Le Chatelier's Principle - Online Tutor, Practice Problems & Exam Prep

Chemical equilibria involve understanding both the equilibrium constant and thermodynamics. While kinetics focuses on the rate of reactions, thermodynamics determines the direction of reaction shifts at equilibrium. According to Le Châtelier's principle, a disturbed system will adjust to restore equilibrium. Key concepts include the equilibrium constant (K), Gibbs free energy (ΔG), and entropy (ΔS). These principles are essential for predicting how changes in concentration, temperature, or pressure affect chemical reactions, enhancing comprehension of dynamic equilibrium in chemical systems.

Thermodynamics

In this video, we're going to continue our discussion of chemical equilibria by taking a look at the equilibrium constant and its relationship to thermodynamics. Earlier, we learned that kinetics, or chemical kinetics, studies the rate at which our reactants change into products. Basically, we're looking at the rate or speed of reactions. In this chapter, we are now going to learn about thermodynamics. And thermodynamics deals with the direction that a chemical reaction at equilibrium will shift. So, thermodynamics deals with the direction a reaction takes, whereas kinetics looks at the rate or speed at which a reaction occurs. You need to take a look at both of them to get a complete idea of a chemical reaction. Now, we're going to say here that Le Chatelier's principle states that once a system is disturbed, it will do whatever it can to get back to equilibrium. So basically, you do something to try to alter the equilibrium state, and the chemical reaction shifts to the left or to the right in order to minimize that effect. Now knowing this, let's try to answer the questions below and look at the different things that we can do to a chemical reaction and, as a result of that change, what happens to our chemical reaction.

The following is an endothermic reaction where Kc = 6.73 x 103.For each of the choices below predict in which direction the reaction will proceed

Now, if we take a look at this example, we'll see how exactly Le Chatelier's Principle helps us understand whether the reaction will shift to the left, to the right, or cause no shift at all. So, here what we're going to say is for the following endothermic reaction, kc=6.73×103. Predict in which direction the reaction will proceed as we do each one of these changes. Now, first things first, we said that this is endothermic. So that means that our enthalpy, our delta H value is positive. Remember, endothermic reactions, they absorb heat. If we're heat will be a reactant. Remember, this is the opposite of an exothermic reaction. Exothermic reaction does the complete opposite. It releases heat and there so reaction shifting to the left or to the right or causing no shift at all.

For a, it says the addition of a catalyst. What we should realize is that a catalyst deals with kinetics. It deals with speed. It lowers the energy of activation in order for the reaction to go faster. We're going to say when it comes to thermodynamics though, a catalyst doesn't affect the direction of the reaction. So the reaction will not shift to the left or to the right because catalysts deal with kinetics. Le Chatelier's principle deals with thermodynamics.

Next, we're going to say decreasing the volume. What we should realize here is volume is the opposite of pressure. If we're decreasing the volume, we're increasing the pressure. In both cases, the reaction shifts to the side with fewer moles of gas, fewer moles of gas. So let's take a look. Here in our equation, we have 7 moles of gas here and we have 8 moles of gas here. So it's going to go to the side with fewer moles of gas, so it's going to shift to the left or as we say, it's going to go in the reverse direction. So that's what we would say for B.

Now, for C, we're removing H₂O gas. Now, we're going to say H₂O gas is a product. Here we're going to say adding, Adding reactant or removing product causes the reaction to shift in the forward direction. It's going to go to the right because you have to think of this in this sense. We're at equilibrium so we're balanced. We're at peace with ourselves as a reaction. Then I come in and I remove some product. Now, we have to get back to this balance that we had in the beginning. So, if I remove something, I have to go towards it to make more of it. So if I'm removing products, how can I remake the product that I just lost? I have to go in the forward direction. Going in the forward direction would mean I'm producing more product. In the same way, if I add reactant, uh-oh, I have too much reactant. I have to get that amount back down to what it was before. So how do I get the reactant amounts down? Also go in the forward direction because going in the forward direction, our reactants would be breaking down in order to build up our products. And just realize, for B and C, if I'm saying that, it shifts to the side with fewer moles when volume is decreased and pressure is increased. Then what would happen if I did the reverse? What would happen if I increase the volume and decrease the pressure? We would expect the opposite to occur.

