Le Châtelier’s Principle in the "Real World"

There are LOTS of examples of how Le Chat’s Principle can be seen in our everyday lives. I’ll present a few here, but chances are your instructor will have other examples.

An example about changing temperature

Soda fizzes because it contains dissolved carbon dioxide gas. We can write the relevant reaction this way:

CO2 (g) ⇄ CO2 (aq) + heat

This reaction is slightly exothermic, so heat is written as a product. This results in three different, but related occurrences.

1. When you open a cold can of soda, less gas escapes (that fssss sound you hear) than if you open a warm soda. The reason is simple, when the soda is warmer (more heat has been given to it) the reaction shifts to the left. Therefore there is more gas at higher pressure in the top of the can when you open it.

2. Soda fizzes in your mouth. Unless you are drinking “body-temperature” soda (37oC or 98oF) your mouth is warmer than the soda. Therefore when the soda touches you, heat will transfer from you to the soda. This will drive the reaction to the left, releasing the CO2 as bubbles on your tongue.

3. Warm soda tastes flatter. When we think about flat soda, what we mean is that it doesn’t fizz as much. This is because as the soda warmed, the reaction above went to the left and as a result the soda contains less gas to be released.

An example about adding and taking away reactants

Oxygen is carried in your bloodstream by a large protein called Hemoglobin. We can think of this process in a simple way with this reaction:

Hb(aq) + O2 (g) ⇄ HbO2 (aq) 

where Hb will stand for hemoglobin, and HbO2 is hemoglobin holding oxygen (and therefore able to “carry” it in the bloodstream).

There are two situations worth thinking about;

1) When blood enters the lungs very few of the hemoglobin molecules are carrying oxygen (very low concentration of HbO2). That means that the backward reaction is VERY slow - hardly happening at all. At the same time, there are LOTS of O2 molecules. That means that the forward reaction can go very quickly. So, the overall effect is that this reaction shifts to the right and oxygen goes IN TO the bloodstream.

2) A short time later, that same blood may have found its way to the pinky toe on your left foot. There, very little oxygen gas is present, so the forward reaction is virtually stopped. As a result the system shifts to the left, releasing oxygen to the cells in your toe.