CHEM 132, Chapter 16

• Chemical kinetics


  The pathway between reactants and products that determines reaction rate
• Thermodynamics


  Predicts the direction in which a process will occur
• First law of thermodynamics

  • Energy can be converted from one form to another but energy cannot be created or destroyed
• Second law of thermodynamics

  • The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process
• Third law of thermodynamics

  • The entropy of a perfect crystalline substance is zero at the absolute zero of temperature
• Entropy (S)


  A measure of the randomness/disorder of a system
• Increased order


  Decreases entropy (S)
• Increased disorder


  Increases entropy (S)
• Spontaneous processes


  Occur without outside intervention and are thermodynamically favored
• Spontaneous processes


  Caused by increase in entropy of the universe
• Formation of solutions


  Increases entropy
• Phases of matter


  Increase entropy from solid to liquid to gas
• In any spontaneous process, there is always an increase in the entropy of the universe
• Delta S-univ >0


  Entropy of universe increases and the process is spontaneous
• Delta S-univ <0


  Entropy of universe decreases, process is spontaneous in opposite direction
• Delta S-univ =0


  Entropy of universe is unchanged and the process has no tendency to occur and system is at equilibrium
• Reaction involves gas molecules

  Change in entropy is dominated by relative number of molecules of gaseous reactants and products
• Number of molecules of gases products is greater than the number of molecules of gaseous reactants

  Entropy of system typically increases
• Entropy of a perfect crystal at 0K is zero
• Entropy of a substance

  Increases with temperature
• Delta S-surr


  Depends on temperature
• At constant pressure, heat flow

  Change in enthalpy (delta H)
• Delta G <0


  Reaction is spontaneous in the forward direction
• Delta G >0


  Reaction is spontaneous in reverse direction
• Delta G =0


  Reaction is at equilibrium
• Standard free energy of formation

  Change in free energy from the formation of 1 mole of the substance from its constituent elements in their standard states
• Equilibrium position represents the lowest free energy value available to a system
• Free energy


  Dependent on pressure of a gas or on concentration of species in solution, so its value changes as the reaction proceeds
• For one mole of ideal gas, S-large volume

  Greater than S-small volume
• Pressure and volume are inversely related


  1. low pressure greater than S-high pressure
• Entropy and free energy of an ideal gas depend on its pressure
• Systems achieve lowest free energy by reaching equilibrium, not by going to completion
• Delta G


  Predicts whether products or reactants are favored under a set of conditions
• Equilibrium is the lowest value of free energy available to system
• When substances undergo chemical reactions

  System proceeds to minimum free energy (equilibrium), which corresponds to delta G =0
• Maximum possible useful work from a process at constant temperature and pressure

  Equal to change in free energy
• Delta G for a spontaneous process


  The free energy to do useful work
• Delta G for a non spontaneous process


  The minimum amount of work that must be used to make the process occur
• Reversible process


  When the universe (system and surroundings) is exactly the same as it was before a cyclic process
• Irreversible process


  When the universe is different after a cyclic process is performed