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Chapter 18

Chapter 18: Free Energy and Thermodynamics

Spontaneous and Nonspontaneous Processes

  • Spontaneous: proceed without any external influence
  • Nonspontaneous: needs an external influence to occur
  • A spontaneous process will drive towards equilibrium
  • These can be fast or slow. We are not talking about kinetics here, that was chapter 14. This is just us talking about IF it will happen, not the speed at which it will.
    • We are speaking about what is thermodynamically favorable

Enthalpy and Entropy

  • When we think about enthalpy, we can have a negative delta H function, exothermic and feels hot, or a positive delta H function, endothermic and feels cold.
  • Entropy is delta S. This is disorder/randomness (easiest way to call it, but it is the cheap and easy way)
    • The number of ways a system can be arranged
    • Boltzmann's Equation: $S = k\times lnW$
      • W is the number of ways that a state can be achieved
  • First law of thermodynamics
    • The total energy in the system and surrounds is constant
  • Second law of thermodynamics
    • The total entropy in the system and its surroundings will always increase for a spontaneous process

Entropy Changes

  • Entropy and temperature
    • As temperature increases, entropy increases
    • The entropy of a pure substance at 0K will be zero
    • There will be a steady increase in entropy with an increase in temperature, punctuated by discontinuous jumps at phase transitions
  • Entropy and state changes
    • Very large increase in entropy as we go from liquid to gas, with a large, but smaller, increase from solid to liquid
    • Very large decrease in entropy as we go from gas to liquid, with a large, but smaller, decrease from liquid to solid
  • Molecularity
    • The change in the number of particles
    • As molecularity increases, entropy increases as well
  • Entropy and state changes calculation
    • $\Delta S = \frac{Z_{rev}}{T}$
      • Entropy will have units of J/k
  • Heat transfer and energy changes of the surroundings
    • delta S of the university is equal to delta S of the system plus delta S of the surroundings
    • If you have an exothermic reaction, heat is going to go from the system to the surroundings
      • Positive delta S
    • If you have an endothermic reaction, heat will go from the surroundings to the system
      • Negative delta S
    • Spontaneous reactions have a delta S of the universe being greater than zero
    • Nonspontaneous reactions have a delta S of the universe being less than zero
    • At equilibrium, delta S of the universe is equal to zero
    • Temperature dependence of delta S of the surroundings
      • At higher temperatures, we see less change in delta S of the surroundings
      • At lower temperatures, we see more change in delta S of the surroundings
        • This is because it is a more meaningful change in energy
    • We can quantify entropy changes in the surroundings
      • delta S of the surroundings is equal to negative delta H of the system over T. This works at a constant P and T


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