In This Lesson:
Equilibrium
(Lesson 4 of 4)
Today is Thursday,
June 11th, 2015
Stuff You Need:
Paper Towel
Pre-Class:
What does it mean for something to be at
equilibrium?
P.S. Get a paper towel and turn in your labs.
Today’s Agenda
• CBSD History
• Equilibrium
• Le Châtelier’s Principle
• Where is this in my book?
– P. 549 and following…
By the end of this lesson…
• You should be able to use Le Châtelier’s
Principle to determine the equilibrium-seeking
response of a reaction to a stress.
Making Reactions Move
• So how can you make a reaction proceed faster?
– Add a catalyst (like an enzyme).
• They lower activation energy – more on this on the next
slide.
– Decrease the size of the reactant particles.
• Higher surface area = better.
– Add more reactant(s).
• That’s an easy one.
– Remove a product(s).
• More reactants will react to take its place.
– Others.
FROM MY BIOLOGY CLASS:
Activation Energy
• Enzymes are catalysts because they catalyze
reactions.
– In other words, they “kick-start” ‘em.
No Enzyme 
Sketch
me!
http://www.saskschools.ca/curr_content/chem30_05/graphics/2_graphics/catalyst1.gif
Enzyme
Sketch
me!
Now then…
• Now that we know about how to get reactions
moving, what happens when they finish?
• Well, in some cases, reactions reach a point of
equilibrium, when the process is done.
– Importantly, there are often some reactants left.
• But what exactly does it mean to be at
equilibrium?
Equilibrium Example
• For many years in CBSD, the population was going
steadily up.
– That’s why we have CB South now.
– In the ‘50s, CB West (then known as Central Bucks
High School) replaced Doylestown High School, which
burned down.
– East opened in 1969.
• Another way to put this is that up until now, CB
was not at equilibrium.
– It got so big that now your district is the largest nonurban district in the state by both students and area.
Equilibrium Example
• Now, however, for the most part CBSD isn’t
growing or shrinking.
• Each year, there’s roughly the same amount of
students in each class.
– In other words, there won’t be a CB North for a
long time.
• Or, another way to put this is that our district
is now at equilibrium.
Equilibrium Example
• But does that mean that each year CB has
EXACTLY the same amount of students? Do
our students never get old and leave?
– No. The numbers still change.
• It’s just that the seniors that graduate each
year are replaced just about equally by the
kindergarteners that enroll each fall.
• Thus, this is actually dynamic equilibrium.
Chemical Equilibrium
• Similarly, at the end of some reactions, there
are both reactants and products leftover.
– And, like students, reactants are becoming
products at the same rate as products are
becoming reactants.
• What this means is that some chemical
reactions can proceed both forward and
backward.
– The products react together to re-form reactants.
• These are called reversible reactions.
Chemical Equilibrium
• A reaction reaches chemical equilibrium
(same as dynamic equilibrium) when the rate
of the forward reaction equals the rate of the
reverse reaction.
– The concentrations of reactants and products is
unchanged.
– Importantly, particles don’t stop moving, so both
reactions are still occurring.
• Just like kids are still enrolling/graduating.
Chemical Equilibrium Example
• 2HgO (s)  2Hg (l) + O2 (g)
• In this reaction, the “” indicates that a
reaction is reversible.
– Sometimes “↔” is used.
Le Châtelier’s Principle
• Le Châtelier’s Principle states
that when a system at
equilibrium is placed under
stress, the system will
undergo a change in a such a
way as to relieve that stress.
– In other words, if you add
more reactant, the system
tries to use it up, and vice
versa.
Henry Le Chatelier
Le Châtelier – Translated
• When you take something away from a system
at equilibrium, the system shifts in such a way
as to replace what you’ve taken away.
• When you add something to a system at
equilibrium, the system shifts in such a way as
to use up what you’ve added.
Le Châtelier - Example
• If I have a bucket of water and scoop water
out of the right side of it with a cup, do I then
have a hole in the water? What happens?
– I don’t have a hole – the other water molecules
shift over to replace what was lost.
• If I pour that water back in, do I have a
mountain of water?
– No, the water spreads out to minimize the effect.
Big Important Note
• Catalysts do increase the rate of reaction, but
they DO NOT change the equilibrium
position.
• Think of a catalyst like getting people on a
seesaw faster:
– Yes, the reaction will tip in one direction or
another faster than without a catalyst.
– But, will the weights of those on the seesaw
change?
• Nope.
Last Thing
• Here are three steps to solve these problems:
1. Identify the change.
– As in, “heat added” or “CO2 removed.”
2. Identify the fix.
– As in, “remove the heat” or “add more CO2.”
3. Identify the shift.
– Which way does equilibrium move to apply the fix.
• Key: Shift toward something to make more.
• Key: Shift away from something to make less.
Le Châtelier’s Principle Practice
• A closed container holds ice and water at
equilibrium.
Ice + Heat  Water
•
•
•
•
•
The temperature is raised.
Change: Heat increased.
Fix: Remove heat.
Shift: Equilibrium shifts right to use up the heat.
The seesaw tips up on the right to tip the left down.
Le Châtelier’s Principle Practice
• A closed container holds N2O4 and NO2 at
equilibrium.
N2O4 (g) + Heat  2 NO2 (g)
•
•
•
•
•
NO2 is added.
Change: NO2 added.
Fix: Remove NO2.
Shift: Equilibrium shifts left to use up the NO2.
The seesaw tips up on the left to tip the right down.
Le Châtelier’s Principle Practice
• A closed container holds water and water
vapor at equilibrium.
Water + Heat  Vapor
• Vapor is removed.
• The equation shifts to the ( right / left ) to
replace the vapor.
Final Example
• A closed container holds N2O4 and NO2 at
equilibrium.
N2O4 (g) + Heat  2 NO2 (g)
• Pressure is increased.
• The equation shifts to the ( right / left ) to
lower the pressure, because there are fewer
moles of gas on that side of the equation.
Final Example
• If the equation shifts to the left, what happens
to the amount of heat we have?
N2O4 (g) + Heat  2 NO2 (g)
• It goes up, since we’re breaking down NO2 to
make more N2O4 and heat is a byproduct.
Think of it this way…
• If the equation shifts to the right, the terms on
the right side of the equation increase.
– The left side decreases.
• If the equation shifts to the left, the terms on
the left side of the equation increase.
– The right side decreases.
• It’s exactly like a seesaw. One side of the
equation goes up, one goes down.
Closure
• Le Châtelier’s Principle worksheet
– All
• Le Châtelier’s Principle Practice worksheet
– All