A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society.
Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.
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Chemical Equilibrium
This unit introduces the concept of chemical equilibrium and how it applies to many chemical reactions. The quantitative aspects of equilibrium are explored thoroughly through discussions of the law of mass action as well as the relationship between equilibrium constants with respect to concentrations and pressures of substances. Much of the discussion explores how to solve problems to find either the value of the equilibrium constant or the concentrations of substances at equilibrium. ICE (initial-change-equilibrium) tables are introduced as a problem-solving tool and multiple examples of their use are included.
From a qualitative standpoint, Le Châtelier’s principle is used to explain how various factors affect the equilibrium constant of a reaction along with the concentrations of all species.
in this unit we will be talking about chemical equilibrium.
The first thing, we need to understand, is what we mean by the term dynamic equilibrium.
So that is the goal of this unit, is to understand what we mean
and specifically how that relates to the forward and reverse reactions.
So when we looked at kinetics, what we were worried is how fast the reaction proceeds.
How long, how much time does it take
to produce a certain amount of product.
How fast are our reactants consumed.
In equilibrium, we are worried about how far the reaction proceeds
because when we first introduce th idea of chemical reactions.
We kind of assume that they go from all reactants to forming all products.
But in reality, for many reactions that is not in fact what happens. We kind of assume that they go from all reactants to forming all products.
But in reality, for many reactions that is not in fact what happens.
What we get is some portion of the reactants are converted to products But in reality, for many reactions that is not in fact what happens.
What we get is some portion of the reactants are converted to products
and then we don't see the formation of products continue to increase.
So this is when we get to the point of chemical equilibrium.
What we have to look at is how do we know we are at equilibrium?
How does that differ between different reactions.
We need understand something about that ratio
of products to reactants.
So we are going to use the term dynamic equilibrium.
The reason we use this, is that we need to make sure
clear that there is change happening. The reason we use this, is that we need to make sure
clear that there is change happening.
Because the word dynamic means just that, it means change.
But we are also looking at equilibrium, so something else also has to be equal.
The things that are equal are the rates of the
forward and reverse reaction.
When we have a reaction that is at chemical equilibrium
so say we have A goign to B.
We use this equilibrium arrow
to represent that in fact A is being converted to B
and B is being converted back to A.
If we are at the point of equilibrium and B is being converted back to A.
If we are at the point of equilibrium
the rate of the forward process, A going to B, is equal to
to the reverse process, B going back to A.
When I get to some concentration of A, we are going to find that it
doesn't change once we are at equilibrium
neither does the concentration of B.
They don't necessarily have to be the same and
in fact they are usually not the same.
So the concentration of A is constant in fact they are usually not the same.
So the concentration of A is constant
the concentration of B is constant
but the individual molecules
that are going back and forth are going to be different.
So what I see is that some molecules are
at A are being concerted to B.
At the same rate that some molecules of B
are being converted back into A.
So we have seen this idea of equilibrium a coupl of times before.
We have looked at thermial equilibrium, if I have two objects
same mass, same substance
at different temperatures, we put them in thermal contact with one another.
what we see is they go to some temperature in the middle.
In this case, because it is the same mass and the same substance they go to what we see is they go to some temperature in the middle.
In this case, because it is the same mass and the same substance they go to
a temperature halfway in between.
Different substance, different temperature
but we see that they get to this point of equilibrium, there is a transfer
in this case, of heat from one object, the hotter one but we see that they get to this point of equilibrium, there is a transfer
in this case, of heat from one object, the hotter one
to the colder one, but then it gets to the point where we
don't see any net change in the temperature.
So what we look at with this, is we look at our process
going on, we look at the different panels to represent what is going on. So what we look at with this, is we look at our process
going on, we look at the different panels to represent what is going on.
We can put a time with this, this time is going to be different for every reaction.
We are not worried about the rate at which this is happening we are just
saying over time a substance reaches equilibrium.
And so I look at my reactants here, we have H2
reacting with Cl2
and I form HCl
and know that there are two molecules of HCl
because I have to have a balanced chemical reaction.
When I look at each panel I can look down here
and in Panel A I see that I have H2
and I have Cl2.
But notice I have none of my products
present initially, because I have combined my reactants.
Now, after some time passes present initially, because I have combined my reactants.
Now, after some time passes
what I see, is that I still have H2 and Cl2
but now I also have
4 [and I am going to call this moles]
so each molecule is representing a mole of product
so I have 4 moles of my HCl2 being produced.
I got onto my panel C
I still have HCl2 I still have H2
but now I have 8 moles
of my HCl
and I continue on to my panel D
now I have 12 moles of my HCl
again I still have H2 and Cl2 present
and I get to my panel E and I still have 12 moles of HCl.
So what has happened, is when I got to panel D
I am now at chemical equilibrium.
So we can say at this point
we are at equilibrium.
Because after we reach this point, we do not see any increase in the concentration of product
we do not see any decrease in the concentration of reactants. Because after we reach this point, we do not see any increase in the concentration of product
we do not see any decrease in the concentration of reactants.
We have reached a constant concentration of all our reactants and products. we do not see any decrease in the concentration of reactants.
We have reached a constant concentration of all our reactants and products.
Now if I were flag a particular Hydrogen molecule.
Say we look at this particular hydrogen molecule
here in panel D is shows us H2 Say we look at this particular hydrogen molecule
here in panel D is shows us H2
perhaps over here
that hydrogen molecule has split and formed
into HCl and there is some other
H2 that has form from other HCls
so we see individual molecules going back and forth
and that is where we get the dynamic part of this
but the concentration of H2 is constant
the concentration of Cl2 is constant
and the concentration of HCl is constant.
But that doesn't mean that they are all exactly the same.
It just means they are constant, their concentrations are not changing.
We can look at this in a tabulate form, we can look at here and again we have our reaction
we can see we start out with H2 and Cl2
and no HCl.
Over time we are losing H2 and no HCl.
Over time we are losing H2
losing Cl2
and we are gaining out HCl
and then again when we get to our point D and E what we see
is that our concentrations become constant.
We still have to acknowledge to Stoichiometry so
as we lose 2 moles, or two molecules of H2
we are gaining 4 molecules of HCl. as we lose 2 moles, or two molecules of H2
we are gaining 4 molecules of HCl.
because that is what the Stoichiometry of our reaction tells us.
We have looked at this in terms of the
qualitative assessments of what is going on
now what we need to be able to do, is to look at the law of mass action.
and actually quantify
these ratios between products and reactants.
That is what we will do int eh next unit.
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Dr. Allison SoultLecturer
Dr. Kim WoodrumSr. Lecturer
