Class Note for CHEM 111 at UMass(3)
Class Note for CHEM 111 at UMass(3)
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Date Created: 02/06/15
ChemisTry and Energy The science of heaT and work Thermodynamics SecTions 61 63 ChemisTs wanT To know abouT The characTerisTics of chemical reacTions a how much producT will form and b how fasT will The reacTion go ChemisTs wanT To know how To change The reacTion condiTions To influence These characTerisTics in a predicTable fashion ChemisTs need To know someThing abouT how energy and chemical reacTions are relaTed For mosT chemical reacTions The imporTanT form of energy is heaT The science of heaT work and energy changes is called Thermodynamics LoTs of useful reacTions give ouT heaT We can cause reacTanTfavored reacTions To become producTfavored by supplying some heaT DefiniTions Energy is The capaciTy To do work move a force Broadly speaking we can classify energy as eiTher kineTic or poTenTial KineTic energy is due To moTion of molecules and aToms random Thermal energy or heaT wiTh regular compressions and rarefacTions sound of macroscopic objecTs mechanical energy of elecTrons moving in an elecTric field elecTrical energy PoTenTial energy is sTored enerqy TriniTroToluene chemical Any objecT ThaT could move in a graviTaTional field graviTaTional OpposiTe charges held aparT elecTrosTaTic Energy can be converTed from one form inTo anoTher buT iT cannoT be creaTed or desTroyed This observaTion is one of The laws of Thermodynamics The ToTal energy of The universe is consTanT ChemisTs are very inTeresTed in heaT energy because This is relaTed closely To The behavior of molecules and The ways in which aToms are held TogeTher in molecules The exTenT To which The molecules of a maTerial are moving abouT is characTerized by a concepT known as TemperaTure We work wiTh a scale on which The freezing of waTer of assigned a value of 0 degrees and The boiling of waTer is assigned a value of 100 degrees When comparing Two objecTs wiTh regard To TemperaTure we can decide which is hoTTer higher TemperaTure and which is colder or cooler lower TemperaTure HeaT energy always passes from The hoTTer objecT To The colder objecT The explanaTion for This phenomenon is based on The moTions of molecules The hoTTer an objecT The more rapid The moTions of iTs molecules When objecTs come inTo conTacT The molecules collide wiTh each oTher and The kineTic energies of The fasTer moving molecules are decreased as The kineTic energies of The slower moving molecules are increased The more molecules and The fasTer They are moving The more heaT energy The objecT possesses Energy UniTs Based on increasing Temper aTur e of waTer calor ie cal amounT of heaT energy needed To raise The Temper aTur e of 1 g of waTer from 145 To 155 degrees Celsius C rafher smaL so chemsfs use kca kIocaore w1c1 are aso known as Caores cap39fa C m ofher can 7 6st usuay de fury ChemisTs also use The Joule J The energy needed To move a force of 1 newTon N Through a disTance of 1 meTer m A newTon is The force ThaT impar Ts an acceler39aTion of 1 m s392 To a 1 kg objecT 1 cal 4184 J 7716 J39oue Is nof very bg efwr and chemsfs use kJ39 Power The r aTe aT which energy is consumed or delivered is known as power The usual uniTs ar e waTTs W 1 W 1 J s391 one Joule per second A 100waTT lighT bulb consumes 100 Joules of energy every second ElecTr iciTy supply companies bill you for The ToTal energy you use measured in kilowaTT hour s kWh So how many J in a kWh HeaT CapaciTy The amounT of heaT required To cause a given change in TemperaTure of an objecT ChemisTs use specific heaT capaciTy or specific heaT given The uniTs J 9391 Kquot1 Where K is The degree kelvin IT is numerically equal To The C buT has a differenT zero The socalled absoluTe zero aT which all molecular and aTomic moTions cease WaTer has a large specific heaT 4184 J 9391 K1 The number looks familiar because of The definiTion of The calorie and The relaTionship beTween The Joule and The calorie 4184 J 1 cal One of The reasons ThaT large bodies of waTer influence The planeT39s weaTher If someThing has a low mass and low specific heaT iT only has a small amounT of heaT To give ouT as iT cools Thus Al foil can be Touched sTraighT from The oven whereas The roasT chicken cannoT QuanTiTaTive basis 639 qm x A 7 Where is The specific heaT q is The amounT of heaT Transferred J m is The mass g and A Tis The TemperaTure change K The symbol A Greek upper case delTa is used To mean change in IT is usually calculaTed as final value minus iniTial value Thus if heaT is supplied The objecT heaTs up A 739gt 0 If heaT is given ouT objecT cools down A 739lt O Cis posiTive so The sign of qdepends on wheTher heaT is supplied To or given ouT by The objecT in quesTion HeaT Taken in q is posiTive HeaT given ouT q is negaTive ChemisTs consider The aboraTory To consisT of a coecTion of sysTems and surroundings A sysTem is The chemical maTeriaI being sTudied and The surroundings are The conTainer lid aboraTory aTmosphere waTer baTh and so on Any process occurring in The sysTem which Takes in heaT from The surroundings is endoThermic A process in The sysTem which gives ouT heaT To The surroundings is exoThermic The sysTem may be connecTed To The surroundings so ThaT work is done on The surroundings or ThaT work is done by The surroundings EnThalpy Work is done by moving forces such as Those exerTed by The gas molecules in The surroundings So if The