CBSE Class 11 Chemistry Exemplar Chapter 7: Equilibrium NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 11 PDF

This comprehensive set of NCERT Solutions for CBSE Class 11 Chemistry Exemplar, Chapter 7: Equilibrium, provides detailed explanations and step-by-step solutions for various multiple-choice questions. The chapter delves into the fundamental concepts of chemical equilibrium, including the relationship between equilibrium constants Kp and Kc, the influence of standard free energy on equilibrium, and the general characteristics of physical and chemical equilibria. It also covers the application of these principles in specific reactions, such as the dissociation of PCl5 and the complex formation involving cobalt ions. These solutions are designed to help students grasp the intricacies of equilibrium, understand how to calculate equilibrium constants, and predict the direction of reactions based on thermodynamic parameters. They serve as an excellent resource for exam preparation, reinforcing theoretical knowledge with practical problem-solving techniques.

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Board	CBSE
Class	Class 11
Subject	Chemistry Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 7

Chapter summary

Chapter 7 of the CBSE Class 11 Chemistry Exemplar focuses on Equilibrium. This NCERT Solutions set covers key concepts such as the relationship between Kp and Kc, the impact of standard free energy on the equilibrium constant, and the general characteristics of systems at equilibrium, including dynamic nature and the role of closed systems. It also addresses specific examples like the dissociation of PCl5 and the color changes in cobalt complex reactions, linking them to enthalpy changes and Le Chatelier's principle. The solutions provide a clear understanding of how to apply equilibrium principles to solve problems.

Learning outcomes

Understand the relationship between Kp and Kc for chemical reactions.
Relate standard free energy change (ΔG°) to the equilibrium constant (K).
Identify the general characteristics of equilibrium in physical processes.
Calculate the equilibrium constant (Kc) for a given reaction using equilibrium concentrations.
Determine the sign of enthalpy change (ΔH) for a reaction based on temperature and color changes.
Analyze the effect of adding substances on equilibrium position and color intensity.

Topics covered

Paper topics

Chemical Equilibrium
Equilibrium Constant (Kc)
Equilibrium Constant (Kp)
Relationship between Kc and Kp
Δn calculation
Standard Free Energy Change (ΔG°)
Equilibrium Constant (K)
Physical Equilibria
Characteristics of Equilibrium
Dynamic Nature of Equilibrium
Closed System Requirement
Le Chatelier's Principle (implied)

Important topics

Relationship between Kp and Kc
Calculating Δn
Effect of ΔG° on K
Characteristics of Physical Equilibria
Calculating Kc from concentrations
Predicting ΔH from temperature effects

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Questions and Solutions

Question 1

We know that the relationship between $K_c$ and $K_p$ is $K_p = K_c (RT)^{\Delta n}$ . What would be the value of $\Delta n$ for the reaction: $NH_4Cl(s) \Longrightarrow NH_3(g) + HCl(g)$ ?

Solution: The relationship between the equilibrium constants

K_p

and

K_c

is given by the equation

K_p = K_c (RT)^{\Delta n}

. Here,

\Delta n

represents the change in the number of moles of gaseous components in a balanced chemical reaction. It is calculated as:

\Delta n = (\text{Sum of moles of gaseous products}) - (\text{Sum of moles of gaseous reactants})

For the given reaction,

NH_4Cl(s) \Longrightarrow NH_3(g) + HCl(g)

, we have:

Number of moles of gaseous products = 1 mole of $NH_3$ + 1 mole of $HCl$ = 2 moles
Number of moles of gaseous reactants = 0 moles (since $NH_4Cl$ is a solid)

Therefore,

\Delta n = 2 - 0 = 2

Question 2

For the reaction, $H_2(g) + I_2(g) \rightleftharpoons 2 HI(g)$ , the standard free energy is $\Delta G^{\ominus} > 0$ . The equilibrium constant (K) would be:

Solution: The relationship between the standard free energy change (

\Delta G^{\ominus}

) and the equilibrium constant (K) is given by the equation:

\Delta G^{\ominus} = -RT \ln K

We are given that

\Delta G^{\ominus} > 0

, which means the standard free energy change is positive. For this equation to hold true with a positive

\Delta G^{\ominus}

, the term

\ln K

must be negative. A negative value for

\ln K

implies that

K

must be less than 1. Therefore, for this reaction, the equilibrium constant

K < 1

Question 3

Which of the following is not a general characteristic of equilibria involving physical processes?

