1. CHEMICAL EQUILIBRIUM Unit 11, Part II
4/15/2017
CHEMICAL EQUILIBRIUM Unit 11, Part II
Pb2+(aq) Cl–(aq) PbCl2(s)
Dr. Mihelcic AP Chemistry 1 1

2. What is equilibrium?
Definition (dictionary.com): a state of rest or balance due to the equal action of opposing forces
Chemical Equilibrium: A process where a forward and reverse reaction occur at equal rates
Not all chemical reactions are reversible!!!

3. General Characteristics of Equilibrium
DYNAMIC (in constant motion)
REVERSIBLE
can be approached from either direction (reaction can run in the forward direction or the reverse direction)
Cobalt Chloride Complex Equilibrium
Pink to blue
Co(H2O)6Cl2 Co(H2O)4Cl H2O
Blue to pink
Co(H2O)4Cl H2O Co(H2O)6Cl2

4. Characteristics of Dynamic Equilibrium
Fe SCN FeSCN2+ +
After a period of time, the concentrations of reactants and products are constant.
The forward and reverse reactions continue after equilibrium is attained.

5. Examples of Chemical Equilibria
Phase changes such as H2O(s) H2O(liq)

6. Examples of Chemical Equilibria
Formation of stalactites and stalagmites
CaCO3(s) + H2O(liq) + CO2(g) Ca2+(aq) + 2 HCO3-(aq)

7. Graphing Dynamic Equilibrium
Equilibrium achieved
Product conc. increases and then becomes constant at equilibrium
Equilibrium achieved when product and reactant concentrations remain constant!!
Reactant conc. declines and then becomes constant at equilibrium

8. The Equilibrium Expression, Keq
Keq = equilibrium constant (for a given T)
Brackets "[ ]" = concentration (molarity)
"a, b, c, and d" = coefficients from balanced equation
The "c" in Kc = concentration
(Kc = a special Keq based on concentration)

9. The Equilibrium Constant for the Synthesis of HI
Note that Equilibrium Constants have NO units!!
Equilibrium achieved
In the equilibrium region

10. SOLIDS – (s) after the formula
There are two cases when a species is not shown in the equilibrium expression:
#1:
SOLIDS – (s) after the formula
#2:
pure LIQUIDS – (l) after the formula

11. Writing Equilibrium Expressions
Solids and liquids NEVER appear in equilibrium expressions.
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

12. Writing Equilibrium Expressions
Solids and liquids NEVER appear in equilibrium expressions.
S(s) + O2(g)  SO2(g)
Burning Sulfur in Oxygen

13. Examples of Equilibrium Expressions:
The oxidation-reduction reaction occurring between iron(III) chloride and tin(II) chloride:
2 FeCl3 (aq) + SnCl2 (aq)  2 FeCl2 (aq) + SnCl4 (aq)
The replacement of silver ions by copper:
Cu (s) + 2 Ag+ (aq)  Cu2+ (aq) + 2 Ag (s)

14. Equilibrium Constants
K is independent of:
Original amounts of reactants or products
Size of container
Pressure
K is dependent on:
Temperature
States of reactants and products

15. Can tell if a reaction is product-favored or reactant-favored
The Meaning of K
Can tell if a reaction is product-favored or reactant-favored
For N2(g) H2(g)  2 NH3(g)
If K is large, the conc. of products is much greater than that of the reactants at equilibrium.
The reaction is strongly product-favored
(right side of equation is favored)

16. The Meaning of K
For AgCl(s)  Ag+(aq) + Cl-(aq)
Kc = [Ag+] [Cl-] = 1.8 x 10-5
If K is small, the conc. of the products is much less than that of the reactants at equilibrium.
The reaction is strongly reactant-favored (left side of equation is favored).
AgCl(s)  Ag+(aq) + Cl-(aq)
is reactant-favored.

