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What Is the Collision Theory?

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  • Written By: Victoria Blackburn
  • Edited By: Jessica Seminara
  • Last Modified Date: 07 November 2016
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In chemistry, the collision theory provides an explanation of why certain chemical reactions take place, and also why the rates of these reactions is different depending on which reaction is taking place. In 1916 and 1918, Max Trautz and William Lewis came up with the collision theory to explain chemical reactions. They found that for a chemical reaction to occur, the molecules had to collide and also there had to be enough energy for the reaction to proceed.

During a chemical reaction, substrate molecules interact to form new products. While there are many different types of chemical reactions, the substrate molecules have to come in contact with each other if any reaction is to occur. The collision theory states that if the substrate molecules do collide, then a chemical reaction could occur, although the collision alone is not a guarantee of a subsequent reaction.

Molecules collide with each other all the time, but chemical reactions don’t always occur. The collision theory states that there are specific factors that determine whether a chemical reaction will occur or not. The two main factors that are considered by this theory are the orientation of the molecules when they collide and also the amount of energy that results.

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When two molecules bump into each other, the orientation of each molecule is important. All molecules have a specific shape, so how they collide affects whether they react or not. If the molecules are not properly aligned, they can bounce off one another. On the other hand, if the molecules are oriented so that the active areas line up, then a chemical reaction can occur as long as the second requirement of the collision theory is met.

As well as having molecules collide in a specific way, the collision theory states that a certain amount of energy is required for the chemical reaction to occur. This energy is referred to as the activation energy, and different amounts of energy are necessary for different reactions. If the energy produced by the collision is less than the required activation energy, then the chemical reaction will not occur. The reason that activation energy is necessary is that chemical bonds within the substrate molecules must be broken.

Certain physical factors can cause an increase in the number of successful collisions that occur. By increasing the concentration, or number, of the substrate molecules, there is a greater chance of molecules colliding with the correct orientation. In addition, increasing the temperature of the solution containing both molecules leads to an increase in the energy with which the molecules collide. This means there is a greater chance of the activation energy threshold being met or exceeded.

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