Today is Thursday, June 11 th, 2015 Pre-Class: What does it mean for something to be at equilibrium? P.S. Get a paper towel and turn in your labs. In This Lesson: Equilibrium (Lesson 4 of 4) Stuff You Need: Paper Towel

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   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 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 non- urban 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) + O 2 (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 “CO 2 removed.” 2.Identify the fix. – As in, “remove the heat” or “add more CO 2.” 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. 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. Ice + Heat  Water

Le Châtelier’s Principle Practice A closed container holds N 2 O 4 and NO 2 at equilibrium. NO 2 is added. Change: NO 2 added. Fix: Remove NO 2. Shift: Equilibrium shifts left to use up the NO 2. The seesaw tips up on the left to tip the right down. N 2 O 4 (g) + Heat  2 NO 2 (g)

Le Châtelier’s Principle Practice A closed container holds water and water vapor at equilibrium. Vapor is removed. The equation shifts to the ( right / left ) to replace the vapor. Water + Heat  Vapor

Final Example A closed container holds N 2 O 4 and NO 2 at equilibrium. 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. N 2 O 4 (g) + Heat  2 NO 2 (g)

Final Example If the equation shifts to the left, what happens to the amount of heat we have? It goes up, since we’re breaking down NO 2 to make more N 2 O 4 and heat is a byproduct. N 2 O 4 (g) + Heat  2 NO 2 (g)

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