Dilution as a Stress to Equilibrium

At first glance, it may appear that dilution should have no effect on equilibrium. Unfortunately, that is incorrect.

Let’s first look at why that idea is wrong.

Why You Might Think Dilution shouldn’t matter

Let’s think about a real reaction for a moment. This is the reaction between Iron III ions and thiocyanate (SCN-1) ions to produce the ion FeSCN+2. Here is the reaction:     Fe+3 + SCN-1 ⇄ FeSCN+2

There is no water in this reaction, so it would seem that adding water would not have any effect. After all, adding water is NOT adding a reactant, nor is it adding a product.

Why Dilution Does Matter

To understand this, we'll use the same reaction. Chemistry teachers like this reaction to demonstrate Le Châtelier’s Principle because an equilibrium can easily be established with enough reactants and products that the solution looks orange.

Here is the reaction written to show those colors:

Then if a stress is applied and the reaction shifts to the right, students can see the solution get redder.

If a stress causes the reaction shifts to the left, the solution gets visibly yellower.

It turns out that when water is added to the mixture, it turns yellow. That means we KNOW that the reaction is shifting to the left. Now we need to understand why.

Let's look at just the Fe+3 ions. When water is added, the concentration of Fe+3 goes down. That means that there will be fewer Fe+3/SCN-1 collisions each moment. As a result the forward reaction will slow down. 

We can work through the same logic with the other components of our reaction. Adding water will decrease the concentration of the SCN-1 ions and will, therefore, slow down the forward reaction. Adding water will decrease the concentration of the FeSCN+2 ions and will, therefore, slow down the backward reaction.

So, there are two factors (decreasing [Fe+3] and decreasing [SCN-1] ) that slow down the forward reaction and one factor (decreasing [FeSCN+2] ) that slows down the backward reaction. That means that the forward reaction is slowed MORE than the backward reaction. 

Both directions get slower, but the forward reaction gets slower-er. The reaction shifts left.

We can expand this idea to say that

Adding water will shift the reaction toward the side with more moles of stuff

This applies to coefficients as well, so the reaction 3 A ⇄ B + C would shift to the left if you added water since there are 3 moles on the left and only 2 moles on the right.