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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , Hydrogen, , , , , , , 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 (H2) molecule and is called, dihydrogen., It forms more compounds than any other element. Do you know that the global, concern related to energy can be overcome to a great extent by the use of hydrogen as, a source of energy?, In fact, hydrogen is of great industrial importance as you will learn in this unit., , 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 1s1., On one hand, its electronic configuration is similar to the outer electronic, configuration (ns1) of alkali metals, which belong to the first group of the periodic, table., On the other hand, like halogens (with ns2np5 configuration belonging to the, seventeenth group of the periodic table), it is short by one electron to the, corresponding noble gas configuration, helium (1s2)., Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to, form uni-positive 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 notpossess metallic, characteristics under normal conditions., In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens, ∆i H, of Li is 520 kJ mol–1, F is 1680 kJ mol–1 and that of H is 1312 kJ mol–1., 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., 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 (H+ ) of ~ 1.5×10–3 pm, size., This is extremely small as compared to normal atomic and ionic sizes of 50 to 200, pm. As a consequence, H+ does not exist freely and is always associated with other, atoms or molecules., , Tel +91 84330 83279, 98376 03674., , Page 1

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Chemistry-Mojo, Chapter-wise Theory, , , Simplifying Chemistry, , Thus, it is unique in behaviour and is, therefore, best placed separately in the periodic, table ., , DI-HYDROGEN (H2), Occurrence,  Di-hydrogen is the most abundant element in the universe 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% by mass) in the, earth’s atmosphere. Of course, in the combined form it constitutes 15.4% 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., Isotopes of Hydrogen, Hydrogen has three isotopes:, protium, deuterium, and tritium ., , , , , , , , , , , 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., Terrestrial hydrogen contains 0.0156% of deuterium., The tritium concentration is very low., Of these isotopes, only tritium is radioactive and emits low energy β – particles (t½,, 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., , PREPARATION OF DIHYDROGEN (H2), There are a number of methods for preparing dihydrogen from metals and metal hydrides., Laboratory Preparation of Dihydrogen, (i) It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid., Zn + 2H+ → Zn2+ + H2, (ii) It can also be prepared by the reaction of zinc with aqueous alkali., Zn + 2NaOH → Na2ZnO2 + H2, Sodium zincate, , Tel +91 84330 83279, 98376 03674., , Page 2

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , Commercial Production of Di-hydrogen, The commonly used processes are out-lined 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 by product 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:, , 2Cl– (aq) → Cl2(g) + 2e–, 2H2O (l) + 2e– → H2(g) + 2OH– (aq), , The overall reaction is :, 2Na+ (aq) + 2Cl– (aq) + 2H2O(l) → Cl2(g) + H2(g) + 2Na+ (aq) + 2OH– (aq), (iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of, catalyst yields hydrogen., CH4 + H2O (g) →, , CO (g) + 3H2(g), , The mixture of CO and H2 is called water gas., , , As this mixture of CO and H2 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'., C(s) + H2O (g) →, , , , CO(g) + H2(g), , The production of di-hydrogen can be increased by reacting carbon monoxide of, syngas mixtures with steam in the presence of iron chromate as catalyst., CO (g) + H2O (g) →, , CO2 (g) + H2 (g), , This is called water-gas shift reaction., Carbon dioxide is removed by scrubbing with sodium arsenite solution., , Tel +91 84330 83279, 98376 03674., , Page 3

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , Presently ~77% of the industrial di-hydrogen is produced from petro-chemicals, 18% from, coal, 4% from electrolysis of aqueous solutions and 1% from other sources., , Physical Properties, 1. Dihydrogen is a, a. colourless, d. combustible gas., 2. It is lighter than air., 3. Insoluble in water., , b. odourless, , c. tasteless, , Chemical Properties, , , The chemical behaviour of di-hydrogen (and for that matter any molecule) is, determined, to a large extent, by bond dissociation enthalpy., , , , The H–H bond dissociation enthalpy is the highest for a single bond between two, atoms of any element., , , , It is because of this factor that the dissociation of di-hydrogen into its atoms is only, ~0.081% around 2000K which increases to 95.5% at 5000K., , , , It is relatively inert at room temperature due to the high H–H bond enthalpy., , , , Thus, the atomic hydrogen is produced at a high temperature in an electric arc or, under ultraviolet radiations., , , , Since its orbital is incomplete with 1s1 electronic configuration, it does combine with, almost all the elements., , , , It accomplishes reactions by, (i) Loss of the only electron to give H+., (ii) Gain of an electron to form H–., (iii) Sharing electrons to form a single covalent bond., , The chemistry of di-hydrogen can be illustrated by the following reactions:, 1. Reaction with halogens:, It reacts with halogens, X2 to give hydrogen halides, HX,, While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst., 2. Reaction with dioxygen:, It reacts with dioxygen to form water. The reaction is highly exothermic., 2H2(g) + O2(g) → 2H2O(l), ∆H = –285.9 kJ mol–1, Tel +91 84330 83279, 98376 03674., , Page 4

