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Equilibrium in Chemical Processes

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Chemical equilibrium is the state of a system in which the reactant and product concentrations do not change over time and the system’s attributes do not change further.

Reactions take place in both forward and reverse directions. When the rates of the forward and reverse reactions are similar in such processes, the concentrations of the reactants and products stay constant. The reaction is said to be in chemical equilibrium at this point. This equilibrium, on the other hand, is thought to be dynamic in nature. This is due to the fact that it comprises a forward reaction in which the reactants react to produce products and a reverse reaction in which the products can react to produce the original reactants.

Why is Chemical Equilibrium called Dynamic Equilibrium?

An equilibrium stage is defined as the point at which the rate of forwarding reaction equals the rate of backward response. The number of reactant molecules changing into products and product molecules converting into reactants is the same at this moment. The same equilibrium can be achieved with the same reactants under comparable conditions anywhere in the world, implying that chemical equilibrium is dynamic.

Dynamic Nature of Chemical Processes in Equilibrium

Take a look at the following reversible reaction–

A + B ⇌ C + D

The products (C and D) build with time, whereas the reactants (A and B) diminish. As a result, the rate of forwarding reaction decreases while the rate of reverse reaction increases. Finally, both reactions take place at the same moment, resulting in a state of equilibrium. This balance can be attained from either direction.

Haber’s Process

Fritz Haber, a German chemist, invented a method for producing ammonia from dinitrogen and dihydrogen. This is known as the Haber process.

N2(g) + 3H2(g)  ⇌ 2NH3(g)

Haber began with known proportions of dinitrogen and dihydrogen, kept the mixture at high temperature and pressure, and measured the amount of ammonia generated at regular intervals. As the reaction progressed, he saw that the composition of the mixture remained constant, despite the presence of some reactants. This indicates that the reaction has achieved equilibrium.

In addition, Haber’s method exemplifies the dynamic character of chemical equilibrium in the following way. Maintaining the same experimental settings as before, hydrogen was substituted with deuterium (D2). As a result, instead of NH3, ND3 is produced. Both reactions, one using H2 and the other involving D2, were allowed to reach equilibrium. When these two mixtures were mixed and left for a while, the concentration of ammonia was found to be the same as previously. Despite this, mass spectrometry confirmed that, in addition to ammonia, all types of deuterium and dihydrogen were present.

Because the forward and reverse processes continue, the scrambling of H and D atoms must be conceivable. If the reaction had ended when it reached equilibrium, there would have been no mixing of isotopes. As a result, chemical reactions attain a state of dynamic equilibrium in which the rates of forwarding and reverse reactions are equal and no net change in composition occurs.

Chemical Equilibrium is Bidirectional

Whether we begin a reaction with reactants or products, equilibrium can be reached on both sides. Take a look at the following reaction.

H2(g) + I2(g) ⇌ 2HI(g)

If we start the reaction with equal initial concentrations of H2 and I2, the reaction will go forward, with the concentrations of hydrogen and iodine decreasing and the concentration of hydrogen iodide increasing until it achieves equilibrium. If we reverse the above reaction, the concentration of hydrogen iodide falls while the concentrations of hydrogen and iodine grow until equilibrium is reached. As a result, if the total number of atoms of an element in a given volume is the same, we get the same equilibrium mixture whether we start with reactants or products.

Conditions for Equilibrium

The process of achieving chemical equilibrium is a dynamic one. Even after the equilibrium state is reached, the forward and reverse processes continue to occur. However, the speeds of the reactions are the same in this case, and there is no change in the relative concentrations of reactants and products for an equilibrium reaction. The following are the criteria and properties of an equilibrium system.

  1. The system must be closed, which means that no substances may enter or exit it.
  2. Equilibrium is a living, breathing thing. Even if we cannot see the reactions, both forward and reverse reactions are occurring.
  3. The rates of forwarding and backward reactions must be equal.
  4. The number of reactants and products does not have to be the same. However, once equilibrium is reached, the amounts of reactants and products remain constant.

Examples of Chemical Equilibrium

  • The forward reaction in chemical reactions converts reactants into products, whereas the backward reaction converts products back into reactants. There are two states, reactants and products, and they both exist in different compositions. 
  • When the reaction begins, the rates of the forward and backward responses may become equal after some time. 
  • Following this, the number of reactants converted will be created again by the reverse reaction, resulting in no change in the concentration of reactants and products. As a result, the reactants and products will be in a state of chemical equilibrium.

Sample Questions

Question 1: What Happens at Chemical Equilibrium?


The pace of advance and backward reactions becomes equal at the chemical equilibrium state, and the concentrations of products and reactants remain constant. During a reversible chemical process, chemical equilibrium occurs when there is no net change in the proportions of reactants and products. Reversible chemical reactions occur when the products, after they have been generated, react with the initial reactants to make the products. In equilibrium, both opposing processes occur at the same rate or velocity, and the quantity of substances involved do not change. In equilibrium, both opposing processes occur at the same rate or velocity, and the quantity of substances involved do not change.

Question 2: What Happens after Equilibrium is Reached?


When equilibrium is attained, solution particles will continue to travel over the membrane in both directions. Despite the fact that nearly an equal number of particles flow in each direction, there is no further change in concentration. Equilibrium is the state of a reversible reaction in which the forward reaction rate matches the backward reaction rate. The reaction does not thereafter come to a halt. Because dissolution is a reversible process, if we add sugar to water indefinitely, we will eventually be unable to add more, but this does not mean that the sugar being added is not being dissolved in water; it is simply the concept of Equilibrium at that time.

Question 3: What is Lechatelier’s principle?


According to Lechatelier’s principle, any changes in the components impacting the equilibrium condition will offset or lessen the total transformation effect. This rule holds true for both chemical and physical equilibrium.

Question 4: What Happens to Equilibrium When the Product is Removed?


If we remove a reactant or product from the equilibrium state, the equilibrium shifts to produce more reactant or product, respectively, to compensate for the loss. When a product is removed from a chemically balanced system, the equilibrium shifts to produce more products in order to compensate for the loss in product concentration. In contrast, removing a reactant causes the system’s equilibrium to shift, allowing more reactants to be created.

Last Updated : 18 Feb, 2022
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