Solution: Let's analyze the general characteristics of equilibria involving physical processes:

(a) Equilibrium is possible only in a closed system at a given temperature: This is a fundamental requirement for equilibrium.
(b) All measurable properties of the system remain constant: At equilibrium, macroscopic properties like pressure, temperature, and concentration do not change over time.
(c) All the physical processes stop at equilibrium: This statement is incorrect. At equilibrium, the forward and reverse processes occur at the same rate, leading to a dynamic state where changes are still happening at the molecular level, but there is no net change in observable properties.
(d) The opposing processes occur at the same rate and there is dynamic but stable condition: This accurately describes the dynamic nature of equilibrium.

Therefore, the statement that all physical processes stop at equilibrium is not a general characteristic.

Question 4

$PCl_5$ , $PCl_3$ , and $Cl_2$ are at equilibrium at 500 K in a closed container and their concentrations are $0.8 \times 10^{-3}$ mol L^-1, $1.2 \times 10^{-3}$ mol L^-1, and $1.2 \times 10^{-3} \text{ mol L}^{-1}$ , respectively. The value of $K_c$ for the reaction $PCl_5(g) \Longrightarrow PCl_3(g) + Cl_2(g)$ will be:

Solution: For the given reversible reaction at equilibrium:

PCl_5(g) \Longrightarrow PCl_3(g) + Cl_2(g)

The equilibrium constant

K_c

is defined as the ratio of the product of the concentrations of the products to the concentration of the reactant, each raised to the power of their stoichiometric coefficients.

K_{c} = \frac{[PCl_{3}][Cl_{2}]}{[PCl_{5}]}

We are given the equilibrium concentrations at 500 K:

$[PCl_5] = 0.8 \times 10^{-3} \text{ mol L}^{-1}$
$[PCl_3] = 1.2 \times 10^{-3} \text{ mol L}^{-1}$
$[Cl_2] = 1.2 \times 10^{-3} \text{ mol L}^{-1}$

Substituting these values into the expression for

K_c

K_{c} = \frac{(1.2 \times 10^{-3} \text{ mol L}^{-1}) \times (1.2 \times 10^{-3} \text{ mol L}^{-1})}{(0.8 \times 10^{-3} \text{ mol L}^{-1})}

K_{c} = \frac{1.44 \times 10^{-6} \text{ mol}^2 \text{ L}^{-2}}{0.8 \times 10^{-3} \text{ mol L}^{-1}}

K_{c} = 1.8 \times 10^{-3} \text{ mol L}^{-1}

Note: The units for Kc in this case are mol L^-1. The question options might vary in units, but the numerical value is derived from the calculation.

Question 5

Which of the following statements is incorrect?

Solution: Let's evaluate each statement:

(a) In equilibrium mixture of ice and water kept in perfectly insulated flask, mass of ice and water does not change with time: This is correct. In a closed, insulated system, the rate of melting equals the rate of freezing, leading to a constant mass of ice and water at equilibrium.
(b) The intensity of red colour increases when oxalic acid is added to a solution containing iron (III) nitrate and potassium thiocyanate: This statement is incorrect. The reaction is $Fe^{3+}(aq) + SCN^{-}(aq) \rightleftharpoons Fe(SCN)^{2+}(aq)$ (red complex). Oxalic acid ( $H_2C_2O_4$ ) reacts with $Fe^{3+}$ ions to form stable complexes (e.g., $[Fe(C_2O_4)_3]^{3-}$ ). This removes $Fe^{3+}$ from the equilibrium, shifting the equilibrium to the left (backward direction), thus decreasing the intensity of the red color.
(c) On addition of catalyst the equilibrium constant value is not affected: This is correct. A catalyst increases the rate of both forward and reverse reactions equally, allowing equilibrium to be reached faster, but it does not alter the equilibrium concentrations or the equilibrium constant.
(d) Equilibrium constant for a reaction with negative $\Delta H$ value decreases as the temperature increases: This is correct. According to Le Chatelier's principle, for an exothermic reaction ( $\Delta H < 0$ ), increasing the temperature shifts the equilibrium to the left (favoring reactants), which corresponds to a decrease in the equilibrium constant.

Therefore, statement (b) is incorrect.

