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General Chemistry

Learn the toughest concepts covered in Chemistry with step-by-step video tutorials and practice problems by world-class tutors

16. Chemical Equilibrium

Le Chatelier's Principle

Thermodynamics and Le Chatelier’s Principle


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in this video, we're gonna continue our discussion of chemical equilibrium by taking a look at the equilibrium constant and its relationship to thermal dynamics. So here earlier, we learned that kinetics or chemical kinetics, study the rate at which are reacting changed into products. So basically, we're looking at the rate or speed of reactions in this chapter. We're now gonna learn about thermodynamics in thermal dynamics, deals with the direction that a chemical reaction at equilibrium will shift. So thermodynamics deals with the directional reaction takes whereas kinetics looks at the right or speed at which your reaction has, you need to take a look at both of them to get a complete idea of a chemical reaction. Now we're gonna stay here that late shuttlers principle solo shot Saliers Principle states that states that once a system which is a chemical reaction, that is that equilibrium is disturbed, it will do whatever it can to get back to equilibrium. So basically, you do something to try toe alternate 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 attempt 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?

According to Le Chatelier’s Principle if a system (chemical reaction) is at equilibrium and we disturb it then the system will readjust to maintain it’s equilibrium state.


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

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Now, if we take a look at this example, we'll see. How exactly does the shotty lawyers principal help us understand? Well, the reaction shift to the left to the right or caused no shift at all. So here, what we're gonna say is for the following Indo thermic reaction. KC equals 6.73 times 10 to the three Predicting which direction the reaction will proceed as we do each one of these changes Now, First things first. We said that this is Endo thermic So that means that our entropy are delta H value is positive. And remember, Endo thermic reactions, they absorb heat and if we're absorbing heat, heat will be a reactant. Remember, this is the opposite of an exile. Thermic reaction Excel thermic reaction. Does the complete opposite releases heat and therefore heat would be a product. So be aware of that. So here it's Endo thermic so Delta H s positive. Now we're talking about the reaction shifting to the left or to the right or causing no shift at all. So for ad says the addition of a catalyst what we should realize is 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 gonna say when it comes toe thermodynamics, though a catalyst doesn't ah fact the direction off the reaction. So the reaction will not shift to the left or to the right, because catalysts deal with kinetics. The shot dealers principal deals with thermal dynamics. Next, we're going to say decreasing the volume. What we should realize here is volume is the opposite of pressure. So for decreasing the volume, we're increasing the pressure. And in both cases, reaction shifts to side with less moles of gas les miles of gas. So let's take a look here in our equation. We have seven moles of gas here and we have eight moles of gas here, so it's gonna go to the side with less moles of gas. So it's gonna shift to the left or as we say, it's gonna go in the reverse direction. So that's what we would say for B. Now for C, we're removing H 20 gas. Now we're gonna sage to oh, gas is a product. So here we're gonna say adding, adding reactant or were moving product causes the reaction to shift in the forward direction. So it's gonna go to the right because the other thing of this, in this sense, were at equilibrium. Sor balance were 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 a removing products, how can I remake the product that I just lost? I have to go in the four direction. Going in the four direction would mean I'm producing Mawr product in the same way. If I add reactant Oh, I have to much reacted. I have to get that amount back down to what it was before. So how do I get the reacting amounts down? Also go in the four direction because going in the four direction are reacting would be breaking down in order to build up our products and just realize for B and C. If I'm saying that, um, it shifts to the side with less moles when volume is decreased in 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 toe occur and same thing here. Let's say we were removing reactant or adding product. Then you would expect the opposite occur. So remember what I'm telling you. One way. Think of it the opposite way also. Now these incredibly important here. We're talking about temperature and it was important that we identified that the reaction was Indo thermic because endo thermic exo, thermic reactions deal with heat and temperature and heat very close together. They're not the same thing, but they're related together. So we're gonna 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 reacting or was it a product? And to know this, we had to figure out what did Endo thermic mean. So we're gonna shift away from the heat. The heat is a reacted, so we're gonna move away from it. So we're gonna go the right direction. So we're gonna go in the four directions. So in the opposite way. If I decrease my temperature, my reaction would shift towards my heat. So remember, if it's true one way, it's the opposite. The opposite way. Okay. Adding an H three. So NH three is reacting and we just said, adding reacted, It'll go in the four direction. Next one is decreasing pressure, which is the same thing, is increasing volume now here, remember? So in this case, reaction shifts to side with more moles of gas. Okay, so it's incredibly important to figure out which side has more moles of gas. We said this earlier that the product side happens toe have eight moles, so it has the most moles of gas. That's why we're going in the four directions. Let's say your professor gave you a different equation where most moles were reacted. 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 mawr Moles of gas. Whether that be the side with products or the side with react, it's here. We're removing water liquid. Okay, 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. Okay, solids and liquids caused no shift, so we've ignored them in the equilibrium. Constant. We've ignored them in the ice charts were ignoring them. Now with shuttlers principle next edition of a precipitate. Remember what's a precipitate? While it's a fancy way of saying a solid. So we're adding a solid. So this would be the same exact answer here is well, remember, solids and liquids caused no shift. Yeah, finally, the addition of an inert gas at constant volume Now an inert gas is what a nerve gas is a noble gas. Okay, and we're assuming if we're adding a nerve gas to our sample that the volume is going to be held constant, okay? And that's because most of the noble gasses are un reactive. So as a result of that, we're also going to say that adding inert gas or noble gasses also causes no shift in our reaction. So again, we're gonna 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 what's gonna 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 reacting, which way will it shift thes air? All the concepts that you really need to know when it comes to the shuttlers 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 the settlers 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?


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. 

When looking at a chemical reaction we look at the two concepts of Kinetics and Thermodynamics to get a total view. However, remember that Le Chatelier’s Principle deals with Thermodynamics and not Kinetics. 


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)            Kp = 3.2 x 10-28


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

A2 (g)  +  B(g)  ⇌  2 AB (g) 

Which container proceeds more to completion? 

The water–gas shift reaction CO1g2 + H2O1g2Δ CO21g2 + H21g2 is used industrially to produce hydrogen. The reaction enthalpy is H = -41 kJ. (b) Could you increase the equilibrium yield of hydrogen by controlling the pressure of this reaction? If so would high or low pressure favor formation of H2(g)?
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