Slide 1EnergyForms of EnergySlide 4KE and PE in ChemistryUnits of EnergyEnergy ChangesExamining Energy ChangesTransferring EnergyTransferring EnergySign ConventionsInternal EnergyInternal EnergyExo and EndoState FunctionsState FunctionsFirst Law of ThermodynamicsEnthalpyThermochemical EquationCalorimetryCalorimetryHess’s LawHess’s LawEnthalpies of FormationEnthalpies of FormationFoods and FuelsFoodsFood LabelsUniform LabelingCarbohydratesCarbohydratesCarbohydratesFatsFatsProteinsCalculating the CaloriesFuelsFuelsFuel ValueHydrogen!Fuel CellsIntegrative ExercisesChapter 6: ThermochemistryThermodynamics: the Science of the Relationships Between Heat and Other Forms of Energy.Thermochemistry: One Area of Thermodynamics That Involves the Quantity of Heat Absorbed or Evolved.01/18/2019 1EnergyThe capacity to do work or to produce heat.Conservation of energy – energy can be converted from one form to another, but cannot be created nor destroyed.01/18/2019 2Forms of EnergyKinetic Energy= ½ mv2Energy of motionPotential Energy= mgh (physics)Energy of positionConversion between KE and PEEx) Tossing a stone into the air (LEP #1)01/18/2019 301/18/2019 4KE and PE in ChemistryKEFound in the various motions and vibrations of the molecules.PEFound in the electrostatic attractions between atoms and ions (chemical bonds and intermolecular forces).01/18/2019 5Units of EnergySI unit is the joule (J)= 1 kg m2 / s2Relatively small unit – will usually use kilojoules (kJ)1 kJ = 1000 JCommonly used non-SI unit is the calorie (cal)= amount of heat required to raise 1g of water by 1oC.1 cal = 4.184J (exact)01/18/2019 6Energy ChangesSystem = the substances undergoing a chemical reaction or a physical change.Surroundings = everything else.Universe = System + SurroundingsWhat we study is the exchange of energy between the system and the surroundings01/18/2019 7SurroundingsSystemSurroundingsSystemExamining Energy ChangesExample: Adding NaOH(aq) to HCl(aq) in a styrofoam cup.System = NaOH, HCl, H2O, and NaCl.Surroundings = cup and everything else.01/18/2019 8Transferring EnergyEnergy may be transferred in one of two forms or a combination of both.Work = Force times distance, where a force is any push or pull on an object.Heat = energy transferred from a hotter object to a colder one.01/18/2019 9Transferring Energy All reactions in the lab thus far have been ones that transfer energy in the form of heat.How can we have a reaction that does work?A car engine does both work and heat.01/18/2019 10Sign ConventionsWork (w) and heat (q) must be defined from the direction of the system (and surroundings).W = positive if work is done by the surroundings on the system and negative if work is done by the system on the surroundings.q = positive if heat is transferred from the surroundings to the system and negative if heat is transferred from the system to the surroundings.See Table 5.1 (p. 171) for summary. 01/18/2019 11Internal EnergyThe internal energy (E) of a system is defined as the sum of both the KE and PE of all the particles of a system.Calculating E for any system is a rather daunting task.When a system undergoes a physical or chemical change, the change in internal energy can be easily found.01/18/2019 12Internal EnergyDE = q + wCombustion of gasoline molecules in a car engine do work on the surroundings and transfer heat to the surroundings.Thus, the internal energy of the system decreases.01/18/2019 13Exo and Endo______________ reactions are reactions that transfer heat from the system to the surroundings.______________ reactions are reactions that transfer heat from the surroundings to the system.What signs (+/-) would be assigned to these definitions? 01/18/2019 14State FunctionsAny property that depends only on its present state and not the past states is called a state function (property).A state function is independent of the path taken.Ex) Change in altitude is a state function.01/18/2019 15State Functions01/18/2019 16First Law of ThermodynamicsThe energy of the universe is constantIn a chemical process, energy can neither be created nor destroyed – that is energy is conserved.Energy lost by the system is gained by that of the surroundings (and vice versa).01/18/2019 17EnthalpyEnthalpy (H) is a state function.Enthalpy is a measurement of the flow of heat in a reaction.Enthalpy is an extensive property.DH = DE + PDV.If no work is done, then DH = DE.01/18/2019 18Thermochemical EquationA reaction with phase labels can now include the quantity of heat as a DH.Ex) N2(g) + 3H2(g)  2NH3(g) ; DH = -92kJLEP #22 NH3(g)  N2(g) + 3H2(g) ?2N2(g) + 6H2(g)  4NH3(g) ?0.50 moles of NH3(g)0.34g NH3(g)01/18/2019 19CalorimetryA simple experiment for determining the change in enthalpy.Measure the temperature change in an insulated cup.Specific heat.q = msDTHeat capacityq = CDT01/18/2019 20CalorimetryCalculating the specific heat of a metal (LEP #3)Calculating the change in enthalpy using a specific heat (LEP #4 and #5)Calculating the change in enthalpy using a heat capacity (LEP #6) by a bomb calorimeter01/18/2019 21Hess’s LawIf a reaction can be written as a series of steps that sum to the overall reaction, then the change in enthalpy for each step will sum to the overall change in enthalpy for the overall reaction.Problem: can not directly measure a change in enthalpy because of issues with a reaction.Ex) 2 C(graphite) + O2(g)  2 CO(g)01/18/2019 22Hess’s LawCan measure these:C(graphite) + O2(g)  CO2(g) ; DH = -393.5kJ2 CO(g) + O2(g)  2 CO2(g); DH = -566.0kJBy manipulating these two equations, we can get them to add to the desired equation…LEP #701/18/2019 23Enthalpies of FormationA set of standard values for various substances can be found in Appendix C.These represent the formation of the substance from the elements in their standard states at 25oC and 1 atmosphere of pressure.Ex) Na(s) + ½ Cl2(g)  NaCl(s) ; DHfo = -410.9 kJ Ex) C(gr) + 2 H2(g)  CH4(g) ; DHfo = -74.8 kJAny element in its standard state will have a DHfo of zero.01/18/2019 24Enthalpies of FormationTo find a DH for a reaction, simply sum all the products and the reactants. Then subtract the reactants from the products.DH = S(DHfo[products]) –