Rate of Reactions
This section is all about how quickly chemical reactions happen and what
is going on at an atomic and molecular level to change the rate. This
all hinges on a theory called "Collision Theory" which explains how
atoms and molecules behave during a reaction.
In a reaction of A+B->AB
An A particle must collide with a B particle with enough speed (this
minimum energy is called the activation energy) for the reaction to happen.
If these atoms only collide rarely, then it will take a long time for the
reaction to take place. Likewise, if they collide with suitable force and with great frequency, then there
will be a faster reaction. There are 4 ways of altering the speed of a
reaction and they are described below:
1) Surface area.
If a reaction is taking place, we have already discussed the fact that the number of collisions per second have a massive baring on the rate. If we have a small surface area, like a lump of limestone, only the particles on the surface are reacting, those in the center have to wait for the outer ones to react and move away before they get their chance so using powdered limestone reacts much faster as there are many more particles on the surface reacting. Imagine it like two armies meeting each other in a battle and they arrive in perfect formation when they meet. Only the ones at the front fight and even those on the second row have to wait for the one in front to fall before they get a go. If they were in a jumbled mass, lots more would be fighting.
Back to these particles, we said that they need to hit each other with sufficient force, with more thermal energy, they move faster. As well as moving faster to ensure a harder collision, they are more likely to hit another particle because of the speed they are moving. Temperature, therefore, increases both the frequency and energy of the collisions between reacting particles.
3) Concentration (or pressure in a gas).
Referring once again to the frequency of collisions, this is exactly what is influenced by concentration. If we increase the concentration, we increase the number of particles within the same unit of volume so they are more likely to to collide and react. In a gas, increasing the pressure brings the particles closer which makes chances of collision higher.
Not just any catalyst will do, these are very specific elements or compounds for each reaction. One catalyst will do nothing for another reaction and vice versa. For a specific reaction, a catalyst lowers the energy (activation energy) required to get the particles reacting and speeds up the reaction. To increase the effect of the catalyst, we increase the surface area as much as possible so they are usually found as powders, pellets or a mesh.