Chemical Equilirbrium Kc value is Temperature Dependent

Añadido: 2026-05-14

[00:00:00]In this lesson, I'm going to teach you about the significance of the KC value.

[00:00:04]Why is it important? What can change the KC value? And the one thing that I want you to take away from this lesson is that KC value is only temperature dependent. What does that mean? Only temperature can affect the KC value.

[00:00:19]These questions are quite difficult. I sometimes think that these questions, the way that they can ask this is even more difficult than the table question.

[00:00:26]And I'm not just saying that. You have to think a little bit differently. So, let's get into it. The first thing I want you to remember is that KC equilibrium constant is calculated by using the concentration of products at equilibrium over divided by the concentration of reactants at equilibrium. It's very important that you understand that it is not just concentration, it's concentration at equilibrium. And it's always going to be products divided by reactants. Remember, reactants are always on the left-hand side of the arrow and my products are always on the right-hand side. And because it tells me, because it uses concentration at equilibrium, it tells me about the position of equilibrium. So, think about it mathematically. If you have more products at equilibrium, if this number is bigger, think maths, if your numerator is bigger, it means that your KC value is going to be bigger. So, the bigger you make the numerator, the more products you have at equilibrium, the bigger the KC value, KC is bigger than one.

[00:01:26]Opposite, in an opposite way, if you have more reactants at equilibrium. So, if your denominator is bigger, the bigger you make your denominator, then the smaller the KC value gets. Think of it as inversely proportional. So, the more reactants you have at equilibrium than products, then your KC value is most likely going to be less than one.

[00:01:48]Another thing that I want you to remember is that the forward reaction, so I've drawn an arrow pointing to the right, representing forward reaction. It would be this one. The forward reaction produces products. The reverse reaction produces reactants. Now, think about it like this. If I favor the forward reaction, so the forward reaction is favored, what that means is that I will produce more products. So, the concentration of the products will increase. This top number here will increase because the forward reaction points towards products, meaning that the forward reaction produces products. So, if we favor the forward reaction, there'll be more products, which means the numerator will increase. And as we said, if the top of the fraction gets bigger, then the Kc value will increase.

[00:02:34]But, remember, the only thing that can do this is a change in temperature. So, if temperature changes in such a way so that the forward reaction is favored, okay? So, for example, maybe the forward reaction is endothermic, and I increase temperature. Remember, an increase in temperature favors the endo- thermic reaction. I'll remind you about that now. Then, the forward reaction will be favored, so concentration of products will increase. So, the numerator increases, so Kc will increase. The opposite is also true. If I had to favor the reverse reaction because of a temperature change, so if I favor the reverse now, it's going to be going this way. It points that way. The reverse reaction produces more reactants. So, then the denominator of this fraction is going to get bigger. If your bottom of your fraction gets bigger, it means that your Kc value gets smaller. It decreases. So, basically the opposite.

[00:03:26]But, remember, the only thing that can do this is a temperature change. So, let's just do a quick recap about temperature.

[00:03:33]If the temperature of the system is changed, so say for example, the temperature increases. In other words, it gets hotter. Then, remember Le Chatelier's principle says that the system will reinstate a new equilibrium by favoring the reaction that opposes the disturbance. So, basically it favors the reaction that goes against what the disturbance was. So, if I make it hotter, the system's going to want to make it colder. So, it will favor the endothermic reaction because that takes in heat, making it colder. I told you in previous videos that I just remember increase, so I N, increase, and endothermic, okay? They both have the the N in it.

[00:04:16]If I decrease temperature, so in other words, it's gets colder, the going to want to favor the reaction that does the opposite. It's going to want to make it hotter. So, it's going to want to favor the exothermic reaction because that's the one that releases heat. So, what you need to remember is if temperature increases, endothermic is favored. If temperature decreases, exothermic is favored. And how do we know which one's exo and which one's endo? When you're given a chemical reaction a reversible, see it's got a double arrow, you get given this piece of information. It's either going to be bigger than zero or less than zero. Just as a recap, if they tell you that your delta H is less than zero, in other words, it's a negative, that refers to the exothermic reaction.

[00:04:58]Exothermic has a negative change in enthalpy or heat of reaction. If delta H is bigger than zero, in other words, it's positive, that should refers to an endothermic reaction. The change in enthalpy or the heat of reaction is positive. So, here you can see that it is less than zero. In other words, it's negative, which tells me exothermic. And this piece of information always refers to the forward reaction. So, the forward reaction is exothermic, so the reverse is the opposite, endothermic. So, you need to just remember that the piece of information given always refers to the forward reaction.

