Explain how pressure can effect the rate of reaction (higher chemistry)

Pressure only effects those elements that are in Gaseous state. Therefor when you increase the pressure in a reaction the gas molecules are closer together, there is an increase in density of molecules. When the molecules are closer together they will collide more often - bang into each other more. More collisions means more chances of a successful reaction. So ultimately your overall rate of reaction will increase meaning that the reaction will be faster. The key thing to remember that a change in pressure only effects those molecules that are in gas state not liquid or solid