Unit 7– Equilibrium
Let’s examine for a moment the types of graphs we saw last unit in the Kinetics section. For
the generic equation:
A —> B
A typical graph of concentration versus time for this reaction might look like this:
A
B
[X]
A
B
time
Notice how we start with some A and no B but by the end, we still have some B and some A
left. This graph can actually be split into 2 distinct regions:
Region 1
[X]
Region 2
time
Notice how in region 1, the concentrations are changing over time. But by the time we get to
region 2, the concentration lines have leveled off. What is happening here? The concentrations
are clearly NOT changing any more. What does that imply about the reaction at region 2?
Common sense would tell us the reaction has stopped; if the concentrations cease to change at
this point, the reaction must not be going any more. There is, however another explanation possible.
To see this, I’d like to use an analogy from our childhoods; a game called ―Red Rover‖. In Red
Rover, 2 lines of people face each other and lock arms and a person from one side is sent over
to the other side to try and break through. The sides alternate in this fashion with kids constantly changing sides until recess is over or there are enough sore necks and broken arms and wrists
that the nurse and the DCFS puts a stop to the game (I didn’t say it was a good game, I just said
it was something from our childhood). Let’s take a look at this in diagram form:
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Game in progress of side A versus side B
Side B
Side A
Side A sends
over a runner
We have this
And now...
Side B
Side A
Side B
Side A
But the round isn’t over until side B
sends one back….
We are now left
with this...
So side B sends
over a runner
And now...
Side B
Side B
Side A
Side A
Notice how we ended up with
the same that we started with!
AP Pirate Chemistry– All text copyright Christopher Smith 2012. All pictures obtained from the web and assumed
to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.
So what does this mean for us? If gives us another possibility for this part of the graph…
Region 1
[X]
Region 2
time
Instead of concluding that the reaction has stopped, we could conclude that the reaction is still
going but the forward reaction is going at the same rate as the reverse reaction. Thus, the system is at equilibrium. Thus, equilibria have two initial requirements:
Equilibrium requirements:
1. Reactions must be reversible
2. Reactions must be dynamic
(they have not stopped, the forward and reverse rates are equal)
Please note, that to be at equilibrium does NOT mean that the two sides have to be equal in
amounts. For example:
Side A
Side B
Sides A and B
are not equal.
There are NOT
the same number
people on each
side. However,
the system can
be at equilibrium. As long as
the same number
of people change
sides at the same
time, the
amounts remain
the same.
Side A
Side B
Note that 4 people from side A
are going to side
B and vice versa.
As long as the
rate of the forward reaction
equals the rate of
the reverse reaction, the system
is at equilibrium.
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to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.
How, then, can we predict this equilibrium? Let’s try a little experiment. Let’s set up two identical graduated cylinders. In one of them, I’m going to put 25 mL of water and the other, I’m
going to leave empty. In the first graduated cylinder (labeled R for reactants) I’m going to put a
straw with a wide diameter, capable of holding lots of liquid. In the second cylinder (labeled P
for products), I’m going to put a straw with a small diameter, capable of holding less liquid.
Then I’m going to transfer water from the first to the second using the big straw and from the
second to the first using the little straw at the same time. What is going to happen?
You might think that eventually all the liquid from the 1st cylinder will end up in the 2nd cylinder because the first straw is bigger and can transfer more. But let’s examine the graph that occurs.
See how the blue straw is bigger than the white straw?
Does that mean that all the liquid will eventually go from the
left to the right?
Click here to see this experiment in
action
Click on ―straws video 1‖
R
P
AP Pirate Chemistry– All text copyright Christopher Smith 2012. All pictures obtained from the web and assumed
to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.
R
P
Trial
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
R
25
21
18
16
14
13
12
11
10
10
10
10
10
10
10
10
10
P
0
4
7
9
11
12
13
14
15
15
15
15
15
15
15
15
15
R —> P
Notice what happened. After a
few exchanges, the amount in
each cylinder is constant. It’s
not changing. What is going on
here?
How could this have happened? As the liquid level in the first cylinder goes down, there is less
for the straw to transfer. Even though the straw has the capacity to move more, it can’t because
there is less of it. At the same time, though, the liquid level in the 2nd cylinder begins to rise.
Even though the straw is smaller in that cylinder and has the capacity to move less, there is
more and more water for it to move so the amount it transfers back is greater and greater.
Eventually, the amount of water moved from the 1st to the 2nd equals the amount moved from
the 2nd to the first and we have thus reached equilibrium. However, there is another side to
this story. Let’s examine a 2nd experiment.
AP Pirate Chemistry– All text copyright Christopher Smith 2012. All pictures obtained from the web and assumed
to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.
We are going to do the experiment a 2nd time but this time we are starting with 25 mL in the
products side of the equation and none in the reactants side. Will the graph be the same?
Click here to see this experiment in
action
Click on ―straws video 2‖
Notice how the straws are the same
but all the liquid starts on the right
instead of the left.
R
P
Trial
0
1
2
3
4
5
6
7
8
9
10
11
R
0
4
7
9
10
10
10
10
10
10
10
10
P
25
21
18
16
15
15
15
15
15
15
15
15
Look at this! The lines ended up
at the exact same volumes
AP Pirate Chemistry– All text copyright Christopher Smith 2012. All pictures obtained from the web and assumed
to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.
Regardless of where we start we end up at 15 mL in the Products and 10 mL in the Reactants.
It seems that no matter where we start, we end up with a ratio of 15 mL/10 mL or 1.5:1. It is at
this ratio where the rate of the forward reaction equals the rate of the reverse reaction. Every
equilibrium reaction reaches a ratio like this at some point. It is called the equilibrium constant, Keq. We have thus determined the last factor about equilibria:
Equilibrium requirements:
3. Equilibria occur when there is a stable ratio of products to reactants, Keq
But how do we know what ratio each reaction will be? How can we develop Keq?
Equilibrium Summary
Equilibrium requirements:
1. Reactions must be reversible
2. Reactions must be dynamic
(they have not stopped, the forward and reverse rates are equal)
3. Equilibria occur when there is a stable ratio of products to reactants, Keq
But how do we know what ratio each reaction will be? How can we develop Keq?
AP Pirate Chemistry– All text copyright Christopher Smith 2012. All pictures obtained from the web and assumed
to be publicly accessible. If you are the owner of a picture and want it removed, please email csmith@d211.org.