Particles must collide
To understand what changes the rate, we first have to look at how a reaction happens at the level of the particles.
When a chemical reaction takes place, the chemical bonds between the reactant particles (atoms or molecules) break and new bonds are built. The result is that different products are formed.
Step 1
The existing bonds break.
Step 2
The freed atoms join up in a new arrangement.
Step 3
Products with new properties are formed.
But first they have to meet
For bonds to break and new ones to form, the reactant particles have to collide with one another. Two particles that never come near each other cannot react.
The key to the whole chapter
A reaction is a series of collisions. So anything that increases the number of collisions in unit time increases the rate of reaction.