Substances

A substance which is in a liquid, aqueous or gaseous state is composed of particles which are in rapid and constant movement.

The rate of a chemical reaction is dependent on:

• collision frequency
• collision energy
• activation energy

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Collision frequency

In order for a chemical reaction to occur between two particles they need to collide. The collision frequency is the number of collisions between particles per unit time within a system. When a chemical reaction occurs it is known as a successful collision.

The collision frequency can be changed by altering the:

• concentration of reactants
• total pressure
• temperature
• size of the reacting particles

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Collision energy

Although a collision is needed for a chemical reaction to occur not all collisions end in a chemical reaction. A lot of the time the particles simply bounce off each other. The particles do not have enough energy to break the required bonds. When this happens it is called an unsuccessful collision.

The combined energy of the particles is called the collision energy. Not all the particles which make up a system carry the same energy. Instead, there is a wide distribution of energies. How the energies are distributed depends on the system’s temperature: the higher the temperature of the system the higher the mean kinetic energy of the particles.

Molecular energy distribution at a characteristic temperature (T₁) can be represented in a graph and is known as a Maxwell-Boltzmann distribution.

At a higher temperature (T₂) the energy distribution will change: the mean energy will increase and the distribution widen. The higher the mean kinetic energy of the particles in a system the higher the collision energy.

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Activation energy

The activation energy is the minimum energy required for the colliding particles in a system to react.

• If the collision energy of the particles is lower than the activation energy then the collision will be unsuccessful.
• If the collision energy of the particles is equal or greater than the activation energy then the collision will be successful.

With the addition of a catalyst it is possible to change the activation energy.

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Speeding up reactions

Reactions can be speeded up in two ways:

• Increasing the frequency of collisions: the higher the frequency of particle collisions the faster the reaction.
• Increasing the successful collision fraction: the larger the fraction of collisions that end in a chemical reaction the faster the reaction. This fraction can be increased by increasing the collision energy or decreasing the activation energy. The successful collision fraction can be depicted graphically: it is the area located under the curve right of the activation energy divided by the whole area of the distribution curve.