A First Look At Equilibrium

In the Martian environment, water will be a precious resource, both for human consumption and as a source of H2 to generate fuel.  An analysis of water evaporating is a good place to start to gain an understanding of equilibrium.

If a glass of water is left standing, after a few days most, if not all, of the water in it will have evaporated.  Yet if the same glass had been covered with plastic wrap or a stopper the water level would not change.

The reason may seem obvious, for there is no way the molecules of water can escape when the glass is sealed.  However, a closer examination of what is happening with the water molecules can help us to understand the concept of chemical equilibrium.  Why do reactions stop before all of the reactant molecules have turned into products – the liquid water in this case into water vapor?

H2O (l) H2O (g)

Click on video icon for an explanation of equilibrium
Water vapor equilibrium
Water vapor equilibrium (higher resolution)  Water vapor equilibrium (lower resolution)  Click to read script

Equilibrium is a state of balance in a closed system.  We recognize a system is in equilibrium because we can observe its constant macroscopic properties such as unchanging color, pressure, or concentration.  A reaction at equilibrium appears to stop before all the reactants turn into products.

Your goal in this chapter should be to understand how reactions approach equilibrium, and the role of reaction rates.

By the time you have completed this chapter you should be able to:

  • describe and give examples of a reversible reaction
  • describe the difference between an open and closed system, and a steady state and an equilibrium
  • describe an equilibrium in terms of observable properties
  • explain equilibrium in terms of equal forward and reverse reaction rates
  • explain why apparently unchanging reactions may not be at equilibrium
  • explain how forward and reverse reaction can be used to prove if a reaction is at equilibrium
  • state what the coefficients in a balanced equation mean, and what they do not tell you
  • carry out simulations of equilibrium using non-chemical analogies, and graph the results
  • carry out and analyze a computer simulation of an equilibrium reaction in the gas phase
  • explain why reactions take some time to reach equilibrium
  • explain what is meant by "dynamic" equilibrium
  • describe how radioactive tracers can be used to demonstrate the dynamic nature of equilibrium