17. Product- or Reactant Favored
4/15/2017
Product- or Reactant Favored
Product-favored
K >> 1
Reactant-favored
K << 1
Dr. Mihelcic AP Chemistry 1

18. Determining an Equilibrium Constant if all Concentrations are known:
Kc = [ NH3]2
[N2] [ H2]3
Can substitute numbers into concentrations; if all concentrations = 2.0M,
Then Kc = (2)2 \ (2) x (2)3 = ¼ or .25
(no units on K)

19. Example: For NH4Cl(s)  NH3(g) + HCl(g) at 500oC, at equilibrium there are 2.0 mol of ammonia, 2.0 mol of hydrochloric acid and 1.0 mol of ammonium chloride present in a 5.0 L container. Calculate the equilibrium constant of the system at this temperature.

20. Example:
For the system 2SO3(g)  2SO2(g) + O2(g) at a given temperature the equilibrium concentrations are [SO2] = [O2] = 0.10M and [SO3] = 0.20M. Calculate the Kc at this temperature.

21. Example: For N2(g) + O2(g)  2NO(g) Kc = 1.0 x 10-30 at 25°C
Calculate the equilibrium concentration of NO(g) if at equilibrium the concentration of N2(g) is 0.04 M and that of O2 is 0.01 M.

22. LE CHATELIER’S PRINCIPLE
“If a system at equilibrium is stressed, the system tends to shift its equilibrium position to counter the effect of the stress.”

23. How does a “stress” influence equilibrium?
The impact of addition of reactants
on reaction rate

24. The Seesaw Analogy

25. The “stresses” (factors) that can cause changes at equilibrium
Changes in amount of species
A. Add reactant; system shifts to right (produces more products)
B. Add product; system shifts to left (produces more reactants)
C. Remove reactant; system shifts to left
D. Remove product; system shifts to right

26. CH4(g) + 2S2(g)  CS2(g) + 2H2S(g)
Example: Predict the direction of shift of the following concentration changes on the reaction:
CH4(g) + 2S2(g)  CS2(g) + 2H2S(g)
(A) Some S2(g) is added.
(B) Some CS2(g) is added.
(C) Some H2S(g) is removed.
(D) Some argon gas (an inert gas) is added.

27. Changes in Pressure/Volume
If P goes down (same as V goes up), system shifts to increased # of moles of gas
If P goes up (same as V goes down), system shifts to decreased # of moles of gas

28. Changes in Temperature
Write heat as a product (exothermic) or reactant (endothermic).
System shifts to get rid of added heat: will shift
LEFT for exo reactions as T goes up
RIGHT for endo reactions as T goes up

29. Changes in Amount of Pure Solid/Liquid or Inert Substance
If a substance is NOT in Keq, changes will have NO EFFECT on equilibrium!

30. (a) CH4(g) + 2S2(g)  CS2(g) + 2H2S(g)
Example Predict the effect of increasing pressure (decreasing volume) on each of the following reactions.
(a) CH4(g) + 2S2(g)  CS2(g) + 2H2S(g)
(b) H2(g) + Br2(g)  2HBr(g)
(c) CO2(g) + C(s)  2CO(g)
(d) PCl5(g)  PCl3(g) + Cl2(g)

31. How can the reaction be shifted to the right?
Le Chatelier Example
EXAMPLE:
CO(g) + 2 H2 (g) <---> CH3OH (g)
How can the reaction be shifted to the right?

32. Example 16.17
What conditions would be most ideal for shifting the following equilibrium as far to the right as possible?
N2(g) + 3H2(g)  2NH3(g) ∆H = negative

33. Haber-Bosch Process for NH3
N2(g) + 3 H2(g)  2 NH3(g) + heat
K = 3.5 x 108 at 298 K

34. Haber-Bosch Ammonia Synthesis
Fritz Haber
Nobel Prize, 1918
Carl Bosch
Nobel Prize, 1931

35. Addition of a Catalyst (NOT in Keq)
Catalytic exhaust system
Add catalyst ---> no change in K
A catalyst only affects the RATE of approach to equilibrium.