How does the reaction rate change with time?
The rate of a reaction decreases as time progresses.
Collision Theory tells us that the rate of reaction depends on the reactant particles colliding with energy in excess of the Activation Energy for the reaction. As a reaction progresses the reactants are being turn junto the products, so the number of reactant particles left is decreasing.
If the amount of reactant particles is decreasing as the reaction progresses, then the chance of successful collisions must also decrease, and ultimately when all the reactant particles have reacted, the reaction must stop and the rate become zero.