ICHAPTER2Environmental Chemistry2.1 INTRODUCTION2.2 STOICHIOMETRY2.3 ENTHALPY IN CHEMICAL SYSTEMS2.4 CHEMICAL EQUILIBRIA2.S ORGANIC CHEMISTRY2.6 NUCLEAR CHEMISTRYPROBLEMSREFERENCESIt often matters muchhowgiven atoms combine, in what arrange-ment, with what others, what impulsetheyreceive, and whatimpart. The same ones make up earth, sky, sea, and stream; thesame the sun, the animals, grain and trees,butmingling and mov-ing in ever different ways.-lucretius(9S-52B.C.)inThe NatureofThings2.1INTRODUCTIONAlmost every pollution problem that we face has a chemical basis.Even the most qual-itative descriptions of such problems as the greenhouse effect, ozone depletion, toxicwastes, groundwater contamination, air pollution, and acid rain, to mention a few,require at least a rudimentary understanding of some basic chemical concepts. And, ofcourse, an environmental engineer who must design an emission control system or awaste treatment plant must be well grounded in chemical principles and the tech-niques of chemical engineering. In this brief chapter, the topics have been selected withthe goal of providing only the essential chemical principles required to understand thenature of the pollution problems that we face and the engineering approaches to theirsolutions.2.2STOICHIOMETRYWhen a chemical reaction is written down, it provides both qualitative and quantita-tive information. Qualitatively, we can see what chemicals are interacting to producewhat end products. Quantitatively, the principle of conservation of mass can be appliedtogive information about how much of each compound is involved to produce the3940Chapter 2 Environmental ChemistryTABLE2.1AtomicNumbersandAtomicWeightsAtomicAtomicAtomicAtomicresults shown. Thebalancingofequationssothatthesamenumberof eachkindofSymbolnumberweightElementSymbolnumberweightatomappearsoneachsideof theequationandthesubsequentcalculations, whichcanElementbeusedtodetermineamountsofeachcompoundinvolved, isknownasstoichiometry.ActiniumAc89227.03MercuryHg80200.59Thefirststepis tobalancetheequation.Forexample,supposewewantto investi- Al1326.98MolybdenumMo4295.94Aluminumgatethecombustionofmethane(CH.),theprincipalcomponentofnaturalgasandaAmericiumAm95243NeodymiumNd60144.24Sb51121.75NeonNelO20.IXmajorgreenhousegas.Methanecombineswith oxygen toproducecarbondioxide andAntimonyAr1839.95NeptuniumNp93237.05water, as the followingreactionsuggests:Argon3374.92NickelNi285X.70ArsenicAsAstatineAt852lONiobiumNb4192.91CH.+02-?CO2+H2OBariumBa56137.33NitrogenN714.01BarkeliumBk97247NobeliumNolO2259Theequationis not balanced.Oneatomofcarbonappearsoneachside, which isBeryliumBe49.01OsmiumOs76190.2fine,buttherearefouratomsof hydrogen on the leftandonly two on the right,andthereBismuthBi83208.98Oxygen0816.00are only twoatomsof oxygen on the left whiletherearethreeon the right. We might tryB510.81PalladiumPd46106.4Borontodoublethewatermolecules ontheright to balance thehydrogenon each side,butBromineBr3579.90PhosphorusP1530.97thentherewould be an imbalance of oxygen with two on the left and four on the right.CadmiumCd48112.41PlatinumPt78195.09Ca2040.08PlutoniumPu94244So try doubling the oxygen on the left. This sort of trial-and-errorapproachto balancingCalciumCaliforniumCf98251PoloniumPo84209simple