The Collision Model

The reaction rate depends on:collision frequencya probability or orientation factoractivation energy (Ea)

The reaction rate increases as the number of collisions between reacting species increase.Concentrationtemperature

The Collision Model

Collisions must occur in a particular orientation for reactions to occur.

For the reaction: Cl. + H - Br H - Cl + Br.

Cl . Br HDesired rxn cannot occur.

Cl . BrH

Cl . Br

H

Desired rxn cannot occur.

Desired rxn can occur.

The Collision Model

Collisions must occur with a specific minimum amount of energy in order for a reaction to take place.

Activation energy (Ea) the minimum energy the

reactants must have for a reaction to occur

the energy difference between the reactants and the transition state

The Collision Model

Transition state:a particular arrangement of atoms of

the reacting species in which bonds are partially broken and partially formed

the state of highest energy between reactants and products

a relative maximum on the reaction-energy diagram.

Reaction Energy Diagrams

Reaction energy diagram:a plot of potential energy changes

that occur as reactants are converted to products

Reaction Energy Diagrams

Given a reaction energy diagram for a chemical reaction, you should be able to identify the reactants, products, transition state, activation energy, the heat of reaction, and whether the reaction is endothermic or exothermic.

Reaction Energy Diagrams

Example: For each reaction energy diagram below, mark the location of the reactants, products and transition state. Identify the magnitude of Ea and Hrxn. Is each reaction endothermic or exothermic?

Arrhenius Equation

Reaction rate increases with temperature because:molecules have more kinetic energymore collisions occurgreater number of collisions occur

with enough energy to “get over the hill” i.e. with energy greater than or

equal to Ea

Arrhenius Equation

The Arrhenius Equation relates the value of the rate constant to Ea and the temperature:

k = Aewhere k = rate constant

Ea = activation energyR = gas constant (8.314 J/mol.K)T = temperature in KelvinA = frequency factor (a

constant)A is related to the frequency of collisions and the probability that the collisions are oriented favorably for reaction.

-Ea/RT

Arrhenius Equation

The activation energy of a reaction can be found by measuring the rate constant at various temperatures and using another version of the Arrhenius equation.

ln k1 = Ea 1 - 1k2 R T2 T1

You do not need to memorize this equation. You must be able to use it, however, to solve for Ea.

Arrhenius Equation

Example: At 189.7oC, the rate constant for the rearrangement of methyl isonitrile to acetonitrile is 2.52 x 10-5 s-1. At 251.2oC, the rate constant for the reaction is 3.16 x 10-3 s-1. Calculate the activation energy for this reaction.

ln k1 = Ea 1 - 1k2 R T2 T1

Arrhenius Equation

Arrhenius Equation

Once you find the value for Ea, you can use the Arrhenius Equation to find the frequency factor (A) for the reaction.

Once you have the value for Ea and A, you can calculate the value for the rate constant at any temperature.

The following two examples illustrate this process.Be prepared for similar problems on

your exam.

Arrhenius Equation

Example: Using the activation energy obtained in the previous example, calculate the value for the frequency factor using k = 2.52 x 10-5 s-1 at 189.7oC. (Note: You could also have used the other set of conditions.)

Arrhenius Equation

Example: Use the value for the frequency factor (A) and the activation energy obtained in the previous two examples to calculate the value of the rate constant at 25oC.