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Chemical Equilibrium- Equilibrium, Types, Practice problems, and FAQs

Chemical Equilibrium- Equilibrium, Types, Practice problems, and FAQs

You might seen people standing in a queue for so many purposes -say to board a bus..

On the arrival of a bus, some people may board the bus from queue , but new people keep joining the queue, and wait for the next bus to come. 
At times, the number of people leaving in the front side may become the same as the the number of people joining at the back, such that the queue's length appears to be constant. This is a state of Equilibrium

Similarly, in chemistry, there are many reactions that do not go for completion and form a mixture containing reactants as well product ia fixed ratio. Such reactions has major implications for the industry's amount of chemical output, purity of products and cost effectiveness of the production etc..

Despite the fact that these reactions appear to be coming to a halt, they actually keep continue. If you could see what was going on at the atomic level, you'd observe that the reactants are producing products at the same rate, as the products are forming reactants

Table of Content

  • Static and Dynamic Equilibrium
  • Chemical equilibrium
  • Characteristics of Chemical Equilibrium
  • Homogeneous equilibrium
  • Heterogeneous Equilibrium
  • Practice Problems
  • Frequently Asked Questions-FAQs

Static and Dynamic Equilibrium

In equilibrium, two equal but opposing forces are at work. 

This can take place in two ways-
The two opposing forces may face each other. Consider a box kept on a table. The box is acted upon by two equal and opposing forces- 1) Gravitational force trying to pull the box down to the ground and 2) the supporting table acting against this gravity with a force acting upwards.

The two forces and the box will be standstill without any movement. As a result, its position does not change over time and said to be static (without any motion). This equilibrium of two opposing forces causing zero movement is called as a static equilibrium.

But is it possible to attain equilibrium, with the two forces still moving? Well definitely yes, it is.

Let's take one bucket that has a tap at the bottom and is filled by the same kind of tap. The rate of inflow of water equals the rate of outflow of water in a bucket. The volume of water in the bucket remains the same. This is an example of dynamic equilibrium.

Based on motion of the acting forces, Equilibrium can be static or dynamic.

Illustration of dynamic equilibrium, bucket fitted with a tap

Equilibrium can also be divided into two other categories based on the nature.:

Physical equilibrium. 

The physical equilibrium is established when the opposing forces involve only physical and not chemical changes of the substances.

Chemical equilibrium

In chemical equilibrium, reactants changes to product (forward reaction) and at the same time product also reacts to form products (backward reaction). So chemical equilibrium is a dynamic equilibrium involving two active opposing reversible reactions.

With the help of a generic reaction, this can be understood. Consider the following reversible reaction between reactant R and product P:

R ⇌ P

Only the reactant is present at first. As a result, the backward response rate (rb) is zero. The product, on the other hand, is generated over time, and its concentration rises with it. As a result, the rate of backward response rises. As a result, the rate of forward reaction (rf) begins to decrease. After a certain time (t), both the forward and backward rates become equal and the reaction is said to attain equilibrium.

At equilibrium, the concentrations of the reactants [R] and concentration of products [P] are constant but not necessarily equal. There are three sorts of concentration-time graphs that can be created.

Case I: If [R] = [P]
Case II: If [R] > [P]
Case III: If [R] < [P]

Characteristics of Chemical Equilibrium

  • The equilibrium in a closed system is dynamic. It can be done in both directions (forward or backward).
  • The existence of a catalyst has no effect on the equilibrium state, but it affects the rate of reaction depending on the catalyst type
  • The forward reaction rate equals the backward reaction rate at equilibrium.
  • When the reactants and products are in equilibrium, their concentrations remain constant.
  • At equilibrium, the free energy change (G) is equal to zero.
  • The system takes its own time to reach equilibrium.

Chemical Equilibrium can be classified into one more category as

  1. Homogeneous equilibrium
  2. Heterogeneous equilibrium

Homogeneous equilibrium

All reactants and products are in the same states of matter in homogeneous equilibrium.


PCl5(g) ⇌ PCl3(g) + Cl2(g)

PCl5(g) (yellow colour) is the sole substance present in the container at first. PCl3(g) decomposes when heated. PCl3(g)(blue colour) and Cl2(g) (orange colour) begins to appear in the beaker. The homogeneous equilibrium is reached after time t.

