Catalysts lower activation energy by offering alternate
route
A catalyst by
definition is a substance that increases the rate of a reaction without being
consumed during the reaction. In general, catalysts do this by lowering the
activation energy of a reaction, as stated in the question.
Let's start with what
activation energy means.
Activation energy is
the minimum energy molecules must have in order to react.
You can think of the molecules as cars. Two cars can collide and
may or may not make dents. Only those collisions that have enough energy will
cause the cars to become dented.
A chemical reaction
happens when two or more species collide and break old bonds and/or form new
bonds. Not all collisions happen with the same energy. Only those collisions
that meet that minimum energy requirement will react to make products.
A catalyst's job is to
lower the amount of energy required for collisions to be successful at making
product. They accomplish this by offering an alternate pathway to form
products. If you have to get from point A to point B which is over a mountain,
you need enough energy to climb the mountain. This would be the activation
energy. If, however, you found a tunnel that went through the mountain instead
of over, it would require less energy to reach point B. This tunnel can be
thought of as a catalyst. It provides an alterative pathway that requires less
energy to get from the starting point to the finish. It is also not consumed
during this process as the tunnel would remain for the next person to utilize.
There are many methods
that catalysts use to provide lower energy pathways.
Some catalysts provide
a charge to a molecule to make it ore attractive to another reactant.
Acids are often used
as catalysts. They can give the reacting species a positive charge. This
charged species is more attractive to a negative or partially negative
reactant. This new attraction increases the chance that the two species will
come together and react. Some catalysts increase the local concentration of
reactants so that they are more likely to collide. If the catalyst has the
ability to bring both reactants close together, it can increase the likelihood
that the two molecules will bump into each other.
If two people go to
the mall, they may or may not bump into each other. If both people like
shopping at the GAP, they are more likely to run into one another while
shopping at the GAP. The store is the catalyst that brings the two people into
close proximity and makes a meeting more likely.
Some catalysts may
even modify the shape of one reactant so that it is more susceptible to a
reaction with the other molecule. Imagine throwing darts at a balloon at the
state fair. If the balloons are not inflated it is much harder to hit your
target. By blowing the balloons up, you increase the chance that the dart will
hit the balloon and you will win a prize.
Source:http://www.ccmr.cornell.edu/education
