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 (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=6  A atoms (red), B atoms (blue), and AB and B2 molecules.  A +B 2 AB + B  After the system is at equilibrium, what will the system look like?

5. Write the K expression  16 = (# AB)(#B)  (#A)(#B 2 )  Let us pretend:  Initial conditionsFinal conditions  A = 99-5=4  B 2 =1212-5=7  AB=00+5=5  B=00+5=5

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

7. To reach equilibrium…  A +B 2 AB + B  For every A that “disappears,”  One AB “appears”.  For every B 2 that “disappears”,  One AB and one B appears.  If we call that number X…

8. A +B 2 AB + B  16 = (# AB)(#B) = (x)(x)  (#A)(#B 2 ) (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 TTTThe tendency of a reaction is indicated by the magnitude of the K AAAA large K indicates a tendency toward products. AAAA K much smaller than one indicated a tendency towards the reactants. HHHHowever, the magnitude of K and the time to reach equilibrium are not related!!!

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

11.  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)RTn/V like mol/liter is C  C is concentration

14. Thus P=CRT and if T is constant:  Kc = (C NH3 ) 2 /(C N2 )(C H2 ) 3  Kp = (P NH3 ) 2 /(P N2 )(P H2 ) 3  K is an equilibrium constant  Kc is about [ ]  Kp is about pressure

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

16. How do Kp and Kc relate??  If C= P/RT  Then for N 2 (g) +3H 2 (g) 2NH 3 (g)