reactions usually converges quickly. In this instance the following is a balancedCarbonC612.01PotassiumK1939.09equationwith thesamenumberof C, H, and°atomson each side of the arrow:CeriumCe58140.12PraeseodymiumPr59140.91CesiumCs55132.90PromethiumPm61145CH4+202-?CO2+2 HzO(2.1)ChlorineCI1735.45ProtactiniumPa91231.04ChromiumCr2451.99RadiumRaXX226.03Thisbalancedchemicalequationcanbereadas follows:Onemolecule ofCobaltCo2758.93RadonRn86222methanereacts with two molecules of oxygen toproduceonemolecule ofcarbondiox-CopperCu2963.55RheniumRe75186.2ideandtwo molecules of water.Itis ofmoreuse, however, to beabletodescribethisCuriumCm96247RhodiumRh45lO2.91reactionintermsofthemassofeachsubstance(thatis, howmanygramsof oxygenareDysprosiumDy66162.50RobidiumRb3785.45requiredtoreactwithhowmanygramsofmethane,andsoon).To do sorequiresthatEinsteiniumEs99254RutheniumRu44lO1.07Er68167.26SamariumSm62150.4weknowsomethingaboutthemassof individualatomsand molecules.ErbiumEuropiumEu63151.96ScandiumSc2144.96Theatomic weightofanatomisthemassoftheatommeasuredinatomic massFermiumFm100257SeleniumSe3478.96units(arnu),whereoneamuis defined to be exactly one-twelfth the mass of acarbonFluorineF919.00SiliconSi1428.09atomhaving sixprotonsandsixneutronsin its nucleus. While thismightsuggestthatifFranciumFr87223SilverAg47107.89welookuptheatomicweight ofcarbonwe wouldexpectto find it to be exactlyGadoliniumGd64157.25SodiumNaII22.99Ga3169.72StrontiumSr3887.6212amu,thatisnotthe case.Allcarbonatomsdohave sixprotons,buttheydonotallGalliumGermaniumGe3272.59SulfurS1632.06have sixneutrons,sotheydonotallhavethesameatomicweight.AtomshavingtheGoldAu79196.97TantalumTa73180.95samenumberofprotonsbutdifferingnumbersofneutronsarecalledisotopes.WhatisHafniumHf72178.49TechnetiumTc4397reportedintablesofatomicweights,suchasTable2.1, istheaveragebasedontherela-HeliumHe24.00TelluriumTe52127.60tiveabundanceofdifferentisotopesfoundinnature.Alsoshownin Table 2.1 is theHolmii.mHo67164.93TerbiumTh65158.93H1l.01ThalliumTI81204.37atomic number,which is thenumberofprotonsin the nucleus. Allisotopesof a givenHydrogen]]4.82ThoriumTh90232.04IndiumIn49elementhavethesameatomicnumber.I53126.90ThuliumTm69168.93lodineThemolecularweight of amoleculeis simply thesumof theatomicweightsofallIr7719222TInSn50118.69lridiumof theconstituentatoms. If we divide the mass of asubstanceby itsmolecularweight,Fe2655.85TItaniumTI2247.90IronW74183.X5the result is the massexpressedinmoles(mol). Usually the mass isexpressedin grams,KryptonKr3683.80TungstenLa57138.91UraniumU92238.03in which casethemolesareg-moles;in like fashion, if the mass isexpressedin pounds,Lanthanum2350.94LawrenciumLr103260VanadiumVthe result would be lb-moles.In this text, all moles will beassumedto be g-rnoles.OnePb82207.2XenonXe54 131.30Leadg-rnolecontains6.02 X 1023molecules(Avogadro'snumber),whileonelb-rnole isLi36.94YtterbiumYb70173.04Lithiummadeup of 2.7x1026molecules.Lu71174.97YttriumY3988.91LutetiumMagnesiumMg1224.31ZincZn3065.38MassManganeseMn2554.94ZirconiumZr409122Moles=(2.2)MendeleviumMd101258Molecularweight42 Chapter 2Environmental ChemistrySection 2.2Stoichiometry43We already knowthatthereare 44 gramspermole of CO" so we do not need to recalculate that.Two moles ofbutane(2 mol x 58g/mol=