Le Châtelier's principle | Reaction rates and equilibrium | High school chemistry | Khan Academy

- [Instructor] Let's imagine a reaction

that is in equilibrium.

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

or you could go the other way around.

C plus D could react to form A plus B.

And we assume that
they've all been hanging

around long enough for
this to be in equilibrium

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

it's happening at the same rate

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

Now what we're gonna do is imagine

what would happen if we
disturb this equilibrium,

and let's say we disturb this equilibrium

by taking some C and D out of,

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

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

Well, that disturbance, first of all,

is going to throw us out of equilibrium,

because now the reaction
that goes from C plus D

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

'Cause I just took C and D out,

they're not going to bump
into each other enough

to now form A and B at the same rate.

So if you think about the net direction

until we hit a new equilibrium,

this is going to happen less.

and this, initially, is going

to be happening at the same amount.

So you're going to have a net direction

until we hit equilibrium again

that goes from A plus B to C plus D.

And then if you wait long enough,

you're going to hit
back at an equilibrium.

Now, let's think about what just happened.

We disturbed the equilibrium
by taking C and D out.

Until we hit our new equilibrium,

we have more of the
reaction going from A plus B

to C plus D on a net basis.

And so it's relieving it.

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

And it's going to reestablish
a new equilibrium.

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

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

then you would have the opposite happen.

But either way, if you disturb it,

the system shifts to
relieve the disturbance

and re-establish equilibrium.

Now this principle, you might imagine,

'cause it's been sitting
here the whole time,

is Le Chatelier's Principle
that describes that.

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

say, concentrations of
reactants or products.

You could be changing other things.

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

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

So A plus B can react to form C,

or C could react to,

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

And let's imagine that
these are all gases.

So let's assume that it's happening

in a container of a certain size.

And let's say that I were to shrink

the volume of that container.

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

Well, if I shrink the
volume of that container,

then you have a situation where A and B

are going to bump into each other more.

They're going to collide
into each other more.

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

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

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

bumping into each other,
colliding each other to form C,

until we hit a new equilibrium.

And notice what is happening there.

When A plus B reacts to form C,

it decreases the number of
particles in the container

and it decreases the pressure.

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

But when you disturb that equilibrium

by changing the volume,

the system shifted to
relieve that disturbance.

In that case, the disturbance
was an increased pressure

and it reestablished the equilibrium.

Let's imagine another reaction.

Let's imagine A plus B.

And let's say this is
an endothermic reaction.

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

Just to make it clear that
this is an endothermic reaction

that could form C plus D.

Or you could have C plus D react

to form A plus B plus energy.

So the reaction that starts with C plus D

and forms A plus B in energy,

well, that's going to be exothermic.

So let's imagine what would happen here,

and let's imagine it's at equilibrium,

but then we disturb that equilibrium.

What happens if we
disturb that equilibrium

by adding more energy over here?

Well, if I add more energy,

it's going to be easier

for this endothermic reaction to occur,

and so it's going to
disturb the equilibrium

in that direction right over there.

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

to form more C and D.

You could imagine the other way.

What happens if I were
to take energy away?

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

So if you were to take energy away,

then the reaction that starts with A and B

is going to happen less.

And so you're gonna have a net direction

with C plus D reacting to form A plus B

until you hit a new equilibrium.

But the important thing to realize here

is in every situation,

whether we're disturbing the equilibrium

by changing concentration,
by changing volume,

and therefore changing pressure,

or you're adding or taking away energy,

which you could do in the form
of changing the temperature,

the system shifts to
relieve that disturbance

and re-establish a new equilibrium,

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