Entropy: Summary
The answers to the ConcepTests are given below and will open in a separate window. 

Key points from this module:
  1. To calculate the entropy change of a system for an irreversible process between two states, create a reversible pathway between the same states and calculate the entropy change for the reversible pathway.
  2. The entropy change of the surroundings is most often Q/Tsurr, where Q is the heat transferred to the surroundings and Tsurr is the surroundings temperature. This is true even if the heat transfer is reversible.
  3. The second law of thermodynamics: the total entropy change of the system plus surrounding is greater than or equal to zero. That is, for a real process to be possible, the total entropy change of the system plus surroundings must be greater than zero.
  4. For a reversible process, ΔSsys + ΔSsurr = 0.
  5. The entropy change of a system for an irreversible process can be negative as long as the entropy change of the surroundings is large enough so that ΔStotal > 0.
  6. The entropy of mixing of ideal gases at constant pressure and constant temperature is positive because the partial pressure of each gas decreases.
  7. For an adiabatic, reversible process, ΔSsystem = 0.
  8. The entropy change for a phase change is ΔH/T.

From studying this module, you should now be able to:
  1. Calculate the entropy change for reversible processes using the heat transferred and the temperature
  2. Calculate entropy changes for mixing ideal gases
  3. Calculate entropy changes for ideal gases when temperature and/or pressure change.
  4. Calculate entropy changes for phase transitions.
  5. Calculate entropy changes for liquids and solids as the temperature changes.
  6. Calculate the entropy change for an irreversible process by devising a reversible process between the initial and final conditions.
  7. Explain why work can be converted continuously completely into heat, but heat cannot be continuously converted completely into work.
  8. Predict if a process is possible based on application of the second law.
  9. Explain the second law in terms of entropy changes for system and surroundings.
  10. Explain why the entropy change is zero for a system that undergoes an adiabatic reversible process.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder