Chemical Equilibrium is one of the most important and conceptual chapters of 10th class Chemistry. In this chapter, students learn about reversible reactions and how certain chemical reactions do not stop completely but continue in both forward and backward directions. When the rate of the forward reaction becomes equal to the rate of the backward reaction, a state called dynamic equilibrium is established. This chapter also explains the Law of Mass Action, equilibrium constant (Kc), its units, and the importance of equilibrium constant in predicting the direction and extent of a reaction. These concepts are very important for board exams as well as for building a strong foundation in Chemistry.
In this PDF, you will find important MCQs, short questions, and long questions that are carefully selected according to the board examination pattern. The questions are designed to help students understand key concepts, prepare effectively for exams, and score high marks. This material will provide complete guidance and revision support for Chapter 9 – Chemical Equilibrium.
Important MCQs – Chapter 18 Pharmacology
1. What happens at dynamic equilibrium?
(a) Only the forward reaction continues
(b) Only products are present
(c) Forward and reverse reactions occur at equal rate ✅
(d) Reaction stops completely
2. Which of the following symbols is used to represent a reversible reaction?
(a) →
(b) ←
(c) ⇌ ✅
(d) =
3. The reaction H₂ + I₂ ⇌ 2 HI is an example of:
(a) Irreversible reaction
(b) Reversible reaction ✅
(c) Exothermic reaction
(d) Endothermic reaction
4. The reaction CaCO3 <-> CaO + CO2 in a closed flask represents:
(a) Static equilibrium
(b) Dynamic equilibrium ✅
(c) Open system
(d) Irreversible reaction
5. The Law of Mass Action was proposed by:
(a) Dalton and Rutherford
(b) Guldberg and Waage ✅
(c) Boyle and Charles
(d) Avogadro and Lavoisier
6. What is the unit of active mass?
(a) mol
(b) mol⁻¹
(c) mol dm⁻³ ✅
(d) g cm⁻³
Short Questions – Chapter 18
1. What are irreversible reactions? Give a few characteristics of them.
Answer:
Irreversible reactions are chemical reactions that proceed only in the forward direction and do not reverse to form the original reactants.
Characteristics:
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Products do not recombine to form reactants.
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The reaction goes to completion.
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Represented by a single arrow (⇒).
Example: 2H₂ + O₂ ⇒ 2H₂O or CaCO₃ ⇌ CaO + CO₂
Explanation:
Irreversible reactions are common in daily life and industrial processes, where the reactants are completely transformed into products and cannot revert under normal conditions.
2. Define chemical equilibrium state.
Answer:
Chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
Explanation:
At chemical equilibrium, the reaction appears to have stopped because there is no net change in concentrations, but both forward and backward reactions continue at the molecular level. This is also called dynamic equilibrium.
3. What is the relationship between active mass and rate of reaction?
Answer:
According to the Law of Mass Action, the rate of a reaction is directly proportional to the active mass (concentration) of the reactants.
Explanation:
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Higher active mass → Faster reaction rate
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Lower active mass → Slower reaction rate
Example:
For the reaction A + B <-> C, the rate of the forward reaction is proportional to [A][B].
4. If the reaction quotient Qc of a reaction is more than Kc, what will be the direction of the reaction?
Answer:
If Qc > Kc, it means there are more products than needed at equilibrium. Therefore, the reaction will proceed in the reverse direction to form more reactants until equilibrium is achieved.
Explanation:
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Forward reaction slows down.
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Backward reaction increases.
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System shifts toward reactants to restore equilibrium.
Example:
For the reaction A + B <-> C + D, if Qc = [C][D]/[A][B] > Kc, more C and D are present than at equilibrium. Hence, reaction shifts backward.
Long Questions – Chapter 18
Q1: Describe a reversible reaction with the help of an example and graph.
Answer:
A reversible reaction is a chemical reaction in which the products can react again to form the original reactants. Unlike irreversible reactions, reversible reactions do not go to completion. Instead, they reach a dynamic equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant, though the reaction continues at the molecular level.
Definition:
A reversible reaction is a reaction that can proceed in both forward and reverse directions under the same conditions.
General Form:
A + B <-> C + D
Example:
Formation of ammonia gas from nitrogen and hydrogen:
N2(g) + 3 H2(g) <-> 2 NH3(g)
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Forward reaction: Nitrogen and hydrogen combine to form ammonia.
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Reverse reaction: Ammonia decomposes back into nitrogen and hydrogen.
Graph Explanation:
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X-axis: Time
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Y-axis: Concentration
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The reactants curve decreases over time.
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The products curve increases over time.
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At dynamic equilibrium, both curves level off, showing constant concentrations.
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This indicates that the forward and backward reactions occur at equal rates, even though the reactions continue at the molecular level.
Key Points for Students:
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Reversible reactions are common in chemical processes.
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Equilibrium can be influenced by temperature, pressure, and concentration.
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Understanding reversible reactions is essential for industrial applications, like ammonia production (Haber process).
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