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HYDROGEN - NCERT

284 CHEMISTRYUNIT 9 After studying this unit, you will beable to present informed opinions on theposition of HYDROGEN in theperiodic table; identify the modes of occurrenceand preparation of dihydrogen ona small and commercial scale;describe isotopes of HYDROGEN ; explain how different elementscombine with HYDROGEN to formionic, molecular and non-stoichiometric compounds; describe how an understanding ofits properties can lead to theproduction of useful substances,and new technologies; understand the structure of waterand use the knowledge forexplaining physical and chemicalproperties; explain how environmental waterquality depends on a variety ofdissolved substances; differencebetween 'hard' and 'soft' water andlearn about water softening; acquire the knowledge aboutheavy water and its importance; understand the structure ofhydrogen peroxide, learn itspreparatory methods andproperties leading to themanufacture of useful chemicalsand cleaning of environment; understand and use certain , electron-deficient, electron-precise, el

solutions and 1% from other sources. 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 ...

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Transcription of HYDROGEN - NCERT

1 284 CHEMISTRYUNIT 9 After studying this unit, you will beable to present informed opinions on theposition of HYDROGEN in theperiodic table; identify the modes of occurrenceand preparation of dihydrogen ona small and commercial scale;describe isotopes of HYDROGEN ; explain how different elementscombine with HYDROGEN to formionic, molecular and non-stoichiometric compounds; describe how an understanding ofits properties can lead to theproduction of useful substances,and new technologies; understand the structure of waterand use the knowledge forexplaining physical and chemicalproperties; explain how environmental waterquality depends on a variety ofdissolved substances; differencebetween 'hard' and 'soft' water andlearn about water softening; acquire the knowledge aboutheavy water and its importance; understand the structure ofhydrogen peroxide, learn itspreparatory methods andproperties leading to themanufacture of useful chemicalsand cleaning of environment; understand and use certain , electron-deficient, electron-precise, electron-rich, hydrogeneconomy, hydrogenation , the most abundant element in the universe and thethird most abundant on the surface of the globe, is beingvisualised as the major future source of has the simplest atomic structure among all theelements around us in Nature.

2 In atomic form it consistsof only one proton and one electron. However, in elementalform it exists as a diatomic (H2) molecule and is calleddihydrogen. It forms more compounds than any otherelement. Do you know that the global concern related toenergy can be overcome to a great extent by the use ofhydrogen as a source of energy? In fact, HYDROGEN is ofgreat industrial importance as you will learn in this OF HYDROGEN IN THE PERIODICTABLEH ydrogen is the first element in the periodic , its placement in the periodic table has been asubject of discussion in the past. As you know by nowthat the elements in the periodic table are arrangedaccording to their electronic has electronic configuration 1s1. On onehand, its electronic configuration is similar to the outerelectronic configuration (ns1) of alkali metals , which belongto the first group of the periodic table.

3 On the other hand,like halogens (with ns2np5 configuration belonging to theseventeenth group of the periodic table), it is short by oneelectron to the corresponding noble gas configuration,helium (1s2). HYDROGEN , therefore, has resemblance toalkali metals, which lose one electron to form unipositiveions, as well as with halogens, which gain one electron toform uninegative ion. Like alkali metals, HYDROGEN formsoxides, halides and sulphides. However, unlike alkalimetals, it has a very high ionization enthalpy and does not2019-20285 HYDROGEN possess metallic characteristics under normalconditions. In fact, in terms of ionizationenthalpy, HYDROGEN resembles morewith halogens, i H of Li is 520 kJ mol 1, F is1680 kJ mol 1 and that of H is 1312 kJ mol halogens, it forms a diatomic molecule,combines with elements to form hydrides anda large number of covalent , in terms of reactivity, it is very low ascompared to of the fact that HYDROGEN , to acertain extent resembles both with alkalimetals and halogens, it differs from them aswell.

4 Now the pertinent question arises aswhere should it be placed in the periodic table?Loss of the electron from HYDROGEN atomresults in nucleus (H+) of ~ 10 3 pm is extremely small as compared to normalatomic and ionic sizes of 50 to 200pm. As aconsequence, H+ does not exist freely and isalways associated with other atoms ormolecules. Thus, it is unique in behaviour andis, therefore, best placed separately in theperiodic table (Unit 3). , is the most abundant element inthe universe (70% of the total mass of theuniverse) and is the principal element in thePropertyHydrogenDeuteriumTritiumRelat ive abundance (%) 15 Relative atomic mass (g mol 1) point / point/ / gL of fusion/kJ mol of vaporization/kJ mol of bonddissociation/kJ mol 1 at enthalpy/kJ mol 11312--Electron gain enthalpy/kJ mol 1 73--Covalent radius/pm37--Ionic radius(H )/pm208solar atmosphere.

5 The giant planets Jupiterand Saturn consist mostly of , due to its light nature, it is much lessabundant ( by mass) in the earth satmosphere. Of course, in the combined formit constitutes of the earth's crust andthe oceans. In the combined form besides inwater, it occurs in plant and animal tissues,carbohydrates, proteins, hydrides includinghydrocarbons and many other of HydrogenHydrogen has three isotopes: protium, 11H,deuterium, 21H or D and tritium,31H or T. Canyou guess how these isotopes differ from eachother ? These isotopes differ from one anotherin respect of the presence of neutrons. Ordinaryhydrogen, protium, has no neutrons,deuterium (also known as heavy HYDROGEN ) hasone and tritium has two neutrons in thenucleus.

