Collision theory explains how chemical reactions occur and why rates of reaction differ. For a reaction to occur the reactant particles must collide. Only a certain fraction of the total collisions cause chemical change; these are called "successful collisions". The successful collisions have sufficient energy (activation energy) at the moment of impact to break the existing bonds and form new bonds, resulting in the products of the reaction.

Overview

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Fundamentally collision theory is based on kinetic theory. At a basis, it assumes chemical species "collide" prior to reacting. Moreover, it postulates that the majority of collisions do not lead to a reaction, but only those in which the colliding species have:

1. A kinetic energy greater than a certain minimum, called the activation energy, E_a
2. The correct spatial orientation (steric factor) with respect to each other.

These collisions which lead to reaction are called effective collisions. The reaction rate, of a reaction, may be defined as the number of effective collisions per unit time. To note, collision theory is most applicable to ideal gas systems; approximations are used otherwise.

Related definitions

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• Concentration: Increase in concentration of reactants increases the collision frequency between the reactants. Thus the effective collision frequency also increases.
• Surface area: Finely divided reactants have larger surface area and therefore are more likely to collide. This increases the effective collision frequency as well.
• Temperature: The kinetic energy of particles follow the Maxwell-Boltzmann distribution. Increase in temperature not only increase average kinetic energy of the reactant particles, but also the fraction of particles having kinetic energy higher than the activation energy. Thus the effective collision frequency increase.

Related derivations

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From collision theory, we are able to derive the following relationship:

$k\; =\; zp*\backslash exp\; \backslash left(\; \backslash frac\{-E\_\{act\}\}\{RT\}\; \backslash right)$

where:

• k is the rate constant
• z is the collision frequency
• p is the steric factor
• E_act is the activation energy of the reaction
• T is the temperature

External links

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• Introduction to Collision Theory

chemistry