Collision Theory and Explaining Chemical Kinetics

Collision Theory

1. Reactions occur because of collisions with one another or the walls of their container.

2. Collisions are only effective if they have sufficient energy and correct alignment.

3. Reaction rate is dependent on

• collision frequency and

• fraction of effective collisions

Can be used to explain rates of reaction and the various factors affecting reaction rate

According to collision theory, not all collisions will result in a reaction. The collisions must have sufficient energy if a new product is to form.

Activation Energy

The minimum amount of energy required to overcome the “energy barrier” in a reaction.

The minimum potential energy that must be achieved by colliding reactants before they can convert to products.

Activated Complex

A very unstable high-energy configuration of atoms that forms if reactants collide with enough energy

Decomposes into the products of a reaction.

Potential Energy Diagram

Chemical Nature of Reactants (Reactant 1)

Reactant 1:

- Some molecules have relatively weak bonds and lower activation energies.

At a particular temperature, a relatively high proportion of the molecules have sufficient energy for an effective collision.

Chemical Nature of Reactants (Reactant 2)

Reactant 2:

- Other molecules have relatively strong bonds and higher activation energies.

At the same temperature, a lower proportion of the molecules have sufficient energy for an effective collision.

Thus Chemical Nature of Reactants affects…

the Fraction of Effective Collisions

Concentration

A higher concentration of reactants means a greater number of particles per unit volume, and thus there is a greater chance of collision of particles in the same fixed space.

Thus Concentration of Reactants affects…

the Collision Frequency

Surface Area

Collisions can only occur at the interface where the two reactants in different phases are in contact.

Increasing the surface area of a reactant allows for more “exposure” between the two phases.

Thus Surface Area…

affects the Collision Frequency

Temperature

At higher temperatures, the average kinetic energy of reacting molecules is higher.

Therefore, the molecules are moving around more quickly and collide more often.

ALSO, at higher temperatures, since molecules are moving more quickly, collisions are more forceful, so a greater fraction of reacting molecules possess the minimum threshold (activation energy) when they collide.

Thus temperature affects…

the Collision Frequency and Fraction of Effective Collisions

Catalysts

A catalyst works by providing an alternative pathway from reactants to products.

The activation energy of the catalyzed reaction is lower than for the uncatalyzed reaction.

So, at a particular temperature, more collisions meet the minimum threshold energy requirement for a successful collision, simply because the energy barrier has been lowered.

Thus catalysts affect…

the Fraction of Effective Collisions