Impact of pH on LeChatelier's Principle

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Le Chatelier's Principle helps chemists understand how the equilibrium will shift when some sort of change is applied to the reaction. While that sounds challenging, it isn't so bad. This lesson will focus on pH and how it can shift equilibrium.

Le Chatelier's Principle Defined

Let's do some dog-mushing chemistry. Imagine the following equation:

Dog-mushing reaction

In this reaction, the dog and the harness are the reactants and the sled dog is the product. Note the double arrow in the equation. This denotes that the reaction can go in the forward and the reverse direction. In other words, the forward reaction is a dog + a harness becoming a sled dog. The reverse reaction is a sled dog becoming a regular dog and a harness.

We can describe this reaction as one that is in equilibrium, meaning the concentration of products and reactants remain the same, and the forward reaction and reverse reactions are happening at the same time.

Can you imagine some more factors that could change our dog mushing reaction? What if we added more harnesses? Or what if we removed some dogs? In order to understand what happens to reactions that are in equilibrium, there's Le Chatelier's Principle, which states that if a reaction in equilibrium is disturbed, the equilibrium will change to counteract that change.

For example, let's say you increase the reactants, or the number of dogs and harnesses. According to Le Chatelier's Principle, this would shift the reaction to favor the products. This makes sense, because if you have a lot more dogs and a lot more harnesses, you're going to get more sled dogs.

What is pH?

Now that you're more familiar with Le Chatelier's Principle and the concept of equilibrium, let's explore pH. You might remember that pH represents how many hydrogen ions are in a solution. If a solution has a bunch of hydrogen ions, it'll have a low pH and it's considered acidic. On the other hand, if a solution has a low amount of hydrogen ions, it has a high pH and it's considered basic. The concentration of hydrogen ions is inversely proportional to the concentration of hydroxide ions. So in other words, as the concentration of hydrogen ions increases, the concentration of hydroxide ions decreases, and vise versa.

pH and Le Chatelier's Principle

Do you remember what happened when we increased the number of reactants in our dog-mushing example? It shifted to the products side with more sled dogs. And if we added a bunch more sled dogs, the reaction would shift towards the reactants, or harnesses and dogs. This is due to Le Chatelier's Principle, which is always trying to counteract the change.

In pH reactions, the same holds true. In order to understand this, let's go over some examples.

Example #1

Let's explore what happens if you increase the pH. When you increase the pH you'll make it more basic by adding hydroxide ions.

Reaction 1

Take a look at the reaction in our first example above. Our first challenge is to see if we can find hydroxide ions in this equation. If you take a look on the product side, you'll see some hydroxide ions, or OH-. By increasing the pH, we increase the number of hydroxide ions. Based on Le Chatelier's Principle, this will cause the equilibrium to shift to counteract that. Therefore it shifts towards the reactants. This is equivalent to adding more sled dogs and the equilibrium shifting towards the harness and the dog.

Now, let's say you increase the H+ ions, thus lowering the pH. The H+ ions are actually represented by H3 O+. By adding H3 O+ ions, the OH- ions get 'used' up in the reaction making water. Therefore, since they are 'used up', there's actually less of them on the product side. According to Le Chatelier's Principle, the equilibrium will counteract to deal with this change. Therefore, it'll shift towards the product side.

How do you feel about this? Let's do two more.

Example #2

Let's check out this reaction:

Example 3

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