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Science Grade XI Chemistry

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Chapter 6: Equilibrium

Grade 11 Science | Chapter 6

Equilibrium

Many reactions reach a balance rather than finishing. This chapter develops dynamic equilibrium, the equilibrium constant, Le Chatelier’s principle, and acids, bases and the pH scale.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: A Reversible Balance

Dynamic Equilibrium

The Equilibrium Constant

Le Chatelier’s Principle

Acids and Bases

The pH Scale

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: A Reversible Balance

Many reactions are reversible: products can re-form reactants. In a closed container such a reaction reaches a balance called equilibrium, where the forward and reverse reactions continue at equal rates. This chapter explains that balance, how it is measured by the equilibrium constant, how it shifts, and how the same ideas describe acids and bases.

Core idea

At dynamic equilibrium the forward and reverse rates are equal, so concentrations stay constant. The equilibrium constant measures the balance, and Le Chatelier’s principle predicts how it shifts.

2. Dynamic Equilibrium

As a reversible reaction proceeds, the forward rate falls while the reverse rate rises, until the two become equal. At this point the reaction has reached dynamic equilibrium: the concentrations of reactants and products no longer change, yet both reactions are still occurring, in balance. Equilibrium can only be reached in a closed system.

Diagram 1 – Reaching Dynamic Equilibrium

Forward and reverse rates meeting at equilibrium over time

Fig 1. The forward rate falls and the reverse rate rises until they become equal; from then on the concentrations stay constant.

3. The Equilibrium Constant

The law of mass action gives the equilibrium constant K_c, the ratio of the product concentrations to the reactant concentrations, each raised to the power of its coefficient. A large K_c means the products are favoured, a small one means the reactants are favoured. The constant depends only on temperature.

4. Le Chatelier’s Principle

Le Chatelier’s principle states that if a system at equilibrium is disturbed, it shifts to oppose the change. Adding a reactant shifts the balance toward products; raising the pressure shifts it toward the side with fewer gas molecules; and changing the temperature shifts it in the direction that absorbs the added heat. This lets chemists control the yield of a reaction.

5. Acids and Bases

An acid donates a hydrogen ion and a base accepts one, in the Bronsted definition. In water, acids increase the hydrogen ion concentration and bases decrease it. The product of the hydrogen and hydroxide ion concentrations, the constant K_w, is fixed at a given temperature, so the two are linked: as one rises, the other falls.

6. The pH Scale

The pH measures acidity as the negative logarithm of the hydrogen ion concentration, pH = minus log[H⁺]. A pH below 7 is acidic, 7 is neutral and above 7 is basic. Because the scale is logarithmic, each unit represents a tenfold change in hydrogen ion concentration.

Diagram 2 – The pH Scale

The pH scale from acidic through neutral to basic

Fig 2. The pH scale runs from acidic below 7, through neutral at 7, to basic above 7, with each unit a tenfold change.

7. Key Reasoning (Principles)

Principle 1: Equilibrium is a balance of equal rates

At dynamic equilibrium the forward and reverse reactions continue at equal rates, so the concentrations stay constant while both reactions still occur.

Principle 2: A disturbed equilibrium shifts to oppose it

Le Chatelier’s principle says a change in concentration, pressure or temperature shifts the equilibrium in the direction that reduces the change.

Principle 3: pH measures acidity logarithmically

Since pH = minus log[H⁺], each pH unit is a tenfold change in hydrogen ion concentration, and pH plus pOH is constant in water.

8. Worked Examples

Example 1

Q: Find the pH of a solution with [H⁺] = 1 × 10^-3 mol/L.

▶ Show Solution

pH = minus log[H⁺] = minus log(10^-3).

= 3.

Answer: 3.

Example 2

Q: Find the pH of a neutral solution with [H⁺] = 1 × 10^-7.

▶ Show Solution

pH = minus log(10^-7).

= 7.

Answer: 7.

Example 3

Q: Is a solution of pH 4 acidic or basic?

▶ Show Solution

A pH below 7 is acidic.

So it is acidic.

Answer: Acidic.

Example 4

Q: If pH = 9, find pOH (given pH + pOH = 14).

▶ Show Solution

pOH = 14 minus pH = 14 minus 9.

= 5.

Answer: 5.

Example 5

Q: Find the pH of [H⁺] = 1 × 10^-5.

▶ Show Solution

pH = minus log(10^-5).

= 5.

Answer: 5.

Example 6

Q: Adding more reactant to an equilibrium shifts it toward which side?

▶ Show Solution

By Le Chatelier, the system opposes the change.

It shifts toward the products.

Answer: Toward the products.

Example 7

Q: A large value of K_c means which side is favoured?

▶ Show Solution

A large K_c means more products.

The products are favoured.

