Higher and foundation tiers
Collision theory- explaining the rates of chemical reactions
Before a chemical reaction can happen between two reacting substances the particles present in each of them must collide with a large enough force to break all the bonds holding the reactant particles together and they may also have to collide in a specific orientation. As an example consider
the reaction between hydrogen and chlorine
to make hydrogen chloride gas. The image below shows a molecule of hydrogen
and a molecule of chlorine reacting to make two new molecules of hydrogen chloride gas.
Before the hydrogen and chlorine gases can
react you need to think about what must happen during the reaction:
- The covalent bond between the hydrogen atoms must break. Initially the two
hydrogen atoms are bonded together in a
small molecule made up of the two hydrogen atoms, but in the product the hydrogen atoms have been separated from each
other and are bonded instead to chlorine atoms.
- A similar argument is true for the chlorine atoms. The covalent
bond holding the two chlorine atoms together must break in order for them to react with the hydrogen atoms and form a new bond with them.
- To break these covalent bonds in the reactants the hydrogen and
chlorine molecules must collide with each
other with a large enough force to provide all the energy needed to break these covalent bonds. This energy is
called the activation energy. Once the covalent bonds holding the
reactant molecules together are broken the individual atoms can form new
covalent bonds to ultimately lead to the formation of the products. If the molecules in the reactants collide with each other with enough force to break the covalent bonds holding the molecules
together it is called a successful collision. This is outlined in the image below:
If the molecules collide with less force and the bonds in the reactants
do not break, this is called an unsuccessful collision. This is outlined in the image below:
High activation energies will effectively stop the reactants turning into products. High activation energies will likely result in very slow reactions while fast reactions; such as explosions are likely to have very low activation energies. Any factor that increases the number of successful collisions in a reaction will increase the rate of that reaction. There are four factors that will affect the rates of chemical reactions; these are:
- Temperature
- Concentration or pressure (in the case of gases reacting).
- Surface area.
- Catalysts.
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