chm115_lecture13

chm115_lecture13 - 15-1Chemistry 115Lecture 13...

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Unformatted text preview: 15-1Chemistry 115Lecture 13 OutlineChapter 5CalorimetryThermochemical EqnsHess’s LawStandard Heats of ReactionHW 6: Due Wed March 2, 11 pm.Recitation: Review Chapter 514-2Where does heat come from in reactions?C8H18+O2→8CO2+ 9H2O + 5450 kJ/mol252Not all bonds have the same energyC-C C-H C-ODifference between breaking and making bonds gives enthalpy change.∆HcombBreaking food bonds: C6H22O1116.2 kJ →4 kcaloil8.8 kcalbutter7 kcal∆H = -5450 kJ/mol lots of energy! (Heat)Answer: The Bonds14-3Heats of CombustionBreaking and Making BondsCompoundethane ethanolmethanemethanolMolecular formulaC2H6C2H5OHCH4CH3OH# of C-C and C-H bonds 7643# of C-O and O-H bonds22ΔHcomb (kJ/mol)-1560-1367 -890-727ΔHcomb(kJ/g)-52-30 -56-2314-4Breaking food bondsFats: 9 kcal/g, or 9 food calories/gSugars: 4 kcal/gProteins: 4 kcal/g(Alcohol: 7 kcal/g)14-5How do we measure heat of combustion ∆Hrxncomb?We use a calorimeter.calorimeter“Universe”watersurroundingsinsulationsystemheatO2rxn“bomb”C6H22O11sugarC12H22O11+ 12O2→12CO2+ 11H2O∆H = -16.2 kJ/gm = -4 kcal/gm or 4 food calories∆H = 16.2 kJ/gm ×392 g/mol= 5540 kJ/molCalorimetrymeasuring heat in caloriesExample:All heat given off by the system is absorbed by the surroundings.∆H universe = 0So water heats up.Change measured as rise of water temperature.14-6Coffee-cup calorimeterFor student experiments14-7A bomb calorimeterGives more accurate results 14-8Heat CapacityHeat flow to an object proportionally increases the temperature of that object :q = constant x ∆TWe call the constant of proportionality the heat capacityof that object.If we divide by the mass, we have the specific heat capacity:Specific heat capacity =qmass x ∆T= cq = c x mass x ∆TSo:If we use moles instead of mass, we have the Molar Heat capacity, Cc for water = 1.00 cal/g-K14-9Example:A chemical engineer studying the properties of fuels placed 1.500g of a hydrocarbon in the bomb of a calorimeter and filled it with oxygen gas. The bomb was immersed in 2.500 L of water and the reaction initiated. The water temperature rose from 20.00 °C to 23.55 °C. What was the heat of combustion per gram of the fuel?...
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chm115_lecture13 - 15-1Chemistry 115Lecture 13...

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