chapt06_lec

chapt06_lec - 6-1Chapter 6Thermochemistry:Energy Flow and...

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Unformatted text preview: 6-1Chapter 6Thermochemistry:Energy Flow and Chemical Change6-2Thermochemistry: Energy Flow and Chemical Change6.1Forms of Energy and Their Interconversion6.2Enthalpy: Heats of Reaction and Chemical Change6.3Calorimetry: Laboratory Measurement of Heats of Reaction6.4Stoichiometry of Thermochemical Equations6.5Hess’s Law of Heat Summation6.6Standard Heats of Reaction (∆Hrxn)6-3Thermochemistry: Energy Flow and Chemical ChangeIn the observation and measurement of a change in energy, we talk about the Systemand the Surroundings.The Systemis what we are studying, e.g. a reaction taking place in a flask.The Surroundingsare the rest of the universe, but generally only the nearby portion that is relevant to the system.6-4Change in energy, ∆E = Efinal- Einitial= Eproducts- EreactantsEnergy diagrams for the transfer of internal energy (E) between a system and its surroundings.A change in energy in the System is always accompanied by an oppositechange in energy of the Surroundings.6-5In a system transferringenergy as heat only.∆E = q+ wHeat (thermal energy) and Work, Two Forms of ∆EThe symbol for heat is q. The symbol for work is w.They can be positiveor negativevalues.Energy flowing into a system is defined as positive.Energy flowing out of a system is negative.∆E = q+ 6-6A system losing energy as work only.∆E = + wWork is done on the surroundings, so work on the system is negative.6-7The Sign Conventions* for q, wand ∆Eqw+=∆E++----+++-depends on sizesof qand wdepends on sizesof qand w* For q: + means system gainsheat; - means system losesheat.* For w: + means word done onsystem; - means work done bysystem.6-8∆Euniverse= ∆Esystem+ ∆Esurroundings = 0Units of EnergyJoule (J)Calorie (cal)British Thermal Unit1 cal = 4.18J1 J = 1 kg*m2/s2 (mass x accel x dist)1 Btu = 1055 JLaw of Conservation of Energy(First Law of Thermodynamics)The total energy of the universe is constant.Energy transfers can occur between the system and surroundings in the form of heat and/or work. But the energy of the system and the energy of the surroundings remains constant; energy is conserved.6-9Sample Problem 6.1Determining the Change in Internal Energy of a SystemPROBLEM:When gasoline burns in a car engine, the heat released causes the products CO2and H2O to expand, which pushes the pistons outward. Excess heat is removed by the car’s cooling system. If the expanding gases do 451J of work on the pistons and the system loses 325J to the surroundings as heat, calculate the change in energy (∆E) in J, kJ, and kcal.SOLUTION:PLAN:Define system and surrounds, assign signs to q and wand calculate ∆E. The answer should be converted from J to kJ and then to kcal.q= - 325Jw= - 451J∆E = q+ w=-325J + (-451J) = -776J-776J103JkJ= 0.776kJ0.776kJ4.18kJkcal= 0.186kcal6-10State functions such as internal energy of a system are dependent only upon the initial state and the final state....
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chapt06_lec - 6-1Chapter 6Thermochemistry:Energy Flow and...

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