1. Equilibrium Expressions mass-action expression
Chemists use an equilibrium expression to mathematically describe a system undergoing a chemical change.

2. the General Expression
Start with a balanced equation
N H NH3
Coefficients
become powers
(exponents)
Then
[products]
[NH3]2
[N2] x [H2]3
= Keq
[reactants]

3. Significance of Keq equilibrium constant tells you if you have
more products OR more reactants at equilibrium
“favors” this means
that at Eq, you have a
greater concentration
of that (those) species.
A small constant favors reactants
> 1
< 1
A large constant favors
products

4. Practice Problems PCl5(g) ↔ PCl3(g) + Cl2(g)
The equilibrium constant for this system equals 35.0 at a certain temperature. If the concentrations of the PCl5 and the PCl3 are mol/L and 0.78 mol/L respectively, what is the concentration of the Cl2?
0.67 mol/L
Answer worked out

5. Practice Problems PCl5(g) ↔ PCl3(g) + Cl2(g)
The equilibrium constant for this system equals 35.0 at a certain temperature. If the concentrations of the PCl5 and the PCl3 are mol/L and 0.78 mol/L respectively, what is the concentration of the Cl2?

6. Use an ICE table and MOLES to help you figure it out!
…………….. Now
A total of 3.50mol of PCl5 is placed into a 0.500L container and heated to 250oC. At equilibrium, the container holds mol of Cl2.
Calculate the equilibrium constant.
Use an ICE table and MOLES to help you figure it out!

7. This is what an ICE table looks like

11. Subtract because reactants are being used up.
Add because products are being formed.

12. This Keq makes sense because it is < 1 and therefore favors the
reactants at equilibrium. Go back and look at the last line of the
ICE table. There is a greater concentration of reactants than products.

13. 2SO2 (g) +  O2 (g) ⇄  2SO3 (g)
Consider the above reaction. When moles of SO2 and moles of O2 are placed into a 1.00 liter container and allowed to reach equilibrium, the equilibrium [SO3] is to be 0.250M. Calculate the Keq value.

15. All these Ks which is which?????????
Ksp = solubility product constant for “insoluble” substances
Ka = relative strength of ionization of an acid
HI = H+ + I- Ka= “very large”/strong
H2S = 2H+ + S-2 Ka=9.5x10-8 /weak
Keq = equilibrium constant; calculated from mass action expression
Larger numbers
favor products

16. THE END