entropy

• entropy: a measure of the dispersal of energy in a system
• how physical state affects entropy:

  • solid particles have very little randomness - lowest entropy
  • gas particles have most random arrangements (more disordered) - highest entropy
• dissolving affects entropy:

  • dissolving a solid increases its entropy
  • particles can move freely, no longer held in one place (more disordered)
• more particles means more entropy:

  • more particles, more ways to arrange them and their entropy
  • eg N2O4 -> 2NO2
• entropy change of system: change between reactants and products
• total entropy change: sum of system and surroundings
• entropy of surroundings changes too as energy is transferred to or from the system
• reactions won't happen unless the total entropy change is positive
• some endothermic reactions are spontaneous (entropy increases enough), for spontaneous reactions free-energy change must be negative or zero
• free energy change depends upon:

  • entropy
  • enthalpy
  • temperature
• even if the free-energy change shows a reaction is feasible, it might have a high activation energy and be slow, so may not be noticeable it is happening
• total entropy change equation:

  $\Delta S_{total}=$$\Delta S_{system}+$$\Delta S_{surroundings}$
• free energy change equation:

  $\Delta G=$$\Delta H-T\Delta S$
• entropy change of surroundings equation:

  $\Delta S_{surroundings}=$$\frac{-\Delta H}{T}$
• entropy change of system equation:

  $\Delta S_{system}=$$\Delta S_{products}-\Delta S_{reactants}$