Warmup 1/14/16 Imagine we combine 2 chemicals and see a reaction. We know at first that chemicals are reacting and combining. But what happens if we wait a few minutes? Are chemicals still reacting? Does it slow down? Objective Tonight’s Homework To define and measure what equilibrium means pp 562: 10, 11, 12

Notes on Chemical Equilibrium After a chemical reaction has happened, we say that it is in equilibrium. Nothing else is happening after that point. Most of the time, a chemical in equilibrium is actually in what’s called dynamic equilibrium.

Notes on Chemical Equilibrium Let’s take a simple example: Na + Cl  NaCl If we put sodium and chlorine in the same container, we get salt. We’ll see them combine and salt form. However, after the reaction stops, it’s never 100% done. There’s always a little more sodium and chlorine to combine and always some NaCl turning back into sodium and chlorine. NaCl amount

Notes on Chemical Equilibrium Let’s do a more complex example: H 2 + I 2 2HI As the reaction starts, H 2 and I 2 combine. However, after a little while, HI starts breaking down to make H 2 and I 2 again.

Notes on Chemical Equilibrium Let’s do a more complex example: H 2 + I 2 2HI As the reaction starts, H 2 and I 2 combine. However, after a little while, HI starts breaking down to make H 2 and I 2 again. Really what we get is two reactions happening at the same time. - Our forward reaction: H 2 + I 2 2HI - And the reverse reaction: 2HI H 2 + I 2 So… how much of what do we get in the end?

Notes on Chemical Equilibrium It depends on the rate of each reaction. Rate of forward reaction = k f [H 2 ][I 2 ] Rate of reverse reaction = k r [HI] 2

Notes on Chemical Equilibrium It depends on the rate of each reaction. Rate of forward reaction = k f [H 2 ][I 2 ] Rate of reverse reaction = k r [HI] 2 When this reaction finally settles down and reaches equilibrium, the rate forward will equal the reverse rate. We can do a bit of math. k f [H 2 ][I 2 ] = k r [HI] 2 k f [HI] 2 _ [HI] 2 _ k r [H 2 ][I 2 ] [H 2 ][I 2 ] = K eq =

Notes on Chemical Equilibrium We call K eq the equilibrium constant. It turns out we can generalize how to get this a bit. Given an equation:mA + nB  sP + rQ K eq = [P] s [Q] r _ [A] m [B] n

Notes on Chemical Equilibrium K eq can tell us a few things about a reaction. If it is much greater than 1, our equation will favor products and react fast. If it is much less than 1, our equation will favor reactants and will barely react.

Notes on Chemical Equilibrium K eq can tell us a few things about a reaction. If it is much greater than 1, our equation will favor products and react fast. If it is much less than 1, our equation will favor reactants and will barely react. If the number is close to 1, we will get a slow- ish reaction that will end with a mix of reactants and products. Not everything will get used up. Every chemical equation has a unique K eq for every temperature.

Practicing Chemical Equilibrium Example: Find the equilibrium constant given the following end concentrations. HCl – MNaOH – M H 2 O – MNaCl – M

Practicing Chemical Equilibrium Example: Find the equilibrium constant given the following end concentrations. HCl – MNaOH – M H 2 O – MNaCl – M First, we have to make our equation. HCl + NaOH NaCl + H 2 O K eq = = K eq = 3.31 [NaCl][H 2 O] [0.655][0.654] [HCl][NaOH] [0.302][0.428]

Practicing Chemical Equilibrium Like we’ve been doing all week, let’s practice as a class. pp 564: 13, 15 pp 570: 41, 42

Exit Question What is the definition of dynamic equilibrium? a) When the concentration of products = reactants b) When a chemical concentration keeps changing c) When the forward rate of a reaction equals the backwards rate d) When a reaction has reached the point where nothing changes e) There’s no such thing as “dynamic equilibrium” f) None of the above