HYDROGEN
PositionofHydrogenin PeriodicTable
*Lightestelementknownhavingatomicnumber1.
* Dihydrogen
*It resemblesbothalkalimetalsandhalogensand
therefore,itspositionisanomalous.
* Inmodernperiodictableit islocatedseparately
Resemblancewithalkalimetals:1.Electronicconfiguration
1H=1s1
11Na=1s2,2s2,2p6,3s1
19K=1s2,2s2,2p6,3s23p6,4s1
2.Electropositivecharacter:H+,Na+,K+etc.
3.Oxidationstate:+1
4.Combinationwithelectronegativeelements:formbinarycompoundswith
electronegativeelementslikealkalimetals.
Halides:HClNaCl,KCletc
Sulphides:H2SNa2S,K2Setc
Resemblancewithhalogens:1.Electronicconfiguration:
Bothcontainoneelectronlessthanthe
noblegasconfiguration
1H=1s1(nearto2He)
9F=1s2,2s2,2p5(nearto8Ne)
17K=1s2,2s2,2p6,3s23p5(nearto18Ar)
2.Non-metalliccharacter:likehalogens,hydrogenis non-metallicinnature.
3.Atomicity:Diatomicmolecules.
4.Formationofsimilartypesofcompounds:
i.Halides:CCl4,SiCl4,GeCl4
ii. Hydrides:CH4,SiH4,GeH4
5.Oxidationstate:–1
Na+1H-1Na+1Cl-1
Differencefromalkalimetals:1)Ionizationenthalpy:-theionizationenthalpyofhydrogenisveryhighin
comparisontoalkalimetals.
2)Non-metalliccharacter:alkalimetalsaretypicalmetalswhilehydrogenis
non-metal
3)Atomicity:hydrogenisdiatomicwhilealkalimetalsaremonoatomic.
4)Natureofcompounds:thecompoundsofhydrogenarepredominantly
covalentwhilethoseofalkalimetalsareionic.Forexample:HCliscovalent
whileNaClisionic.
Theoxidesofalkalimetalsarebasicwhilehydrogenoxideisneutral.
Differencefromhalogens:1)Lesstendencyforhydrideformation:Hydrogenhaslesstendencytotakeup
electrontoformhydrideion(H-)ascomparedtothehalogenswhichfrom
halideions(X-)veryeasily.
2)Absenceofunsharedpairsofelectrons:
3)Natureofoxides:Theoxidesofhalogensareacidicwhilehydrogenoxideis
neutral.
OccurrenceofHydrogen:
* Hydrogen,themostabundantelementintheuniverseandthe
thirdmostabundantonthesurfaceoftheglobe,is beingvisualised as
themajorfuturesourceofenergy
Preparation:
Methodsforcommercialproductionofdihydrogen
1.Electrolysisofwater
2H2O(l)2H2(g)O2(g)
* Thehydrogenpreparedbythismethodisofveryhighpurity.However,this
methodisnotcommonlyusedbecauseit isveryexpensive.Thismethodiscanbe
usedonlyatthoseplaceswheretheelectricityischeap.
*Bythereactionofsteamoncoke:- C
+H2O(g)CO+H2
Watergas
*SincethemixtureofCOandH2isusedforthesynthesisofmethanolanda
numberofhydrocarbons,it isalsocaledsynthesisgasorsyngas.
*Theprocessofproducingsyngasfromcoalorcokeiscaledcoalgasification.CO+
H2+H2O CO2+2H2
Watergassteam
* Thisreactioniscaledwatergasshiftreaction.
PropertiesofHydrogen:* PhysicalProperties:-
1)Itis slightlysolubleinwater(about2%)
2)Itis highlycombustibleandthereforeshouldbehandledcarefully.
3)Itlightestsubstance.TheweightofonelitrehydrogenatNTPis
only0.0899g.
*Chemicalproperties:-Notveryreactiveduetohighbond
dissociationenergy
(435.88kJmol-1at298.2K)
(i) Combustion:-It burnswithpaleblueflame
2H2(g)+O2(g)2H2O(l)
(i) Reactionwithmetals:-ReactivemetalslikeNa,K,Ca,Liandformhydrides.Ca
+H2CaH2
* MetalslikePt,Pd,Ni(elementsofdblock)forminterstitialhydridesbyabsorbing
largevolumeofhydrogen.Suchhydrogeniscalled‘occludedhydrogenandthis
propertyofadsorptionofagasbyametaliscalledocclusion.
(iii) Reactionwithmetaloxides:-Hydrogenreducesoxidesoflessactivemetalsto
correspondingmetal.