sysTem expands iT has To do work on The surroundings To Them push back The change in energy of The sysTem is The sum of The heaT Transferred q and The work done w AE qw This is anoTher sTaTemenT of The law of conservaTion of energy or The firsT law of Thermodynamics w has a sign JusT as qdoes To indicaTe wheTher The sysTem loses energy by doing work w is negaTive or wheTher The sysTem gains energy by having work done on iT w is posiTive Many sysTems operaTe aT consTanT pressure planTs animals and chemical processes in vessels open To The lab aTmosphere The heaT Transfer aT consTanT pressure is The enThalpy change AH ie by definiTion AH 2 zip so aT consTanT pressure A5 AH w For many chemical sysTems and surroundings w is small compared wiTh AH The values for chemical processes are always given on a per mol basis Thus when we wriTe H20I H20g AH 44 kJ aT 25 C This is inTerpreTed as meaning The heaT required To vaporize 1 mol of waTer aT 25 C aT consTanT pressure is 44 kJ This is an endoThermic process The reverse process The condensaTion of waTer vapor is exoThermic and AH 44 kJ aT 25 C The heaT released when a ThundersTorms worTh of waTer condenses aT 25 C is equivalenT To ThaT released when 35000000 kg of dynamiTe explodes EnThalpy Changes for Chemical ReacTions The producTs are The final sTaTe of The sysTem and The reacTanTs are The iniTial sTaTe of The sysTem Thus AH HproducTs 39 reacTanTs Many reacTions are exoThermic ie AHlt 0 H2 9 05 02 9 H20 9 AH 2418 kJ Meaning ThaT when one mol of waTer vapor is formed by The reacTion of 1 mol of hydrogen gas wiTh half a mol of oxygen gas 2418 K7 are given ouT The reacTion is exoThermic On The oTher hand for The reacTion H20 g H2 g 0502 g AH 2418 kJ Meaning ThaT when one mol of waTer vapor decomposes To give one mol of hydrogen gas and half a mol of oxygen gas 2418 kJ are Taken in from The surroundings The reacTion is endoThermic When dealing wiTh Thermodynamic quanTiTies The equaTions are always undersTood To represenT moles IT is imporTanT ThaT The sTaTe of The reacTanTs and producTs be specified because of The heaTs associaTed wiTh changes of sTaTe H20I H2 9 029 AH2858 kJ More energy is needed To decompose 1 mol of liquid waTer as heaT is needed To conver T iT To a vapor So we have To add The heaT of vaporizaTion of waTer 44 kJ mol39l If we know AHfor a r eacTion we can calculaTe how much heaT is produced or Taken in for The r39eacTion of any quanTiTy of r eacTanT If we know AHfor The componenT processes or r eacTions we can calculaTe AHfor The overall process by adding TogeTher The individual AHvalues Session 17 Hess s Law and Standard Enthalpies of Formation Sections 67 and 68 Chemists want to know AHfor as many reactions as possible It would be very time consuming to measure them all Hess s law states that if a reaction can be written as the sum of several other reactions then the AHfor that reaction is the sum ofthe AHvalues for each constituent reaction eg the combustion of carbon to give carbon monoxide The reaction would proceed to give carbon dioxide even ifthe amount of oxygen was restricted If we know the AHforthe reactions C 02 002 AH 3935 kJ CO 12 02 002 AH 2830 kJ we can calculate AHfor the reaction C 12 02 9 CO as follows C02 9 CO 1202 reverse the sign of AHso AHforthis reaction is 2830 kJ Now add this reaction to C 02 9 C02 AH3935 kJ to give C 02 C02 9 C02 CO 1202 cancel the C02 and 1202 C 1202 9 CO AH39352830kJ 1105 kJ Example 68 Find enthalpy change for reaction between solid carbon and gaseous hydrogen to form methane Get at this from a knowledge ofthe enthalpies of combustion of C to form carbon dioxide 3935 kJ of H2 to form liquid water 2858 kJ and of methane to form carbon dioxide and liquid water 8903 kJ wowl Why so big State Functions Enthalpy is a state function Its value depends only on the state and not how the state was formed For a chemical reaction the AHdepends only on the products and reactants not how the products are formed from the reactants Other state functions are volume temperature and pressure Unlike the situation for these state functions we are usually interested in the change in enthalpy for a chemical reaction rather than the absolute enthalpy of any particular set of reactants or products Chemists take Hess s law one stage further with a procedure that allows the enthalpy change for almost any reaction to be calculated Standard Enthalpies of Formation The standard refers to the standard state of an element or compound The most stable form at a pressure of1 bar atmosphere and 25 C The AHfor a reaction in which all reactants and products are in their standard states is called the and is given the symbol AHU The for the formation of1 mol ofa compound from the relevant elements is called the standard molar enthalpy of formation AllFf AH ins just a special case of AHfor a reaction Values for many compounds are tabulated From these we can calculate the AHfor a reaction ifwe know the AIF values ofthe reactants and the products Note that AIF for an element is zero AH Jmm Z AIFfproducts Z AIFf reactants Example 69 Decomposition of nitroglycerin to give nitrogen oxygen carbon dioxide and water CaHsN033 1 9 N2 t 02 t 002 g t H20g Calculate the enthalpy change per g of nitroglycerin AHU f for nitrogylcerine is 7364 kJ mol391 AHaf for co2 is 3935 kJ mol391 AH Jf for H20g is 2418 kJ mol391 molar mass of nitroglycerin is 2271 g mol391
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