Topic 6.2 – Collision Theory

According to the kinetic theory, all matter consists of particles (atoms or molecules) that are in constant motion.

These molecules move randomly as a result of the kinetic energy that they possess.

However, because of the random nature of these movements and collision, not all particles in a substance have the same kinetic energy at any one time, but instead have a range of kinetic energies that are close to each other.

So, a convenient way to describe the kinetic energy of a substance is to take the average of these values (average kinetic energy).

The average kinetic energy of a substance is directly related to its absolute temperature.

Increasing the temperature causes an increase in the average kinetic energy of the particles of a substance.

The kinetic energies of the particles in a substance are expressed by the Maxwell-Boltzmann distribution curve

The Maxwell-Boltzmann distribution curve shows the number of particles that have a particular kinetic energy plotted against the values for kinetic energy.

The area under the curve represents the total number of particles in the sample.

When reactants are placed together, the kinetic energy that their particles possess causes them to collide with each other.

The energy of these collision may cause bonds of reactants to break and new bonds to form.

So, molecules can only react if they collide with each other. However, not all collisions will lead to the formation of

products.

For collisions to be successful, the particles must collide with sufficient energy and with the correct orientation.

In order for a collision to lead to a reaction, the particles must have a certain minimum value for their kinetic energy. This energy is necessary to overcome the repulsion between molecules and to break bonds in the reactants before they can react.

The energy required is known as the activation energy- defined as the minimum value of kinetic energy which particles must have before they are able to react.

When the activation energy is supplied, the reactants achieve the transition state, from which products can form.

Only particles which have kinetic energy values greater than the activation energy will be able to achieve reaction. Particles with kinetic energy values lower than the activation energy will not have enough energy to achieve reaction.

The rate of the reaction depends on the proportion of particles that have values of kinetic energy greater than the activation energy, the greater the number of particles with enough energy to react, the faster the reaction go.

Effective Collision

Ineffective Collision

For molecules to react, they also need to collide with the correct orientation

The rate of reaction will depend on frequency of collisions which occur between particles possessing: 1. values of kinetic energy greater than the

activation energy 2. appropriate collision geometry or

orientation

Reaction 1 Reaction 2Reaction 3

Temperature- increasing the temperature increases the rate of all reactions.

Temperature is defined as the measure of the average kinetic energy of the particles in a substance.

So, if the temperature is increased, the average kinetic energy of the particle increases, which means that a larger number of particles will have energies greater than the activation energy.

There will also be an increase in the number of collisions, which would cause an increase in the number of successful collisions and hence an increase in the reaction rate.

Many reactions double their reaction rate for every 10oC increase in temperature.

At higher temperature, a larger number of molecules have higher energy.

Increasing the concentration of reactants increases the rate of the reaction. This is because as the concentration increases, the frequency of collisions increase too.

This can be observed from a graph as a reaction progresses. As reactants are used up, their concentrations decrease and the rate of the reaction decreases.

Decreasing the particle size increases the rate of reaction because this increases the total surface area, which allows the particles to have more space to make contact, which increases the probabilities for collisions.

For reactions involving gases, increasing the pressure increases the concentration.

An increase in concentration increases the frequency of collisions, which increases the rate.

A catalyst is a substance that increases the rate of the reaction without itself undergoing a permanent change.

Catalysts work by lowering the activation energy of a

reaction.

This means that without increasing the temperature, a larger number of particles will now have values of kinetic energy greater than the activation energy, increasing the number of successful collisions.

Because of their ability to increase the rate of reaction, catalysts play an essential role in many industrial processes. Without catalysts, many reactions would proceed to slowly.

Every biological reaction is controlled by a catalyst, known as an enzyme. There are thousand of different enzymes, since every enzyme is specific to a particular reaction

The catalyst (in the form of platinum and palladium) is coated onto a ceramic honeycomb or ceramic beads that are housed in a muffler-like package attached to the exhaust pipe.

Catalytic Converters

The catalyst helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen.