Chem 1211 1st Edition Lecture 15 Outline of Last Lecture I Specific Heat II State Functions and Changes of State III Thermochemical Equations Outline of Current Lecture I Thermochemical equations continued Current Lecture I Thermochemical Equations Continued a Thermochemical equations balances chemical reaction plus the delta Hrxn b C5H12 8 O2 5 CO2 6 H2O 3523 kJ c Heat released during reaction 3523 kJ assuming stoichiometric units are moles 1 mole of C5H12 reacts with 8 moles of O2 to produce 5 moles of CO2 6 moles of H2 O d Change in enthalpy heat e Exothermic heat produced f ConcepTest What is the enthalpy change for the first reaction i 2 C s O2 g 2 CO g delta Hrxn ii 2 CO g O2 g 2 CO2 g delta Hrxn 566kJ iii C s O2 g CO2 g delta Hrxn 393kJ 1 ANSWER 221 kJ 2 Reasoning a Double everything in the third equation gives you 786 kJ b Delta H1 Delta H2 Delta H3 c 566 786 d 220 g Example Hess s Law i A B C delta H x ii 2 A 2 B 2 C delta H 2x These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute iii 5 A 5 5C delta H 5x 1 C A B delta H x h Hrxn n deltaH products n deltaH reactants i n stoichiometric coefficients i ConcepTest Write the equation for the reaction of aluminum with oxygen to form Al2O3 i ANSWER 4 Al s 3 O g 2 Al2O3 s j Bond something that holds elements together i When you break them it costs energy k ConcepTest Calculate the enthalpy change for the reaction in which 30 0 g of aluminum reacts with oxygen to form Al2O3 at 25 degrees Celsius and one atmosphere i 4 Al s 3 O g 2 Al2O3 s delta Hf Al2O3 1676 kJ mol ii ANSWER Use stoichiometry Convert 30 grams of Al to 1 11 moles Al It takes two moles of Al to get one mole of Al2O3 according to the equation so divide 1 11 by 2 to get 555 moles Al 2O3 Multiply 555 by 1676 kJ moles to get your answer approx 930 2 kJ l ConcepTest For this reaction which delta Hf values do we need from the table i C3H8 g 5 O2 g 3 CO2 g 4 H2O l 1 Propane CO2 and water need to know all molecules
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