1. Chemical Equilibrium Collision theory Rates of reactions
Reversible reactions
Chemical equilibrium

2. A. Collision Theory
Reaction rate depends on the collisions between reacting particles.
Successful collisions occur if the particles...
collide with each other
have the correct orientation when they collide
have enough kinetic energy when they collide

3. Unsuccessful Collisions
A. Collision Theory
Particle Orientation
Required Orientation
Unsuccessful Collisions
Successful Collision

4. Activation energy: minimum energy required for a reaction to occur
Exothermic
Endothermic
Activation energy
Time
Energy
Time
Energy
Energy of reaction

5. B. Reaction Rates SURFACE AREA 2. CONCENTRATION of reactants
The more collisions, the faster the reaction rate
Increase collisions by changing the…
SURFACE AREA
More surface area = more collisions
Increase surface area by crushing or dissolving
2. CONCENTRATION of reactants
More particles = more collisions

6. 50 mph “high-speed crash”
3. TEMPERATURE (T) of reactants
Increase temperature; particles collide with more energy; increases the rate of the reaction.
5 mph “fender bender”
50 mph “high-speed crash”
4. Presence of a CATALYST
speeds up a reaction, but is not consumed in the reaction.
Lowers the activation energy.
Activation energy
Time
Energy
Activation energy with catalyst

7. C. Reversible Reactions
Some reactions are reversible:
They go forward (“to the right”) :
A + B → C + D
and backwards (“to the left”) :
A + B ← C + D
Written with a two way arrow: A + B ↔ C + D
Examples:
Boiling and condensing
Freezing and melting

9. Chemical Equilibrium

10. C. Reversible Reactions
At Equilibrium
The rates of the forward & reverse reactions are equal
The AMOUNTS of the reactants and product DO NOT HAVE TO BE THE SAME
*In fact they can be dramatically different.
A + B ↔ C + D A B + C D +

11. D. Equilibrium Expression
We can write an expression for the position of equilibrium
Consider this hypothetical rxn…
The EQUILIBRIUM EXPRESSION is written
wA + xB
yC + zD
K =
[C]y
[D]z
[A]w
[B]x
The concentration (Molarity) or pressure of each substance is raised to a power equal to it’s coefficient
Products / reactants

12. D. Equilibrium Expression
Keq=
[C]y
[D]z
[A]w
[B]x
NOTE: pure solids and liquids ARE NOT included in the equilibrium expression. Only gases and aqueous solutions are used.

13. H2O (g) + C (s) ↔ H2 (g) + CO (g)
Practice:
Write the equilibrium expression for the following reaction:
H2O (g) + C (s) ↔ H2 (g) + CO (g)

14. E. Equilibrium Constant
The equilibrium constant or Keq is the numerical value obtained when you solve the expression
a value of Keq > 1 means that products are favored
(there will be more product than reactant)
Keq < 1 than reactants are favored
(there will be more reactant than product)

15. Sample Problem 1 N2O4(g) 2NO2(g)
A liter of gas mixture at 10C at equilibrium contains mol N2O4 & mol NO2.
Write the Keq expression
Calculate Keq for the reaction.

16. Sample Problem 1 [NO2]2 Keq= [N2O4]1 [.030M]2 Keq= [.0045M]1
The equilibrium expression is:
[NO2]2
Keq=
[N2O4]1
Calculating Keq is equal to:
Keq= 0.20
Keq < 1, therefore reactants are favored
[.030M]2
[.0045M]1
Keq=

17. CO2(g)+ H2(g)<==> CO(g) + H2O(g)
Sample Problem 2
A mixture at equilibrium at 827°C contains M CO2, M H2, M CO, and M H2O.
CO2(g)+ H2(g)<==> CO(g) + H2O(g)
Write the equilibrium expression for the above reaction.
Calculate Keq at this temp
Are the reactants or products favored in this reaction?

18. F. Le Chatelier’s Principle
* Le Chatelier’s Principle is about reducing stress – a stress applied to a chemical equilibrium
Relax! Reduce stress brought on by chemical equilibrium with me, Henri Le Chatelier!
(1850 – 1936)

19. F. Le Chatelier’s Principle
When a stress is applied to a system (reaction) at equilibrium, the system responds to relieve the stress.
A stress is a change in:
Concentration
Temperature
Volume

20. F. Le Chatelier’s Principle: Concentration
Ex: Co(H2O)62+(aq) + 4 Cl-(aq) ↔ CoCl42- (aq) + 6 H2O (g) (pink) (blue) Stress Result
Add Cl- •Forward reaction favored
• Uses up extra Cl-
• More CoCl42- will form
Add H2O • Reverse reaction favored
• Use up extra H2O
• More Co(H2O)62+ will form

21. F. Le Chatelier’s Principle: Temperature
Ex: Co(H2O) Cl1- ↔ CoCl H2O (g) (pink) (blue) ∆H = kJ/mol
This reaction is endothermic. Consider “heat” as a chemical.
Heat + Co(H2O) Cl1- ↔ CoCl H2O (g)
(pink) (blue)
Stress Result
Increase Temp •Forward reaction favored; shifts forward to reduce extra heat
•More CoCl42- will form
Decrease Temp •Backward rxn favored; shifts backward to replace “lost” heat
•More Co(H2O)62+ will form

22. F. Le Chatelier’s Principle: Volume
Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g) (1 + 3 = 4 moles of gas) ↔ (2 moles of gas) Stress Result
Decrease Volume •Forward reaction favored
•shifts to side with fewer moles of gas
Increase Volume •Backward reaction favored
• shifts to side with more moles of gas

23. F. Le Chatelier’s Principle: Catalysts
MnO2
Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g)
A catalyst increases the forward and backward rates equally,
It will not shift the equilibrium.

24. Practice: Le Châtelier’s Principle
Explain how the following changes will affect the position of the equilibrium below
A (g) + B(aq) ↔ C(s) ∆H = -453 kJ/mol
Increase pressure of A
Increase temperature
Add a catalyst
Increase concentration of substance B
Increase amount of substance C

25. Shift to product side
Shift to reactant side
No shift
No shift; solids do not affect equilibrium