Reaction Rates and Chemical Equilibria

Bettelheim, Brown, Campbell and Farrell

Chapter 7

Chemical Kinetics

• Chemical kinetics:Chemical kinetics: the study of the rates of chemical reactions

Example, the concentration might increase from 0 to 0.12 mol/L over a 30 minute time period

CH3-Cl I- CH3-I Cl-+ +Chloro-methane

Iodo-methane

30 min(0.12 mol CH3I/L) - (0 mol CH3I/L)

=0.0040 mol CH3I/L

min

Reaction Rates

• The rates of chemical reactions are affected by several factors– Molecular collisions– Activation energy– Nature of the reactants– Concentration of the reactants– Temperature– Presence of a catalyst

Molecular Collisions

– Two species, A and B (molecules or ions), must collide in order to react

– Most collisions do not result in a reaction– A collision that does result in a reaction is

called an effective collisioneffective collision– Effective collisions must have

• Enough energy to reach the activation energy• Correct orientation of A and B at the time of

collision

Molecular Collisions• EEaa Activation energy: Activation energy: the minimum energy

required for a reaction to take place– Most reactions involve breaking covalent

bonds initially– Energy is required to break covalent bonds– Energy comes from the collision between A

and B

– If the collision energy is large, there is sufficient energy to break the necessary bonds, and reaction takes place

– If the collision energy is too small, no reaction occurs

Molecular Collisions• Orientation at the time of collision

– Colliding particles must be properly oriented for bond breaking and bond making

– In reaction between H2O and HCl, the oxygen of H2O must collide with the H of HCl so that the new O-H bond can form and the H-Cl bond can break

+ +

H2O + HCl H3O+ Cl-+

Energy Diagrams

Exothermic

Energy Diagrams

Endothermic

Energy Diagrams

• Transition state:Transition state: maximum on an energy diagram

+ +

H2O + HCl H3O+ Cl-+

+ -

transition state

Factors Affecting Rate

Factors Affecting Rate• Catalyst:Catalyst: a substance that increases the rate of a

chemical reaction without itself being used up

Reversible Reactions

• Equilibrium:Equilibrium: a dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction– No change in concentration of either reactants

or products– Reaction is still taking place but the rates of

the two reactions are equal

forward reactionreversereaction

CO(g) + H2O(g) CO2(g) + H2(g)

Equilibrium Constant, K

• For the general reaction

– the equilibrium constant isaA + bB cC + dD

K =[C]c[D]d

[A]a[B]b

CO(g) +H2O(g) CO2(g) +H2(g)

[CO2][H2]

[CO][H2O]K =

Equilibrium Constant ExpressionKeq

aA + bB → cC + dD reactants products

Keq = [C]c [D]d

[A]a [B]b

For equilibrium constant expressions, we use coefficients as the powers.

Equilibrium Constants

• Problem:Problem: when H2 and I2 react at 427°C, the following equilibrium is reached

– Equilibrium concentrations are [I2] = 0.42 mol/L, [H2] = 0.025 mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K

– Solution:Solution:

I2(g) +H2(g) 2HI(g)

Equilibrium Constants

• Problem:Problem: when H2 and I2 react at 427°C, the following equilibrium is reached

– Equilibrium concentrations are [I2] = 0.42 mol/L, [H2] = 0.025 mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K

– Solution:Solution:

I2(g) +H2(g) 2HI(g)

[HI]2

[I2][H2]K = = (0.76 M)2

(0.42 M) x (0.025 M)= 55

Coefficients from different sources

Equilibrium constants:

coefficients are the powers in equation

Reaction Rate equations:

coefficients are NOT the powers

(must be determined experimentally)

What does Keq mean?At equilibrium:

Keq > 100: mostly products present

Keq < 0.01: mostly reactants present

0.01 < Keq < 100: significant amounts of both products and reactants present

Keq value changes at different temperatures

Equilibrium and Rates

• There is no relationship between a reaction rate and the value of K– Reaction rate depends on the activation

energy of the forward and reverse reactions; these rates determine how fast equilibrium is reached but not its position

• LeChatelier’s Principle:LeChatelier’s Principle: when a stress is applied to a chemical system at equilibrium, the position of the equilibrium shifts in the direction to relieve the applied stress

Le Chatelier’s Principle

When system at equilibrium is “disturbed” or “stressed” the system moves in the direction that relieves the stress

Types of “stress”:

Add or remove reactant

Add or remove a product

Change temperature

Ways to stress system

Increase the concentration of a chemical

Decrease the concentration of a chemical

Heat the system (add heat to the system)

Cool the system (remove heat energy)

Le Chatelier’s Principle

2 O2 + N2 ↔ 2 NO2

colorless colorless red/brown

Add O2: Turns Redder

Concentration increases on left

System makes more product to relieve stress

System “shifts” to right

Le Chatelier’s Principle

2 O2 + N2 ↔ 2 NO2

colorless colorless red/brown

Remove O2: Turns less red

Concentration decreases on left

Some product changes to reactants to relieve stress

System “shifts” to left

• Can treat heat as if it were a reactant or product for LeChatelier

A + B ↔ C + heat