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Tips for Le Chateliers Principle - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tips, and tricks for Le Chateliers Principle. You remember that Le Chateliers Principle tells you how the system will react due to a stress that changes the equilibrium position.

So here we have an example equation. So don't forget, make sure you always balance your equations. We're going to balance the equation for the forward synthesis of Ammonia. If you notice the forward reaction, the one that proceeds from left to right, it has heat as your product. So that's exothermic forward reaction.

So if you take a look here, at the factors, the first factor that affects is concentration. So basically the key is, if you add a certain thing, then the shift is going to go to the opposite side.

So for example is you add more N2, what happens is the system is going to have more N2 to react with the H2, and then it's going to produce it's going to produce more NH3 as a product. So as a result, you would shift to the opposite side. So if you increase N2, and it's on the left side of your equation, it will shift to the right to make more.

Now if you remove, then it's going to shift toward the side you took it from. So in this case, instead of adding N2, if we take away some N2, what's going to happen is the system is going to want to make some more N2 to replace the N2 that we took away from. So it's going to shift towards the left, since we took away some of the reactant from the left side.

Conversely, if I take away some NH3, then my system is going to shift back towards the NH3. So to the right to make more NH3 to compensate for what was taken away. So that's concentration.

The second thing that we'll take a look at is temperature. So temperature is average Kinetic energy, and that's heat which is total Kinetic energy. So if you add or if you increase the temperature, then what's going to happen is, the system is going to do a concentration. It is going to shift towards the opposite side where your heat is.

So in this example, if we increase the temperature, it's like we're increasing the heat. So it's going to shift towards the opposite side. So it's going to shift towards the left and produce more N2, produce more H2, and use up some NH3.

Now if you remove heat, or you lower the temperature, then you're going to shift towards the side that you took it away from. Towards the side with the heat.

In this case if we decrease the temperature, it's like taking away the heat. It's going to shift towards the side where the heat's on. So in this case it's the right side. So it's going to shift to make more NH3. So it's going to shift to the right. Then, you're going to use up some N2, and use up some H2. So that's temperature.

So keep in mind the tricks are tricks are concentration and temperature. If you add, you shift to the opposite side, of where you're saying that you're taking away from or where the heat is. If you remove, you shift back towards the side to make up for what you took away.

For pressure, if you increase the pressure, then the system is going to want to decrease that pressure. How does it decrease? So it shifts towards the side with less moles. Because then with less moles, there's less particles to move around. So if we increase the pressure, which side has less moles? Well, let's take a look here. So we have 4 moles of gas. Remember, it has to be gas on the left side, and two moles of gas on the right side.

So in this case, the system would shift to the right, because there's two moles of gas on the right side, so to relieve some of the pressure, it's going to make less molecules. So there's two moles of gas on the right side. If I decrease the pressure, then the system is going to want to compensate for the decrease in pressure by making more molecules to bounce around, and increase the pressure. So it's just to the side with more moles.

Then pressure is kind of related to volume. If the volume affects the pressure, then you'll follow this rule here. But if the volume does not affect the pressure, then you don't care about the volume change.

Then a couple of things. So there are a few things that don't affect equilibrium. Things that don't affect are; catalysts. Catalysts they only speed up reaction. They don't change the quantities or change any of the concentrations at all.

Then number two, adding an inert gas. So an inert gas is a non-reactive gas. So it could be one that does not participate in your reaction, or could be like a noble gas like Argon, or Neon or anything like that. So those things don't affect equilibrium. So they don't shift any of your equilibrium position that you have going on.

So if you remember concentration, temperature, and pressure and then you remember the things that don't affect it, it will make things a lot easier for you with Le Chatelier's Principle.

Just use a little common sense, and think about if you add stuff to one side, then the system should shift to the opposite side to release some of that stress. Or if you take away some, well, what's it going to do? It's going to want make some more from the side that you took it from.

So hopefully these tips, and tricks will help you with Le Chateliers Principle.