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Class 11 Science (PCM)

Chemistry Part 1

Equilibrium

Equilibrium

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Study Action Plan (Sorted by Difficulty)
Buffer Solutions
Accuracy: 0%
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High
Ionisation Constants of Weak Acids and Weak Bases
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High
Ionisation of Water, Kw, pH and pOH
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High
pKa, pKb and Strength of Acids/Bases
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Difficulty Level:
High
Reaction Quotient (Qc)
Accuracy: 0%
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High
Acids and Bases: Arrhenius, Bronsted-Lowry and Lewis Concepts
Accuracy: 50%
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Total Correct1
Total Incorrect1
Difficulty Level:
Medium
Heterogeneous Equilibria
Accuracy: 50%
Total Attempted2
Total Correct1
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Difficulty Level:
Medium
Le Chatelier's Principle
Accuracy: 50%
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Difficulty Level:
Medium
Equilibrium Constant Expressions
Accuracy: 67%
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Difficulty Level:
Medium
Applications of Equilibrium Constant
Accuracy: 100%
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Difficulty Level:
Low
Dynamic Chemical Equilibrium
Accuracy: 100%
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Difficulty Level:
Low
Equilibrium in Physical Processes
Accuracy: 100%
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Difficulty Level:
Low
Gibbs Free Energy and Equilibrium
Accuracy: 100%
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Difficulty Level:
Low
Temperature Dependence of Equilibrium
Accuracy: 100%
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Difficulty Level:
Low

Learning Objectives

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Learning Objectives

  • Understand the dynamic nature of chemical equilibrium and how it is established when the rate of the forward reaction equals the rate of the reverse reaction.
  • Apply the law of mass action to write equilibrium constant expressions for various chemical reactions.
  • Calculate equilibrium constants (KcK_c and KpK_p) and understand their dependence on stoichiometric coefficients.
  • Predict the effects of changes in concentration, pressure, and temperature on the position of equilibrium using Le Chatelier's Principle.
  • Analyze the temperature dependence of equilibrium constants and how it affects the direction of exothermic and endothermic reactions.
  • Explore heterogeneous equilibria involving multiple phases and understand the exclusion of pure solids and liquids in equilibrium constant expressions.
  • Use the reaction quotient (QcQ_c) to predict the direction of reaction shifts to reach equilibrium.
  • Relate Gibbs free energy change (ΔG\Delta G) to the equilibrium constant to determine reaction spontaneity.
  • Investigate equilibrium in physical processes, such as phase changes and solubility, including the application of Henry's law.
  • Utilize equilibrium constants to predict the extent of reactions, direction of reaction shifts, and calculate equilibrium concentrations from initial data.
  • Classify acids and bases using Arrhenius, Bronsted-Lowry, and Lewis concepts, and calculate ionization constants for weak acids and bases.
  • Explain the ionization of water, the ionic product (KwK_w), and the relationship between pHpH and pOHpOH.
  • Evaluate the strength of acids and bases using pKapK_a and pKbpK_b values.
  • Understand the function and calculations related to buffer solutions and their resistance to pHpH changes.

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Revision Notes & Summary

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Chapter Notes

Dynamic Chemical Equilibrium

  • Definition: A state where the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products.
  • Dynamic Nature: At equilibrium, reactions continue to occur at the molecular level, but no net change is observed in the concentrations.
  • Law of Mass Action: The rate of a chemical reaction is proportional to the product of the masses of the reacting substances, each raised to a power equal to the coefficient in the balanced chemical equation.

Equilibrium Constant Expressions

  • Kc: Equilibrium constant in terms of concentration.
  • Kp: Equilibrium constant in terms of partial pressures.
  • Expression: For a reaction aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dD, the equilibrium constant is given by: Kc=[C]c[D]d[A]a[B]bK_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}
  • Dependence on Stoichiometry: The powers in the expression are the stoichiometric coefficients from the balanced equation.

Le Chatelier's Principle

  • Principle: If a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the system will adjust to counteract the effect of the change and a new equilibrium will be established.
  • Concentration: Adding a reactant or product shifts the equilibrium to consume the added substance.
  • Pressure: Increasing pressure shifts the equilibrium towards the side with fewer moles of gas.
  • Temperature: Increasing temperature favors the endothermic direction.

Temperature Dependence of Equilibrium

  • Exothermic Reactions: Increase in temperature decreases KK.
  • Endothermic Reactions: Increase in temperature increases KK.

Heterogeneous Equilibria

  • Definition: Equilibria involving more than one phase.
  • Expression: Pure solids and liquids do not appear in the equilibrium constant expression.

Reaction Quotient (Qc)

  • Definition: A ratio similar to the equilibrium constant but for non-equilibrium conditions.
  • Use: Predicts the direction of the reaction shift to reach equilibrium by comparing QcQ_c with KcK_c.

