scale use has led to tremendous increase in the industrial production of
(i) 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.
(ii) It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.
(iii) It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc.
(iv) It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils, fats, etc.
(v) 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.
9.8 HEAVY WATER,
It 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. Its physical properties are given in Table 9.3. It is used for the preparation of other deuterium compounds, for example:
9.9 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, are shown in Table 9.5.
From this table it is clear that 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
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
Nowadays, it is also used in fuel cells for generation of electric power. It is expected that economically viable and safe sources of dihydrogen will be identified in the years to come, for its usage as a common source of energy.
Table 9.5 The Energy Released by Combustion of Various Fuels in Moles, Mass and Volume
Energy released on combustion in kJ state |
Dihydrogen (in gaseous state) |
Dihydrogen (in liquid) |
LPG | Octane (in liquid state) |
|
---|---|---|---|---|---|
per mole | 286 | 285 | 2220 | 880 | 5511 |
per gram | 143 | 142 | 50 | 53 | 47 |
per litre | 12 | 9968 | 25590 | 35 | 34005 |
Summary
Hydrogen is the lightest atom with only one electron. Loss of this electron results in an elementary particle, the proton. Thus, it is unique in character. It has three isotopes, namely : protium
Hydrogen is the most abundant element in the universe. In the free state it is almost not found in the earth’s atmosphere. However, in the combined state, it is the third most abundant element on the earth’s surface.
Dihydrogen on the industrial scale is prepared by the water-gas shift reaction from petrochemicals. It is obtained as a byproduct by the electrolysis of brine.
The
Though dihydrogen is rather inactive at room temperature because of very high negative dissociation enthalpy, it combines with almost all the elements under appropriate conditions to form hydrides. All the type of hydrides can be classified into three categories: ionic or saline hydrides, covalent or molecular hydrides and metallic or non-stoichiometric hydrides. Alkali metal hydrides are good reagents for preparing other hydride compounds. Molecular hydrides (e.g.,
Among the other chemical reactions of dihydrogen, reducing reactions leading to the formation hydrogen halides, water, ammonia, methanol, vanaspati ghee, etc. are of great importance. In metallurgical process, it is used to reduce metal oxides. In space programmes, it is used as a rocket fuel. In fact, it has promising potential for use as a non-polluting fuel of the near future (Hydrogen Economy).
Water is the most common and abundantly available substance. It is of a great chemical and biological significance. The ease with which water is transformed from liquid to solid and to gaseous state allows it to play a vital role in the biosphere. The water molecule is highly polar in nature due to its bent structure. This property leads to hydrogen bonding which is the maximum in ice and least in water vapour. The polar nature of water makes it: (a) a very good solvent for ionic and partially ionic compounds; (b) to act as an amphoteric (acid as well as base) substance; and (c) to form hydrates of different types. Its property to dissolve many salts, particularly in large quantity, makes it hard and hazardous for industrial use. Both temporary and permanent hardness can be removed by the use of zeolites, and synthetic ion-exchangers.
Heavy water,
Hydrogen peroxide,