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 heat will be a reactant. Remember, this is the opposite of an exothermic reaction, which does the complete opposite. It releases heat and there 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. So we're going to say when it comes to thermodynamics though, a catalyst does not affect the direction of the reaction. So, the reaction will not shift to the left or to the right because a catalyst deals 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. 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.
Now, for C, we're removing H2O gas. Here we're going to say, adding a reactant or removing a 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. Next, D is incredibly important. Here we're talking about temperature. It was important that I identified that the reaction was endothermic because endothermic and exothermic reactions deal with heat. Temperature and heat are very closely related. We're going to say increasing the temperature, the reaction shifts away from heat. That's why it was important for us to identify which 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.
Now, adding NH3. NH3 is a reactant and we just said adding a reactant, it'll go in the forward direction. The next one is decreasing pressure, which is the same thing as increasing volume. Now here, remember, in this case, the reaction shifts to the side with 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 the 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 is 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, which 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 a noble gas, and 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 happens 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.
