Boltzmann Distribution and Activation energy
First Boltzmann Distribution!
Low energy collisions will not result in a reaction. For a reaction to occur, the particles need to collide with a minimum energy called Ea. This is the activation energy.
Why doesn't an exothermic reaction just gradually take place and why is an Activation energy needed?
Take methane. It needs a spark to react with oxygen. Without the spark the two gases would not react together. This is because as the two gas molecules approach one another, the electrons repel each other. The reaction occurs when there is enough energy to overcome this repulsion and distribute the electrons. This is how the bonds and broken and then remade as the electrons create molecules again.
Orientation also plays a key role. Even if there is enough energy for a reaction, a molecule still has to hit another at the right angle in order for them to react together. This makes successful reactions quite rare.
Catalysts work by providing a different reaction pathway and therefore lowering the activation energy needed to get the reaction started.