We have read about equilibrium in physical systems such as a vessel containing hot water kept at room temperature attains ambient temperature with time. Similarly, chemical systems also attain a state of equilibrium. In this section, we will learn about the equilibrium in a chemical process.
Consider a reversible chemical reaction as shown in the example below. The reaction occurs in both, the forward and the reverse directions. As the reaction proceeds, the reactants A and B deplete to form products C and D, leading to a decrease in the rate of forward reaction and increase in the rate of the reverse reaction, after which the reverse reaction dominates and the products C and D deplete to form the original reactants A and B.
The concentration of the reactants and the products change as the reaction proceeds until the rate of forward and the reverse reactions become equal. When the rate of the forward and the reverse reactions become equal to each other, the reaction attains a stage of equilibrium. This equilibrium is dynamic in nature as during this stage, the forward and the backward reaction still takes place, only the total concentration of the reactants and the products remains constant.
We can understand the concept of dynamic equilibrium more clearly using the following example. The synthesis of ammonia in industries involves a process termed as the Haber’s Process. The following reaction takes place:
In this process, Hydrogen and Nitrogen are made to contact at high temperature and pressure. The amount of the reactants and the products is noted with respect to time, during the reaction. The results incurred are shown graphically in the image below.
We can see from the figure that, the rate of forward reaction is initially high and decreases as the time of the reaction increases, whereas the rate of backward reaction is initially low and it increases as the reaction proceeds. After a certain time, the rate of forward reaction becomes equal to the rate of the backward reaction. This stage is known as the equilibrium stage. At this point, the amount of reactants being converted to products becomes equal to the amount of products being converted back to the reactants.
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