READ: Collision Theory

READ: Collision Theory

Collision Theory

Consider the chemical reaction CH4+2O2→CO2+2H2O. In the reactants, the carbon atoms are bonded to hydrogen atoms, and the oxygen atoms are bonded to other oxygen atoms. The bonds that are present in the products cannot form unless the bonds in the reactants are first broken, which requires an input of energy called activation energy (energy necessary to initiate a chemical reaction).

The energy to break the old bonds comes from the kinetic energy (energy of motion) of the reactant particles. The reactant particles are moving around at random with an average kinetic energy related to the temperature. If a reaction is to occur, the kinetic energy of the reactants must be high enough that when the reactant particles collide, the collision is forceful enough to break the old bonds. Once the old bonds are broken, the atoms in the reactants would be available to form new bonds. At that point, the new bonds of the products could be formed. When the new bonds are formed, potential energy (stored energy) is released. The potential energy that is released becomes kinetic energy that is absorbed by the surroundings (primarily the products, the solvent solution if there is one, and the reaction vessel).

How Reactions Occur

We know that a chemical system can be made up of atoms (H₂, N₂, K), ions (NO₃^-, Cl^-, Na⁺), or molecules (H₂O, CH₃CH₃, C₁₂H₂₂O₁₁). We also know that in a chemical system, these particles are moving around at random. The collision theory – (a model that explains that reactants must collide in order to react) explains why reactions occur between these atoms, ions, and/or molecules at the particle level. The collision theory provides us with the ability to predict what conditions are necessary for a successful reaction to take place. These conditions include:

• The particles must collide with each other.
• The particles must have proper orientation.
• The particles must collide with sufficient energy to break the old bonds.

The rate of the reaction depends on the fraction of molecules that have enough energy and that collide with the proper orientation.

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