The collision theory was put forth in the years 1916 and 1918, by Max Trautz and William Lewis respectively. The basis of this theory and the explanation about how it works, lies in the understanding of the molecular behavior of the particles reacting with each other, and the alignment of atoms that takes place during chemical reactions. The factors that have an effect on the collision theory are temperature and particle density.
It is based on the principle that for a chemical reaction to take place, the particles involved in the reaction must collide with each other. Although the theory proposes that collision of particles is mandatory for the reaction to take place, according to certain facts, it isn't necessary that all the reacting particles possess the required energy to connect effectively during the collision.
Working of Collision
For a reaction to take place, energy is considered to be the basic need. During the chemical reaction, existing bonds by which the reactants are joined to each other break down and new bonds are formed. A minimum amount of energy is necessary for the above process to take place. This energy is termed as 'activation energy'. Also, during a chemical reaction, energy is released as the reactants collide with each other. It should be noted that, the rate of chemical reaction depends upon the temperature and the density of the reacting particles. However, even after an increase in temperature, there is not much effect on the rate of collision of the reacting particles.
The collision theory is based on the assumption that whenever the nuclei of two reactants come close to each other and within a certain specified limit, collision between these particles takes place. For example, a chemical reaction of two particles, 'A' and 'B' with each other. A 'cross-section' area is the area surrounding the particle 'A', where it collides with the particle 'B'. The reaction is successful only if the collision takes place in this cross-section.
To understand the orientation of the process of collision, consider a chemical reaction that takes place between ethylene (CH2=CH2) and HCl. The reaction between the molecules of these elements results in the formation of chloroethane (CH3CH3Cl). In the above reaction, the double bond between carbon atoms is broken and results in the formation of a single bond. For the chemical reaction to be successful, hydrogen atoms must be aligned in such a manner that they bond with carbon atoms only. Any other form of alignment or orientation won't result in the formation of chloroethane. Thus, the orientation of molecules or atoms is of utmost importance in the collision, and subsequently for the reaction to be fruitful.