The collision theory of reactivity.


Collision theory is a model used by chemists to explain the condition in which reactions take place and how to modify the rate of reaction.

Molecules randomly collide with each other, upon collision a successful chemical reaction is contingent on there being both:

‣Sufficient kinetic energy of molecules which gives energy to break chemical bonds.
‣Correct orientation of reactant molecules upon collision.

Excuse the poor resolution there are few decent diagrams available.

Methods by which an increase in the rate of reaction may be induced

Increasing the concentration of a solution of reactants increases reaction rate as the average distance between molecules is reduced. This makes collisions much more likely.The same can also be said for the pressure of gases, as pressure increases there are more reactant molecules per unit volume, and thus affects the rate of reaction in the same way as an increase in solution concentration. The likelihood of appropriate orientation also increases. This is a way to increase the rate of chemical reaction.A good example of collision theory is the oxyacetylene torch, where the reactants do not react unless ignited and under low pressure there is incomplete combustion, however when the pressure at which the gases are outputted is increased the rate of reaction  increase significantly, giving a flame of temperature of over 3500°C.

However in order for the now more likely collisions to result in successful reactions, sufficient kinetic energy must be provided to the reactant molecules. As kinetic energy is linked to temperature, an increase in temperature leads to a greater probability that molecules will have the requisite kinetic energy for a successful reaction, thus the rate of reaction will increase.

Of course this can also be used in the inverse in order to inhibit chemical reaction for applications such as storage or safety.

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