Chemical Equilibrium

Chemical Equilibrium is a condition of a reversible chemical reaction, where the rate of forward and backward reactions are equal. In Chemical equilibrium, the concentrations of the products and reactants remain constant i.e. it does not change with time. This is a state after which there are no observable changes in the properties and the system becomes constant. In this state, the forward reaction proceeds at the same rate as the reverse reaction.

In this article, we will learn more about chemical equilibrium, its types, its examples, and more, so Let’s get started.

What is Chemical Equilibrium?

In Chemistry, Chemical Equilibrium is defined as a condition in which the concentration of reactant and product becomes equal and does not change with time. In this state, the rate of the forward reaction becomes equal to the rate of the reverse reaction and there is no net change in the amount of substance involved. It is a reversible chemical reaction in which the products when formed react again to produce the original reactants.

Definition of Chemical Equilibrium

The state of chemical equilibrium is defined as a condition where there is no further change in the concentration of the reactants and the products and the system attains a state of equilibrium.

When a state of chemical equilibrium is achieved, the continuous interchanging of molecules between the reactants and the products becomes equal, due to which the rate of forward and backward reactions becomes equal, and such a state is known as a state of dynamic equilibrium.

For any reaction in chemical equilibrium, we can break it in two reactions as,

H₂(g) + I₂(g) ⇌ 2HI(g)

• Forward Reaction H₂(g) + I₂(g) → 2HI(g)

• Reverse Reaction 2HI(g) → H₂(g) + I₂(g)

The image of the chemical equilibrium achieved is shown in the image added below,

Examples of Chemical Equilibrium

One of the example of a Chemical Equilibrium is shown below:

N₂O₄ (g) ⇌ 2NO₂ (g)

Under suitable conditions N₂O₄ starts decomposing to 2NO₂

N₂O₄ (g) → 2NO₂ (g)

When enough 2NO₂ is formed, the backward reaction takes place and 2NO₂ reacts to form N₂O₄

2NO₂ (g) → N₂O₄ (g)

Slowly, the amount of N₂O₄ decomposing to 2NO₂ becomes equal to the amount of 2NO₂ reacting to form N₂O₄, Hence the chemical equilibrium is attained.

N₂O₄ (g) ⇌ 2NO₂ (g)

Here, the double arrow shows the process is in dynamic equilibrium.

There are many more examples of chemical reactions attaining the state of chemical equilibrium, few of them are:

• CaCO₃(s) ⇌ CaO(s) + CO₂(g)

• 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂ (g)

• H₂(g) + I₂(g) ⇌ 2HI(g)

• 2H₂(g) + O₂(g) ⇌ 2H₂O(g)

Types of Chemical Equilibrium

Chemical equilibrium is broadly classified into two types namely:

• Homogeneous Equilibrium
• Heterogeneous Equilibrium

Homogeneous Equilibrium

Homogeneous Equilibrium is a chemical equilibrium state in which the reactants and the products are all in the single (same) phase. Its examples commonly include gas-phase or solution reactions. Examples of Homogeneous Equilibrium are as follows:

Both reactants and products are in the gaseous phase:

H₂(g) + I₂(g) ⇌ 2HI(g)

CO(g) + Cl₂(g) ⇌ COCl₂(g)

Both reactants and products are in aqueous solution:

H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃^–(aq)

CH₃COOH(aq) ⇌ CH₃COO^– (aq) + H⁺(aq)

Heterogeneous Equilibrium

Heterogeneous Equilibrium is a chemical equilibrium state which includes more than one phase. Here, the reactants and the products are in different phases. Its examples commonly include solids-gases or solids-liquids reactions. Examples of Heterogeneous Equilibrium are as follows:

Example involving solids-gases reaction:

3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂ (g)

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

Example involving solids-liquids reaction:

Cu(s) + 2Ag⁺(aq) ⇌ Cu²⁺(aq) + 2Ag(s)

CaCO₃(s) + 2HCl(aq) ⇌ CaCl₂(aq) + CO₂(g) + H₂O(l)

Conditions for Chemical Equilibrium

Chemical equilibrium is attained when the forward and reverse reaction rate is the same. It is a dynamic process. The conditions under which a system can attain chemical equilibrium are:

• The net force and torque on the system should be zero.
• The system should be in a state of constant velocity i.e. the number of forward reactions must be equal to the number of backward reactions.
• No new substances should be added or removed from the system, i.e. the system should be closed.
• The reaction should take place under constant temperature and pressure.
• There should be no net change in the amount of substance involved.

Factors Affecting Chemical Equilibrium

Factors that affect chemical equilibrium are:

Change in Temperature

The change in temperature certainly affects the chemical equilibrium. When the temperature is increased, the equilibrium shifts to the direction of an endothermic reaction whereas the decrease in temperature will shift the equilibrium in the direction of exothermic reactions.

Change in Volume

When the volume of the gaseous mixture at equilibrium is increased, it decreases the number of moles per unit volume of reactants and products and the equilibrium will shift towards the direction of a larger number of gas molecules.

However, if the volume of the gaseous mixture at equilibrium is decreased, the equilibrium will shift toward the direction of a smaller number of gas molecules.

Change in Pressure

Pressure and volume are inversely proportional to each other, hence when pressure is increased, the volume will decrease and vice versa. So, the change in pressure shows the opposite effect as that of the change in volume.

