chap 18

• Thermodynamics


  Concerned with energy changes - the flow of energy from one substance to another
• First Law of Thermodynamics


  • Internal energy may be transferred as heat, q, or work, w, but cannot be created or destroyed
• Internal energy (E)


  A state function which means that its value does not depend on how the change from one state to another was carried out
• Energy flows into a system


  DE is positive
• Energy flows out of a system


  DE is negative
• Work is done on a system


  The system gains and stores energy
• The system does work on the surroundings

  The system loses some energy
• 1. V work


  The work done by a system depends on the volume change and the external pressure
• qv


  Heat at constant volume
• Enthalpy (H)


  More convenient for reactions carried out at fixed pressure
• Volume change occurs for the system


  The internal energy and enthalpy changes are different
• Spontaneous change


  A change that occurs by itself (without continuous outside assistance)
• Most, but not all, exothermic reactions are spontaneous
• Entropy (S)


  Used in thermodynamics to describe the number of equivalent ways that the energy can be distributed
• Entropy change (DS)


  An event that is accompanied by an increase in the entropy of the system will have a tendency to occur spontaneously
• Factors affecting entropy change


  • Volume increase
  • Temperature increase
  • Solid to liquid to gas
• Reaction that increases the number of particles in the system


  Tends to have a positive entropy change
• Second Law of Thermodynamics


  Whenever a spontaneous event takes place in the universe, the total entropy of the universe increases
• Gibbs free energy (G)


  Used to determine if events are spontaneous
• Spontaneous change at constant T and P

  Accompanied by a decrease in the free energy of the system (DG < 0)
• Third Law of Thermodynamics

  At absolute zero the entropy of a perfectly ordered crystalline substance is zero
• Standard entropy (S°)


  Entropy of 1 mol of a substance at 298 K (25 °C) and 1 atm
• Standard entropy of formation (DSf°)

  Calculated standard entropy change for the formation of 1 mol of a compound from its elements
• Molecular motion


  Translational, vibrational, rotational
• Entropies


  Molar entropy values of substances in their standard states
• Standard entropies tend to increase with increasing molar mass
• Larger and more complex molecules have greater entropies
• Types of molecular motion


  • Translational
  • Vibrational
  • Rotational
• Standard entropies of formation

  Not tabulated, must be calculated when needed
• Standard free energy change

  Determined at 298 K and 1 atm
• Thermodynamic reversibility


  A system is changed in such a way that the system and surroundings can be restored to their original state by exactly reversing the change
• Reversible process


  Produces the maximum amount of work that can be achieved by the system on the surroundings
• Irreversible processes


  Cannot be undone by exactly reversing the change to the system
• Spontaneous processes are irreversible
• Free energy change


  Provides a limit to the amount of available energy in a reaction
• Maximum amount of energy produced by a reaction that can be theoretically harnessed as work


  Equal to DG
• Equilibrium


  When the free energy change is zero
• No work can be done by a system at equilibrium because the available (free) energy is zero
• Only one temperature is possible for a phase change at equilibrium
• Free energy change diagrams


  The minimum on the curve indicates the composition of the reaction mixture at equilibrium