Knowledge database: 5.2 Le Chateliers principle

La Chatelier's principle tells us that if one changes the conditions in a system that is in the state of equilibrium, the balance moves in the direction that favors the establishment of the previous conditions. Thanks to this principle, we can improve the yield of chemical reactions in various ways. Some of the most common are described here:

For reactions that produce heat (we call them exothermic), if one reduces the temperature at which the reaction is carried out, the balance is moved to the side of the products and the result will therefore be a greater amount of products. Conversely, if the exothermic reaction is carried out at higher temperatures, the balance moves to the side of the reactants, and as a result, one will have a lower yield.

There are also endothermic reactions which consume energy, and they therefore favor higher temperatures, and in these cases, the balance is moved to the side of the products. When a endothermic reaction is carried out at lower temperatures, the balance is moved to the side of the reactants, and therefore the result is, once again, a lower yield.

As for the concentration, a simple rule applies: The balance shifts to the side of the reaction where one reduces the molar concentration of a certain substance. For example, if one reduces the concentration of one of the reactants, the balance moves to the side of the reactants, and thereby one gets a lower yield. On the other hand, if the concentration of one of the products is reduced (if, for example, one continously collects it out of the reaction in some way), the balance moves to the side of the products and the result is a higher yield.

A special case is when we work with gaseous substances:

In this case, it depends on which side we have more moles of gaseous particles. In the reaction 3H2 + N2 <-> 2NH3 we can see that the sum of moles of gaseous particles on the left side equals 4, while the right side equals 2. The following rule applies: by increasing pressure, the balance moves toward the side where there is a smaller sum of moles of gaseous particles. This rule one can further visualize like this: by increasing the pressure on the side where there are more moles of gas, the gas particles become too closely packed, so the balance is moved to the side of the reaction where there are less moles of gaseous substances.