|kinetic molecular theory||medical dictionary|
However, depending on the conditions, only a small fraction of the collisions are effective in producing a reaction. There are several constraints. In order for a reaction to occur, bonds initially are broken, which requires energy. This energy depends on the type of the reaction and comes from the kinetic energies that the molecules possess before the collision. It is called the activation energy. Increasing the temperature increases the kinetic energies and more collisions will occur. In adition, at a higher temperature a greater number of the reacting molecules might possess an energy equal to or greater than the activation energy. However the molecules must also collide in a specific orientation, called the steric factor in order for a reaction to occur.
A reaction will only be successful, if the collision has enough energy to be either equal to or greater than the activation energy and if the orientation of the collision allows for correct bond formation. These factors are in the Arrhenius equation: k = zp The rate constant k is proportional to the Arrhenius factor A. A is the product of the collision frequency z, and the steric factor p. The fraction of collisions with sufficient energy to produce a reaction are in the term of the equation.
(09 Jan 1998)
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