chem chapter 18

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chem chapter 18
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2014-02-06 16:21:24
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  1. internal energy U
    is the sum of the kinetic and potential energies of the particles making up the sustem
  2. kinetic energy includes
    the energy of motion of eleectrons nuclei and molecules
  3. potential energy includes
    • chemical bonding of atoms
    • attractions between molecules
  4. internal energy is a state function which is
    a property of a system that depends only on its present state which is completely determined by variables such a s temperature and pressure
  5. calculate the change in internal energy
    deltaU = Uf-Ui
  6. two types of echanges of energy
    heat and work
  7. heat
    the energy that moves into or out of the system because of a temperature difference between the system and its surroundings
  8. work
    • the energy exchange that results when a force F moves an object through a distance d; 
    • w=df
  9. the first law of thermodynamics
    • deltaU = q+w
    • the change in internal energy of a system equals q+w
  10. subscript p indicates
    process occurs at constant pressure (pressure of the atmosphere)
  11. formula for work in an open vessel
    w=-P(deltaV)
  12. relating work and internal energy
    deltaU = qp +w = qp -P(deltaV)
  13. enthalpy
    the quantity U+PV 

    all are state functions
  14. enthalpy formulas
    deltaH =Hf -Hi

    deltaH = (deltaU)=P(deltaV)
  15. spontaneous process
    a physical or chemical change that occurs by itself
  16. non spontaneous
    reactions go in the opposite direction
  17. entropy (S)
    is a thermodynamic quantity that is a measure of how dispersed the energy of a system is among the different possible ways that a system can contain energy
  18. units for entropy
    • joules per kelvin 
    • state function
  19. calculate entropy
    deltaS=Sf-Si
  20. the entropy of a solid is _________ than the entropy of a liquid
    • lower
    • less movement in molecules
    • fixed therefore energy is fixed
  21. second law of thermodynamics
    the total entropy of a system and its surroundings always increases for a spontaneous process
  22. difference between energy and entropy (energy dispersion)
    • energy can not be created nor destroyed
    • entropy can be created
  23. heat flow is also
    a flow of entropy because it is a dispersal of energy into the flask or outside of it 

    deltaS = entropy created +q/T
  24. the quantity of entropy created during a spontaneous process
    is positive
  25. restated second law of thermodynamics
    for a spontaneous process at a given temperature T the change in entropy of the system is greater than the heat divided by the absolute temperature q/T
  26. under equilibrium conditions n
    • no significant amount of entropy is created 
    • ex water and ice at 0 degrees C and vaporization of liquid 

    DeltaS =q/T (equilibrium process)
  27. to determine if a reaction is spontaneous
    • delta S > DeltaH/T
    • at constant pressure and temperature 

    also if DeltaH-T(deltaS) is negative then it is spontaneous as written 

    if it equals zero then it is at equilibrium
  28. third law of thermodynamics
    a substance that is perfectly crystalline at 0 K has an entropy of zero 

    should have perfect order
  29. standard entropy / absolute entropy
    Sis the entropy value for the standard state of the species
  30. entropy increases when:
    • 1. a reaction in which a molecule is broken into two or more smaller molecules
    • 2. a reaction in which there is an increase in moles of gas
    • 3. a process in which a solid changes to liquid or a liquid changes to a gas
  31. free energy G
    thermodynamic quantity defined by the equation G=H-TS

    this quantity gives a direct criterion for spontaneity of reaction
  32. values of G
    • negative -- spontaneous
    • positive -- non spontaneous
    • zero -- equilibrium
  33. standard free energy of formation
    the free energy change that occurs when 1 mol of substance is formed from its elements in their stables states at 1 atm and at a specified temperature (usually 25 degrees C or 298 K)
  34. DeltaGas a criterion for spontaneity -
    when it is a large negative number more than -10 KJ
    the reaction is spontaneous as written and reactants transform almost entirely to products when equilibrium is reached
  35. DeltaG0 as a criterion for spontaneity -when it is a large positive number more than 10 KJ
    the reaction is non spontaneous as written and the reactants do not give significant amounts of products at equilibrium
  36. DeltaG0 as a criterion for spontaneity -when it is a small positive or small negative number less than -10 KJ and 10 KJ
    the reaction gives an equilibrium mixture with significant amounts of both reactants and products
  37. spontaneous reactions are used to
    obtain useful work -- energy that can be used directly to move objects
  38. to obtain maximum useful work
    no entropy is produced

    wmax=DeltaG
  39. free energy
    the maximum energy available to do useful work
  40. thermodynamic equilibrium constant K
    the equilibrium constant in which the concentrations of gases are expressed in partial pressures in atmospheres whereas the concentration of solutes in liquid solutions are expressed in molarities
  41. obtain delta G from the following equation
    DeltaG=-RTlnK

    • G -- free energy
    • R -- constant 8.3145 
    • T -- temperature
    • K -- constant
  42. H is negative 
    S is positive
    G is negative
    spontaneous at all T
  43. H positive 
    S negative
    G positive
    nonspontaneous at all T
  44. H negative 
    S negative 
    G either positive or negative
    • spontaneous at low T 
    • non spontaneous at high T
  45. H positive
    S positive
    G either positive or negative
    • spontaneous at high T
    • non spontaneous at low T

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