This model is the sister model to LeChatelier's Principle 1. In LeChatelier's Principle 1, we saw how changes to variables such as temperature, volume, and concentration affected the rate at which a chemical reaction reached an equilibrium state. LeChatelierŐs Principle 2 models the same phenomenon based upon a physical separation.

LeChatelier's Principle 2 investigates some laboratory methods that chemists use to purify chemicals. Most of these methods are based upon physical properties of molecular separation. The same principles that affect chemical equilibrium affect physical equilibrium as well. By changing the variables of temperature, volume, and concentration, we can affect not only the speed at which a system reaches equilibrium, but also the nature of the distribution ratio. In this model, we watch how these factors affect the physical distribution of red molecules that are considered "dissolved" in a blue solvent.


Setup the model by pressing either the SETUP-RANDOM or the SETUP-SIDE buttons at the top of the Interface window. SETUP-RANDOM distributes all the molecules randomly around the screen. SETUP-SIDE distributes the blue molecules evenly, while placing the red molecules on the right side of the screen.

Press bounce to watch the molecules move about the screen as they achieve equilibrium. By pressing DO-PLOT you can track the relative concentrations of each color molecule on each side of the central divider. If the red line dips below 0, there are more red molecules on the left side of the divider than on the right. If it rises above 0, there are more red molecules on the right side of the divider than on the left. The blue line plots the same relationship for blue molecules.

You can add more red molecules to the right side of the screen by pressing ADD RED. 

Similarly, you can shrink or expand the right side of the box with the buttons SHRINK-RIGHT and EXPAND-RIGHT, respectively. You can alter the temperature of the system at any time with the TEMP slider.

Finally, to change the size of the yellow connection window, move the WINDOW slider to your desired height and then press the CHANGE WINDOW button. 


Pay attention to the plot window and compare it what you see in the graphics screen. Is there an equal number of blue and red molecules on each side of the divider according to the plot and according to what you perceive in the graphics screen?


Run the model with several different states for each variable. Do you observe similar equilibrium effects to those seen in LeChatelier's Principle 1? Are there significant differences?

Does the temperature affect the system in the same way it affected the chemical reaction in LeChatelier's Principle 1? Why or why not?

How does changing the concentration affect the rate at which the molecules achieve equilibrium? Does this make sense?


The system we have established here always comes to an approximately identical equilibrium state, no matter how you change the variables. In the lab, this is not useful to chemists, who want to separate one type of molecule from another. Can you extend the model to separate all of the red molecules from the blue molecules?

Try adding another color of molecule to the system and randomly distributing all the molecules around the screen. Can you devise a way to separate the new molecules from the red molecules?