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 = -[A]t

rate = [B]t

[A] = change in concentration of A over time period t

[B] = change in concentration of B over time period t

Because [A] decreases with time, [A] is negative.

3

A B

rate = -[A]

t

rate = [B]

t

Every reaction has its own rate

4

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)

time

393 nmlight

Detector

[Br2] Absorption

red-brown

t1< t2 < t3

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

Exothermic Reaction Endothermic Reaction

The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

A + B AB C + D++

Transition State (AB )

• One or more original bonds are partially broken and one or more new bonds may be in the process of formation

++

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

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.

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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)

Chemical equilibrium

N2O4 (g)

H2O (g)

2NO2 (g)

NO2

12

N2O4 (g) 2NO2 (g)

Start with NO2

equilibrium

13

N2O4 (g) 2NO2 (g)

= 4.63 x 10-3K = [NO2]2

[N2O4]

aA + bB cC + dD

K = [C]c[D]d

[A]a[B]bK = Equilibrium Constant

Equilibrium Expressions

• Problems 7.3, 7.4, 7.5 pages 224 and 225

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K >> 1

K << 1

Lie to the right Favor products

Lie to the left Favor reactants

Equilibrium Will

K = [C]c[D]d

[A]a[B]baA + bB cC + dD

15

Le Châtelier’s Principle

• Changes in Concentration• Change in Pressure• Change in Temperature• Use of a catalyst

• LeChatelier’s Principle: When a stress is placed on a system in equilibrium, the system responds to minimize the stress.

• What’s a stress?

16

Le Châtelier’s Principle

• Changes in Concentration continued

Change Shifts the Equilibrium

Increase concentration of product(s) Left (Reactants)

Decrease concentration of product(s) Right (Products)

Decrease concentration of reactant(s)

Increase concentration of reactant(s) Right (Products)

Left (Reactants)

aA + bB cC + dD

AddAddRemove Remove

N2 (g) + 3H2 (g) 2NH3 (g)

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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

Decrease volume

Increase volume Side with most moles of gas

Side with fewest moles of gas

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Le Châtelier’s Principle• Changes in Temperature

Change Exothermic Rx

Increase temperature K decreases

Decrease temperature K increases

Endothermic Rx

K increases

K decreases

colder hotter

N2O4 (g) 2NO2 (g)

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Catalyst lowers Ea for both forward and reverse reactions.

Catalyst does not change equilibrium constant or shift equilibrium.

• Adding a Catalyst• does not change K• does not shift the position of an equilibrium system• system will reach equilibrium sooner

Le Châtelier’s Principle

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Le Châtelier’s Principle - Summary

Change Shift EquilibriumChange 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