Addition or Removal of a Reactant or Product

You can see from the title that this page (arguably) covers 4 different stresses. However you will see, as you work your way down this page, they are all, essentially, the same stress.

Also, remember as we look at these “stresses” remember that there are two different questions you might have to answer: WHAT DIRECTION will the reaction shift, and WHY will that shift occur.

Let’s tackle the “WHAT DIRECTION” question first. In this case we can use the simple idea:

If it was added, use it. If it was taken, replace it.

If reactant is ADDED, the reaction will shift forward (the direction that USES what was added)

If a product is ADDED, the reaction will shift backwards (the direction that USES what was added)

If a reactant is TAKEN away, the reaction will shift backwards (this will REPLACE what was taken)

If a product is TAKEN away, the reaction will shift forwards (this will REPLACE what was taken)

Putting those four ideas together, you could also say that the reaction will shift away from things you add and toward things you take.

Now let's look at WHY these shifts occur:

Addition of a Reactant

When a reactant is added, there will be more reactant collisions each moment. Since reactions require collisions (collision theory), there will be more opportunity to react each moment and the forward reaction will go faster. Since we haven’t changed the rate of the backward reaction, we will be making product faster than we are using it and the reaction will shift to the right.

Addition of a Product

When a product is added, there will be more product collisions each moment. Since reactions require collisions (collision theory), there will be more opportunity to react each moment and the backward reaction will go faster. Since we haven’t changed the rate of the forward reaction, we will be making reactant faster than we are using it and the reaction will shift to the left.

Removal of a Reactant

When a reactant is removed, there will be fewer reactant collisions each moment. Since reactions require collisions (collision theory), there will be less opportunity to react each moment and the forward reaction will go slower. Since we haven’t changed the rate of the backward reaction, we will be making product slower than we are using it and the reaction will shift to the left.

Removal of a Product

When a product is removed, there will be fewer product collisions each moment. Since reactions require collisions (collision theory), there will be less opportunity to react each moment and the backward reaction will go slower. Since we haven’t changed the rate of the forward reaction, we will be making reactant slower than we are using it and the reaction will shift to the right.

A really important note:

"Le Châtelier’s Principle" is NEVER an answer to a WHY question. Molecules don't know about the principle and they don't feel any need to obey or satisfy it.