Chemical Equilibrium for Multiple Reactions: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Chemical equilibrium corresponds to the composition that has the lowest Gibbs free energy.
  • The Gibbs free energy for a reaction changes with temperature, and the heat of reaction determines how it changes with temperature.
  • Most reaction systems involve multiple reactions.
  • Equilibrium compositions can be determined using either equilibrium constants and extents of reaction or by minimizing Gibbs free energy.
  • When multiple reactions occur simultaneously, minimization of Gibbs free energy may be an easier method to determine equilibrium compositions. This approach does not require the individual reactions to be identified, just the species.
  • Calculated equilibrium compositions depend on which components (or which reactions) are included in the calculation. Often some reactions or products are not included in the equilibrium calculations because their rates of reaction are too low; i.e., kinetics can limit which reactions or products are included in equilibrium calculations.
From studying this module, you should now be able to:
  • Calculate Gibbs free energy of formation at standard conditions as a function of temperature.
  • Calculate Gibbs free energy as a function of mole fraction and pressure.
  • Use minimization of Gibbs free energy to determine equilibrium compositions for one or more simultaneous reactions.
Additional Resources:

Screencast: Gibbs free energy of a chemical reaction as a function of temperature
Plots the Gibbs free energy divided by RT (nG/RT) for the water-gas shift reaction (CO + H2O –> CO2 + H2) at 1-bar pressure as a function of fractional conversion at several temperatures. The spreadsheet used is Plot Gibbs Free Energy vs Reaction Extent.

NIST Chemistry WebBook 
Excellent source of heat capacities, heats of formation, and Gibbs free energies of formation.

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder