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NEET 2022 | Class 12

NEET Important Chapter - Equilibrium

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What is Equilibrium in Chemistry?

What is Equilibrium in Chemistry?

Last updated date: 23rd Jul 2024
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Equilibrium is the state of a process in which the system's properties, such as temperature, pressure, and concentration, do not fluctuate over time. There are two opposing processes in any process that reaches equilibrium. It approaches equilibrium when the concentrations of the two opposing processes are equal.

The equilibrium is known as Physical Equilibrium because the opposing mechanisms simply require physical changes. When chemical processes are in opposition to one another, chemical equilibrium occurs.

Important Topics of Thermodynamics

  • Equilibrium

  • Law of Mass Action

  • Equilibrium Constant

  • Le-Chatelier's Principle

  • Free Energy

  • Ionic Equilibrium

Important Definitions of Thermodynamics

Reversible and Irreversible Reactions

  • Reversible Reactions: Reversible reactions are those in which the reactants and products are not totally changed.

  • An acid and a base, one or both of which are weak, neutralise each other in a reversible reaction.

  • When a weak acid (CH3COOH) reacts with a strong base, this is the result (NaOH).
    CH3COOH + NaOH ⇌ CH3COONa + H2O

  • Irreversible reactions: Irreversible reactions are those in which all of the reactants are converted into products.

  • The neutralisation reaction between a strong acid (HCl) and a strong base is an irreversible reaction (NaOH).
    NaOH + HCl → NaCl + H2O.

Equilibrium: It’s Dynamic Character

  • When the concentrations of reactants and products do not change over time, they are said to be in equilibrium. The concentrations of the reactant and product become constant.

  • A chemical equilibrium is dynamic in nature, meaning that reactions occur at the same rate in both directions.

  • As a result, the amount of product produced reacts backwards to produce reactants, resulting in no change in reactant or product concentration over time.

Law of Mass Action and Equilibrium Constant (Factors Affecting Equilibrium)

  • “At any given moment, the product of the molar concentrations of the reactants at a fixed temperature determines the pace of a chemical reaction."

  • Consider a simple reversible process at a specific temperature,
    aA + bB ⇌ cC + dD.

  • Therefore at equilibrium ,
    Forward reaction rate = Backward reaction rate
    $\dfrac{k_{f}}{k_{b}}= K_{c} = \dfrac{[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}$; Kc stands for equilibrium constant.

Relation Between the Three Constants - Kp, Kc and Kx

  • The values of Kp can be obtained as follows:
    Kp = Kc (RT)Δn
    Kp = Kc (P)Δn
    Δn = The number of moles of gaseous products equals the number of moles of gaseous reactants in a chemical equation.

  • These equations are also well known as chemical equilibrium formulas.

Relationship Between Equilibrium Constant and G°

  • The relation relates ΔG for a reaction under any situation to ΔG°,
    ΔG = ΔG° + 2.303 RT log(Q)

  • The standard free energy change of a reaction and its equilibrium constant are related at temperature T by the relationship,
    ΔG° = - 2.303 RT logK

  • For any general reaction,
    aA + bB ⇌ cC + dD;
    The rate constant is given as,
    K = ((aC)c(aD)d)/((aA)a(aB)b); The activity of the reactants and products is represented by the letter a. It has a lower unit count.

  • For pure solids and liquids, use the following formula:
    a = 1,

  • For gases, use:
    a = gas pressure measured in atm.

  • For components in solution: a = molar concentration.

Le-Chatelier's Principle

  • Changes in any of the components that form a system's equilibrium conditions cause the equilibrium to shift, reducing or neutralising the effect of the change.

  • Application of Le-Chatelier's principle:
    In chemical, physical, and everyday life systems that are in a state of equilibrium, the Le-Chatelier idea is vitally essential.

Applications to The Chemical Equilibrium




Synthesis of Ammonia i.e. Haber’s Process

N2 + 3H2 ⇌ 2NH3 + 23 kcal

- High-pressure situations (Δn<0).
- Low Temperature.
- Excess amounts of N2 and H2.
- Removal of NH3 favours forward reaction.

Formation of Sulphur Trioxide

2SO2 + O2 ⇌ 2SO3 + 45 kcal (exothermic)

- High-pressure situations (Δn<0).
- Low Temperature.
- Excess amounts of SO2 and O2, favours forward reaction.

Nitric Oxide Synthesis

N2 + O2 ⇌ 2NO - 43.2 kcal

- Low Temperature.
- Excess amounts of N2 and O2, favours forward reaction.
- Because there is no change in volume during the reaction, i.e. Δn = 0,  pressure has no effect on equilibrium.

Nitrogen Dioxide Formation

2NO + O2 ⇌ 2NO2 + 27.8 kcal. 

- High Pressure
- Low Temperature
- Excess amounts of NO and O2 favours the reaction in forward direction.

Phosphorus Pentachloride Dissociation

PCl5 ⇌ PCl3 + Cl2 - 15 kcal.

- Low pressure or high volume of the container,
Δn > 0,
- High Temperature
-  Excess of PCl5.

Solved Examples from the Chapter

Question 1: For each reaction, write the equilibrium constant expression. Are these expressions for equilibrium constants equivalent? Explain.

(a) N2O4(g) ⇌ 2NO2(g) 

(b) ½ N2O4(g) ⇌ NO2(g)


  • For the given reaction,
    N2O4(g) ⇌ 2NO2(g);
    - The Rate Constant is given as,
    K = [NO2]2/[N2O4].

