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

Le Chatelier's principle explains how a chemical reaction at equilibrium responds to disturbances. Adding reactants or removing products shifts the reaction to the right, increasing product amounts. Conversely, removing reactants or adding products shifts it to the left, increasing reactant amounts. Changes in pressure and volume affect the side with more or fewer moles of gas. Temperature changes depend on whether the reaction is exothermic or endothermic, shifting away from heat when temperature increases. Adding inert gases under constant volume causes no shift, while solids and catalysts do not affect equilibrium position.

Le Chatelier's Principle

So Le Chatelier's principle states that if we have a chemical reaction at equilibrium and we do something to disturb that equilibrium, the reaction will adjust by shifting either in the forward direction or the reverse direction in order to minimize any changes so that it stays at equilibrium. Now, there are many different things that we could do to try to disrupt this equilibrium. We'll go through each one of the scenarios and see what effects that has on the chemical reaction itself. Firstly, we can talk about reactants and products. So we're going to say here if you are adding reactants or removing products, that means that our reaction is going to shift to the right which means the forward direction. We'd be shifting this way. That's because if you're adding reactants, you need to get rid of the excess reactants you've just added, and the only way to do that is to move in the forward direction to get rid of them. In the opposite way, if you remove any products, you need to remake the products that you've just lost. How would you do that? Again, you'd move in the forward direction or to the right to replenish and remake those products. So if we're shifting to the right, that means that wherever we're moving, we'll be increasing in amount. So here we'd expect our products to be increasing to makeup for what we lost and we'd expect here our reactants to be decreasing just in case we added reactants. So just remember, adding reactants or removing products, we shift to the right. Now if we do the opposite, we should expect the opposite result. If I remove my reactants, I need to remake the reactants I've just lost. How would I do that? My reaction would have to move in the reverse direction or to the left to remake the reactants I've just lost. If I add additional products, how do I get rid of those additional products that I've added? Again, I'd move in the reverse direction to get rid of them. So again, under this situation, we're going to shift to the left or the reverse direction. Again, wherever we're heading, we'll be increasing in amount. So here my reactants would be increasing in amount as a result, and my products would be decreasing in amount. So remember, adding or removing products will have some type of effect in terms of my reaction. Remember these two principles, whether you're adding or removing reactants or products, determines which direction it won't shift. So come back, click on the next video and see what happens when we adjust the pressure or volume.

Le Chatelier's Principle

Now, here we're going to adjust the pressure and volume and see what changes it has on the chemical reaction. Here we're going to say if we're decreasing pressure, that's the same thing as increasing volume because they're inversely proportional. They're basically opposites of one another. If we're decreasing pressure or increasing volume, the reaction will shift to the side with more moles of gas. So if we take a look here, we can see that we have 2 moles of gas, 1 mole of solid. So overall here we have 2 moles of gas. And on this side, we have 3 moles of gas, 1 mole of solid. So overall, we have 3 moles of gas. So again, if we're decreasing pressure or increasing volume, we're going to shift to the side with more moles of gas. So in this example, we'd shift in the forward direction. Remember, the product side is increasing that means that my reactant side is decreasing. Then if we do the opposite, here we're increasing the pressure which means we're decreasing the volume, so the reaction would shift to the side with fewer moles of gas. Again, we still have 2 moles of gas overall here, 3 moles of gas here. We're going to shift to the side with fewer moles of gas, so the side that has 2 moles of gas in this example. Wherever we're shifting, we'll be increasing an amount. And if that's increasing an amount, that means the other side is decreasing in amount. So remember, if we're affecting pressure and volume, we're going to look and see what sides have either more moles of gas or fewer moles of gas. If both sides have an equal number of gas molecules, then no shift will occur because again, we're always looking to see do we shift to the side with more moles or fewer moles of gas. If both sides are equal, there's no direction we can shift. Now that we've seen this one, we'll move on to the effects of temperature in terms of shifting a chemical reaction. Click on to the next video and see how temperature plays a role in Le Chatelier's principle.

Le Chatelier's Principle

Now whenever we're dealing with temperature and how it affects Le Chatelier's principle, we first have to determine if our reaction is exothermic or endothermic. So, an exothermic reaction has a negative delta H (enthalpy). Here, heat would be a product because, in an exothermic process, we release heat, so heat will be a product. If delta H is positive, that means we're dealing with an endothermic process, so that means heat will be a reactant. In our endothermic process, we absorb heat, so heat will be a reactant. Now, if we're increasing the temperature, if you're increasing the temperature, the reaction will shift away from heat. And again, you need to determine if it's an exothermic or endothermic process so you can see what side heat is on. In this example, we're looking at an endothermic process because heat is a reactant. We're increasing the temperature, so we're going to shift away from heat, so we move to the right side because that'll be away from the heat source. Wherever we're shifting to will be increasing the amount, and then this side would have to be decreasing in amount. So again, you can't determine which way a reaction will shift until you first figure out if it is endothermic or exothermic. Now here, if we're decreasing the temperature, the reaction will shift towards heat. So heat here is still on the reactant side, so this is still an endothermic process. So here we would shift this way towards my heat. So that would mean that this here would be increasing and this side here would have to be decreasing. So whether temperature is increasing or decreasing is not enough information. You must first determine if it is exothermic or endothermic. Place the heat on the appropriate side, whether it's a reactant for endothermic or whether it's a product for exothermic. Once you determine the side the heat is on, then you can look and see what effect increasing or decreasing the temperature will have in terms of shifting this chemical reaction based on the principle. A few factors down, we'll go over inert gases in the next video. So click on the next video and see what happens when we're dealing with inert gases and let's shout out to Le Chatelier's principle.