[00:05:33]There we go.

[00:05:35]So, how are they going to ask this in exams? So, here's a basic question, but it is still exam level. It's still kind of tricky. It says here, the temperature increases once equilibrium has been reached. So, this reversible reaction is allowed to reach a certain equilibrium, and once it's reached that equilibrium, then the temperature increases.

[00:05:56]They ask, "What happens to the KC value?" Now, remember, immediately when they give you a reaction like this, you need to look at this information, and this information says your change in enthalpy, your heat of your reaction is bigger than zero, it's positive. And remember that refers to an endothermic reaction. So, your forward reaction is endothermic, meaning that your reverse reaction is the opposite. So, your reverse reverse reaction is exothermic. So, this always tells me information about the forward reaction. Remember, the forward reaction goes this way, it produces products, the forward reaction makes products, the reverse reaction makes reactants. The next thing you need to consider is that when temperature increases, which reaction is favored? So, you can study this. Remember, an increase in temperature favors the endothermic reaction. So, that's what we say there.

[00:06:47]An increase in temperature favors the endothermic reaction. In this particular reaction, can you see that the endothermic reaction is the forward reaction?

[00:06:56]So, that means that the forward reaction is favored.

[00:07:00]If the forward reaction is favored, like I've said here, the forward reaction makes products. So, what we can say is that more products are formed, or the concentration of products increases.

[00:07:11]And what does that mean for the KC value? Remember, KC is equal to concentration of products divided by concentration of reactants. So, if I make more products because I've favored the forward reaction, if this number gets bigger, then the KC value will increase.

[00:07:29]And that would be our full answer, how we answer in an exam question, and this could easily be three to four marks.

[00:07:36]Obviously, if the opposite happens, then the reverse is true. Okay, so for example, if the temperature decreases, remember? So, let's change this. Let's use a blue pen.

[00:07:48]If the temperature had to decrease, then you would say a decrease in temperature, what does that favor? If we make it colder, then it will favor the exothermic reaction, okay? Because it wants to make it hotter, it wants to release heat. And in this case, the reverse reaction is exothermic. So, the reverse reaction will be favored.

[00:08:09]Therefore, if the reverse is favored, the reverse makes reactants, therefore more reactants are formed, or the concentration of reactants increases, and therefore, think about it, if the reactants gets bigger, if the denominator gets bigger, the KC value will decrease. It'll get smaller. So, if we change temperature in the other direction, then the opposite's going to happen. So, let's take this up a level. Let's make Let's ask a more difficult question. So, the question, there's the reaction, there's the question. What must I do to temperature to decrease the KC value? So, pause the screen and try it and see what you can get.

[00:08:45]The first place that I would start is to look at this given information, which says that the change in enthalpy or the heat of the reaction is bigger than zero, so it's positive, which means endothermic. So, again, the forward reaction is endothermic, and the reverse reaction is therefore exothermic.

[00:09:02]Just to remind you, the forward reaction makes these products. The reverse reaction, going this way, makes the reactants.

[00:09:10]So, how do we decrease the KC value?

[00:09:13]Just remind yourself how to calculate KC. So, to make this value smaller, to decrease this value, we must make the reactants bigger, okay? Or another way to think of it is we can make the reactants bigger, or we can make the products smaller. But either way, think about it like this. If we want to make the reactants bigger, we have to favor the reverse reaction. The reverse reaction makes more reactants and therefore you will have less products.

[00:09:44]If we're making more reactants, so following the reverse reaction, we're making more reactants, the reactants this denominator gets bigger, so the KC value will get smaller. I hope that makes sense. Now, how are we going to So that means that we need to favor the reverse reaction. How are we going to favor the reverse reaction? The reverse reaction is exothermic. So what triggers favoring of an exothermic reaction? What makes us want to favor the exothermic reaction? Remember this, the exothermic reaction gets favored if temperature decreases.

[00:10:18]So I'll start off by saying to make the KC value smaller, we need to increase the reactants concentration, which means we need to favor the reverse reaction. I then state that the reverse reaction is exothermic. We got that from this information here. The reverse reaction is exothermic. And then we can say a decrease in temperature will favor the exothermic reaction. Often in exams these questions will first say, what must I do to temperature? Must I increase it, decrease it, or remain the same? So you'll start off your answer up here by saying decrease and then they'll ask for an explanation. Then you'll say the following.

[00:10:55]In the next video I'm going to do two more of these questions, exam questions.

[00:10:59]I'll see you then. Bye everyone.