Entropy: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • 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.
  • 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.
  • 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.
  • For a reversible process, ΔSsys + ΔSsurr = 0.
  • 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.
  • The entropy of mixing of ideal gases at constant pressure and constant temperature is positive because the partial pressure of each gas decreases.
  • For an adiabatic, reversible process, ΔSsystem = 0.
  • The entropy change for a phase change is ΔH/T.
From studying this module, you should now be able to:
  • Calculate the entropy change for reversible processes using the heat transferred and the temperature
  • Calculate entropy changes for mixing ideal gases
  • Calculate entropy changes for ideal gases when temperature and/or pressure change.
  • Calculate entropy changes for phase transitions.
  • Calculate entropy changes for liquids and solids as the temperature changes.
  • Calculate the entropy change for an irreversible process by devising a reversible process between the initial and final conditions.
  • Explain why work can be converted continuously completely into heat, but heat cannot be continuously converted completely into work.
  • Predict if a process is possible based on application of the second law.
  • Explain the second law in terms of entropy changes for system and surroundings.
  • 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