CH131_12B

# CH131_12B - Exam 2 CH 131 Tuesday Oct 23 A D last name CAS...

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Exam 2 CH 131 Tuesday Oct. 23 A - D last name CAS 224 E - Z last name SCI 107 Useful Information will be posted on course website/course documents

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Chapter 12 (see HW assignment) Thermodynamics: study of inter-conversion of forms of energy When chemical activity is the vehicle for energy changes => thermochemistry Energy changes may be detected in 2 ways: 1. Appearance of heat 2. Performance of work Automobile engine => example of chem. energy transformed to both heat and work The relationship between heat and work is essential to considerations of energy changes. So let’s carefully define what we mean by heat & work…
Work : most common type is mechanical work => work = external force on a body x distance body moves due to force W = Fd In order to describe the sign convention: w > 0 or < 0 first define system and surroundings System : part of universe we are interested in; contains matter characterized by familiar parameters such as m, V, T, P Surroundings: rest of universe that system can exchange energy and matter with. Partitioning the universe is arbitrary but usually it is obvious for a given situation. Sign convention: work done by system on surroundings w < 0 work done on system by surroundings w > 0

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Pressure-volume work: w = –P ext V Neg. sign needed to be consistent w/ work sign convention => work done on system by surroundings > 0 Note: external pressure not necessarily pressure of gas that enters into above work expression For example; if P ext = 0, say in outer space, no useful (PV) work can be done => called free expansion of a gas W = 0 Units of work (energy) 1 Joule = 1 kg m 2 sec -2 Historical unit 1 cal = 4.184 J = energy needed to raise the temperature of H 2 O 1 o C; but J used preferentially now. Several forms of energy: electrical (coulomb), chemical, radiative, heat…may be viewed as manifestation of internal kinetic (due to motion) or potential (due to position in space) energy of particles
When work done on system => internal energy of system increases For a chemical system, molecular levels take up this energy. When heat added to system => internal energy of system increases. Overall, energy must be conserved: statement of First Law First Law of Thermodynamics => basically restatement of conversation of energy E = q + w q = heat transferred in or out of system w = work done by or to system Signs are critical to using the First Law; remember E = E f – E i If heat flows into system q > 0 E increases If heat flows out of system q < 0

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CH131_12B - Exam 2 CH 131 Tuesday Oct 23 A D last name CAS...

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