Chemistry Collision Theory

The collision theory is based on the kinetic theory and assumes a collision between reactants before a reaction can take place. Moreover, it postulates that the majority of collisions do not lead to a reaction, but only those in which the colliding species have:

 
  • a kinetic energy greater than a certain minimum, called the activation energy, Ea

  • the correct spacial orientation with respect to each other.
 

The animation below demonstrates these two points by considering the reaction between hydrogen and iodine to produce hydrogen iodide:

H2(g) + I2(g) --> 2HI(g)

hydrogen molecule, H2 iodine molecule, I2 hydrogen iodide molecule, HI
H2 molecule
I2 molecule
HI molecule

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If either or both of these two pre-requisites, correct reaction geometry and sufficient kinetic energy, are not met, then the collision does not lead to the formation of products and the reactants simply "bounce" off each other.

However as the temperature increases, not only does the number of collisions per second increase (and hence the number of collisions in the correct orientation per second) but also the proportion of collisions that have a kinetic energy equal to or greater than Ea, the activation energy. The combination of these two effects, and particularly the second, is used to explain the marked effect of tempearture on reaction rate in the collsion theory.