Le Chatelier's Principle

So now, when we talk about the addition of an inert gas, we're really referring to a noble gas being added to our mixture. And remember, our noble gases are just the elements from group 8a on the periodic table. Now, here we're going to say if you're adding a noble gas under constant volume, we're going to say there will be no shift in the equilibrium position. So our reaction will not shift in the forward direction or the reverse direction. Direction. So we'd say here that this would result in no change. And also here, no change. If you add it under a constant pressure though, when we add it under constant pressure the reaction will shift to the side with more moles of gas. So here, adding it under constant pressure, here we have 2 moles of gases which we figured out earlier. Here we have 3 moles of gases. So we'd shift aside with more moles of gases, so we'd shift that way. Wherever we're shifting, that'd be increasing. And over here, this would be decreasing. Now, when they're talking about the addition of inert gas, by default they're usually talking about adding it under constant volume. So you would expect no shift in the equilibrium position. If they want to be more specific, they would have to tell you under constant pressure so that you would know to do it under the second situation. For the most part, when they say the addition of inert gas or noble gas, that means that no change in the equilibrium position will occur. Now, finally we look at liquids, solids, and catalysts. We're going to say here their addition, there will be no shift in the equilibrium position. That's because when it comes to liquids and solids, they're not included in terms of equilibrium. Remember, any time we do an equilibrium expression, we always make sure to ignore solids and liquids. That's because they don't influence equilibrium amounts within our calculations. A catalyst only helps speed up the chemical reaction by lowering the activation energy. Rates deal with chemical kinetics, but when we're talking about shifting, that's covering chemical thermodynamics. Those are 2 separate ideas. A catalyst only affects rate. It doesn't affect position. The addition of any of these 3, we're going to say will cause no change. So the addition or removal of them causes no change. So these are all the factors that we can manipulate in order to cause our chemical reaction to shift in the forward direction for the production of additional products or in the reverse direction for the production of additional reactants. Keep in mind each one of these changes and the different guidelines that are associated with each.

Le Chatelier's Principle

Here it says, the following data was collected for the following reaction at equilibrium. Here we're told that at 55 degrees Celsius, our equilibrium constant k is 4.7×10-7. And at 100 degrees Celsius, k now becomes 1.9×10-2. We're asked which of the following statements is true. All right. Remember, the only real factor that can affect my equilibrium constant is temperature. Here we see that our temperature is increasing. So, temperature is increasing. Now, we have rules in terms of Le Chatelier’s principle that discuss what happens when we affect the temperature. And according to Le Chatelier’s principle, it says that when we increase temperature, we're going to shift away from heat. That's what the increasing temperature is telling us. Now we have to figure out what side heat is on. How can we determine which side heat is on? We can determine that by figuring out which way my chemical reaction will shift. When we increase the temperature, k actually increases. Now it's 1.9×10-2. We're going to say here if you're increasing your equilibrium constant k, that gives us a clear indication of which direction our reaction is shifting. So think about it. K=productsreactants. The only way for k to increase is for my product amount to go up, to increase, and my reactant amount to decrease. When k is increasing, that means products are increasing and reactants are decreasing. So think about it. How can that happen in terms of shifting in a chemical reaction? Well, if my products are increasing, that must mean my reaction is shifting towards them. So we must be heading in the forward direction so that my products are increasing and so that this side here is decreasing. So looking at k, whether it’s increasing or decreasing gives me the direction involved. We figured out that we're heading in the forward direction. Going back to temperature, we would be shifting away from heat. So, if we're shifting in the forward direction, that must mean that heat is a reactant. It's on the reactant side so we'd normally shift away from it with the increase in temperature. If heat is a reactant, that must mean my reaction is endothermic, a positive delta H value here. A is out. The enthalpy change for delta H is equal to 0. So if delta H is equal to 0, that means that we're thermally neutral. If we're thermally neutral, that would mean that increasing the temperature or decreasing the temperature would cause no change in my equilibrium constant. So k would stay the same exact value if we were indeed thermally neutral. Since k is changing when I change temperature, we know that delta H cannot be equal to 0. And then finally, we did have enough information. We figured out that the reaction is endothermic. So again, the two things that we look at are the change in temperature to determine what we're following in terms of Le Chatelier’s principle. And then we look at k. Is it increasing or decreasing? This helps us to determine which direction my chemical reaction will shift. Then apply Le Chatelier’s principle with temperature to determine what side the heat is on. Now that we've seen this, we'll go on to example 2 in terms of Le Chatelier’s principle. Go ahead and try to see if you can get the answer. If you get stuck, don't worry. Just come back to the next video and see how I approach example 2.