6 In the year 1934, an Americanscientist, Harold C. Urey, got Nobel Prize forseparating HYDROGEN isotope of mass number2 by physical predominant form is HYDROGEN contains ofdeuterium mostly in the form of HD. Thetritium concentration is about one atom per1018 atoms of protium. Of these isotopes, onlytritium is radioactive and emits low energy particles (t , years).Table Atomic and Physical Properties of Hydrogen2019-20286 CHEMISTRYS ince the isotopes have the same electronicconfiguration, they have almost the samechemical properties. The only difference is intheir rates of reactions, mainly due to theirdifferent enthalpy of bond dissociation ( ). However, in physical properties theseisotopes differ considerably due to their largemass OF DIHYDROGEN, H2 There are a number of methods for preparingdihydrogen from metals and metal Preparation ofDihydrogen(i)It is usually prepared by the reaction ofgranulated zinc with dilute + 2H+ Zn2+ + H2(ii)It can also be prepared by the reaction ofzinc with aqueous + 2 NaOH Na2 ZnO2 + H2 Sodium Production ofDihydrogenThe commonly used processes are outlinedbelow:(i)Electrolysis of acidified water usingplatinum electrodes gives HYDROGEN .

7 ()()()Electrolysis222 Tracesofacid/base2 HOl2 HgOg +(ii)High purity (> ) dihydrogen isobtained by electrolysing warm aqueousbarium hydroxide solution between nickelelectrodes.(iii)It is obtained as a byproduct in themanufacture of sodium hydroxide andchlorine by the electrolysis of brinesolution. During electrolysis, the reactionsthat take place are:at anode: 2Cl (aq) Cl2(g) + 2e at cathode: 2H2O (l) + 2e H2(g) + 2OH (aq)The overall reaction is2Na+ (aq) + 2Cl (aq) + 2H2O(l) Cl2(g) + H2(g) + 2Na+ (aq) + 2OH (aq)(iv)Reaction of steam on hydrocarbons or cokeat high temperatures in the presence ofcatalyst yields HYDROGEN .++ ++1270Kn2n222 NiCHnHOnCO(2n1) ,()()()()1270K422 NiCHgHOgCOg3Hg+ +The mixture of CO and H2 is called watergas.

8 As this mixture of CO and H2 is used forthe synthesis of methanol and a number ofhydrocarbons, it is also called synthesis gasor 'syngas'. Nowadays 'syngas' is producedfrom sewage, saw-dust, scrap wood,newspapers etc. The process of producing'syngas' from coal is called 'coal gasification'.()()()()1270K22 CsHOgCOgHg+ +The production of dihydrogen can beincreased by reacting carbon monoxide ofsyngas mixtures with steam in the presence ofiron chromate as catalyst.()()()()673K222catalystCOgHOgCO gHg+ +This is called water-gas shift dioxide is removed by scrubbing withsodium arsenite ~77% of the industrialdihydrogen is produced from petro-chemicals,18% from coal, 4% from electrolysis of aqueoussolutions and 1% from other OF PropertiesDihydrogen is a colourless, odourless,tasteless, combustible gas.

9 It is lighter thanair and insoluble in water. Its other physicalproperties alongwith those of deuterium aregiven in Table PropertiesThe chemical behaviour of dihydrogen (and forthat matter any molecule) is determined, to alarge extent, by bond dissociation H H bond dissociation enthalpy is thehighest for a single bond between two atomsof any element. What inferences would youdraw from this fact ? It is because of this factorthat the dissociation of dihydrogen into itsatoms is only ~ around 2000K whichincreases to at 5000K. Also, it isrelatively inert at room temperature due to the2019-20287 HYDROGEN high H H bond enthalpy. Thus, the atomichydrogen is produced at a high temperaturein an electric arc or under ultravioletradiations.

10 Since its orbital is incomplete with1s1 electronic configuration, it does combinewith almost all the elements. It accomplishesreactions by (i) loss of the only electron togive H+, (ii) gain of an electron to form H , and(iii) sharing electrons to form a single covalent chemistry of dihydrogen can beillustrated by the following reactions:Reaction with halogens: It reacts withhalogens, X2 to give HYDROGEN halides, HX,()()()22 HgXg2 HXg(XF,Cl,Br,I)+ =While the reaction with fluorine occurs even inthe dark, with iodine it requires a with dioxygen: It reacts withdioxygen to form water. The reaction is (g) + O2 (g) 2H2O(l); H= kJ mol 1 Reaction with dinitrogen: With dinitrogenit forms ammonia.()()()673K,200atm223Fe13 HgNg2 NHg; + = H This is the method for the manufacture ofammonia by the Haber with metals: With many metals itcombines at a high temperature to yield thecorresponding hydrides (section )H2(g) +2M(g) 2MH(s);where M is an alkali metalReactions with metal ions and metaloxides: It reduces some metal ions in aqueoussolution and oxides of metals (less active thaniron) into corresponding metals.


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