Chemistry- SPONTANEITY NOTES

  • Once a wound-up toy is released, it runs until the stored energy is released
    • The toy never rewinds itself up
    • Human intervention is necessary to run it again
  • Rewinding of the spring is nonspontaneous

Spontaneous vs. Nonspontaneous

  • A spontaneous process is one that occurs in a system left to itself once started
    • No action from outside the system is necessary to make the process continue
      • Ex. rusting of the nail
  • Nonspontaneous processes will not occur unless external action is continuously applied
    • Ex. rusted nail becoming shiny again

Spontaneous Processes:

  • Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures
  • Above O, it is spontaneous for ice to melt
  • Below, the reverse process is spontaneous
  • Many exothermic reactions are spontaneous
  • But not really…
    • Melting of ice at room temperature
    • Evaporation of water
    • Dissolving of ammonium nitrate in water
      • All endothermic and spontaneous

Entropy:

  • The thermodynamic property related to how the energy of a system is distributed among available microscopic energy levels

Entropy on the Molecular Scale:

  • Temperature is a measure of the average kinetic energy of the molecules in a sample
  • Molecules exhibit several types of motion (microrotations):
    • Vibrational: periodic motion of atoms within a molecule
    • Rotational: rotation of the molecule about an axis or rotation about bonds
    • Translational: movement of the entire molecule from one place to another
  • Nature spontaneously proceeds towards the states that have the highest probabilities of existing
  • The driving force for a spontaneous process is an increase in the entropy of the universe
  • The number of microstates, and therefore the entropy, tends to increase with increases in…
    • Temperature
    • Volume
    • Number of independently moving molecules

Predict the sign of S for each of the following, and explain:

  1. Evaporation of alcohol - +S
  2. Freezing of water - -S
  3. Compressing an ideal gas at constant temperature - -S
  4. Heating an ideal gas at constant pressure - +S
  5. Dissolving NaCl in water - +S

Entropy and Physical States:

  • Entropy increases with the freedom of motion of molecules
  • Generally, when a solid is dissolved in a solvent, entropy increases
  • In general, entropy increases when…
    • Gases are formed from liquids and solids
    • Liquids or solutions are formed from solids
    • The number of gas molecules increases
    • The number of moles increases

The Third Law of Thermodynamics:

  • The entropy of a pure crystalline substance at absolute zero is 0