Three things must happen for a reaction to occur.
1) Molecules must collide.
2) Molecules must collide with enough energy to begin to break the old bonds so new bonds can form. (Remember activation energy)
3) Molecules must collide with the correct orientation.
How can we speed up reactions?
1) Increase concentration. This increases the number of collisions. The more collisions per second, the faster the reaction.
2) Increase temperature.
This increases the number of collisions
because the molecules are moving faster. It
also means that more of the collisions have enough energy for a reaction to
occur. A higher % of collisions
will have enough energy. A rule of
thumb is that at about room temperature, an increase in 10°
C will double the rate.
3) Use a catalyst. A
catalyst lowers the barrier to the reaction.
It lowers the activation energy. A
catalyst speeds up a reaction but is unchanged at the end of the reaction.
Please watch Animation
11.4: Catalysts.
In the first animation
we are looking at the decomposition of peroxide;
Iodide (I-) catalyzes this reaction.
After the iodide has catalyzed one reaction it is unchanged so it can
catalyze another, and another, and another and so on.....
By lowering the energy barrier to the reaction,
more of the collisions will be successful and the reaction will occur faster.
Enzymes are examples of catalysts in our body.