Theories of Reaction Rates

Rates of Chemical Reactions

Beyond Collision Theory

For a collision to be effective, it must satisfy both of these criteria:

1. Correct orientation of reactants

2. Sufficient collision energy

Orientation of Reactants

Reactants must have the correct collision geometry.

If they do not, the reaction will be unable to proceed.

Activation EnergyThe activation energy (Ea) of a reaction is the minimum collision energy that is required for a successful reaction.

The collision energy depends on the total kinetic energy of the colliding particles.

Activation EnergyNotice the following on the graph:

1. At either temperature, only a small fraction of collisions have sufficient kinetic energy (Ea) to result in a reaction.

2. As the temperature of a sample increases, the fraction of collisions with sufficient energy increases significantly.

Transition State Theory

This is a theory that is used to predict what happens when molecules collide in a reaction.

Essentially, it shows how a molecule’s kinetic energy is converted to potential energy when it collides with another reactant.

This can all be shown using potential energy diagrams.

Remember that there is no way to predict the activation energy of a reaction from its enthalpy change.

Potential Energy Diagrams

Potential Energy Diagrams

The top curve on a potential energy diagram represents the transition state. The type of molecule that exists at the transition state is known as the activated complex.

The activated complex is a transitional species that is neither product nor reactant and is highly unstable.

Transition State

Drawing a Potential Energy

DiagramCarbon monoxide reacts with nitrogen dioxide to form carbon dioxide and nitric oxide (NO). Draw a potential energy diagram to illustrate the progress of the reaction. Label the axes, the transition state, and the activated complex. Indicate the activation energy of the forward reaction Ea(fwd) = 134kJ, as well as ∆H = -226kJ. Calculate the activation energy of the reverse reaction, Ea(rev), and show it on the graph.

Drawing a potential energy

diagram

HomeworkPractice Problems 13, 14, and 16 on page 294

Section review: #3, 6, and 7 on page 296