  • Similarly, for the reaction,
    ½ N2O4(g) ⇌ NO2(g);
    - The Rate Constant is given as,
    K = [NO2]/[N2O4]½.

  • Although the equilibrium constant formulations for the same species involved have a 2:1 ratio of concentration for the products to the concentration for the reactants to achieve the K value for a. To get the K value for b, we'd have to square it.

Key points to remember: The equilibrium constant is given by K = Reactants/Products.

Question 2: What happens to the magnitude of the equilibrium constant if the forward response rate is doubled, given K = kf/kr? What happens if the reverse reaction rate for the total reaction is reduced by a factor of three?


  • The magnitude of the equilibrium constant is doubled when the forward reaction rate is doubled. 

  • ∴ Given that K = kf/kr,
    K’ = 2(kf/kr) = 2K.

  • The magnitude of the equilibrium constant is raised by a factor of three if the reaction rate of the reverse reaction for the overall reaction is described by a factor of three.

  • ∴ Given that K = kf/kr,
    K’ = kf/(1/3)kr = 3(kf/kr) = 3K.

Key points to remember: It is important to note the magnitude by which an individual rate constant increases or decreases. Depending on the individual rate constants, the overall rate constant is impacted. 

Solved Examples from Previous Year Questions

Question 1: In a buffer solution containing equal concentration of B and HB, the Kb for B is 10–10. The pH of buffer solution is 

(a) 10

(b) 7

(c) 6

(d) 4


  • Here, the given value of rate constant is:
    Kb = 10-10.

  • ∴ Ka = 10-4. Deriving from the Ka value,
    pKa = 4.

  • Now, the value of pH is given as
    pH = pKa + log(1);
    pH = pKa = 4.

  • As a result, option (d) is the correct answer.

Question 2: If the equilibrium constant for N2(g) + O2(g) ⇌ 2NO(g) is K, the equilibrium constant for 1/2N2(g) + 1/2O2(g) ⇌ NO(g) will be

(a) 1/2K 

(b) K

(c) K2

(d) K1/2


  • The given reactions is:
    N2(g) + O2(g) ⇌ 2NO(g);

  • Hence, the equilibrium constant for the above reaction is
    K = [NO]2/[N2][O2] = K

  • Similarly for the reaction,
    1/2N2(g) + 1/2O2(g) ⇌ NO(g);

  • Equilibrium Constant is given as: K = [NO]/[N2]1/2[O2]½

  • This equilibrium constant therefore on expansion is,
    K = ([NO]2/[N2][O2])½ = (K)½  

  • As a result, option (d) is the correct answer.

Question 3: For the reversible reaction, N2(g) + 3H2(g)⇌ 2NH3(g) + Heat the equilibrium shifts in forward direction

(a)  by increasing the concentration of NH3(g)

(b)  by decreasing the pressure

(c)  by decreasing the concentrations of N2(g) and H2(g)

(d)  by increasing  pressure  and  decreasing temperature


  • Any change in the reaction's concentration, pressure, or temperature changes the direction of equilibrium.

  • The principle of Le-Chatelier governs this shift in equilibrium direction.

  • Equilibrium moves in the opposite direction to undo the change, according to Le-principle. Chatelier's

  • The given reaction
    N2(g) + 3H2(g)⇌ 2NH3(g)

  • When pressure is increased, equilibrium shifts forward, reducing the number of moles; however, when temperature is reduced, equilibrium shifts forward, increasing the number of moles.

  • As a result, option (d) is the correct answer.

Practice Questions

Question 1: If a small amount of heat is added to ice ⇌ liquid equilibrium in a sealed container, the following results will occur:
(a) There will be increase in pressure

(b) There will be increase in temperature

(c) There will be decrease in temperature

(d) There will be no change in pressure and temperature

Answer: (d) There will be no change in pressure and temperature

Question 2: Among the salt solutions listed below, the solution that is most fundamental in nature is,

(a) Ammonium Chloride

(b) Ammonium Sulphate

(c) Ammonium Nitrate

(d) Sodium Acetate. 

Answer: (d) Sodium Acetate. 


It is considered to be in equilibrium when the observable properties of a process, such as colour, temperature, pressure, concentration, and so on, do not change.

The term 'equilibrium' means that the reactants and products in a chemical process are in a state of balance. When both ice and water are present at the same time, the equilibrium state can be seen in physical processes like the melting point of ice at 0°C.

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FAQs on NEET Important Chapter - Equilibrium


1. What does it mean to be in a state of equilibrium?

A state of equilibrium or a stable situation in which opposing forces cancel each other out and no changes occur is known as equilibrium. When supply and demand are equal in economics, equilibrium is reached. This is an example of balance when you are calm and stable.

2. Give some examples of equilibrium.

A chemical reaction in which the rates of forward and reverse reactions are the same, is called equilibrium reaction. In general equilibrium can be understand with the help of following examples:

  • An illustration of a balanced book resting on a table. 

  • A vehicle that travels at a constant speed.

All these are some examples of equilibrium. In the chemical equilibrium reaction, the amount of product getting converted into reactant is the same as the amount of reactant converting back into product.

3. What is the difference between equilibrium and balance?

Equilibrium is the state of a system in which competing effects are balanced, resulting in no net change; balance is an uncountable state in which opposing forces harmonise; equilibrium is the condition of a system in which competing influences are balanced, resulting in no net change.