Question 6

When hydrochloric acid is added to cobalt nitrate solution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink. On the basis of this information mark the correct answer. $[Co(H_2O_6)]^{3+}(aq) + 4CI^-(aq) \Longrightarrow [CoCI_4]^{2-}(aq) + 6H_2O(l)$

Solution: The observation is that adding HCl (which provides

Cl^-

ions) to the cobalt nitrate solution (containing

[Co(H_2O_6)]^{3+}

ions, which are pink) at room temperature causes the mixture to turn blue. This indicates that the forward reaction, forming the blue complex

[CoCl_4]^{2-}

, is favored under these conditions. When the mixture is cooled, it turns pink again, indicating that the reverse reaction (formation of the pink

[Co(H_2O_6)]^{3+}

complex) is favored at lower temperatures. The reaction is:

[\text{Co } (\text{H}_2\text{O})_6]^{3+}(\textit{aq}) + 4\text{CI}^-(\textit{aq}) \Longrightarrow [\text{CoCI}_4]^{2-}(\textit{aq}) + 6\text{H}_2\text{O} (\textit{l})

Since cooling favors the reverse reaction and heating (or addition of

Cl^-

which shifts equilibrium to the right) favors the forward reaction, this implies that the forward reaction absorbs heat. According to Le Chatelier's principle, if a change in temperature causes a shift in equilibrium, the reaction is endothermic if heat is absorbed (forward reaction) and exothermic if heat is released (forward reaction). In this case, the forward reaction is favored by higher temperatures (implied by the blue color formation at room temperature and reversal upon cooling), meaning it absorbs heat. Therefore, the reaction is endothermic. An endothermic reaction has a positive enthalpy change (

\Delta H > 0

Common mistakes

Incorrectly calculating Δn for the Kp-Kc relationship, especially with solid reactants/products.
Confusing the relationship between ΔG° and K; assuming ΔG° > 0 implies K > 1.
Misinterpreting the dynamic nature of equilibrium, thinking processes stop.
Errors in calculating Kc due to incorrect substitution of concentrations or units.
Incorrectly predicting the shift in equilibrium based on Le Chatelier's principle when a substance is added that reacts with a reactant/product.

Revision tips

Memorize the formula relating Kp and Kc and practice calculating Δn correctly for various reaction types.
Understand the direct relationship between ΔG° and K, and how the sign of ΔG° dictates whether K is greater or less than 1.
Review the characteristics of equilibrium, particularly the dynamic nature and the requirement for a closed system.
Practice calculating Kc with given concentrations and ensure correct units are used.
Focus on how temperature changes affect equilibrium for endothermic (ΔH > 0) and exothermic (ΔH < 0) reactions, and how this relates to color changes in specific examples.

Practice MCQs

Q1. For the reaction NH₄Cl(s) ↔ NH₃(g) + HCl(g), what is the value of Δn used in the relationship Kp = Kc(RT)²⁰?

Q2. If the standard free energy change (ΔG°) for the reaction H₂(g) + I₂(g) ↔ 2HI(g) is positive (ΔG° > 0), what can be concluded about the equilibrium constant (K)?

Q3. Which of the following is NOT a general characteristic of equilibria involving physical processes?

Q4. For the reaction PCl₅(g) ↔ PCl₃(g) + Cl₂(g), if at equilibrium [PCl₅] = 0.8 x 10⁻³ mol L⁻¹, [PCl₃] = 1.2 x 10⁻³ mol L⁻¹, and [Cl₂] = 1.2 x 10⁻³ mol L⁻¹, what is the value of Kc?

Q5. Which statement about equilibrium is incorrect?

Q6. When HCl is added to a cobalt nitrate solution, it turns blue. Cooling the mixture makes it pink again. What does this indicate about the reaction [Co(H₂O)₆]³⁺(aq) + 4Cl⁻(aq) ↔ [CoCl₄]²⁺(aq) + 6H₂O(l)?

Frequently asked questions

What is the fundamental relationship between Kp and Kc?

The relationship is given by Kp = Kc(RT)²⁰, where Δn is the difference between the moles of gaseous products and gaseous reactants.

How does standard free energy change (ΔG°) relate to the equilibrium constant (K)?

They are related by the equation ΔG° = -RT ln K. A positive ΔG° indicates K < 1, a negative ΔG° indicates K > 1, and ΔG° = 0 indicates K = 1.

What are the key characteristics of equilibrium in physical processes?

Equilibrium is possible only in a closed system at a constant temperature, all measurable properties remain constant, and the opposing processes occur at equal rates, maintaining a dynamic but stable condition.

How is the equilibrium constant (Kc) calculated?

Kc is calculated by dividing the product of the concentrations of the gaseous products raised to their stoichiometric coefficients by the product of the concentrations of the gaseous reactants raised to their stoichiometric coefficients.

Can a catalyst affect the equilibrium constant?

No, a catalyst speeds up both the forward and reverse reactions equally, helping the system reach equilibrium faster, but it does not change the value of the equilibrium constant itself.

How can we determine if a reaction is endothermic or exothermic using observable changes?

By observing how temperature changes affect the equilibrium. If heating favors the forward reaction (e.g., color change) and cooling favors the reverse, the reaction is endothermic (ΔH > 0). If cooling favors the forward reaction, it's exothermic (ΔH < 0).

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