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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
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
they've all been hanging

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around long enough for
this to be in equilibrium

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so that the reaction that goes
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
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
solution of some kind.

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So I just one time reduced
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
that goes from C plus D

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to A plus B isn't going to
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
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
back at an equilibrium.

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Now, let's think about what just happened.

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We disturbed the equilibrium
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
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
we took some C plus D out.

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And it's going to reestablish
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
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
here the whole time,

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is Le Chatelier's Principle
that describes that.

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And it's not just by
disturbing it by changing,

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say, concentrations of
reactants or products.

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You could be changing other things.

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So for example, let's imagine
the reversible reaction,

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let's say A plus B, and let's
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,
break up into A plus B.

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And let's imagine that
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
to happen in that situation?

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Well, if I shrink the
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
into each other more.

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

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

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

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bumping into each other,
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
particles in the container

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and it decreases the pressure.

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And so, eventually you're
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
relieve that disturbance.

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In that case, the disturbance
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
an endothermic reaction.

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

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Just to make it clear that
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
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
disturb the equilibrium

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in that direction right over there.

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And so you're going to have
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
to take energy away?

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Well, you need energy for A and
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,
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
of changing the temperature,

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the system shifts to
relieve that disturbance

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and re-establish a new equilibrium,

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which, once again, is Le
Chatelier's Principle.