I'm going to demonstrate for chemical equilibrium
How the equilibrium shifts for a gas phase reaction
Namley nitrogen plus hydrogen to make ammonia
As we change temperature, pressure, or by
by adding moles of one of the reactants or products
And we'll use LeChatelier's Principle to predict
the direction of a reaction, takes place
Which direction the reaction will change
And then I'll use an interactive simulation
to show the actual numbers and behavior
Of course we can always do the calculation which
says the equilibrium constant is equal to the
mole fraction of ammonia squared, the mole fraction of
nitrogen, first power, the mole fraction of
hydrogen cubed and the pressure squared
Where the equilibrium constant only changes
as we change the temperature
So from this we can get the values
and that's what we'll use in this simulation
And for this we're assuming ideal gasses
and that's an approximation, which gets worse
as we go to higher pressures but to demonstrate
behavior we'll use ideal gasses even at high pressure
So the simulation is available on the Wolfram
Demonstration Projects site. Can search under
chemical equilibrium to find it and here's the location
So let's look at this simulation
So we're gonna use, it's actually bar graph to represent
the amount of each component and we also have
the numbers listed. So what I'm starting out with is
by adding one mole of ammonia to my system
let it go to equilibrium at 150 bar pressure
and 500 K. Here is the distribution of
nitrogen hydrogen and ammonia, and you notice
mostly ammonia. Indeed this reaction is run at high
pressures so that the equilibrim concentration
of ammonia is high
Let's look at tempreature first
So if I were to raise the temperature and if we were
to predict by LeChatelier' Principle we need to know
something about the reaction
Well this reaciton is exothermic so then we should be
able to predict this , if I raise the temperature for
an exothermica reaction, the equilibrium conversions
is gonna decrease, in other word the amount of
ammonia should go down
as I raise the temperature and indeed
you notice that's what's happening, so as I lower the
temperature I get more ammonia
Almost complete, the numbers are not zero
but just to the significant figures
so let's go back to a value where we have both of these
So let's ook at pressure, andwe wanna predict the
direction of reaction as a fucntion of changing
the pressure. While we could go back and look at
our equilibrium expression. And this says as we raise
the pressure, since K is a constant
as we raise the pressure, we must increase the mole
fraction of ammonia to have the same equilibrium value
And so as we raise the pressure
you'll notice that we are making more and more ammonia
and maybe I should do this at high temperature
to make it easier to see
So, now as I raise th pressure you can see
we're making more ammonia and less nitrogen
and hydrogen, again what we would predict from
LeChatelier's Principle. So let's go back to values
and let's look at if we add moles of ammonia to this system
Well LeChatelier's Principle says we add something
to the right side, namely ammonia, we would
shift the reaction to the left
And so we expect to make more nitrogen and hydrogen
So, I add ammonia indeed, of course I have
more ammonia but i'm making more nitrogen and hydrogen
So all the ammonia that I add does not stay as
ammonia but some of it reacts to nitrogen and hydrogen
Suppose we add hydrogen to the system, we expect
the reaction to shift to the right
again from LeChatelier's Principle. So if I add
hydrogen, notice that the amount ofammonia increases
but now I have reacted not all of the nitrogen
just it's too small to see on this scale
Likewise, if I add nitrogen, I expect the reaction
to shift to the right
and indeed adding nitrogen, notice the hydrogen
decreases and the ammonia increases
So LeChatelier's Principle predicts the behavior
that we see, and the actual calculations allow us
to get these numbers and so we can use this simulation
to quickly visualize the behavior that we expect to see