Le Chatelier's Principle

The principle of Le Chatelier allows us to predict how we can shift an equilibrium into a desired direction. This knowledge is crucial for chemists to influence reaction equilibria — ultimately to maximize the yields of all chemical compounds produced worldwide.

Le Chatelier’s principle states that:
A system at equilibrium, when subjected to a disturbance, responds in a way to minimize the effect of this disturbance.

Sounds cryptic? Well, I might be biased, but I think simulation models are the best way to make sense of Le Chatelier's principle.

Let me introduce you to my Le Chatelier simulator. In this model, all that marbles (atoms if you like) basically do is to randomly hop from on position to another:

This alone cannot teach us anything about Le Chatelier's principle. We have to add some kind of reaction to the model. Let's say that if two marbles meet, they can form a dimer, and with some probability dimers can split up into separate marbles again.

With this reaction added, our model looks like this:

Now we have some kind of reaction going on in our model, and an equilibrium between single marbles and dimers can form. All that is left to do is to change some conditions and observe how this equilibrium changes. In the following we will look at three types of changes to learn more about Le Chatelier's principle: A change in volume / pressure, a change in concentration, and a change in temperature.

Change in Pressure / Volume

In the following simulation, you can change the volume (size of the grid) with the two buttons below. The graph keeps track of the number of dimers. Observe the number of dimers for a while, and then make a drastic increase in volume. How does the number of dimers behave? Play with the volume and observe how it influences the reaction equilibrium.

You have probably observed that with an increased volume, the number of dimers goes down. If you instead decrease the volume, the number of dimers goes up. Let's see how this fits to the principle of Le Chatelier: A system at equilibrium, when subjected to a disturbance, responds in a way to minimize the effect of this disturbance.

If we disturb the system by reducing the volume, the initial effect is that it gets pretty crowded. According to Le Chatelier's principle the system should respond in a way to minimize this effect, and it does! More of the single marbles react to dimers, as if the system tried to avoid this crowded situation.

In the opposite case, if we increase the volume, the initial effect is lots of void space. Again the system reacts as if it wanted to minimize this effect. The equilibrium is shifted away from dimers towards single marbles, which can fill more of that void space.

Ok, but why?

Of course a system like this has no will. It doesn't want to minimize anything. So, why does it behave like this? I think the beauty of the simulation model is that we can easily see what is going on.

First of all we can see that an equilibrium is not really a static thing. Even at equilibrium dimers are constantly formed and split. Equilibrium only means that this both happens at the same speed. If the number of dimers that are formed in a given time is equal to the number of dimers that are split in the same time, the number of dimers stays constant. Any change that favors the forward or backward reaction over the other will therefore shift the equilibrium!

The splitting of a dimer is hardly dependent on the accessible space. All a dimer needs to split in the model is itself and a tiny bit of space to fit the resulting two single marbles. It doesn't matter if there is a large empty space around it or not.

To form a dimer, however, two marbles have to find each other. The accessible space does matter a lot in this case. Marbles confined in a small space, are much more likely to encounter each other compared to the same marbles in a larger space. Reducing the volume, therefore, strongly favors the formation of dimers.

In summery, if we reduce the volume, we make it more likely for marbles to encounter each other, and to form dimers, but we don't influence the splitting of dimers much. Consequently, the equilibrium will shift to more dimers.

How does that translate to the real world?

We can apply the same idea not only to dimerizations, but also to more complicated reactions. In general, a reduction of the volume will favor the side of the reaction with fewer (gas phase) components, and vise versa. It gets a bit more complicated if you have a mix of gas and liquid or solid components. The above model doesn't cover that, but in general, when it comes to Le Chateliers principle and a change in volume, you can ignore all the solid and liquid components. A change in volume / pressure has such a little effect on the solid and liquid components compared to the effect it has on the gas phase components, that you can safely ignore it. Therefore, in practice you only need to determine which side has fewer gas phase components to predict which side will be favored by a reduction in volume.

Change in Concentration

Another way to influence an equilibrium is to alter the concentration of one of the involved components. Let's consider a different type of marbles (green marbles) that cannot react with themselves. The only way they can form a dimer, is to react with a blue marble:

In the following simulation you can add and remove blue marbles. Observe how this influences the number of dimers.

Adding blue marbles increases the number of dimers. This is again inline with the principle of Le Chatelier. The effect of our disturbance is that it gets pretty crowded with single blue marbles. The equilibrium responds in a way to minimize this effect. By shifting towards more dimers, it reduces the number of single blue marbles. In general, increasing the concentration of one of the reactants will will push the equilibrium towards the other side of the reaction equation.

If we remove blue marbles, on the other hand, the effect is that single blue marbles become quite rare. Again, the the system responds in a way to minimize this effect. By splitting up dimers it resupplies single blue marbles. In general, decreasing the concentration of one of the reactants will pull the equilibrium towards that side.

Ok, but why?

The reasoning why this happens is very similar to the "change in volume" case. With more blue marbles around, the chance for a green marble to bump into a blue marble and consequently to form a dimer is increased. The chance for dimers to split, on the other hand, is hardly affected by the number of blue marbles.

By adding blue marbles we promote the reaction towards dimers, but we don't influence the backwards reaction much. Consequently, we shift the equilibrium towards dimers. The opposite applies if we remove blue marbles.

Change in Temperature

Finally, let's add energy to our considerations to understand the influence of temperature. Let's assume that two marbles forming a dimer is an exothermic reaction. So, when a dimer is formed, it is initially rich in energy. I show that as a vibrating dimer. In the next step, the dimer can donate its unit of energy (shown as a glowing white dot) to some other marble or dimer.

In the following simulation, just like marbles move randomly, units of energy can now also hop randomly to adjacent marbles.

Additionally to the grid where our reaction takes place, I added a hot and a cold grid acting as heat baths. In the hot grid, on the right, a lot of energy is roaming around. In the cold grid on the left, no energy is present at the beginning.

You can use the two buttons below to bring our reaction grid into contact with either heat bath. This allows energy transfer to happen between the grids. Observe how contact with the hot and the cold grid influences the number of dimers! You might want to speed up the simulation with the slider below, because it can take a while until the system equilibrates to either temperature.

Upon contact with the hot grid, energy tends to flow into the reaction mix, allowing more dimers to split. The number of dimers goes down.

Upon contact with the cold grid, on the other hand, energy is drained from the reaction mix. It becomes less likely that dimers encounter the necessary energy required to split. The number of dimers goes up.

Let's again check how this fits to Le Chatelier's principle! When we heat something up, the effect of our disturbance is that energy becomes more abundant. To minimize the effect of this disturbance the system would have to absorb some of that energy, and that is exactly what we can see in the simulation! Heat favors the endothermic reaction (the reaction that absorbs energy).

On the other hand if we cool the system down, the exothermic reaction, the one that releases energy, is favored.

Conclusions

As so many things in chemistry, Le Chatelier's principle is a statistical phenomenon. By changing conditions like volume, concentration, or temperature, we influence the probabilities of forwards and backwards reactions, allowing us the move reaction equilibria towards our desired products.