Collision Theory

Chemical reactions are all around us and they can get pretty complicated. To help us better understand scientific phenomena like chemical reactions, scientists create theories

We are going to consider one of these theories called collision theory.

What are we going to learn?

  1. The two tenets of collision theory
  2. Factors that can affect the rate of a reaction

Two Tenets of Collision Theory

1. Particles have to collide with enough energy in order to react.


Every chemical reaction has to overcome a minimum amount of energy in order to take place. This minimum amount of energy is called the Activation Energy.


So when reacts collide with enough energy to get “over the hill” the reaction will take place.


2. Particles have to collide with the right orientation.



Factors that can Affect the Rate of a Reaction

So, now that we know how to make a chemical reaction happen, how to we make the reaction faster? Well there are 4 variables that affect the rate of a chemical reaction.

  1. Temperature
  2. Particle Size
  3. Concentration
  4. Catalyst


In general, the higher the temperature of a chemical reaction, the faster the reaction. For example, charcoal reacts with oxygen. This reaction is incredibly slow. If you add some heat the reaction will go much, much quicker.

CharParticle Size

We all know that wood burns. We can build campfires with it and toast marshmallows. However, you can’t built a campfire with just one huge log. You need to start with small piece of wood for kindling. The small pieces of kindling will ignite much easier than large pieces of wood. The smaller the particle size, the faster the chemical reaction.



Let’s stick with the fire theme here. If you light a match you will produce a small flame at the tip of the match head. The flame will slowly burn until it reaches your finger where you will blow it out. The match is reacting with the oxygen in the air. Oxygen makes up about 20% of air. So we can say that the concentration of oxygen in air is 20%. What if we lit that match in an oxygen concentration of 100%? It would burn much faster! The higher the concentration of the reactants, the faster the chemical reaction.



A catalyst is a substance that will cause a reaction to go faster without changing into something else. So a catalyst does not become part of the products and it isn’t a reactant. Catalysts just speed reactions up. In biological systems, catalysts are called enzymes. Enzymes are molecules that speed up all of the many chemical reactions that occur inside our body everyday. More specifically, catalysts lower the activation energy of a reaction. Lowering the activation energy makes the chemical reaction must easier and thus faster.




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