The rate of a chemical reaction typically decreases over time due to the diminishing concentration of reactants, which reduces the frequency of successful collisions among the reactant particles.

In a chemical reaction, reactants are converted into products. As the reaction proceeds, the concentration of the reactants declines because they are consumed to form the products. This reduction in concentration leads to fewer successful collisions between reactant particles, ultimately slowing the reaction rate.

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants. This relationship can be expressed in the rate equation, which in its simplest form is given by:

In this equation, $[A]$ and $[B]$ represent the concentrations of the reactants, and $k$ is the rate constant. As the concentrations of $[A]$ and $[B]$ decrease, the overall rate of the reaction also decreases.

Moreover, the frequency of successful collisions between reactant particles plays a critical role in determining the reaction rate. For a reaction to occur, particles must collide with sufficient energy, known as the activation energy, and in the correct orientation. As the concentration of reactants diminishes, there are fewer particles available to collide, leading to a decrease in the frequency of successful collisions.

In addition to concentration, the rate of reaction can also be influenced by factors such as temperature, pressure, and the presence of catalysts. However, assuming these external conditions remain constant, the primary reason for the decrease in reaction rate over time is the reduction in reactant concentration.

In summary, as a chemical reaction progresses and reactants are transformed into products, the concentration of the reactants decreases. This decline in concentration results in fewer successful collisions between reactant particles, which consequently slows down the reaction rate. This explains why the rate of a reaction decreases over time.