Ma PChem 9/28, 9/30, 10/2
Ma PChem 9/28, 9/30, 10/2 CHEM 345
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This 3 page Class Notes was uploaded by Kayli Antos on Friday October 2, 2015. The Class Notes belongs to CHEM 345 at Towson University taught by Dr. Ma in Summer 2015. Since its upload, it has received 15 views. For similar materials see Physical Chemistry in Chemistry at Towson University.
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Date Created: 10/02/15
P Chem Ma Fall 2015 9 The First Law Of Thermodynamics gtllt Derivative Problems x 1 x 1 m For when nab 1f 2 xquot dx xquot1x2 x quot1 x x1 n1 1 n1 X 1 X m For when n 1f 2 dx 1n 2 x1 x x1 T hermochemistry And Reaction Enthalpy z Standard States il For pure solids or liquids the standard state is at a pressure of one bar ll For pure gases the standard state is for hypothetical ideal gases at a pressure of one bar L When elements are in their most stable allotropic form at a pressure of one bar they can be given an arbitrary enthalpy and energy of zero il When expressing standard state values give the symbol and superscript of zero and a subscript of the temperature Like many other values we cannot know the absolute values which is why the lowest level is assigned zero and the difference in states in found z Heat Of Formation Standard Molar Enthalpy Of Formation il This is the change in enthalpy during the formation of one mole of a pure substance in its standard state from components that are also in their standard states il Symbolized as Arm il AH Z uAfm products Z uAfmUeactants where u is the stoichiometric coefficient Standard Enthalpy Of Combustion Acm ll This is the change in standard enthalpy for each mole of a combustible substance Thermochemieal Equation L A chemical equation that is accompanied by the change in enthalpy for said reaction The Enthalpy Of Phase Transitions Phase Changes L The opposite phase changes between the same two states will have the same absolute value for change in enthalpy but different signs il Melting vaporizing and sublimation all have positive values for change in enthalpy and can be written as AquHO AvapHO and AsubHO respectively L The change in enthalpy for freezing condensing and deposition are all negative values 22 22 22 i i 22 i i i 22 i 22 49 i i g i i The AHO term is used for one mole of a substance at constant temperature and pressure at the standard state The enthalpy change of melting and vaporization or freezing and condensing equal the enthalpy change of sublimation or deposition Calculation Of AU From AH Of A Reaction AH AU APV since pressure if often constant this can be written as AH AU PAV We can assume that solids and liquids do not contribute to the volume of a system and that gasses have ideal gas behavior With these assumptions we can then write AH AU AngasRT and since the temperature is usually constant the equation becomes AH AU RTAngas To find the change in energy the equation can be rearranged to AU AH RTAngas Solving Problems qP is equal to AH0 at constant pressure Temperature Dependence Of Reaction Enthalpy A small change in pressure produces an even smaller change in enthalpy that is so small that it is negligible so we can say that pressure is constant We know AH qp 11f fpdT AH can be represented as HT2 HT1 because enthalpy is a state function T T For a pure substance HT2 HT1 n IT deT or HT2 HT1 IT deT To find substance i in a mixture if T1To T2T T T HTi HT1L39 ITO CPidT 0r HTi HT1i ITO CPidT For a general reaction of aA bB cC dD the reaction enthalpy at temperature T is AHT c m d w a m b m T T T C HTO39C ITO Cp39CdT d HTO39D ITO CP39D 61HT039A ITO Cp39AdT T b Hm fTO CPIBdT AHT CHTW dHTOD aHTOIA 1917503 c fT 5Pch d fT EplDdT a fT EPAdT b fT EPleT AHTO L cc39m dim aim bc39PBdT Kirchoffs Law Equation AHTO IT AdeT 2 AHT If EP is a constant for each substance then AHT AHTO To AHT AH298 If standard state P1bar AH AH 98 ACP T 298 G The Second Law Of Thermodmamics The Probability OfA State Microstate z The molecules in a system can move into different arrangements z There are many more disordered microstates than there are ordered microstates
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