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- [Instructor] Let's imagine a reaction

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that is in equilibrium.

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So A plus B, they can[br]react to form C plus D,

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or you could go the other way around.

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C plus D could react to form A plus B.

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And we assume that[br]they've all been hanging

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around long enough for[br]this to be in equilibrium

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so that the reaction that goes[br]from A plus B to C plus D,

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it's happening at the same rate

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as the reaction from C plus D to A plus B.

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Now what we're gonna do is imagine

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what would happen if we[br]disturb this equilibrium,

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and let's say we disturb this equilibrium

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by taking some C and D out of,

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let's say this was a[br]solution of some kind.

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So I just one time reduced[br]the concentration of C and D.

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Well, that disturbance, first of all,

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is going to throw us out of equilibrium,

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because now the reaction[br]that goes from C plus D

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to A plus B isn't going to[br]be able to happen as often.

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'Cause I just took C and D out,

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they're not going to bump[br]into each other enough

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to now form A and B at the same rate.

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So if you think about the net direction

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until we hit a new equilibrium,

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this is going to happen less.

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and this, initially, is going

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to be happening at the same amount.

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So you're going to have a net direction

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until we hit equilibrium again

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that goes from A plus B to C plus D.

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And then if you wait long enough,

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you're going to hit[br]back at an equilibrium.

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Now, let's think about what just happened.

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We disturbed the equilibrium[br]by taking C and D out.

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Until we hit our new equilibrium,

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we have more of the[br]reaction going from A plus B

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to C plus D on a net basis.

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And so it's relieving it.

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It's relieving the fact that[br]we took some C plus D out.

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And it's going to reestablish[br]a new equilibrium.

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If we took A plus B out, or A and B out,

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or even just one of them, A or B out,

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then you would have the opposite happen.

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But either way, if you disturb it,

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the system shifts to[br]relieve the disturbance

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and re-establish equilibrium.

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Now this principle, you might imagine,

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'cause it's been sitting[br]here the whole time,

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is Le Chatelier's Principle[br]that describes that.

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And it's not just by[br]disturbing it by changing,

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say, concentrations of[br]reactants or products.

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You could be changing other things.

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So for example, let's imagine[br]the reversible reaction,

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let's say A plus B, and let's[br]say these are all gases.

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So A plus B can react to form C,

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or C could react to,

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I guess you could imagine,[br]break up into A plus B.

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And let's imagine that[br]these are all gases.

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So let's assume that it's happening

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in a container of a certain size.

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And let's say that I were to shrink

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the volume of that container.

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What do you think is going[br]to happen in that situation?

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Well, if I shrink the[br]volume of that container,

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then you have a situation where A and B

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are going to bump into each other more.

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They're going to collide[br]into each other more.

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And so you are going to have a[br]net direction go in that one.

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You'll still have some C reacting[br]to break up into A and B,

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but you're going to have[br]more A and B reacting,

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bumping into each other,[br]colliding each other to form C,

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until we hit a new equilibrium.

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And notice what is happening there.

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When A plus B reacts to form C,

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it decreases the number of[br]particles in the container

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and it decreases the pressure.

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And so, eventually you're[br]going to hit a new equilibrium.

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But when you disturb that equilibrium

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by changing the volume,

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the system shifted to[br]relieve that disturbance.

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In that case, the disturbance[br]was an increased pressure

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and it reestablished the equilibrium.

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Let's imagine another reaction.

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Let's imagine A plus B.

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And let's say this is[br]an endothermic reaction.

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So I'm gonna treat energy[br]really as a reactant here.

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Just to make it clear that[br]this is an endothermic reaction

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that could form C plus D.

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Or you could have C plus D react

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to form A plus B plus energy.

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So the reaction that starts with C plus D

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and forms A plus B in energy,

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well, that's going to be exothermic.

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So let's imagine what would happen here,

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and let's imagine it's at equilibrium,

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but then we disturb that equilibrium.

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What happens if we[br]disturb that equilibrium

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by adding more energy over here?

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Well, if I add more energy,

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it's going to be easier

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for this endothermic reaction to occur,

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and so it's going to[br]disturb the equilibrium

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in that direction right over there.

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And so you're going to have[br]that energy really get used up

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to form more C and D.

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You could imagine the other way.

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What happens if I were[br]to take energy away?

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Well, you need energy for A and[br]B to react to form C plus D.

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So if you were to take energy away,

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then the reaction that starts with A and B

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is going to happen less.

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And so you're gonna have a net direction

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with C plus D reacting to form A plus B

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until you hit a new equilibrium.

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But the important thing to realize here

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is in every situation,

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whether we're disturbing the equilibrium

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by changing concentration,[br]by changing volume,

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and therefore changing pressure,

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or you're adding or taking away energy,

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which you could do in the form[br]of changing the temperature,

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the system shifts to[br]relieve that disturbance

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and re-establish a new equilibrium,

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which, once again, is Le[br]Chatelier's Principle.