CH 301 Chapter 6 notes part 2v3

CH 301 Chapter 6 notes part 2v3 - CH301 Notes Chapter 6...

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CH301 Notes Chapter 6 part 2 WORK for a chemical reaction: For a chemical reaction, expansion work done may be found using: w = -P Δ V = - Δ nRT and the reaction! There are 3 ways to find w ; which is used depends on the information given: ALL for isothermal processes! For an ideal gas expanding vs. constant P ext : w = -P ext Δ V (if P ext = 0 ; a vaccum; w = 0) For an ideal gas expanding reversibly: w = -nRT ln (V f /V i ) For a chemical reaction: w = - Δ nRT (find Δ n from equation) Example: Consider this reaction at 200 º C 2NO (g) + O 2(g) --> 2NO 2(g) Origin of Internal Energy, U. T is proportional to <KE> of gas molecules and v rms As T increases, U increases. KE due to Translational, Vibrational and Rotational motion Each motion has a set of energy levels associated with it. At ~298K, molecules in lowest vibrational levels – ignore. Vibration and rotation are meaningless for single atoms. Each motion has different forms: modes. Per mole of molecules , each mode contributes 1/2RT to the internal energy. Vibrational : modes are complex –not discussed. Translation : modes are motions in x,y,z directions. Three modes total, no matter the sample . Rotation : modes are rotation around x,y,z axes. A linear molecule has TWO modes . A non-linear molecule has THREE nodes . Atoms: NO rotational modes! U m : Molar internal energy ALL HAVE UNITS OF J/mol MONOATOMIC GAS: U m = 3/2 RT (translational) GAS of LINEAR MOLECULES : U m = 3/2 RT (translational) + RT (rotational) = 5/2 RT GAS of NON-LINEAR MOLECULES : U m = 3/2 RT (translational) + 3/2RT (rotational) = 3 RT Contribution to U m also from intermolecular interactions - depends on spacing ( sample volume.) Enthalpy Chemistry is done in open beakers at atmospheric pressure. Pressure changes are small – so assume constant pressure. Enthalpy change Δ H: amount of heat transferred in to or out of a system undergoing a chemical or physical change at constant pressure . Also in use: “heat change” or “heat of….(process)” We CANNOT measure absolute values of H, so: Δ H = H final – H initial or Δ H = H products – H reactants Δ H determines change in heat content at constant pressure . Shows if process (physical change (e.g. condensation) or a chemical reaction (e.g. combustion)) produces or consumes heat. If Δ H < 0 the process is EXOTHERMIC . If Δ H > 0 the process is ENDOTHERMIC . Relationship of Δ H and Δ U Total amount of heat energy that system can provide to surroundings at constant T & P: Δ H= Δ U + P Δ V Δ H = change in enthalpy of system Δ U = change in internal energy of system P Δ V = work done by system Relation between Δ H, Δ U and n : see later in notes: Δ H= Δ U + Δ nRT
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Energy changes at constant P,T: Δ H= Δ U + P Δ V But Δ U = q + w so: Δ H = q + w + P Δ V If system only does expansion work, w = -P ext Δ V giving Δ H = q - P ext Δ V + P Δ V If open to the atmosphere, P ext = P At constant pressure Δ H = q.
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This note was uploaded on 09/02/2009 for the course CHE 301 taught by Professor Fatimafakhreddine during the Spring '08 term at University of Texas at Austin.

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CH 301 Chapter 6 notes part 2v3 - CH301 Notes Chapter 6...

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