A chemical reaction rate quantifies the speed at which a chemical process occurs, measured by observing the change in concentration of reactants or products over a specific period. The speed of a chemical reaction is not constant; it typically changes as the reaction progresses.
The Role of Reactant Concentration
One primary reason for a decreasing reaction rate is the reduction in reactant concentration. As a chemical reaction proceeds, reactants are consumed to form products. This directly leads to a decrease in their concentration over time. With fewer reactant molecules present, opportunities for them to interact become less frequent.
Consider a crowded room where people are constantly bumping into one another. If the room gradually empties, the chances of people colliding significantly decrease. Similarly, in a chemical reaction, a lower concentration of reactant molecules means fewer potential encounters between them. This reduction in the number of available reactant particles directly translates to a slower overall reaction rate. The rate of a reaction is directly proportional to the concentration of the reactants involved.
Understanding Collision Theory
Building upon the concept of reactant concentration, collision theory explains the molecular basis for changes in reaction rates. For a chemical reaction to take place, reactant molecules must collide with sufficient energy, known as the activation energy, and with the correct molecular orientation for bonds to break and new ones to form.
As the concentration of reactants diminishes, the frequency of molecular collisions decreases. Fewer collisions mean a reduced number of instances where molecules meet with the necessary energy and proper alignment. This decline in effective collisions directly results in the reaction proceeding at a slower pace. The rate of reaction is dependent on how often and how effectively these molecular interactions happen.
The Influence of Reversible Reactions
Many chemical processes are reversible reactions, meaning that the products formed can also react with each other to reform the original reactants. As a forward reaction progresses and products accumulate, the reverse reaction, where products convert back to reactants, begins to occur. The rate of this reverse reaction increases as the concentration of products rises.
While the rate of the forward reaction slows due to reactant depletion, the rate of the reverse reaction simultaneously speeds up. Eventually, chemical equilibrium is reached. At equilibrium, the rate at which reactants form products becomes equal to the rate at which products reform reactants. There is no net change in the concentrations of reactants or products, even though both forward and reverse reactions continue actively.