Same thing here. Let's say we were removing reactant or adding product. Then you would expect the opposite to occur. Remember, what I'm telling you one way, think of it the opposite way also. Now, D is incredibly important. Here we're talking about temperature. And it was important that we identified that the reaction was endothermic because endothermic and exothermic reactions deal with heat. And temperature and heat, very close together. They're not the same thing but they're related together. We're going to say increasing the temperature, reaction, shifts away from heat. That's why it was important for us to identify what side the heat was on. Was it a reactant or was it a product? And to know this, we had to figure out what did endothermic mean. So we're going to shift away from the heat. The heat is a reactant so we're going to move away from it. So we're going to go in the right direction. So, In the opposite way, if I decrease my temperature, my reaction would shift towards my heat. Remember, if it's true one way, it's the opposite, the opposite way. Okay, adding NH₃. So NH₃ is a reactant and we just said adding reactant, it'll go in the forward direction. Next one is decreasing pressure which is the same thing as increasing volume. Now here, remember, so in this case, the reaction shifts to the side with more moles of gas. Okay, so it's incredibly important you figure out which side has more moles of gas. We said this earlier that the product side happens to have 8 moles so it has the most moles of gas. That's why we're going in the forward direction. Let's say your professor gave you a different equation where most moles were reactants. Well, that would mean in that case that your reaction would be going in the reverse direction because when you're decreasing your pressure or increasing your volume, your reaction must shift to the side with more moles of gas, whether that be the side with products or the side with reactants. Here, we're removing water liquid. Now, what have we been saying this whole time? We're dealing with equilibrium constants. We're dealing with ice charts. We've been saying this whole time, what two phases do we ignore? We ignore solids and we ignore liquids. So adding or removing them does nothing to shift the reaction. Solids and liquids cause no shift. So we've ignored them in the equilibrium constant. We've ignored them in the ice charts. We're ignoring them now with Le Chatelier's principle. Next, the addition of a precipitate. Remember, what's a precipitate? Well, it's a fancy way of saying a solid, so we're adding a solid. This would be the same exact answer here as well. Remember, solids and liquids cause no shift. Finally, the addition of an inert gas at constant volume. Now, an inert gas is what? An inert gas is a noble gas. We're assuming if we're adding an inert gas to our sample that the volume is going to be held constant. And that's because most of the noble gases are unreactive. As a result of that, we're also going to say that adding inert gas or noble gases also causes no shift in our reaction. Again, we're going to say adding a catalyst, adding a solid, adding a liquid, adding a noble gas which we assume will be done at a constant volume, then we're going to say there is no shift in the reaction. And as long as you can remember what are the, what's going to happen when I mess around with the temperature, when I play around with the pressure and volume, and when I add and remove products and reactants, which way will it shift? These are all the concepts that you really need to know when it comes to Le Chatelier's principle. It's quite a few but as long as you just remember the basic principles, you'll be able to answer any type of Le Chatelier's question you'll see on the exam.

Consider the reaction below:

CH₄ (g)  +  F₂ (g)  ⇌  CF₄ (g)  + HF (g)   H = + 38.1 KJ/mol

The following changes will shift the equilibrium to the left except one. Which one would not cause a shift to the left?

• A. Add some CF₄.
• B. Remove some F₂.
• C. Decrease the Temperature.
• D. Decrease the container volume.
• E. Increase the partial pressure of HF.

The following data was collected for the following reaction at equilibrium 

2 A (g) + 3 B (g)  ⇌ C (g)     

At 25 ^oC K is 5.2 x 10^-4 and at 50 ^oC K is 1.7 x 10^-7. Which of the following statements is true?

a) The reaction is exothermic.

b) The reaction is endothermic.

c) The enthalpy change, ΔH, is equal to zero.

d) Not enough information is given. 

• A. The reaction is exothermic.
• B. The reaction is endothermic.
• C. The enthalpy change, ΔH, is equal to zero.
• D. Not enough information is given.

Which direction will the following reaction (in a 10.0 L flask) proceed if a catalyst is added to the system? 

CaCO₃ (s)  ⇌ CaO (s) + CO₂ (g)            K_p = 3.2 x 10^-28

• A. To the right.
• B. To the left.
• C. The equilibrium position will not change but the rate will increase.
• D. The equilibrium position will not change but the concentrations of everything will increase.

Consider the following gas reaction of A₂ ( shaded spheres) and B₂ ( unshaded spheres)

A₂ (g)  +  B₂ (g)  ⇌  2 AB (g) 

Which container proceeds more to completion? 

• A. A
• B. B

Here’s what students ask on this topic:

What is Le Chatelier's Principle and how does it apply to chemical equilibrium?

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust itself to counteract the disturbance and restore a new equilibrium. This principle applies to chemical equilibrium by predicting how changes in concentration, temperature, or pressure will affect the position of the equilibrium. For example, if the concentration of a reactant is increased, the system will shift towards the products to reduce the effect of the added reactant. Similarly, if the temperature is increased for an exothermic reaction, the system will shift towards the reactants to absorb the added heat.

How does a change in concentration affect a system at equilibrium according to Le Chatelier's Principle?

According to Le Chatelier's Principle, changing the concentration of reactants or products in a system at equilibrium will cause the system to shift in a direction that counteracts the change. If the concentration of a reactant is increased, the equilibrium will shift towards the products to consume the added reactant. Conversely, if the concentration of a product is increased, the equilibrium will shift towards the reactants to reduce the concentration of the product. This shift helps the system to re-establish equilibrium.

How does temperature affect chemical equilibrium according to Le Chatelier's Principle?

Temperature changes affect chemical equilibrium based on whether the reaction is exothermic or endothermic. According to Le Chatelier's Principle, increasing the temperature of an exothermic reaction (which releases heat) will shift the equilibrium towards the reactants to absorb the excess heat. Conversely, increasing the temperature of an endothermic reaction (which absorbs heat) will shift the equilibrium towards the products to absorb the added heat. Decreasing the temperature will have the opposite effects, shifting the equilibrium towards the side that releases heat.

What role does pressure play in shifting the equilibrium of a gaseous reaction according to Le Chatelier's Principle?

For gaseous reactions, pressure changes can shift the equilibrium according to Le Chatelier's Principle. If the pressure is increased by decreasing the volume, the equilibrium will shift towards the side with fewer moles of gas to reduce the pressure. Conversely, if the pressure is decreased by increasing the volume, the equilibrium will shift towards the side with more moles of gas. This shift helps the system to counteract the change in pressure and re-establish equilibrium.

How does Le Chatelier's Principle explain the effect of a catalyst on a chemical equilibrium?

Le Chatelier's Principle states that a system at equilibrium will adjust to counteract disturbances. However, the addition of a catalyst does not shift the position of the equilibrium. Instead, a catalyst speeds up the rate at which equilibrium is reached by lowering the activation energy for both the forward and reverse reactions. This means that while a catalyst helps the system reach equilibrium faster, it does not change the concentrations of reactants and products at equilibrium.