Changing the concentration

As the concentration increases in a chemical and substance eg.acid and magnesium, the frequency of collisions between acid particles and magnesium particles increases, so therefor the rate of reaction increases too.

The effect of temperature on rate of reaction

As the temperature increases so does the average kinetic energy of the reacting particles.This means the particles move about faster and the frequency of collisions is increased. It also means the particles hit each other harder, so a greater proportion of collisions will have energy equal to or greater than activation energy and will lead to a successful reaction. These two factors mean that rate of reaction increases with temperature.

Adding a catalyst to the equation

Catalysts work by making collisions that take place more successfully  - they provide a different route with a lower activation energy, meaning more collisions have enough energy for a reaction to occur.It doesn't increase the rate of reaction but makes the particles that do collide more successful.

Changing the pressure

Two gases with a higher pressure react faster because there are more particles in a certain volume so frequency of collisions is greater, so therefore the rate of reaction increases with pressure. However, increasing the pressure has little effect on the rate of reaction between two solids because the particles do not move in a solid.
High Pressure + Low pressure