Why does the rate of reaction decrease over time?
Because the rate of reaction is intrinsically dependent on the reactant concentration leftover.
Recall that for the reaction
#aA + bB -> cC + dD#,
a rate law is written as:
#r(t) = -1/a(Delta[A])/(Deltat) = -1/b(Delta[B])/(Deltat) = 1/c(Delta[C])/(Deltat) = 1/d(Delta[D])/(Deltat)#
Thus, a rate of reaction has units of
Since the rate is based on the change in concentration over time, which has to do with how quickly the reactants react, there has to be a relationship between rate and how much concentration of reactants are left.
As the concentration of reactants decreases, the reactants have a harder time trying to find each other to react, so the rate of reaction is slower for lower concentrations.
Thus, as time passes and concentrations of reactants drop, the rate of reaction drops.