8.4 Types of Equilibrium

This topic explains the two types of chemical equilibrium based on the physical states of reactants and products: homogeneous equilibria and heterogeneous equilibria.

Definition

Homogeneous Equilibria: An equilibrium system in which all of the reactants and products are in the same phase.

Heterogeneous Equilibria: Equilibria which involve more than one phase.

Main Idea

• Homogeneous Equilibria — all reactants and products are in the same phase.
• Heterogeneous Equilibria — reactants and products are in more than one phase.

If pure solids or pure liquids are involved in an equilibrium system, their concentrations are not included in the equilibrium constant expression. This is because the change in concentration of any pure solid or liquid has no effect on the equilibrium system.

Core Concept

• Homogeneous: Same phase (all gases or all aqueous).
• Heterogeneous: More than one phase (solid + gas, liquid + gas, etc.).
• Pure solids and pure liquids: Not included in equilibrium constant expressions.

Detailed Explanation

Homogeneous Equilibria

An equilibrium system in which all of the reactants and products are in the same phase.

Examples:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g) (i)

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) (ii)

2NO_(g) + Cl_2(g) ⇌ 2NOCl_(g) (iii)

CH₃COOH_(g) + C₂H₅OH_(g) ⇌ CH₃COOC₂H₅(g) + H₂O_(g) (iv)

Heterogeneous Equilibria

Equilibria which involve more than one phases are called heterogeneous equilibria.

Examples:

CaCO_3(s) ⇌ CaO_(s) + CO_2(g)

C_(s) + H₂O_(g) ⇌ CO_(g) + H_2(g)

3Fe_(s) + 4H₂O_(g) ⇌ Fe₃O_4(s) + 4H_2(g)

Pure Solids and Pure Liquids

• If pure solids or pure liquids are involved in an equilibrium system, their concentrations are not included in the equilibrium constant expression.
• This is because the change in concentration of any pure solid or liquid has no effect on the equilibrium system.

Examples:

i) 2H₂O_(g) ⇌ 2H_2(g) + O_2(g)

K_c = [H₂]²[O₂]

K_p = P_H2² × P_O2

ii) 3Fe_(s) + 4H₂O_(g) ⇌ Fe₃O_4(s) + 4H_2(g)

K_c = [H₂]⁴[H₂O]⁴

K_p = P_H2⁴P_H2O⁴

Concept Assessment Exercise 8.3

Write K_c and K_p expressions for each of the following reactions:

• FeO_(s) + CO_(g) ⇌ Fe_(s) + CO_2(g)
• P_4(s) + 5O_2(g) ⇌ P₄O_10(s)
• CH_4(g) + 4Cl_2(g) ⇌ CCl_4(l) + 4HCl_(g)

Intermediate Level

Types of Equilibrium

Homogeneous Examples

N₂ + 3H₂ ⇌ 2NH₃ \] \[ 2SO₂ + O₂ ⇌ 2SO₃ \] \[ 2NO + Cl₂ ⇌ 2NOCl \] \[ CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Heterogeneous Examples

CaCO₃ ⇌ CaO + CO₂ \] \[ C + H₂O ⇌ CO + H₂ \] \[ 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂

Pure Solids and Liquids

• Not included in K_c or K_p.
• Their concentration is constant.

Example: 3Fe_(s) + 4H₂O_(g) ⇌ Fe₃O_4(s) + 4H_2(g) K_c = [H₂]⁴[H₂O]⁴

Concept Assessment Exercise 8.3

• FeO_(s) + CO_(g) ⇌ Fe_(s) + CO_2(g)
• P_4(s) + 5O_2(g) ⇌ P₄O_10(s)
• CH_4(g) + 4Cl_2(g) ⇌ CCl_4(l) + 4HCl_(g)

Important Points

• 1. Two types: Homogeneous and Heterogeneous.
• 2. Homogeneous: Same phase.
• 3. Heterogeneous: More than one phase.
• 4. Pure solids and liquids: Not included in K expressions.
• 5. Examples: N₂ + 3H₂ ⇌ 2NH₃ (homogeneous); CaCO₃ ⇌ CaO + CO₂ (heterogeneous).
• 6. Concept Assessment 8.3: Three reactions.

Remember

• - Homogeneous: Same phase
• - Heterogeneous: More than one phase
• - Pure solids/liquids: Not included in K
• - Examples: N₂ + 3H₂ ⇌ 2NH₃ (homogeneous); CaCO₃ ⇌ CaO + CO₂ (heterogeneous)
• - Concept Assessment 8.3: Three reactions

Common Mistakes

• 1. Including pure solids/liquids in K expressions: They are not included.
• 2. Confusing homogeneous and heterogeneous: Same phase vs. more than one phase.
• 3. Forgetting to balance equations: Essential for correct K expressions.
• 4. Mixing up K_c and K_p: K_c uses concentrations; K_p uses partial pressures.

Quick Revision

• Two types: Homogeneous and Heterogeneous.
• Homogeneous: Same phase.
• Heterogeneous: More than one phase.
• Pure solids and liquids: Not included in K expressions.
• Examples: N₂ + 3H₂ ⇌ 2NH₃ (homogeneous); CaCO₃ ⇌ CaO + CO₂ (heterogeneous).
• Concept Assessment 8.3: Three reactions.

60-Second Revision

• Homogeneous: Same phase
• Heterogeneous: More than one phase
• Pure solids/liquids: Not included in K
• Examples: N₂ + 3H₂ ⇌ 2NH₃ (homogeneous); CaCO₃ ⇌ CaO + CO₂ (heterogeneous)
• Concept Assessment 8.3: Three reactions

QUALITY VALIDATION

• Topic: 8.4 Types of Equilibrium
• Chapter: 8 (Chemical Equilibrium)

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• Source scope followed: PASS
• Knowledge level matched: PASS
• Important concepts covered: PASS
• Definitions checked: PASS
• Equations checked: PASS
• Derivations checked: N/A
• Examples checked: PASS
• Unsupported knowledge added: NO
• Unnecessary repetition: NO
• Notes format maintained: PASS
• Important source content omitted: NO

• Source limitations:
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