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Jump to one of the following chemical equilibrium parts:   Choose One Introduction to Chemical Equilibrium Part 1- Basics of Dynamic Equilibria Part 2- Mass Action Equation and the Equilibrium Constant Part 3- Concentration and Pressure Part 4- The Reaction Quotient Part 5- Le Châtelier's Principle I- Concentration Part 6- Le Châtelier's Principle II- Temperature Part 7- Le Châtelier's Principle III- Pressure Part 8- Le Châtelier's Principle III- Summary Part 9- Calculations I- Equilbrium Constant Part 10- Calculations II- Equilibrium Concentrations Part 11- Calculations III- Using Approximations Part 12- Thermodynamic Equilibrium Constants Part 13- Quizzes

There are a number of movies available on this page. I've given you a choice of files in most cases since movies can be very large and take forever to download. If you have a good connection (you're at achool, ISDN, fast modem connection, etc.) then choose the "high res" file. If you have a slow connection then choose the "low res" file.

Let's restate Le Châtelier's Principle-

...if an outside influence upsets an equilibrium, the system undergoes a change in a direction that counteracts the disturbing influence and, if possible, returns the system to equilibrium.

and three methods to stress a system that's in chemical equilibrium-

1. a change in concentration(s)
2. a change in temperature
3. a change in pressure

Temperature Changes and Chemical Equilibrium

The following reaction shows distinct color changes when the original solution is heated and when it is cooled.

You can see in the 5^oC color that it's split between pink in the portion of the test tube in the cold water bath and the original purplish color in the portion of the tube that's not in the cold water bath. Let's take a look at the chemical equation with some solution color information added.

I didn't add any chemicals to shift the equilibrium but I did add heat by raising the temeprature and I did remove heat by lowering the temperature. The temperature effect upon equilibrium can be explained much like the way we did it for changes in concentration(s). Let's take one more look at the chemical equation but let's add in heat to one side or the other like we did for thermodynamics. But to which side do we add heat? To the reactants side or the products side? You have enough information to figure it out. Click on the equation you think is the correct one.

Make sure you try the above exercise before moving on!

Hopefully you can see what I mean when I say we can treat temperature changes like concentration changes. Raising the temperature is the same as adding heat and lowering the temperature is the same as removing heat. So, let's summarize-

1. An increase in temperature, the addition of heat, to a system at equilibrium causes a shift to the opposite side. In other words, a temperature increase for an endothermic reaction results in a shift towards the products, and for an exothermic reaction results in a shift towards the reactants.
2. A decrease in temperature, the removal of heat, to a system at equilibrium causes a shift to the same side that heat appears. In other words, a temperature decrease for an endothermic reaction results in a shift towards the reactants and for an exothermic reaction results in a shift towards the products.