HYDROGEN - NCERT

1 276 CHEMISTRY. UNIT 9. HYDROGEN . d he 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. is After studying this unit, you will be able to present informed opinions on the bl position of HYDROGEN in the periodic table; HYDROGEN has the simplest atomic structure among all the identify the modes of occurrence pu elements around us in Nature. In atomic form it consists and preparation of dihydrogen on of only one proton and one electron. However, in elemental a small and commercial scale;. describe isotopes of HYDROGEN ;. form it exists as a diatomic (H2) molecule and is called explain how different elements dihydrogen .

2 It forms more compounds than any other be T. combine with HYDROGEN to form element. Do you know that the global concern related to ionic, molecular and non- energy can be overcome to a great extent by the use of re o R. stoichiometric compounds; HYDROGEN as a source of energy? In fact, HYDROGEN is of describe how an understanding of great industrial importance as you will learn in this unit. its properties can lead to the production of useful substances, POSITION OF HYDROGEN IN THE PERIODIC. tt E. and new technologies;. understand the structure of water TABLE. and use the knowledge for HYDROGEN is the first element in the periodic table. C. explaining physical and chemical However, its placement in the periodic table has been a properties; subject of discussion in the past.

3 As you know by now explain how environmental water that the elements in the periodic table are arranged no N. quality depends on a variety of dissolved substances; difference according to their electronic configurations. 1. between 'hard' and 'soft' water and HYDROGEN has electronic configuration 1s . On one learn about water softening; hand, its electronic configuration is similar to the outer acquire the knowledge about 1. electronic configuration (ns ) of alkali metals , which belong . heavy water and its importance;. understand the structure of to the first group of the periodic table. On the other hand, 2 5. HYDROGEN peroxide, learn its like halogens (with ns np configuration belonging to the preparatory methods and seventeenth group of the periodic table), it is short by one properties leading to the electron to the corresponding noble gas configuration, 2.

4 Manufacture of useful chemicals helium (1s ). HYDROGEN , therefore, has resemblance to and cleaning of environment;. alkali metals, which lose one electron to form unipositive understand and use certain terms , electron-deficient, electron- ions, as well as with halogens, which gain one electron to precise, electron-rich, HYDROGEN form uninegative ion. Like alkali metals, HYDROGEN forms economy, hydrogenation etc. oxides, halides and sulphides. However, unlike alkali metals, it has a very high ionization enthalpy and does not 276 C:\ChemistryXI\Unit-9\Unit-9(6).pmd , , HYDROGEN 277. possess metallic characteristics under normal solar atmosphere. The giant planets Jupiter conditions.

5 In fact, in terms of ionization and Saturn consist mostly of HYDROGEN . enthalpy, HYDROGEN resembles more However, due to its light nature, it is much less 1. with halogens, i H of Li is 520 kJ mol , F is abundant ( by mass) in the earth's 1. 1680 kJ mol 1 and that of H is 1312 kJ mol . atmosphere. Of course, in the combined form Like halogens, it forms a diatomic molecule, it constitutes of the earth's crust and combines with elements to form hydrides and the oceans. In the combined form besides in a large number of covalent compounds. water, it occurs in plant and animal tissues, However, in terms of reactivity, it is very low as carbohydrates, proteins, hydrides including d compared to halogens.

6 Hydrocarbons and many other compounds. Inspite of the fact that HYDROGEN , to a Isotopes of HYDROGEN he certain extent resembles both with alkali HYDROGEN has three isotopes: protium, 1 H, 1. metals and halogens, it differs from them as 2 3. deuterium, 1H or D and tritium, 1H or T. Can well. Now the pertinent question arises as you guess how these isotopes differ from each where should it be placed in the periodic table? other ? These isotopes differ from one another is Loss of the electron from HYDROGEN atom in respect of the presence of neutrons. Ordinary +. results in nucleus (H ) of ~ 3 pm size. HYDROGEN , protium, has no neutrons, This is extremely small as compared to normal deuterium (also known as heavy HYDROGEN ) has bl atomic and ionic sizes of 50 to 200pm.

7 As a one and tritium has two neutrons in the +. consequence, H does not exist freely and is nucleus. In the year 1934, an American always associated with other atoms or scientist, Harold C. Urey, got Nobel Prize for molecules. Thus, it is unique in behaviour and pu is, therefore, best placed separately in the periodic table (Unit 3). separating HYDROGEN isotope of mass number 2 by physical methods. The predominant form is protium. be T. dihydrogen , H2 Terrestrial HYDROGEN contains of deuterium mostly in the form of HD. The re Occurrence tritium concentration is about one atom per o R. 18. dihydrogen is the most abundant element in 10 atoms of protium.

8 Of these isotopes, only the universe (70% of the total mass of the tritium is radioactive and emits low energy . universe) and is the principal element in the particles (t , years). tt E. Table Atomic and Physical Properties of HYDROGEN Property HYDROGEN Deuterium Tritium C. 15. Relative abundance (%) 10. 1. Relative atomic mass (g mol ) no N. Melting point / K Boiling point/ K 1. Density / gL . 1. Enthalpy of fusion/kJ mol - 1. Enthalpy of vaporization/kJ mol - Enthalpy of bond 1. dissociation/kJ mol at - Internuclear distance/pm - 1. Ionization enthalpy/kJ mol 1312 - - 1. Electron gain enthalpy/kJ mol 73 - - Covalent radius/pm 37 - - . Ionic radius(H )/pm 208.

9 277 C:\ChemistryXI\Unit-9\Unit-9(6).pmd , , 278 CHEMISTRY. Since the isotopes have the same electronic 1270K. C n H2n nH2O nCO (2n 1)H2. configuration, they have almost the same 2 Ni chemical properties. The only difference is in , their rates of reactions, mainly due to their CH4 ( g ) + H2 O ( g ) . 1270K. Ni CO ( g ) + 3H2 ( g ). different enthalpy of bond dissociation (Table ). However, in physical properties these The mixture of CO and H2 is called water isotopes differ considerably due to their large gas. As this mixture of CO and H2 is used for mass differences. the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas d PREPARATION OF dihydrogen , H2 or 'syngas'.

10 Nowadays 'syngas' is produced There are a number of methods for preparing from sewage, saw-dust, scrap wood, he dihydrogen from metals and metal hydrides. newspapers etc. The process of producing 'syngas' from coal is called 'coal gasification'. Laboratory Preparation of dihydrogen C ( s ) + H2O ( g ) . 1270K. CO ( g ) + H2 ( g ). (i) It is usually prepared by the reaction of The production of dihydrogen can be is granulated zinc with dilute hydrochloric increased by reacting carbon monoxide of acid. syngas mixtures with steam in the presence of + 2+. Zn + 2H Zn + H2 iron chromate as catalyst. bl (ii) It can also be prepared by the reaction of zinc with aqueous alkali.