Stresses to Equilibrium

Remember that Le Châtelier’s Principle says:

When a system is at equilibrium and a stress is applied to the system,

the system will shift in the direction that relieves the stress.

There are a number of “stresses” that an equilibrium system can be subjected to. These include:

1. Addition of a reactant or product

2. Removal of a reactant or product

3. Increase in the amount of solvent (dilution)

4. Change in temperature

5. Change in pressure (for gases) - this can occur in several ways

1. Addition or removal of a reactant or product

2. Addition of another gas

3. Change in temperature

4. Change in volume

We’ll tackle each of these separately. 

One last note before we go on. There are two VERY different types of questions about Le Châtelier’s Principle that you could be asked. 

• What direction will the reaction shift if you… This question only requires that you can predict the direction of shift. It does not require a deep understanding of Le Châtelier’s ideas and is, therefore, the easier (and more) common question.

• WHY does the reaction shift in the direction that it does when you … This question requires you to understand the various factors that affect rates of reactions and to be able to apply them to this situation. For some of the “stresses” this is not difficult. For others, especially for temperature, it is surprisingly complex. We will answer both of these questions for each stress.