A chemical reaction rate is a measure of how quickly reactants are consumed or how rapidly products are formed over a specific period of time. The reaction is fastest when it first begins and then progressively slows down as time passes. To understand this deceleration, it is necessary to examine the molecular processes that govern the reaction speed.
The Role of Reactant Depletion
The most direct explanation for the progressive slowdown of a reaction lies in the continuous consumption of the starting materials, known as reactants. The rate of a chemical process is highly dependent on the concentration of the reactants present in the system. As the reaction proceeds, reactant molecules are transformed into product molecules, meaning the initial supply of starting material diminishes.
This decrease in reactant concentration directly reduces the material available to undergo the chemical transformation. For most reactions, the speed is directly proportional to the amount of reactant concentration. The mathematical relationship between concentration and rate confirms that as the concentration term decreases, the resulting rate must also decrease. This continuous depletion of the necessary starting molecules is the primary factor driving the overall slowdown of the reaction speed.
Collision Frequency and the Rate Decrease
The macroscopic phenomenon of reactant depletion is fundamentally linked to the microscopic behavior of molecules through a concept called collision theory. For a reaction to occur, the particles of the reactants must physically collide with one another. The rate of the reaction is directly proportional to how often these particles collide.
At the beginning of the process, the high concentration of reactants ensures that the particles are densely packed, leading to a high frequency of collisions per second. As the reaction progresses and concentration drops, the particles become more spread out within the same volume. This lower particle density means that the chance of any two reactant particles coming close enough to collide is significantly reduced.
Fewer total collisions per unit of time automatically translates to fewer collisions that meet the necessary energy and orientation criteria to form products. Therefore, the mechanical basis for the rate decrease is the reduction in collision frequency caused by the decreasing concentration of reactant molecules.
The Influence of Reversible Reactions and Equilibrium
A final consideration in the observed rate decrease is the concept of reversibility, as many chemical processes do not go fully to completion. In a reversible reaction, the products formed can also react with one another to turn back into the original reactants. This process is known as the reverse reaction, which proceeds simultaneously with the forward reaction that creates the products.
When the reaction begins, the concentration of products is zero, so the reverse reaction rate is also zero. However, as the forward reaction progresses, products accumulate within the system, causing the rate of the reverse reaction to begin increasing. This product buildup means that the products are now colliding with each other more frequently, converting back into the original reactants.
The net reaction rate, which is the speed that is actually observed, is the difference between the forward reaction rate and the reverse reaction rate. Since the forward rate is constantly decreasing due to reactant depletion and the reverse rate is constantly increasing due to product buildup, the net rate continuously slows down. Eventually, the forward rate and the reverse rate become equal to one another.
At this point, the reaction has reached a state of dynamic equilibrium, where the reactants are being converted into products at the exact same speed that products are converting back into reactants. Though molecular activity has not stopped, there is no longer any net change in the concentrations of reactants or products. The reaction appears to have ceased because the net speed has dropped to zero, further contributing to the overall decrease in the observable rate.