IB Chemistry - Kinetics

IB Chemistry home > Syllabus 2016 > Kinetics > Particle motion, energy and collision

Syllabus ref: 6.1

There are few forces that can act over a distance, such as the electromagnetic force and gravity, so it seems likely that for particles to break bonds and make new ones there must be direct contact between them - they must collide in order to react.

This concept is called collision theory

Kinetic theory

All particles have energy, which is expressed in the microscopic world as motion.

The concept of 'heat' is a purely human artifice to explain the energy of an environment or object.

When particles gain energy they move more. We detect this motion using our senses.

The particles colliding with our nerve cells cause them to vibrate more, which our brain interprets as hotter. We are well adapted to the world in which we live and have senses to detect particular motion.


Energy distribution in particles

In any sample of matter we are dealing with fantastically large numbers of particles and it is reasonable to suppose that the energy of these particles is distributed amongst them in a random fashion.

There is a massive number of possible ways that the energy can be distributed and the most probable energy distribution is likely to be adopted.

This type of system was analysed by Maxwell and Boltzmann, who formulated the hypothesis that energy is distributed statistically over all of the available particles.

They drew up an energy distribution curve that now bears their names, the Maxwell-Boltzmann energy distribution curve.

The curve demonstrates that there are very few particles (in comparison to the whole) with low or high energies. The majority of the particles have energy somewhere in the mid-range.

The area under the curve represents the total number of particles. As a sample is heated up the curve becomes broader and flatter (red line).


Chemical reactions

A chemical reaction is a process in which new substances are formed. For this to happen, bonds must break and new bonds form. This is often represented by the equation:

Reactants Products


Particle collisions

In order for chemical reactions to occur the reactant particles must come into contact in the correct orientation and with sufficient energy to cause bonds to break and initiate the process of chemical change.

One thing that should be taken into consideration at this point, is that collisions essentially only ever involve two particles. A three-particle collision is extremely rare in comparison to two-particle collisions.

It may be that in a two particle collision only one of the particles actually changes in any way, but nevertheless the collision is important to provide the energy required for change to take place.

A collision that leads to some kind of chemical change is called an 'effective collision'.

Any factor that results in a greater number of collisions AND/OR more effective collisions increases the rate of the chemical change.