Therefore, with an increase in pressure, the equilibrium will shift towards the direction of a smaller number of gas molecules, and with a decrease in pressure, the equilibrium will shift towards the direction of a larger number of gas molecules.

Change in Concentration

A change in concentration of reactant or product changes the rate of equilibrium. If the concentration of reactants is increased, the equilibrium shifts forward, and the formation of the product increases. If the concentration of the product is increased, the equilibrium shifts backward, causing more formation of reactants.

Effect of Catalyst

The addition of a catalyst does not affect equilibrium, however, it is helpful to speed up the reaction. Catalyst favors both forward and backward reactions equally and thus there is no net effect on the equilibrium.

Effect of Addition of Inert Gas

If inert gas is added at a fixed volume, the total pressure will increase and there will be no effect on chemical equilibrium.

However, if the inert gas is added at a fixed pressure, this will increase the volume and the equilibrium will shift in the direction of the increased number of gas molecules.

Read More,

• Factors Affecting Chemical Equilibrium
• Laws of Chemical Equilibrium

Equilibrium Constant

The equilibrium Constant of any chemical reaction can be described as the ratio of the concentration of the products to the concentration of the reactants. It is denoted by K or K_C. It is useful in determining the chemical behavior as well as the direction of the reaction.

Equilibrium Constant Formula

The equilibrium constant of a chemical reaction can be calculated by the concentration of products and reactants.

Suppose the Chemical Reaction is,

aA + bB ⇌ cC + dD

For this, the equilibrium constant can be given by:

K_C = ([C]^c × [D]^d)/ ([A]^a × [B]^b)

where,

• K_c is Equilibrium Constant
• C ,D is Concentration of Formed Products
• A, B is Concentration of Involved Reactants
• c, d, a, b is Respective Moles of Product and Reactants Involved in Equation
• If Value of K_c is less than 1, it indicates more reactants in the mixture
• If Value of K_c is more than 1, it indicates the formation of products is more

Importance of Chemical Equilibrium

Chemical Equilibrium allows chemical reactions to take place at a constant rate. It has its importance and uses in various fields such as:

Industrial Processes

It is used in the production of various chemicals and fuels. Equilibrium helps in maximizing the net yield and efficiency of the product. A few examples of its uses in industrial processes are:

• It is used in the preparation of sulphuric acid with the help of the contact process.
• It is used in the preparation of ammonia by Haber’s process.

Biological Processes

Various biological process like transportation of gases within the organism, pH regulation, enzyme-catalyst reaction, etc. involves the use of chemical equilibrium. It is useful in maintaining balance and promoting growth in living organisms. It is also helpful while studying the functioning of organisms.

Chemical Equilibrium Practice Problems

Problem 1: Classify the following equilibrium reactions as homogeneous or heterogeneous.

• 2Hg(l) + O₂(g) ⇌ 2HgO(s)
• H₂(g) + I₂(g) ⇌ 2HI(g)
• CH₃COOH(aq) ⇌ CH₃COO^– (aq) + H+(aq)
• C(s) +O₂(g) ⇌ CO₂(g)
• CaCO₃(s) ⇌ CaO(s) + CO₂(g)

Problem 2: Consider the equilibrium equation: H₂(g) + I₂(g) ⇌ 2HI(g)

If 2 moles of H₂ and 1 mole of I₂ are placed in a 1-liter vessel. How many moles of each substance is present at equilibrium at 458℃, if K_c at 458℃= 49.7?

Problem 3: Consider the reaction: 2Hg(l) + O₂(g) ⇌ 2HgO(s)

If 10 moles of Hg, and 5.6 moles of O₂ attain equilibrium at 350℃. If 20 moles of HgO are obtained, find the equilibrium constant.

Problem 4: The equilibrium constant at 500℃ for the reaction A(g) +B₂(g) ⇌ AB₂(g) is 0.04. A catalyst is added the rate of equilibrium is increased by five times. Find the change in equilibrium constant.

Chemical Equilibrium Frequently Asked Questions (FAQs)

What is Chemical Equilibrium in Chemistry?

In Chemistry, Chemical Equilibrium is defined as a condition in which the concentration of reactant and product becomes equal and does not change with time.

What is Forward and Backward Reaction?

• Forward Reaction: It is the reaction in which the reactants decomposed to form the product.

• Backward Reaction: It is also known as reverse reaction, in this reaction the product are converted back to the original form of the reactants.

What are Laws of Chemical Equilibrium?

The law of chemical reaction states that the rate of forward reaction must be equal to the rate of backward reaction. It is in compliance with law of mass action.

Why is Chemical Equilibrium Called Dynamic Equilibrium?

In chemical equilibrium, there is continuous flow of equal amount of forward and backward reaction even after attaining equilibrium, hence it is known as dynamic equilibrium.

What is Effect of Addition of Inert Gas at Constant Volume during a Chemical Equilibrium Reaction?

The state of equilibrium remains undisturbed on the addition of inert gases at constant volume as it does not take part in the reaction, hence addition of inert gas cause no effect in chemical equilibrium state.

What are Factors that Affect Chemical Equilibrium?

Factors that affect chemical equilibrium are:

• Change in Concentration
• Change in Temperature
• Change in Pressure
• Change in Volume

What is Effect of Addition of a Catalyst on a Chemical Equilibrium?

The addition of catalyst no effect on chemical equilibrium, it only speeds up the reaction.