Chemical Reactions Thermochemistry Part 2 Prepare for Recitation December 1st ALEKS Objective 13 due November 29th ANGEL Quiz 13 December 1st Lecture 37 November 16th Lecture 38 November 18th Thanksgiving Break Lecture 39 November 28th Read Ch 4 6 10 5 Reactions 5 3 5 7 8 8 Thermo Additional Preparation Packet Rxns 12 18 BLB 10 57 BLB 5 4 17 29 37 39 53 55 83 85 Packet 5 1 7 BLB 8 65a 67ac 92a 18 72ab 74 Packet 8 12 14 CH110 FA11 SAS 1 Chemical Reactions Thermochemistry Part 2 KEY QUESTIONS THE BIG PICTURE What 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 chemistry Remember 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 By the end of Today s Lecture you should know Hess s Law Thermodynamic cycles Standard enthalpy of reaction Bond energy CH110 FA11 SAS 2 Hess s Law Hess s Law Reason Bottom line of Hess s Law You do not need to measure Hrxn for every reaction use Hfo for products and reactants from tables and calculate Remember Hrxn is for the i e a given stoichiometry The magnitude of Hrxn depends on the CH110 FA11 SAS 3 Hess s Law Example Smog H1 H2 N2 g 2 O2 g Hrxn 2 NO2 g N2 g 2 O2 g 2 NO2 g H Find two or more steps with known enthalpy and calculate Hrxn as Hrxn N2 g O2 g 2 NO g 2 NO g O2 g 2 NO2 g N2 g 2 O2 g 2 NO2 g H1 180 kJ H2 112 kJ Hrxn sum of H values gives H of CH110 FA11 SAS 4 Standard Enthalpy of Reaction Hrxno Hrxno n Hfo products m Hfo reactants an application of Hess s law EXAMPLE What is Hrxno for the reaction C2H5OH l 3 O2 g 2 CO2 g 3 H2O l Hfo values kJ mol and stoichiometries C graphite O2 g CO2 g Hfo H2 g O2 g H2O l Hfo C2H5OH l 3 H2 g O2 g 2 C graphite Hfo Hrxno CH110 FA11 SAS 5 Practice Problem Given the following 2SO2 g O2 g 2S s 3 O2 g 2SO3 g 2SO3 g H 196kJ 790kJ What is Hrxn for the following reaction S s O2 g SO2 g A B C D E 986 kJ 594 kJ 594 kJ 986 kJ 297 kJ CH110 FA11 SAS 6 Where Does the Energy Come From BOND ENTHALPY Hrxn depends on the breaking and forming of bonds Bond breaking is always endothermic Bond formation always exothermic CH110 FA11 SAS 7 Covalent Bond Length and Energy BOND LENGTH atom 1 atom 2 BOND ENERGY bond length As the size of the atoms increases the bond gets and bond Bond energy C H C Cl C Br kJ mol 413 328 276 Bond length pm 110 176 196 As number of bonds increases the bond gets and bond Bond energy kJ mol Bond length pm C C C C C C 348 614 839 154 134 121 CH110 FA11 SAS 8 Practice Problem Given the bond enthalpies in kJ mol C C 348 C O 799 C H 413 O H 463 Which bond is the strongest A B C D E C C C H C O O H I don t have enough information to answer this question CH110 FA11 SAS 9 Estimating Hrxn from Bond Dissociation Energies Hrxn D all bonds broken D all bonds formed EXAMPLE R C C H Br Br R Br2 H R C C H H R Hrxn CH110 FA11 SAS 10 Does this Actually Work What is the heat of combustion of CH4 CH4 2 O2 CO2 2 H2O Calc Hrxn Actual Hrxn calorimetry 802 kJ mol CH110 FA11 SAS 11 Does this Actually Work What is the heat of reaction for the decomposition of hydrogen peroxide 2 H2O2 O2 2H2O Calc Hrxn Actual Hrxn 196 kJ mol CH110 FA11 SAS 12 Demo Peroxide Decomposition 2 H2O2 l 2 H2O l O2 g Hrxn 196 kJ mol Add 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 MnO2 CH110 FA11 SAS 13 Practice Problem You have the following reaction C2H4 HBr C2H5Br Hrxn 57 kJ And the bond dissociation energies in kJ mol are C C 348 C C 614 C H 413 C Br 276 What is the bond dissociation energy of the H Br bond CH110 FA11 SAS 14 Standard Enthalpies of Formation CH110 FA11 SAS 15 Bond Enthalpies and Bond Lengths CH110 FA11 SAS 16 What You Should Know Hess 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 reaction CH110 FA11 SAS 17
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