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Chapter 7 – Equilibrium


You must have often heard the term chemical Equilibrium. But, do you know what it exactly means? Chemical equilibrium is not an individual entity; it is a complex topic and involves the fine-tuning of various concepts. Also, to solve the various numericals of chemical stability, you need to have a fair idea of the factors affecting chemical equilibrium. 

At the end of this session, you shall have a fair idea about the:-

  • The exact meaning of chemical equilibrium
  • Different types of chemical equilibrium
  • Factors affecting the chemical equilibrium of a system

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Chemical equilibrium is a state. It is that specific state or condition of a chemical system in which the concentration of the reactants and products’ concentration remains equilibrium. The term “remains in equilibrium” means that:- 

  • Both entities (reactants and products) will not change concerning time.
  • Both commodities (reactants and products) will not change concerning chemical properties. 

Now, there must be an immediate question in your mind after this definition – when will such a state arise in a chemical system? The answer is:- When the rate of a forward chemical reaction (the rate of product formation from the reactants) equals the rate of reverse, or backward chemical reaction of the system, such a state of chemical equilibrium shall be attained. There shall be zero change in the concentrations of reactants and products in the form of equal rates of forward and reverse reaction (i.e., chemical equilibrium). 

In the world of physical chemistry, chemical equilibrium is often referred to as Dynamic Equilibrium.

Primarily, there are two types of chemical equilibrium based on the reactant and product phases. They are:- 

  • Homogeneous Equilibrium
  • Heterogeneous Equilibrium

In the case of Homogeneous Chemical Equilibrium, the phase of the reactants and products remains the same. There are two subtypes under this:-

  • Chemical reactions are featuring the same number of products and reactant molecules. E.g.:-

N2 (gas) + O2 (gas) ⇆ 2 NO (gas)

Here, the reactant molecules – Nitrogen and Oxygen – are in the gaseous phase. The product molecule – Nitric Oxide is also in the gaseous phase. Hence it qualifies as a homogeneous equilibrium system. 

Secondly, The number of nitrogen molecules on the reactant side is 2, equal to the number of nitrogen molecules on the product side, which is also 2. The same applies to the oxygen molecules. There are two oxygen molecules on the reactant side and 2 Oxygen molecules on the product side. 

  • Chemical reactions are featuring an unequal number of product and reactant molecules. E.g.:

2SO2 (gas) + O2 (gas) ⇆ 2SO3 (gas)

Here, the reactant molecules – sulfur dioxide and Oxygen – are in the gaseous phase. The product molecule – sulfur trioxide is also in the gaseous phase. Hence it qualifies as a homogeneous equilibrium system.

Secondly, the number of product molecules is not equal to the number of total reactant molecules. Hence it is a different category of homogeneous equilibrium.

In this type of equilibrium system, the phases of reactants and products are different from each other. Let us have a look at the example for a better understanding.

CaCO3 (solid) ⇆ CaO (solid) + CO2 (gaseous)

Here, Calcium carbonate that is the reactant is in the solid phase, whereas the product Carbon dioxide is in the gaseous phase. That means this reaction qualifies as a heterogeneous equilibrium system.

Now, let us have a look at the various factors affecting chemical equilibrium.

Before going into the factors, there is a theory you need to understand. This principle is proposed by Le-Chatelier and explains the influence of physical and chemical factors on a chemical equilibrium. 

Le- Chatelier’s principle says that:

If there is any change in the factors affecting the equilibrium condition, the system will counteract or reduce the effect of the transformation. The factors can be both physical and chemical“. 

Here is a list of the factors that influences a chemical equilibrium:-

A change in concentration of product or reactant is compensated by the reaction that either consumes (if reactant or product is added) or replenishes (if reactant or product is removed), changing the concentration of reactant product, the composition of the chemical equilibrium changes.

Change in pressure affects gaseous reaction by rendering inequality between the number of gaseous reactants and products. However, in a heterogeneous state, the alteration in the tension between the liquid and solid can be ignored.

With the increase in temperature, there is a considerable decrease in an exothermic reaction’s equilibrium constant. With an increase in temperature, there is an increase in the equilibrium constant of an endothermic reaction.

The presence of a catalyst does not make any difference in the chemical equilibrium of a reaction. There is no effect of a catalyst on the number of reactants or product molecules. Catalyst only speeds up a chemical reaction. 

  • The addition of inert gases does not affect chemical equilibrium.
    1.  What is the equilibrium process?
      Ans. It is an equilibrium state in which the forward process equals the reverse process.
    2. What are the types of equilibrium?
      Ans. Heterogeneous chemical equilibrium
      Homogeneous chemical equilibrium
    3. What is an equilibrium in physics ?
      Ans. In physics, equilibrium is a stable phase where two opposing forces neutralize each other.
    4. What is an example of equilibrium?
      Ans.  N2 + H2 ⇄ 2NH3
    5. What is the importance of equilibrium?
      Ans. Chemical equilibrium is essential for various industrial processes like:- 

      • Contact process for the preparation of sulfuric acid
      • Haber’s process for the preparation of ammonia

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