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , 3. Reaction with dinitrogen:, With dinitrogen it forms ammonia., , This is the method for the manufacture of ammonia by the Haber process., 4. Reactions with metals:, With many metals it combines at a high temperature to yield the corresponding hydrides, H2(g) +2M(g) → 2MH(s);, where M is an alkali metal., 5. 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., , 6. 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., , 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.,  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, Tel +91 84330 83279, 98376 03674., , Page 5

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Chemistry-Mojo, Chapter-wise Theory, , , , , Simplifying Chemistry, , purposes., Atomic hydrogen atoms (produced by dissociation of di-hydrogen 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., Di-hydrogen 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., , WATER, , , , , , , A major part of all living organisms is made up of water., Human body has about 65% and some plants have as much as 95% 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., , Physical Properties of Water, , , , , , , , , , It is a colourless and tasteless liquid., 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 H2S and H2Se., 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., , Structure of Water, , , , , , , , , In the gas phase water is a bent molecule with a bond angle of 104.5°, and O–H bond, length of 95.7 pm ., It is a highly polar molecule., 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., , Tel +91 84330 83279, 98376 03674., , Page 6

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , In winter season ice formed on the surface of a lake provides thermal insulation which, ensures the survival of the aquatic life., 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 NH3 and a base with H2S., , The auto-protolysis (self-ionization) of water takes place as follows :, , (2) Redox Reactions Involving Water:, Water can be easily reduced to di-hydrogen by highly electropositive metals., , Thus, it is a great source of dihydrogen., Water is oxidised to O2 during photosynthesis., , With fluorine also it is oxidised to O2., , (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., , Tel +91 84330 83279, 98376 03674., , Page 7

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , (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., Q. How many hydrogen-bonded water molecule(s) are associated in CuSO4.5H2O?, Ans. Only one water molecule, which is outside the brackets (coordination sphere), is, hydrogen-bonded. The other four molecules of water are coordinated., , 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 hydrogen carbonate, 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 (C17H35COONa) 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:, (i) temporary hardness, and (ii) permanent hardness., , Temporary Hardness,  Temporary hardness is due to the presence of magnesium and calcium hydrogen, carbonates.,  It can be removed by :, (i) Boiling: During boiling, the soluble Mg(HCO3)2 is converted into insoluble, Mg(OH)2 and Ca(HCO3)2 is changed to insoluble CaCO3.,  It is because of high solubility product of Mg(OH)2 as compared to that of MgCO3,, that Mg(OH)2 is precipitated.,  These precipitates can be removed by filtration. Filtrate thus obtained will be soft, water., , Tel +91 84330 83279, 98376 03674., , Page 8

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , (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., 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 hexa-metaphosphate (Na6P6O18), commercially, called ‘calgon’, when added to hard water, the following reactions take place., , (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 (NaAlSiO4) can be written as NaZ., When this is added in hard water, exchange reactions take place., , Permutit/zeolite is said to be exhaustedwhen all the sodium in it is used up. It is regenerated, for further use by treating with an aqueous sodium chloride solution., , Tel +91 84330 83279, 98376 03674., , Page 9

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , HYDROGEN PEROXIDE (H2O2), Hydrogen peroxide is an important chemical used in pollution control treatment of domestic, and industrial effluents., Preparation, It can be prepared by the following methods., Acidifying barium peroxide and removing excess water by evaporation under reduced, pressure gives hydrogen peroxide., , Physical Properties,  In the pure state H2O2 is an almost colourless (very pale blue) liquid.,  H2O2 is miscible with water in all proportions and forms a hydrate H2O2.H2O, (m.p = 221K).,  A 30% solution of H2O2 is marketed as ‘100 volume’ hydrogen peroxide. It means, that one millilitre of 30% H2O2 solution will give 100 mL of oxygen at STP.,  Commercially marketed sample is 10 V, which means that the sample contains 3%, H2O2., Structure,  Hydrogen peroxide has a non-planar structure.,  The molecular dimensions in the gas phase and solid phase are shown below., , Chemical Properties, It acts as an oxidising as well as reducing agent in both acidic and alkaline media., a. Reaction with KI, , b. Reaction with PbS, , c. Reaction with acidified FeSO4, , Tel +91 84330 83279, 98376 03674., , Page 10

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Chemistry-Mojo, Chapter-wise Theory, , Simplifying Chemistry, , d. Reaction with Chlorine :, , Strength of Hydrogen peroxide, , Tel +91 84330 83279, 98376 03674., , Page 11