Why does a reaction's rate decrease over time?

A reaction's rate decreases over time because the concentration of reactants decreases, reducing the frequency of successful collisions.

In a chemical reaction, reactants are transformed into products. As the reaction progresses, the concentration of the reactants decreases because they are being used up to form the products. This decrease in concentration reduces the number of successful collisions between reactant particles, which in turn slows down the rate of the reaction.

The rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants. This relationship is described by the rate equation, which in its simplest form is: Rate = k[A][B], where [A] and [B] are the concentrations of the reactants, and k is the rate constant. As the concentrations of [A] and [B] decrease, the rate of the reaction decreases.

The frequency of successful collisions between reactant particles is also a key factor in the rate of a reaction. 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 decreases, there are fewer particles available to collide, reducing the frequency of successful collisions.

Furthermore, the rate of reaction is also influenced by temperature, pressure, and the presence of catalysts. However, assuming these conditions remain constant, the decrease in reactant concentration is the primary reason why a reaction's rate decreases over time.

In summary, as a reaction progresses and reactants are converted into products, the concentration of reactants decreases. This reduction in concentration leads to fewer successful collisions between reactant particles, which in turn slows down the rate of the reaction. This is why a reaction's rate decreases over time.