Le Chatelier's Principle

In which of these gas-phase equilibria is the yield of products increased by increasing the total pressure on the reaction mixture? All right. Here they're saying that we're increasing the pressure. Remember, increasing the pressure is the same thing as decreasing the volume. We're going to shift to the side with fewer moles of gas. We want to increase the amount of products made, so we want to shift towards the products. So if we take a look here, in the first one, we have 2 moles of gases as reactants and we have 2 moles of gases as products. Because both sides have the same number of moles, increasing or decreasing the pressure will cause no shift because we can't go to the side with more or fewer moles of gas.

Next, this one has 3 moles of gas. This side only has 2 moles of gas. So we'd shift to the side with fewer moles of gas since the pressure is increasing. So we would shift towards the product side so the product would be increasing. This is an answer. Next, we have 2 moles of gas here for reactants and we have 3 total moles of gas here as products. We'd shift this way towards the side with fewer moles of gas. So in this case, reactants would actually be increasing, not products. Here we have 1 mole of gas and here we have 2 moles of gas. So like in option C, we'd shift to the side with fewer moles of gas, so reactants would be increasing again.

Then, here we have 2 moles of gas. Here we have 3 moles of gas. Again, we shift to the side with fewer moles of gas, so we'd be shifting towards the reactant side. So the reactants again would be increasing. Therefore, the only option here where the products are being increased because we're shifting towards them is option B. Remember, increasing the total pressure is the same thing as decreasing the total volume. We would shift to the side with fewer moles of gas.

The reaction 2 NaHCO₃ (s) ⇌ Na₂CO₃ (s) + H₂O (g) + CO₂ (g) is endothermic. What would you do in order to maximize the yield of Na₂CO₃ (s)?

• A. Lower the temperature and increase the volume of the container.
• B. Raise the temperature and add CO₂(g) and H₂O(g).
• C. Lower the temperature and decrease the volume of the container.
• D. Add CO₂(g) and H₂O(g) and increase the volume of the container.
• E. Increase the volume of the container and raise the temperature.

Here’s what students ask on this topic:

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

Le Chatelier's Principle states that if a chemical reaction at equilibrium is disturbed, the system will adjust to minimize the disturbance and re-establish equilibrium. This can involve shifting the reaction in the forward or reverse direction. For example, adding reactants or removing products shifts the reaction to the right (forward direction), increasing product amounts. Conversely, removing reactants or adding products shifts the reaction to the left (reverse direction), increasing reactant amounts. This principle helps predict how changes in concentration, pressure, volume, and temperature will affect the position of equilibrium in a chemical reaction.

How do changes in pressure and volume affect a chemical reaction at equilibrium according to Le Chatelier's Principle?

Changes in pressure and volume affect a chemical reaction at equilibrium based on the number of moles of gas on each side of the reaction. Decreasing pressure (or increasing volume) shifts the reaction toward the side with more moles of gas. Conversely, increasing pressure (or decreasing volume) shifts the reaction toward the side with fewer moles of gas. If both sides have an equal number of gas molecules, no shift occurs. This adjustment helps the system counteract the change in pressure or volume and maintain equilibrium.

How does temperature affect the equilibrium position of a chemical reaction according to Le Chatelier's Principle?

Temperature changes affect the equilibrium position based on whether the reaction is exothermic or endothermic. For an exothermic reaction (negative ΔH), heat is a product. Increasing temperature shifts the reaction to the left (reverse direction), away from the heat. Decreasing temperature shifts it to the right (forward direction), toward the heat. For an endothermic reaction (positive ΔH), heat is a reactant. Increasing temperature shifts the reaction to the right, away from the heat. Decreasing temperature shifts it to the left, toward the heat. Identifying the reaction type is crucial to predicting the shift.

What is the effect of adding an inert gas on the equilibrium position of a chemical reaction?

Adding an inert gas, such as a noble gas, to a chemical reaction at constant volume does not affect the equilibrium position. This is because inert gases do not react with the reactants or products and do not change the partial pressures of the gases involved in the reaction. However, if an inert gas is added at constant pressure, the reaction will shift toward the side with more moles of gas to counteract the change in volume. Generally, the addition of inert gases under constant volume results in no shift in equilibrium.

Do catalysts affect the equilibrium position of a chemical reaction according to Le Chatelier's Principle?

No, catalysts do not affect the equilibrium position of a chemical reaction. Catalysts work by lowering the activation energy, thereby increasing the rate at which equilibrium is reached. However, they do not change the concentrations of reactants or products at equilibrium. Catalysts affect the kinetics (rate) of the reaction but not the thermodynamics (equilibrium position). Therefore, the addition of a catalyst will speed up both the forward and reverse reactions equally, without shifting the equilibrium position.