1. Chapter 16 Chemical Equilibrium

2. Reversible reactions
Reversible reaction – a reaction which can also convert products back to reactants
Reaction rate for the forward and reverse reactions dependent on concentration

3. When the rate of the forward reaction equals the rate of the reverse reaction, the reaction is in a dynamic equilibrium
rateforward = ratebackwards
Rates are equal, but concentration is not
Dynamic equilibrium

4. Chapter 14, Figure 14.3
A Population Analogy for Chemical Equilibrium

7. Recap
When a reaction is at equilibrium, what can be said about the rate of the forward reaction and the rate of the reverse reaction?
Are the concentrations of the reactants and products the same at equilibrium?

8. The equilibrium constant expression - K
At equilibrium, the concentration of the reactants and products can be related through the equilibrium constant K
Remember: k = rate constant, K = eq. constant
The equilibrium constant expression - K

9. Writing out the K expression

10. Questions

11. Large K value When K >> 1 Rxn is product favored
[P]’s higher than equilibrium
Large K value

12. Small K value When K << 1 Rxn is reactant favored
[R]’s is higher than equilibrium
Small K value

13. Medium K value??? When K ≈ 1 Neither direction is favored
Rxn proceeds about ½ way
Calculations must be done to figure out whether the [R]’s or the [P]’s is higher

14. K remains the same for a reaction at a specific temperature, independent of initial concentrations

15. Heterogeneous equilibrium
Pure solids and liquids excluded in the equilibrium constant expression

16. Chapter 14, Figure 14.6
Heterogeneous Equilibrium

17. Practice
Write the equilibrium constant expression for the following reactions
C(s) + 2H2(g) CH4(g)
HF(aq) + H2O(l) H3O+(aq) + F-(aq)
2KClO3(s) KCl(s) + 3O2(g)

18. Reaction Quotient (Q)
Similar to K, but it can be at anytime during a reaction (not just equilibrium)
Can determine direction of reaction when compared to the K value, heading towards products or heading towards reactants

19. Chapter 14, Figure 14.7
Q, K, and the Direction of a Reaction

20. Summary of meaning Q When Q = K, reaction at equilibrium
Chapter 14, Unnumbered Table, Page 630
Summary of meaning Q
When Q = K, reaction at equilibrium
When Q > K, reacts toward reactants, to the left
When Q < K, reacts towards products, to the right

21. Practice
Right before a reaction starts, and if there are no products yet, what is the Q value?
When the Q value is lower than K, which way will the reaction proceed, toward products on the right, or toward reactants on the left?
If a reaction is proceeding to the left, is Q larger or smaller than K?

22. Problems
At a certain temperature, K = 8.5x10-3. A reaction mixture contains [NH3] M and [H2S] 0.166M. What is the value of Q?
If the reaction proceeds to equilibrium, will it produce more reactants or products?

23. ICE Tables
Method to determine concentrations of reactants and products at equilibrium if given initial concentrations and K
Initial, Change, and Equilibrium (ICE)
Example reaction
A B
[A]
[B]
Initial
1.0
0.00
Change -x
+2 x
Equilibrium
1.0 - x 2x
Solve for x,
May need to use quadratic formula

24. ICE example H2 (g) + I2(g) 2 HI (g)
What are the equilibrium concentrations of the reactants and products if you start with 1.50 M H2 and 1.50M I2 and have an equilibrium constant of 1.68?
K = 1.68
[H2]
[I2]
[HI]
Initial
1.50
0.00
Change -x
+2x
Equilibrium x
1.50-x 2x

25. This is the change in concentration. Use correct x value
[H2] at equilibrium = 1.50 – x
1.50-(0.589) = M possible = answer
1.50-(-2.762) = M not possible
[HI] at equilibrium = 2x
2(0.589) = M possible = answer
2(-2.762) = M not possible

26. ICE table Review
Set up general ICE table and fill in Initial concentrations of products and reactants.
Fill in the Change with the term –x for reactant and +x for product, then multiply it by the coefficient of the molecule
Fill in the Equilibrium values in terms of the Initial concentrations plus the Change term
Then write the K expression. Then plug in the Equilibrium terms into the K expression.
Solve for x. May need to use quadratic formula.

27. Practice
A rxn mixture that originally contains 11 mol A and 0 mol B in a 1.0 L container is allowed to reach equilibrium. What is [A] and [B] at equilibrium?
A(g) B(g) K = 10
A rxn mixture that originally contains [X] 5.0 M and [Y] 6.0M and [Z] 0.0 M. What is [X] , [Y] , [Z] at equilibrium?
X(g) + Y(g) Z(g) K = 2

28. Le Châtelier’s Principle

32. Le Châtelier’s Principle
Le Châtelier's Principle - if a system at equilibrium is disturbed, the position of equilibrium will shift to minimize the disturbance
Concentration
Pressure/Volume
Temperature
Concentrations of all the chemicals will change until equilibrium is re-established
The new concentrations will be different, but the equilibrium constant (K) will be the same
Le Châtelier’s Principle

33. Le Châtelier’s: Concentration

35. Le Châtelier’s: Pressure/volume

37. Le Châtelier’s: Temperature
If the temp of a system at equilibrium is changed, the system will shift in a direction to counter that change
A new K is established at the new temp
Consider whether an endothermic or an exothermic reaction

38. Endothermic reaction

39. Exothermic reaction

40. Le Châtelier’s: Catalysts
Catalysts provide an alternative, more efficient mechanism
Speed up reaction by lowering the Ea
Result in the same ratios of products and reactants at equilibrium
Do not affect the position of equilibrium
K stays the same

41. Questions
The reaction 2 SO2(g) + O2(g) Û 2 SO3(g) is exothermic. How will each of the following changes affect the equilibrium concentrations of each gas once equilibrium is re-established?
Adding more O2 to the container
Removing SO3
Compressing the gases
Cooling the container
Doubling the volume of the container
Warming the mixture
Adding SO3
Adding a catalyst to the mixture

42. Chapter 16 Summary Reversible reactions go to equilibrium
Equilibrium constant K and how to write the expression
Heterogeneous equilibrium
Reaction quotient Q, direction of a reaction
ICE tables, how to calculate Eq concentrations
Le Châtelier’s principle, disturbing the Eq
Chapter 16 Summary