1. Equilibrium Constant
Chapter 15
Part 2

2. Review question (conceptual) 2A B
Which of the following must be true of this equilibrium?
(a) K > 1 (b) K = 1 (c) K < 1 (d) K = 0 (e) K < 0

3. Applications of equilibrium
Knowing the equilibrium constant allows us to make predictions.
The tendency for the reaction to occur.
If the given concentrations represent equilibrium.
Equilibrium position

4. Assume a Rxn with K=16
A atoms (red), B atoms (blue), and AB and B2 molecules.
A +B AB + B
After the system is at equilibrium, what will the system look like?

5. Write the K expression 16 = (# AB)(#B) (#A)(#B2) Let us pretend:
Initial conditions Final conditions
A = =4
B2 = =7
AB= =5
B= =5

6. Do the new conditions represent equilibrium?
(5) (5) = 0.9
(4) (7)
Last I looked 0.9 did not equal 16
Therefore this system is not at equilibrium.
The system needs to move toward products to achieve equilibrium.

7. To reach equilibrium… A +B2 AB + B For every A that “disappears,”
One AB “appears”.
For every B2 that “disappears”,
One AB and one B appears.
If we call that number X…

8. A +B2 AB + B 16 = (# AB)(#B) = (x)(x) (#A)(#B2) (9-x)(12-x)
Using Trial & Error
X>5 but less than 9
Try 8
(x)(x) = (8)(8) = 64/4 = 16
(9-x)(12-x) (9-8)(12-8)

9. Extent of a reaction
The tendency of a reaction is indicated by the magnitude of the K
A large K indicates a tendency toward products.
A K much smaller than one indicated a tendency towards the reactants.
However, the magnitude of K and the time to reach equilibrium are not related!!!

10. Reaction Quotient
The Reaction Quotient is used to determine the shift towards or from equilibrium.
It is the same expression, but is not at equilibrium.
It is used for the initial concentrations.

11. Reaction Quotient When Q = K, the system is at equilibrium.
When Q > K, the system will shift towards the left forming reactants.
When Q < K, the system will shift towards the right forming products.

12. Equilibrium and Gas mixtures

13. Equilibrium and Pressure
Remember [ ] is not just an aqueous situation.
Gas has volume (moles/liter)
Molarity applies to both aqueous and gas mixtures.
PV=nRT
P=(n/V)RT n/V like mol/liter is C
C is concentration

14. Thus P=CRT and if T is constant:
Kc = (CNH3)2/(CN2)(CH2)3
Kp = (PNH3)2/(PN2)(PH2)3
K is an equilibrium constant
Kc is about [ ]
Kp is about pressure

15. P NOCl = 1.2 atm P NO = 5.0 x 10 -2 atm P Cl2 = 3.0 x 10 -1 atm
2NO(g) + Cl2(g) 2NOCl (g)
At T = 25° C
P NOCl = 1.2 atm
P NO = 5.0 x atm
P Cl2 = 3.0 x atm
Kp = ??
1.9 x 10 3

16. How do Kp and Kc relate??
If C= P/RT
Then for N2(g) +3H2(g) NH3(g)