1. Chemical Equilibrium Collision theory Rates of reactions Catalysts
Reversible reactions
Chemical equilibrium
Le Chatelier’s Principle
Concentration
Temperature
Volume

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
have enough kinetic energy to break bonds

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. A. Collision Theory Activation Energy depends on reactants
low Ea = fast rxn rate Ea

6. 16.2: Rates of Reactions 1. SURFACE AREA
Chemical kinetics: the study of the rate (the speed) of a reaction
Rate of a chemical reaction depends on:
1. SURFACE AREA
2. CONCENTRATION of reactants
3. TEMPERATURE (T) of reactants
4. Presence/absence of a CATALYST

7. SURFACE AREA Surface Area high SA = fast rxn rate
more opportunities for collisions
Increase surface area by…
using smaller particles
dissolving in water

8. Effect of Concentration on Rate
increasing concentration of reactants results in more collisions.
More collisions = increased rate of reaction

9. Effect of Temperature on Rate
Increasing T increases particle speed.
Faster reactants means more collisions have the activation energy, which increases the rate of the reaction.

10. Analogy: 2-car collision
Temperature
Analogy: 2-car collision
5 mph “fender bender”
50 mph “high-speed crash”

11. Effect of Catalysts on Rate
A catalyst:
A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.)
Lowers the activation energy of the reaction.
Activation energy
Time
Energy
Activation energy with catalyst

12. Catalysts
Enzyme Catalysis

13. 16.1: Reversible Reactions
* Thus far, we have considered only one-way reactions: A + B → C + D
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 & condensing
Freezing & melting

15. Chemical Equilibrium

16. Reversible Reactions
At chemical equilibrium there is no net change in the actual amounts of the components of the system.
And although the rates of the forward & reverse rxns are equal at chemical equilibrium, the concentrations of the components on both sides of the chem-ical eqn are not necessarily the same.
*In fact they can be dramatically different.

17. Consider a set of escalators as being like the double arrows in a dynamic equilibrium.
The # of people using the up escalator must be the same as the # of people using the down escalator for the # of people on each floor to remain at equilibrium
However, the # of people upstairs do not have to equal the # of people downstairs
Just the transfer between floors must be consistent

18. Examples of irreversible reactions:
Striking a match / burning paper
Dropping an egg
Cooking (destroys proteins)

19. 16.3: Chemical Equilibrium
For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established.
Both the forward and backward reactions continue, but there is a balance of products “un-reacting” and reactants reacting.
A + B ↔ C + D A B + A B + A B + C D + C D + C D +

20. Equilibrium Expression
Chemist’s generally express the position of equilibrium in terms of numerical values
These values relate the amounts of reactants to products at equilibrium
Consider this hypothetical rxn…
wA + xB
yC + zD
Where “w” mols of reactant A and “x” mols of reactant B react to give “y” mols of product C and “z” mols of product D at equil.

21. Equilibrium Expression
We can write a mathematical expression to show the ratio of product concentrations to reactant concentrations called an equilibrium expression
[C]y
[D]z
[A]w
[B]x
K =
The concentration or pressure of each substance is raised to a power equal to the # of mols of that substance in the balanced rxn eqn.
The square brackets indicate concentration in Molarity (mol/L)

22. Equilibrium Expression
Keq=
[C]y
[D]z
[A]w
[B]x
The resulting ratio of the equilibrium is called the equilibrium constant or Keq
The Keq is dependent on the temp
If the temp changes so does the Keq
NOTE: pure solids and liquids cannot change concentration, therefore they ARE NOT included in the equilibrium expression

23. Equilibrium Constant
Equilibrium constants provide valuable chemical information
They show whether products or reactants are favored in a rxn
always written as a ratio of products over reactants
a value of Keq > 1 means that products are favored
Keq < 1 than reactants are favored

24. products favored at equilibrium reactants favored at equilibrium
Keq > 1
products favored at equilibrium
Keq < 1
reactants favored at equilibrium

25. Sample Problem 1 N2O4(g) 2NO2(g)
Dinitrogen tetroxide (N2O4), a colorless gas, and nitrogen dioxide (NO2), a brown gas, exist in equilibrium with each other according to the following eqn:
N2O4(g) NO2(g)
A liter of gas mixture at 10C at equilibrium contains mol N2O4 & mol NO2. Write the Keq expression and calculate Keq for the reaction.

26. Analyze: list what we know
Known:
[N2O4] = .0045mol/L
[NO2] = .030mol/L
Unknown:
Keq expression = ?
Keq = ?
At equilibrium, there is no net change in the amount of N2O4 or NO2 at any given instant

27. Calculate: solve for unknowns
The only product of the rxn is NO2, which has a coefficient of 2 in the balanced eqn
The only reactant N2O4 has a coefficient of 1 in the balanced eqn
The equilibrium expression is:
[NO2]2
[.030M]2
[.0045M]1
Keq=
Keq=
[N2O4]1
Keq is equal to: Keq= 0.20
Keq < 1, therefore rxn doesn’t favor products

28. CO2(g)+ H2(g)<==> CO(g) + H2O(g)
Classwork:
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 rxn.
Calculate Keq at this temp?
More CO2 is added to the system, which direction will the reaction shift?
Are the reactants or products favored in this reaction?

29. * 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)

30. 16.4: Le Chatelier’s Principle
When a stress is applied to a system (i.e. reaction) at equilibrium, the system responds to relieve the stress.
A stress is a change in:
Concentration
Temperature
Volume

31. 16.5: Stress: Change Concentration
Ex: Co(H2O) Cl- ↔ CoCl H2O
(pink) (blue)
Stress Result
Add Cl1- Forward rxn favored
Shifts forward to use up extra Cl-
More CoCl42- will form
Add H2O Backward rxn favored
Shifts backward to use up extra H2O
More Co(H2O)62+ will form

32. 16.7: Stress: Change Temperature
Ex: heat + Co(H2O) Cl1- ↔ CoCl H2O
(pink) (blue)
This reaction is endothermic. For Le Chatelier’s principle, consider “heat” as a chemical.
Stress Result
Increase T Forward rxn favored; shifts forward to reduce extra heat
More CoCl42- will form
Decrease T Backward rxn favored; shifts backward to replace “lost” heat
More Co(H2O)62+ will form

33. 16.6: Stress: Change Volume
Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g)
(1 + 3 = 4 moles of gas) ↔ (2 moles of gas)
Stress Result
Decrease V Forward rxn favored; shifts forward to side with fewer moles of gas (reduces # of molecules packed into this smaller volume)
Increase V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules)

34. 16.7: Catalysts & Equilibrium
MnO2
Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g)
Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.