1. Reaction Rates and Chemical Equilibrium
Chapter 7
Reaction Rates and Chemical Equilibrium

2. Chemical Kinetics Thermodynamics – does a reaction take place?
Kinetics – how fast does a reaction proceed?
Reaction rate is the change in the concentration of a reactant or a product with time (M/s).
A B
rate = -
D[A] Dt
D[A] = change in concentration of A over
time period Dt
rate =
D[B] Dt
D[B] = change in concentration of B over
time period Dt
Because [A] decreases with time,
D[A] is negative.

3. A B
Every reaction has its own rate
rate = -
D[A] Dt
rate =
D[B] Dt

4. Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)
red-brown
Br2 (aq) + HCOOH (aq) Br- (aq) + 2H+ (aq) + CO2 (g)
time
393 nm
light
Detector
t1< t2 < t3
D[Br2] a D Absorption

5. Collision Theory
Collision theory: In order for a reaction to take place, the following conditions must be met:
1. Two molecules must collide
2. The molecules must have the proper orientation during collision
3. The molecules must collide with enough energy
Effective Collision: a collision that results in chemical reaction

6. Importance of Molecular Orientation
effective collision
ineffective collision

7. A + B AB C + D +
Exothermic Reaction
Endothermic Reaction
The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

8. Transition State (AB ) + +
One or more original bonds are partially broken and one or more new bonds may be in the process of formation

9. Rate of Reaction The rate of reaction depends on several conditions:
1. Nature of Reactants
2. Concentrations
3. Temperature
3. Catalysts-speeds up a reaction by providing an alternate pathway that has a lower activation energy

10. Chemical Reaction
A reversible reaction is one which moves in both the forward and backward direction.
Both forward and backward reactions have different initial rates.
Eventually the forward reaction and backward reactions have the same rate. When this happens, there is no more change in concentration. This is called equilibrium.

11. Chemical equilibrium is achieved when:
Equilibrium is a state in which there are no observable changes as time goes by.
Chemical equilibrium is achieved when:
the rates of the forward and reverse reactions are equal and
the concentrations of the reactants and products remain constant
Physical equilibrium
H2O (l)
H2O (g)
NO2
Chemical equilibrium
N2O4 (g)
2NO2 (g)

12. N2O4 (g) NO2 (g)
equilibrium
Start with NO2

13. Equilibrium Expressions
aA + bB cC + dD
K =
[C]c[D]d
[A]a[B]b
K = Equilibrium Constant
N2O4 (g) NO2 (g)
K =
[NO2]2
[N2O4]
= 4.63 x 10-3
Problems 7.3, 7.4, 7.5 pages 224 and 225

14. Equilibrium Will K = [C]c[D]d [A]a[B]b aA + bB cC + dD K >> 1
Lie to the right
Favor products
K << 1
Lie to the left
Favor reactants

15. Le Châtelier’s Principle
LeChatelier’s Principle: When a stress is placed on a system in equilibrium, the system responds to minimize the stress.
What’s a stress?
Changes in Concentration
Change in Pressure
Change in Temperature
Use of a catalyst

16. Le Châtelier’s Principle
Changes in Concentration continued
Remove
Add
Add
Remove
aA + bB cC + dD
N2 (g) + 3H2 (g) NH3 (g)
Change
Shifts the Equilibrium
Increase concentration of product(s)
Left (Reactants)
Decrease concentration of product(s)
Right (Products)
Increase concentration of reactant(s)
Right (Products)
Decrease concentration of reactant(s)
Left (Reactants)

17. Le Châtelier’s Principle
Changes in Volume and Pressure
A (g) + B (g) C (g)
Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas

18. Le Châtelier’s Principle
Changes in Temperature
Change
Exothermic Rx
Endothermic Rx
Increase temperature
K decreases
K increases
Decrease temperature
K increases
K decreases
N2O4 (g) NO2 (g)
colder
hotter

19. Le Châtelier’s Principle
Adding a Catalyst
does not change K
does not shift the position of an equilibrium system
system will reach equilibrium sooner
Catalyst lowers Ea for both forward and reverse reactions.
Catalyst does not change equilibrium constant or shift equilibrium.

20. Le Châtelier’s Principle - Summary
Change
Shift Equilibrium
Change Equilibrium
Constant
Concentration
yes no
Pressure
yes* no
Volume
yes* no
Temperature
yes
yes
Catalyst no no
*Dependent on relative moles of gaseous reactants and products