ch19d1_reduced

ch19d1_reduced - Chapter 19 Chemical Thermodynamics...

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1 Chapter 19 Chemical Thermodynamics Kinetics How fast a rxn. proceeds Equilibrium How far a rxn proceeds towards completion Thermodynamics Study of energy relationships & changes which occur during chemical & physical processes
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2 A) First Law of Thermodynamics Law of Conservation of Energy : Energy can be neither created nor destroyed but may be converted from one form to another. Energy lost = Energy gained by system by surroundings 1) Internal Energy, E E = total energy of the system Actual value of E can not be determined Only ) E
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3 2) Relating ) E to Heat & Work 2 types of energy exchanges occur between system & surroundings Heat & Work + q : heat absorbed , endo thermic ! q : heat evolved , exo thermic + w : work done on the system ! w : work done by the system a) First Law ) E = q + w
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4 3) System and Surroundings System = portion we single out for study - focus attention on changes which occur w/in definite boundaries Surroundings = everything else System : Contents of rx. flask Surround. : Flask & everything outside it Agueous soln. rx : System : dissolved ions & molecules Surround : H 2 O that forms the soln.
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5 B) Thermodynamic State & State Functions Thermodynamic State of a System defined by completely specifying ALL properites of the system - P , V , T , composition , physical st. 1) State Function prop. of a system determined by specifying its state . depends only on its present conditions & NOT how it got there ) E = E final ! E initial independent of path taken to carry out the change - Also is an extensive prop .
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6 C) Enthalpy In ordinary chem. rx., work generally arises as a result of pressure - volume changes Inc. vol. & system does work against pressure of the atmosphere Constant Pressure w = ! P ) V Negative because work done by system
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7 ) E = q ! P ) V 1) ) E at Constant Volume ) E = q v 2) ) E at Constant Pressure : ) E = q p ! P ) V q p = ) E + P ) V
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8 3) Enthalpy , H H = E + P V Change in enthalpy at constant P is: ) H = ) E + P ) V & ) H = q p Can think of as heat content state fnc. & is extensive
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9 D) Enthalpies of Reaction ) H rxn = H products ! H reactants 2 H 2 ( g ) + O 2 ( g ) 6 2 H 2 O( R ) ) H = ! 572 kJ Thermochemical eqn. Physical states are given and energy associated w. rx. written to right - MUST give physical states If product is H 2 O( g ), ) H = ! 4 84 kJ ) H corresponds to molar quantities given in eqn. as written H 2 ( g ) + ½ O 2 ( g ) 6 H 2 O( R ) ) H = ! 286 kJ
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10 1) Guidelines a) Enthalpy is extensive Multiply a rxn by some factor the ) H is multiplied by that factor b) ) H reverse = ! ) H forward c) Enthalpy is a state function ) H depends on the states of reactants and products .
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11 E) Hess’s Law ) H is a state fnc. Same whether the process occurs as a single step or as a series of steps The ) H rxn is the sum of the ) H ’s for the individual steps. ) H rx = G ) H steps Steps * Add chem. eqn’s for steps to get overall rxn .
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ch19d1_reduced - Chapter 19 Chemical Thermodynamics...

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