Jacqueline Spivey

**Ph.D.,U.C.Santa Cruz**

Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

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**Equilibrium changes** are caused by Le Chatlier s Principle and depend upon the stress applied. Le Chatelier s Principle predicts the direction of change. For changes in volume, it is important to note that volume and pressure are inversely related. The effect of a change in concentration would increase or decrease in molecular collisions and increase or decrease the rate of forward and reverse reactions accordingly. The result from the effect of a change in temperature can either be exothermic, in which energy is released or endothermic, in which energy is consumed.

So this segment let's go ahead and build more on what we know about Le Chateliers Principle. So first of all let's just kind of recap what it is and it's basically stating what happens to an equilibrium system in the event of stress in the form of some change in concentration of reactants or products, change in volume which is also affects the pressure or a change in temperature. So let's start with concentration as it's kind of the easiest one. Okay so basically the effect of a change in concentration so remember when I'm saying concentration I mean the units of reactants or products in mols per liter. So let's take this scenario here where everything we have a homogeneous equilibrium right so all of the reactants and products are in the gaseous phase okay so let's say the equilibrium condition is defined by the concentrations of our reactants a and b and our product c. So what would happen here if we increased the concentration of a by 1.0 molar? So basically the system is going to want to compensate for that so there's going to be an increase in the molcular collisions happening here as a result of an increased concentration of the reactants, so a and b are going to be having more collisions and as a result to that creating more of the product c. So our forward reaction for a while is going to increase as a result to that making, there's going to be more c so then eventually the back reaction will also increase it's rate and make more of a and b until the system kind of mellows out and is back at equilibrium.

But at that point you're still going to have more of your product c left. So basically we say then that the equilibrium has shifted to the right and that's where the new equilibrium position lies. That does not have effect though on our equilibrium constant k, so k does not change that's important to note here. So let's talk about the effect of a change in volume, so first let's recall from understanding gas laws that if you have a decrease in volume that's going to increase your pressure. Right so you change the special dimensions it gets smaller, you have the same number of molecules present so the pressure will increase. So conversely if the volume goes up, then the pressure will decrease. So remember that volume and pressure are inversely related. So based on Le Chateliers Principle again it'll state for us that whatever stressors added to the system that the system is going to shift in a direction that's going to reduce that stress. So in this case it's going to reduce the pressure added to the system. So then when the volume of the gaseous reaction at equilibrium is decreased then the system will shift in the direction giving less gas molecules. So here again simple equation, homogeneous equilibrium everything is in the gaseous phase, so we've got our reactants a and b and our product d.

Okay so here on the left if we look at our stoichiometric efficients here we've got 1 and 1 so 2 total mols of reactant here and on the product side there we have 3. So if we were to decrease our volume basically increasing our pressure then our equilibrium is going to shift to the left. Right because on the left hand side we have 2 mols on the right hand side we have 3 mols. Conversely if we increase the volume make more space in our reaction vessel, then the pressure will go down and so the tendency of the system is to want to make more mols of product. Okay so then it'll shift to the right because we've got 3 mols of products here versus our 2 mols of reactants. So the last stressor that we want to talk about is the effect of a change in temperature. So the most important thing to note here is that the value of k the equilibrium constant does in fact change with temperature and a problem usually when you're given a setoff conditions to solve an equilibrium problem, when they define k for you they will tell you at what temperature that k actually holds to. So this is point where people always get tripped up k changes with temperature but that's the only, this is the only stressor for which k is changed.

Okay so again Le Chateliers Principle will predict the direction of change. So whether the equilibrium is shifted forward or whether it's shifted backward. So how do you know that though? So for a change in temperature you need to discuss whether or not the reaction is an exothermic reaction or an endothermic reaction. So let's just remember what those mean for a second, so with an exothermic reaction, exo outside that means that the reaction produces heat right so it goes forward and as a result of going forward it produces heat. If it's an endothermic reaction it means that heat is actually required in order to carry out the reaction. Okay so energy is consumed in order to make the reaction go forward.

Okay so here again based on this equilibrium, if we have an exothermic reaction where energy is released then we treat heat as a product. Okay so in this generic equation here again everything is a homogeneous equilibrium we've got our reactants a and b in the gaseous phase and our product d and we also have the product heat. So Le Chatelier will tell us if we add heat into the system as a product in order to compensate for that addition, it's going to want to shift the equilibrium back to the left. Right so then the equilibrium shifts to the left to compensate for the fact that we basically added products to our system in the form of heat. The other side of that is if it's endothermic, where you're consuming energy so in order for this reaction a plus b to form 3b heat is actually required so putting heat into the system making heat a reactant means that now the system is going to compensate for the fact that you added another reactant on the lest hand side. So it's going to want to shift forward or to the right in order to compensate for this addition of heat again here this is a homogeneous equilibrium and so all these then rules apply.

The most important thing though again is to remember that k does change with temperature and only with temperature and that is Le Chateliers Principle.