Gibbs Free Energy and Equilibrium

  • Relationship: ΔG=ΔG∘+RTln⁡Q\Delta G = \Delta G^\circ + RT \ln Q
  • Equilibrium: At equilibrium, ΔG=0\Delta G = 0 and Q=KQ = K, so ΔG∘=−RTln⁡K\Delta G^\circ = -RT \ln K.

Equilibrium in Physical Processes

  • Examples: Solid-liquid, liquid-vapour, solid-vapour, solid-in-liquid, and gas-in-liquid equilibria.
  • Vapour Pressure: At equilibrium, the vapour pressure is constant.

Applications of Equilibrium Constant

  • Extent of Reaction: Large KK indicates products predominate; small KK indicates reactants predominate.
  • Direction: Compare QQ with KK to predict the direction of the reaction.
  • Equilibrium Concentrations: Calculate using initial concentrations and KK.

Acids and Bases: Arrhenius, Bronsted-Lowry and Lewis Concepts

  • Arrhenius: Acids produce H+H^+ in water, bases produce OH−OH^-.
  • Bronsted-Lowry: Acids are proton donors, bases are proton acceptors.
  • Lewis: Acids accept electron pairs, bases donate electron pairs.

Ionisation Constants of Weak Acids and Weak Bases

  • Ka and Kb: Measure the strength of weak acids and bases.
  • Ostwald's Dilution Law: Relates degree of ionization to concentration.

Ionisation of Water, Kw, pH and pOH

  • Kw: Ionic product of water, Kw=[H+][OH−]Kw = [H^+][OH^-].
  • pH and pOH: pH+pOH=pKwpH + pOH = pKw.

pKa, pKb and Strength of Acids/Bases

  • Relation: pKa=−log⁡KapKa = -\log Ka, pKb=−log⁡KbpKb = -\log Kb.
  • Strength: Lower pKapKa or pKbpKb indicates stronger acid or base.

Buffer Solutions

  • Definition: Solutions that resist changes in pH upon addition of small amounts of acid or base.
  • Components: Typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid.
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Practice Test – MCQs, True/False

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Multiple Choice Questions

A.

The equilibrium will shift to the right, favoring the formation of HIHI.

B.

The equilibrium will shift to the left, favoring the formation of H2H_2 and I2I_2.

C.

There will be no change in the equilibrium position.

D.

The equilibrium constant KcK_c will increase.
Correct Answer: A

Solution:

According to Le Chatelier's principle, increasing the concentration of H2H_2 will shift the equilibrium to the right to counteract the change, thus favoring the formation of HIHI.

Chapter Concept:

Ionisation of Water, Kw, pH and pOH

A.

ΔG=0\Delta G = 0 and K=1K = 1

B.

ΔG<0\Delta G < 0 and K>1K > 1

C.

ΔG>0\Delta G > 0 and K<1K < 1

D.

ΔG=0\Delta G = 0 and K≠1K \neq 1
Correct Answer: A

Solution:

At equilibrium, the Gibbs free energy change ΔG=0\Delta G = 0, and the equilibrium constant KK is 1, indicating no net change in the system.

Chapter Concept:

Gibbs Free Energy and Equilibrium

A.

Acids are substances that increase the concentration of H+H^+ ions in aqueous solution.

B.

Bases are substances that increase the concentration of OH−OH^- ions in aqueous solution.

C.

Acids are substances that donate protons.

D.

Bases are substances that accept electron pairs.
Correct Answer: A

Solution:

According to the Arrhenius concept, acids increase the concentration of H+H^+ ions, while bases increase the concentration of OH−OH^- ions in aqueous solutions.

Chapter Concept:

Acids and Bases: Arrhenius, Bronsted-Lowry and Lewis Concepts

A.

The reaction is endothermic.

B.

The reaction is exothermic.

C.

The reaction is unaffected by temperature.

D.

The reaction has reached dynamic equilibrium.
Correct Answer: B

Solution:

The equilibrium constant KcK_c decreases with an increase in temperature for exothermic reactions. This indicates that the reaction releases heat.

Chapter Concept:

Applications of Equilibrium Constant

A.

The equilibrium will shift to the right, favoring the formation of NH3NH_3.

B.

The equilibrium will shift to the left, favoring the decomposition of NH3NH_3.

C.

The equilibrium will remain unchanged.

D.

The reaction will stop completely.
Correct Answer: B

Solution:

Increasing the temperature of an exothermic reaction shifts the equilibrium to the left, favoring the decomposition of NH3NH_3 to N2N_2 and H2H_2.

Chapter Concept:

Reaction Quotient (Qc)

A.

KcK_c will increase.

B.

KcK_c will decrease.

C.

KcK_c will remain the same.

D.

The effect on KcK_c cannot be determined.
Correct Answer: A

Solution:

For an exothermic reaction, decreasing the temperature increases the equilibrium constant KcK_c, as the reaction shifts to the right, favoring the formation of N2O4N_2O_4.

