Zumdahl s Chapter 6 Thermochemistry Chapter Contents Energy E Chemical Energy Matter Energy Chemist s Enthalpy Enthalpy H Calorimetry CV dE dT Hess s Law State Functions Standard Enthalpies of Formation Hf Elements Compounds Ions in Solution Energy Sources Efficiencies Environmental concern POTENTIAL ENERGY KINETIC ENERGY Fixed Total Energy Chemical Energy Exothermicity Release of energy usually as heat from chemical reactions whose products are at lower potential energy than their reactants If total energy is conserved and it comes as kinetic plus potential energy E KE PE then the lowered product potential mean raised product kinetic energies and heat flows out Chemists use enthalpy when discussing thermicity POTENTIAL ENERGY KINETIC ENERGY Fixed Total Energy Consumption of Energy Endothermicity If a reaction s products are higher in potential energy than its reactants its kinetic energy must be lower after the reaction Heat flows in to equalize temperatures Again chemists use enthalpy instead of energy Both kinds of chemical energy presume the conservation of total energy Conservation of Energy Energy can be converted from one form to another but can be neither created nor destroyed Empirical observations verify e g Count von Rumford boring out cannon observed the relationship between the mechanical energy of the drill and the frictional heat of the cannon True even of atomic energy if mass is E E mc2 Actually E m02c4 p2c2 Einstein Valid for both matter and light Light photon m0 rest mass is strictly zero for light So E pc but light has momentum p h deBroglie was also correct for both light and matter So E h c h for photons Just as we always knew it was Matter s Energy Expression E m02c4 p2c2 but p mv for matter E m02c4 m2v2c2 but m m0 1 v c 2 Lorentz which explains why you can t go even as fast as c E m02c4 m02v2c2 1 v c 2 E m0c2 1 v c 2 at all v even v c Indeed given Lorentz E mc2 for matter at all v At Garden Variety Velocities E m0c2 1 v c 2 when v c or v c 1 2 1 2 ignorable terms So E m0c2 1 v2 c2 or E m0c2 m0v2 for matter at low velocity E rest mass energy kinetic energy What happened to potential energy PE Conservation of Mass eh Potential energy is an algebraic shorthand for changes in mass that occur with the juxtapositions of matter and fields Use it PE c2 m0 implies that atomic weights vary in compounds Are we worried Formation of 1 mole of water yields 286 000 J m0 2 86 105 J 3 108 m s 2 3 2 10 12 kg With 1 8 10 2 kg mol we won t miss m0 w E Surroundings System q Pedestrian Energy Far from Einsteinian Esoterica we can observe energy changes in systems due to two gross causes Heat q flowing into the system raises its E Work w done on the system raises its E Together these macroscopic components imply E q w and when we isolate a system from surroundings then E 0 Internal Energy s Components HEAT Thermal energy flow q C T By Kinetic Theory T is proportional to kinetic energy q By Quantum Theory heat associates with changes in energy level populations q WORK Organized rather than chaotic molecular motion Comes in many forms By Quantum Theory work associates with changes in the energy levels themselves w Work Work Work Work Electrical work drags a charge Q through an electrical potential difference V so Q V Surface work stretches surface tension over larger areas so work is A Newtonian work pushes an object a distance x against a force F so its work is F x Pressure volume work compresses a volume with a pressure P so it is P V Pressure Volume Work Inescapable when doing chemistry under the relentless atmospheric pressure if Vgas changes during a reaction gas V n For constant Pext w PextdV Pext V But if Pext is always the system s Pinternal w PdV nRT V 1dV nRT ln V1 V2 which uses the Ideal Gas Law to track Pext and assumes constant T and is called reversible work Calorimetry We can use E C T to infer E from observed T if C heat capacity is known Conceptually C measures the system s number of energy modes that can hide thermal energy Since E changes with V too we must fix V in order to measure E by heat capacity so E CV T qV and we should work in fixed V bombs Enthalpy H a chemist s energy Since PV work is inevitable in reactions open to the omnipresent atmosphere we ll be doing a lot of PV calculations unless Define H E PV then H E PV But E qV bomb qP P V At constant P So H qP P V P V V P qP Now we get out of the bomb and onto the bench Practical E vs H Relation Chemical energies E are best measured in bomb calorimeters but enthalpies H are most conveniently used So relate them H E PV qV bomb RT ngas Which makes the quite defensible assumption that all gases are ideal enough Since their non idealities can be determined we can spruce this up at will Temperature Dependence of H Just as E CV T for constant CV H CP T for constant CP So we can extrapolate H at T other than at 25 C from standard tables if we know CP But neither C is truly independent of T so H CP T dT and it s so common We find tables of CP a bT c T 2 Molar C J mol K vs Specific Heat C J g K Hess s Law State functions are thermodynamic variables like energy or enthalpy that have the same value when you return the system to the same state same P V T n Hess Enthalpy changes between reactants and products are not dependent upon how the reaction is brought about Otherwise return to reactants wouldn t undo H Hess s Joyful Consequences C2H6 3 5 O2 2 CO2 3 H2O H1 Then Hess s Law guarantees that 2 CO2 3 H2O C2H6 3 5 O2 has H1 Even though the latter s not a feasible reaction Since H is extensive scales with of moles 4 CO2 6 H2O 2 C2H6 7 O2 is 2 H1 Which permits us to do algebra with reactions Chemical Algebra Suppose calorimetry gave us the following C2H6 3 5 O2 2 CO2 3 H2O H1 C2H4 3 O2 2 CO2 2 H2O H2 H2 O2 H2O H3 If we reverse the first reaction and add C2H4 H2 C2H6 results and has a Where H H2 H3 H1 H We find ethene s hydrogenation enthalpy without having to hydrogenate ethene Just torch it Chemical Reference Points We just used the fully oxidized forms of the compounds to do thermochemistry on a reaction that didn t even involve oxygen And we did it because calorimetry is easy But Hess lets us use any consistent reference …