Hydrogenationofoils:-
Vegetableoilsarepolyunsaturatedinnature.TheC=Cbondsin oils
caneasilyundergooxidationandtheoilbecomesrancidi.e.,
unpleasantintaste.Hydrogenationreducesthenumberofdouble
bondsbutcompletely.
UsesofHydrogen:1.asareducingagent.
2.Inthemanufactureofvanaspatifat,ammonia,metalhydrides,methanol,
fertilizerssuchasureaetc.
3.Inthemanufactureofsyntheticpetrol.
4.Intheatomichydrogentorchandoxyhydrogentorchesforcuttingandwelding.
Dihydrogenisdissociatedwiththehelpofanelectricarcandthe
hydrogenatomsproducedarealowedtorecombineonthesurfaceto
bewelded.Hightemperatureofabout4000kisgenerated.
5.Inthefuelcellforgeneratingelectricalenergy.
Hydrides:UndercertainconditionsH2combineswithalmostalltheelements,except
noblegasestoformcompoundscalledhydrides.
* Therearethreetypesofhydrides,theyare
(i) Ionicorsalinehydrides
(i) Covalentormolecularhydrides(iii) Metallicornon-stoichiometrichydrides(i)
Ionicorsalinehydrides:* ThesearethecompoundsofH2formedwithmostofthes-blockelements
whicharehighlyelectropositive.
(ii) Covalentormolecularhydrides:-Thesearethecompoundsofhydrogen
formedwithmostofthep-blockelements
* Electrondeficient:-Thehydrideswhichdonothavesufficientnumberofelectrons
toformnormalcovalentbondsiscalledelectrondeficienthydride.Forexample,
hydrideofgroup13(BH3,AlH3,etc.).TheyareknownasLewisacidsi.e.,electron
acceptors.Tomakeuptheirdeficiencytheygeneralyexistin polymericformssuch
asB2H6,Al2H6,etc.
*Electronprecise:-
• Thehydrideswhichhavesufficientnumberofelectrons
requiredforformingcovalentbondsiscalledelectronprecisehydride.For
example,hydridesofgroup14(CH4,SiH4,GeH4,SnH4,PbH4etc.)theyhave
tetrahedralgeometry.
*Electronrichhydrides:-
Thehydrideswhichhaveexcesselectronsasrequired
toformnormalcovalentbondsis calledelectronrichhydride.Forexample,
hydridesofgroup15to17(NH3,PH3,H2O,H2S,H2Se,H2Te,HFetc.)
#Metallicornon-stoichiometrichydrides:* Theseareformedbymanyd-blockandf-blockelements
* Thesehydridesconductsheatandelectricitythoughnotefficient.
Water:-
Water!Itis themajorpartofalllivingorganisms.wateris alsoknownas
theriveroflife.
* Humanbodyhasabout65%andsomeplantshaveasmuchas95%water.
STRUCTUREOFWATER:* Inagasphasewateris bentmoleculewithabondangleof104.5andO-H
bondlengthof95.7pmItis highlypolarmolecule.
Structureofice:Icehasahighlyordered3D
hydrogenbondedstructure.Eachoxygenatomis
surroundedtetrahedrally byfour other four
otheroxygenatomsatadistanceof276pm
ChemicalPropertiesofwater:AMPHOTERICNATURE:-
It hastheabilitytoactasanacidaswellasbase
.itactsasanacidwithNH3andasabasewithH2S
H2O(l)+NH3(aq)OH-(aq)+NH4+(aq)
H2O(l)+H2S(aq)H3O+(aq)+HS- (aq)
REDOXREACTIONSINVOLVINGWATER:-
* WatercanbeeasilyreducedtoH2byhighlyelectropositivemetals
2H2O(l)+2Na(s)2NaOH(aq)+H2(g)
HYDROLYSISREACTION:-
*Duetohighdielectricconstant,ithasavery
stronghydratingtendency.itdissolvesmanyioniccompounds
P4O10(s)+6H20(l)4H3PO4(aq)
SiCl4(l) +2H2O(l)SiO2(s)+4HCl(aq)
HYDRATESFORMATION:Fromtheaqueoussolutionsmanysaltscanbe
crystallizedashydratedsalts.It ofdifferenttypes.
(1) Coordinatedwatere.g.,[Cr(H2O)6]3+3Cl(2)Interstitialwatere.g.,BaCl2.2H2O
(3)hydrogen-bondedwatere.g.[Cu(H2O)4]2+4SO2-.H2Oin
CuSO4.5H2O