Enduring Understanding 5.E.1: Entropy

  • Entropy can be understood, qualitatively, as the amount of disorder in a system.
  • Chemical and physical processes can result in a change in entropy, denoted as ΔS.
  • Entropy increases as particles become more randomly dispersed.
  • Phase changes from solid to liquid, or solid to gas, or liquid to gas, all involve an increase in entropy - the particles have more freedom to move.
  • Reactions that produce an increase in the number of particles - more particles in products than reactants - result in an increase in entropy.
  • Example: A → 2B would likely involve an increase in entropy, while 2B → A would involve a decrease in entropy.
  • Increasing the volume of a gas sample (at constant temperature) increases the entropy of the gas.
  • Increasing the temperature of a gas increases the dispersion of the kinetic energy of the gas particles (See Maxwell-Boltzmann Distribution). This results in an increase in entropy; therefore increasing temperature of a gas increases the entropy.
  • Sample Question: Which of the following transformations would involve an increase in entropy, and which would involve a decrease?
  • CO2(s) → CO2(g)
    N2(g) + 3H2(g) → 2NH3(g)
  • The sublimation of carbon dioxide, CO2(s) → CO2(g), is a solid changing into a gas. The particles have much more freedom to move, so this is an increase in entropy.
  • The reaction of nitrogen and hydrogen gas to form ammonia, N2(g) + 3H2(g) → 2NH3(g), involves four gas molecules becoming two gas molecules. A reaction that decreases the number of gas particles involves a decrease in entropy.
  • Enthalpy and entropy can be used to determine if a given chemical reaction or process is favored (equilibrium lies towards products) or disfavored (equilibrium lies towards reactants).

Related Links:
Chemistry Quizzes
AP Chemistry Notes
Reaction Energies

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