Chapter Concept:

Reaction Quotient (Qc)

A.

KcK_c increases with an increase in temperature.

B.

KcK_c decreases with an increase in temperature.

C.

KcK_c remains constant regardless of temperature changes.

D.

The effect of temperature on KcK_c cannot be determined.
Correct Answer: B

Solution:

For exothermic reactions like the synthesis of ammonia, the equilibrium constant KcK_c decreases as the temperature increases.

Chapter Concept:

Equilibrium Constant Expressions

A.

The equilibrium will shift to the right, favoring the formation of N2O4N_2O_4.

B.

The equilibrium will shift to the left, favoring the formation of NO2NO_2.

C.

The equilibrium will remain unchanged.

D.

The equilibrium constant will decrease.
Correct Answer: B

Solution:

Increasing the temperature favors the endothermic direction. Since the decomposition of N2O4N_2O_4 into NO2NO_2 is endothermic, the equilibrium shifts to the left.

Chapter Concept:

Reaction Quotient (Qc)

A.

1.0×10−41.0 \times 10^{-4}

B.

1.0×10−31.0 \times 10^{-3}

C.

1.0×10−51.0 \times 10^{-5}

D.

1.0×10−61.0 \times 10^{-6}
Correct Answer: A

Solution:

The ionization constant KaK_a is given by Ka=cα2K_a = c \alpha^2, where cc is the initial concentration and α\alpha is the degree of ionization. Substituting the given values: Ka=0.1×(0.01)2=1.0×10−4K_a = 0.1 \times (0.01)^2 = 1.0 \times 10^{-4}.

Chapter Concept:

Ionisation Constants of Weak Acids and Weak Bases

A.

The equilibrium will shift to the right, producing more CO2CO_2.

B.

The equilibrium will shift to the left, consuming CO2CO_2.

C.

The equilibrium will not change.

D.

The equilibrium constant will decrease.
Correct Answer: B

Solution:

Increasing the pressure by decreasing the volume will shift the equilibrium to the left, consuming CO2CO_2, as the system tries to decrease the number of moles of gas to counteract the pressure increase.

Chapter Concept:

Equilibrium in Physical Processes

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True or False

Correct Answer: True

Solution:

The reaction quotient QcQ_c is calculated similarly to the equilibrium constant KcK_c, but with concentrations that are not necessarily at equilibrium. By comparing QcQ_c with KcK_c, we can predict whether the reaction will proceed in the forward or reverse direction to reach equilibrium.

Chapter Concept :

Reaction Quotient (Qc)

Correct Answer: True

Solution:

The equilibrium constant KcK_c is a characteristic of the reaction at a given temperature and does not depend on the initial concentrations of reactants and products.

Chapter Concept :

Applications of Equilibrium Constant

Correct Answer: True

Solution:

The equilibrium constant KcK_c is a characteristic value for a given reaction at a specific temperature and does not depend on the initial concentrations of reactants and products.

Chapter Concept :

Equilibrium Constant Expressions

Correct Answer: True

Solution:

In heterogeneous equilibria, the concentrations of pure solids and liquids are constant and do not appear in the equilibrium constant expression.

Chapter Concept :

Heterogeneous Equilibria

Correct Answer: True

Solution:

Le Chatelier's Principle states that if a system at equilibrium is disturbed by a change in concentration, temperature, or pressure, the system will shift its equilibrium position to counteract the effect of the disturbance. Thus, increasing the concentration of a reactant will shift the equilibrium to the right, consuming the added reactant.

Chapter Concept :

Le Chatelier's Principle

Correct Answer: False

Solution:

In heterogeneous equilibria, the concentrations of pure solids and liquids do not appear in the equilibrium constant expression because their concentrations are constant.

Chapter Concept :

Equilibrium Constant Expressions

Correct Answer: True

Solution:

According to the principle of temperature dependence of equilibrium, the equilibrium constant for an exothermic reaction decreases with an increase in temperature.

Chapter Concept :

Temperature Dependence of Equilibrium

Correct Answer: True

Solution:

Dynamic equilibrium is characterized by equal rates of forward and reverse reactions, leading to constant concentrations of reactants and products.

Chapter Concept :

Dynamic Chemical Equilibrium

Correct Answer: True

Solution:

According to the principles of chemical equilibrium, for an exothermic reaction (where ΔH\Delta H is negative), the equilibrium constant KK decreases as the temperature increases. This is because increasing the temperature shifts the equilibrium towards the reactants to absorb the excess heat, according to Le Chatelier's principle.

Chapter Concept :

Temperature Dependence of Equilibrium

Correct Answer: True

Solution:

The equilibrium constant KcK_c is a characteristic of the reaction at a given temperature and does not depend on the initial concentrations of reactants and products.

Chapter Concept :

Equilibrium in Physical Processes

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