Explain in terms of collision theory, how and why a reactions's rate changes as the reaction progresses

The rate of a chemical reaction is the speed with which the reactants are converted to products while the collision theory is used to explain why chemical reactions occur at different rates. It states that for a reaction to proceed, the reactants particles must collide and the more successful collisions there are per unit of time, the faster the reaction will be. As the reaction progresses, the products builds up in the system and the chance that the reactants will collide amongst themselves reduces as the product molecules begin to build up while the molecules of the reactants are deprived. therefore the rate of reaction changes.