Chemical Reactions: Thermochemistry Part 2Chemical Reactions: Thermochemistry Part 2Hess’s LawHess’s Law Example: SmogStandard Enthalpy of Reaction ΔHrxnoPractice ProblemWhere Does the Energy Come From?Covalent Bond Length and EnergyPractice ProblemEstimating ΔHrxn from Bond Dissociation EnergiesDoes this Actually Work?Does this Actually Work?Demo: Peroxide DecompositionPractice ProblemStandard Enthalpies of FormationBond Enthalpies and Bond LengthsWhat You Should KnowChemical Reactions:Thermochemistry Part 2CH110 FA11 SAS 1Prepare for Recitation December 1stALEKS Objective 13 due November 29thANGEL Quiz 13, December 1stLecture 37: November 16thLecture 38: November 18th(Thanksgiving Break)Lecture 39: November 28thRead: Ch. 4.6,10.5 (Reactions); 5.3 – 5.7, 8.8 (Thermo)Additional Preparation: Packet Rxns: 12-18BLB 10: 57BLB 5: 4,17,29,37,39, 53,55,83,85; Packet 5: 1-7BLB 8: 65a,67ac,92a; 18: 72ab,74; Packet 8: 12-14CH110 FA11 SAS 2By the end of Today’s Lecture you should know:Hess’s LawThermodynamic cyclesStandard enthalpy of reactionBond energyKEY QUESTIONS: THE BIG PICTUREChemical Reactions:Thermochemistry Part 2What is Hess’s Law?How do I determine or predict the enthalpy of a reaction?How does the enthalpy of a reaction relate to the actual chemistryRemember: Energy is stored in molecules. What are the microscopic connections?How do thermodynamic cycles allow me to predict the outcome of unknown reactions?What is the relationship between bond length and bond strength?Hess’s LawCH110 FA11 SAS 3Reason: Bottom line of Hess’s Law:You do not need to measure ∆Hrxnfor every reaction…use ∆Hfofor products and reactants (from tables) and calculate.Remember∆Hrxnis for the ________________(i.e., a given stoichiometry.) The magnitude of ∆Hrxndepends on the _________________.Hess’s Law:Hess’s Law Example: SmogCH110 FA11 SAS 4N2(g) + 2 O2(g) → 2 NO2(g) ∆H = ?Find two (or more) steps with knownenthalpy and calculate ∆Hrxnas: N2(g) + O2(g) → 2 NO(g) ∆H1= 180 kJ2 NO(g) + O2(g) → 2 NO2(g) ∆H2= – 112 kJN2(g) + 2 O2(g) → 2 NO2(g) ∆Hrxn= sum of ∆H values gives ∆H of ___________N2(g) + 2 O2(g)2 NO2(g)∆H1∆H2∆Hrxn∆Hrxn=Standard Enthalpy of Reaction ΔHrxnoCH110 FA11 SAS 5∆Hrxno= Σn ∆Hfo(products) – Σm ∆Hfo(reactants)an application of Hess’s lawEXAMPLE:What is ∆Hrxnofor the reaction C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)∆Hfovalues (kJ/mol) and stoichiometries∆Hrxno=C(graphite) + O2(g) → CO2(g)ΔHfo=H2(g)+ ½ O2(g) → H2O(l)ΔHfo=C2H5OH(l) → 3 H2(g)+ ½ O2(g) + 2 C(graphite) ΔHfo=Practice ProblemCH110 FA11 SAS 6Given the following:What is ΔHrxnfor the following reaction?S(s) + O2(g) → SO2(g)A) -986 kJB) -594 kJC) +594 kJD) +986 kJE) -297 kJ∆H2SO2(g)+ O2(g) → 2SO3(g) −196kJ2S(s) + 3 O2(g) → 2SO3(g) −790kJWhere Does the Energy Come From?CH110 FA11 SAS 7Bond formation always exothermic∆Hrxndepends on the breaking and forming of bonds –BOND ENTHALPY = Bond breaking is always endothermicCovalent Bond Length and EnergyCH110 FA11 SAS 8BOND LENGTHbond lengthBOND ENERGY• As number of bonds increases, the bond gets ______ and _______.atom 1atom 2bond Bond energykJ/molBond length pmCC348 154C=C614 134C≡C839 121bondBond energykJ/molBond length pmCH413 110CCl328 176CBr276 196• As the size of the atoms increases, the bond gets ______ and ______ .Practice ProblemCH110 FA11 SAS 9Given the bond enthalpies (in kJ/mol):Which bond is the strongest?C-C 348 C=O 799C-H 413 O-H 463A) C-CB) C-HC) C=OD) O-HE) I don’t have enough information to answer this question.Estimating ΔHrxnfrom Bond Dissociation EnergiesCH110 FA11 SAS 10∆Hrxn= D(all bonds broken) – D(all bonds formed)EXAMPLEC CHH=R R′C CHHR R′+ Br2BrBr∆Hrxn=Does this Actually Work?CH110 FA11 SAS 11Calc. ∆Hrxn= What is the heat of combustion of CH4?CH4+ 2 O2 CO2+ 2 H2OActual ∆Hrxn(calorimetry) = -802 kJ/molDoes this Actually Work?CH110 FA11 SAS 12Calc. ∆Hrxn= What is the heat of reaction for the decomposition of hydrogen peroxide?2 H2O2 O2+ 2H2OActual ∆Hrxn= -196 kJ/molDemo: Peroxide DecompositionCH110 FA11 SAS 132 H2O2(l) → 2 H2O(l) + O2(g) ∆Hrxn= -196 kJ/molAdd KI to catalyze the reaction:I-(aq) + H2O2(l) → H2O(l) + IO-(aq)IO-(aq) + H2O2(l) → H2O(l) + O2(g)+ I-(aq)2 H2O2(l) → 2 H2O(l) + O2(g)Can also catalyze the reaction with MnO2Practice ProblemCH110 FA11 SAS 14You have the following reaction:C2H4+ HBr C2H5Br ΔHrxn= -57 kJAnd the bond dissociation energies (in kJ/mol) are:C-C 348 C=C 614 C-H 413 C-Br 276What is the bond dissociation energy of the H-Br bond?Standard Enthalpies of FormationCH110 FA11 SAS 15Bond Enthalpies and Bond LengthsCH110 FA11 SAS 16What You Should Know CH110 FA11 SAS 17Hess’s Law and Enthalpy of Reaction• What is Hess’s Law? Why is it OK to construct arbitrary thermodynamic cycles?• What is the standard enthalpy of a reaction and how can I calculate it?Bond Dissociation Energies• Where does the energy in a chemical reaction come from?• What is the relationship between bond length and bond energy? What determines bond length?• What are bond dissociation energies and how can I use them to predict the enthalpy of a
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