Homogeneous Equilibrium 

Other examples

H2(g) + Cl2(g) ⇌ 2HCl(g)
C2H5OH(l) + CH3COOH(l) ⇌ CH3COOC2H5(l) + H2O(l)


  • Gases always exist in homogeneous equilibrium because they are miscible in each other.
  • Liquid may either exist in homogeneous equilibrium or in heterogeneous equilibrium depending upon the miscibility of liquids present.
  • Solids always exist in heterogeneous equilibrium because they are immiscible in each other.

Heterogeneous Equilibrium 

The reactants and products are in more than one phase in heterogeneous equilibrium.


In the reaction, CaCO3(s)⇌CaO(s)+CO2(g), the reactants and products are in different phases (solid and gas).

Related Video

Practice Problems:

Q1. The production of ND3 is carried out under the same partial pressure and temperature circumstances as for NH3. The equilibrium mixtures of H2, N2, NH3, and D2, N2, and ND3 are mixed and set aside for a while. Will equilibrium mixtures react and if yes then how? 

Solution: Yes, The equilibrium mixtures will react and form a number of new species.

N2(g) + 3D2(g) ⇌ 2ND3(g)
N2(g) + 3H2(g) ⇌ 2NH3(g)

When the two mixtures are combined, along with these reactions, H2 and D2 combine to produce HD via the reaction:
H2(g) + D2(g) ⇌ 2HD(g)

The produced HD also reacts with N2 to form NHD2 and NH2D, as seen below:
3HD(g) + N2(g) ⇌ NHD2(g) + NH2D(g)

Q2. Which of the following graphs most accurately depicts the equilibrium A ⇌ B?

Solution:  In graph (A), (B) and (C), a drop in the reactant concentration and an increase in the product concentration are the same.

Graph (D) cannot be the proper graph because the concentrations of both the reactant and the product are falling.

Hence, Right answers are (A), (B) and (C).

Q3. Which of the following statement(s) about the reaction is/are correct?

(A) It is possible to attain equilibrium.
(B) At equilibrium, the amount of reactants and products can differ.
(C) Equilibrium that is homogeneous
(D) Equilibrium in a heterogeneous environment 

Solution: Because equilibrium can be attained in reversible reactions. As a result, the (A) is correct.
(B) Because the concentrations of the reactants and products might differ at equilibrium, this is also a true statement.
(C) It is a homogeneous equilibrium since all of the reactants and products are gaseous.
Option (D) is thus incorrect
As a result, the correct answers are (A), (B), and (C).

Q4. How many of the reactions below are homogenous and reversible?

  • N2(g) + 3H2(g) ⇌ 2NH3(g)
  • H2(g) + Cl2(g) ⇌ 2HCl(g)
  • CaCO3(s) ⇌ CaO(s) + CO2(g)
  • NH4HS(s) ⇌ NH3(g) + H2S(g)

Solution: If all the reactants are in the same phase, then reaction is known as in homogeneous equilibrium. In (A) and (B), all the reactants and products are in the same phase. Hence these reactions are in homogeneous equilibrium and reversible in nature.

Frequently Asked Questions-FAQs

Question 1. The allotropic transformation of graphite(s) into a diamond(s) exists in heterogeneous equilibrium. How?

Answer. Despite having both reactant and product in the same phase, the reaction exists in heterogeneous equilibrium because solids are always immiscible in nature.

Question 2. What are three things that can cause the chemical equilibrium to be disrupted?

Answer. Adding or removing reactants or products, changing the overall pressure or volume, and changing the temperature of the system are three forms of stressors that can change the composition of an equilibrium system.

Question 3. Can you justify the equilibrium state of a girl jogging on a treadmill?

Answer. In this scenario, the female is running at the same speed but in the opposite direction as the treadmill. As a result, the girl's position does not vary over time.

Question 4. What is the basic requirement for chemical equilibrium in a system?

Answer. Chemical equilibrium is a state in which no net change in the amounts of reactants and products occurs during a reversible chemical reaction. 

Related Topics

Le-Chatelier’s Principle

Reversible and Irreversible reactions

Physical equilibrium-Solid Liquid Equilibrium,Liquid-Vapour Equilibrium,Solid-Vapour Equilibrium

Law of mass action and Equilibrium constant

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