In this screencast I want to demonstrate an example where a Le Chatelier's principle does
not work. We can use Le Chatelier's principle to predict chemical equilibrium for example
the gas phase reaction to form ammonia from nitrogen and hydrogen. There is another screencast
that discusses behavior as we change temperature, pressure, and the number of moles of the various
species. This is usually pretty good at predicting the behavior we would expect but it is not
always and I want to show an exception to this behavior. I will use an interactive simulation
to show that. We can always predict it by using the equilibrium constant and the calculation
where the equilibrium constant is the mole fraction of ammonia squared divided by the
mole fraction of nitrogen and the mole fraction of hydrogen cubed times the pressure squared.
This equilibrium expression we are assuming standard states of one bar and the pressure
then must be in units of bar so that K is dimensionless. What I use to show the behavior
where the violation occurs in Le Chatelier's principle is an interactive Mathematica simulation.
This simulation is available on the Wolfram demonstration project site and you do not
need to own a copy of Mathematica. You just need to download a free player to be able
to use this simulation. Let's look at this simulation. What I am showing is essentially
a bar graph of the number of moles of nitrogen, hydrogen and ammonia at equilibrium. This
is a case where I added one mole of ammonia and we are given pressure and temperature
and let it go to equilibrium. This is the number of moles at equilibrium from that one
mole. A previous screencast demonstrated the behavior where we added nitrogen, hydrogen
or ammonia. These sliders here allow us to add additional moles. What I want to look
at is the unusual case where I am adding nitrogen. As I add nitrogen as we predict the number
of moles of ammonia would increase and the number of moles of hydrogen would decrease.
Let's look at that again. You can see I add nitrogen. The number of moles of hydrogen
decrease and the number of moles of ammonia increase. As I add more nitrogen now something
has changed. Now you will notice the number of moles of nitrogen is decreasing and the
number of moles of hydrogen is increasing. This is in contrast to what we would expect from
Le Chatelier's principle. Adding nitrogen which is a reactant molecule on the left side and
we are increasing the hydrogen number of moles and decreasing the number of moles of ammonia.
The question is, why is this happening? If we go back and look at our equilibrium expression
as we add nitrogen we are decreasing the mole fraction of hydrogen. The mole fraction of
hydrogen is cubed in this expression so this term when hydrogen is a small number and nitrogen is a
relatively big number so when the nitrogen hydrogen ratio is relatively large
then we can get the effect that adding nitrogen lowers the hydrogen concentration. If we lower
the mole fraction then the equilibrium has to have some ammonia react to increase the hydrogen, to satisfy the
equilibrium expression. So it violates the behavior we expect from a simple application of Le Chatelier's
principle. The exception happens because we have 3 moles of hydrogen to one mole of nitrogen
reacting.