Answer: The products.

Example 8

Q: By how much does [H⁺] change for each pH unit?

▶ Show Solution

The scale is logarithmic, so each unit is a factor of ten.

Answer: Tenfold.

Example 9

Q: In the Bronsted sense, what does an acid do?

▶ Show Solution

An acid donates a hydrogen ion.

Answer: It donates a hydrogen ion.

Example 10

Q: On what does the equilibrium constant depend?

▶ Show Solution

The equilibrium constant depends only on temperature.

Answer: Only on temperature.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. At dynamic equilibrium the forward and reverse rates are: (a) zero (b) equal (c) increasing (d) opposite

2. The equilibrium constant depends only on: (a) pressure (b) volume (c) temperature (d) catalyst

3. Le Chatelier’s principle predicts the equilibrium will: (a) stop (b) oppose the change (c) reverse fully (d) vanish

4. A pH of 7 is: (a) acidic (b) neutral (c) basic (d) impossible

5. pH equals: (a) log[H⁺] (b) minus log[H⁺] (c) [H⁺] (d) 1/[H⁺]

▶ Reveal Answers

1. (b) equal.

2. (c) temperature.

3. (b) oppose the change.

4. (b) neutral.

5. (b) minus log[H⁺].

Set B – Short Answer (Understanding)

1. Define dynamic equilibrium.

2. What does the equilibrium constant measure?

3. State Le Chatelier’s principle.

4. Define an acid and a base in the Bronsted sense.

5. Write the formula for pH.

▶ Reveal Answers

1. A balance where forward and reverse reactions occur at equal rates, keeping concentrations constant.

2. The ratio of product to reactant concentrations at equilibrium, showing which side is favoured.

3. A system at equilibrium shifts to oppose any change imposed on it.

4. An acid donates a hydrogen ion; a base accepts one.

5. pH = minus log[H⁺].

Set C – Application and Reasoning

1. Find the pH of [H⁺] = 1 × 10^-2.

2. If pH = 11, find pOH.

3. Raising pressure shifts equilibrium toward which side for gases?

4. Is pH 6 acidic, neutral or basic?

5. Why does the equilibrium constant not change when a catalyst is added?

▶ Reveal Answers

1. pH = 2.

2. pOH = 14 minus 11 = 3.

3. Toward the side with fewer gas molecules.

4. Acidic, since it is below 7.

5. A catalyst speeds both forward and reverse reactions equally, so the balance, and K_c, is unchanged.

Set D – Higher Order (Challenge)

1. A solution has pH 3. Find its [H⁺].

2. Explain why raising the temperature of an exothermic equilibrium lowers the yield.

3. Compare the [H⁺] of a pH 2 and a pH 5 solution.

4. Explain why equilibrium is described as dynamic, not static.

5. Adding a product to a system at equilibrium has what effect?

▶ Reveal Answers

1. [H⁺] = 1 × 10^-3 mol/L.

2. Added heat shifts the exothermic equilibrium backward (to absorb the heat), reducing the amount of product.

3. pH 2 has [H⁺] = 10^-2 and pH 5 has 10^-5, so pH 2 is a thousand times more concentrated.

4. Because both forward and reverse reactions continue at equal rates, even though the concentrations stay constant.

5. By Le Chatelier, it shifts the equilibrium back toward the reactants.

Chapter Summary

Dynamic Equilibrium

Forward and reverse rates equal; concentrations constant in a closed system.

Equilibrium Constant

K_c is the ratio of products to reactants; depends only on temperature.

Le Chatelier

A disturbed equilibrium shifts to oppose the change.

Acids and Bases

An acid donates a hydrogen ion; a base accepts one.

pH = minus log[H⁺]; below 7 acidic, 7 neutral, above 7 basic.

Logarithmic

Each pH unit is a tenfold change in [H⁺].

Quantity

Unit

Symbol

Equilibrium

equal rates

–

minus log[H+]

–

Neutral pH

–

8-Point Exam Quick-Check

Dynamic equilibrium: forward and reverse rates equal, concentrations constant.

Equilibrium constant Kc compares products to reactants; depends only on temperature.

Le Chatelier: a disturbed equilibrium shifts to oppose the change.

An acid donates a hydrogen ion; a base accepts one.

pH = minus log of the hydrogen ion concentration.

Below 7 acidic, 7 neutral, above 7 basic.

pH plus pOH is constant (14 in water at room temperature).

Each pH unit is a tenfold change in hydrogen ion concentration.

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Class 11 Chemistry Chapter 6: Equilibrium, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Chemistry syllabus and develops chemical equilibrium, covering reversible reactions and dynamic equilibrium, the equilibrium constant and law of mass action, Le Chatelier’s principle, Bronsted acids and bases, and the pH scale, with two diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.