9.2 Conditions for Equilibrium

This topic explains the conditions that must be met for a chemical equilibrium to be established and maintained.

Definition

Equilibrium is achieved when pure reactants, pure products, or a mixture of reactants and products are first placed in a closed container. In each such case, forward and backward movement in the tank occurs at the same speed. This leads to a situation where the concentrations of reactants and products remain the same indefinitely, as long as the following physical conditions are met.

Simple explanation: Equilibrium is reached when the forward and reverse reactions happen at the same rate, and the amounts of reactants and products stay constant.

Main Idea

For equilibrium to be established and maintained, the following three physical conditions must be met:

• The concentration of reactant or product remains unchanged.
• The temperature of the system remains constant.
• The pressure or volume of the system remains constant.

Simple explanation: For equilibrium to exist, the amounts, temperature, and pressure/volume must all stay constant.

Core Concept

The Three Conditions for Equilibrium

Simple explanation: If any of these conditions change, the equilibrium will shift.

Why a Closed System Is Needed

• No reactants or products can escape.
• No new substances can enter.
• The system can reach a stable balance.

Simple explanation: In an open system, substances can escape, so equilibrium can't be maintained.

Detailed Explanation

Condition 1: Constant Concentration

The concentration of reactant or product remains unchanged.

• The rate of the forward reaction equals the rate of the reverse reaction.
• So the concentrations of reactants and products stop changing.
• They remain constant over time.

Important: "Constant" does not mean "equal." The concentrations of reactants and products are constant, but they are not necessarily equal to each other.

Simple explanation: At equilibrium, the amounts of reactants and products stay the same — but they don't have to be equal.

Condition 2: Constant Temperature

The temperature of the system remains constant.

• Equilibrium is temperature-dependent.
• If the temperature changes, the equilibrium shifts to a new position.
• This is because temperature affects the rates of forward and reverse reactions differently.

Example: In the cobalt chloride equilibrium:

CoCl₂ · 6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O

• Heating shifts the equilibrium to the right (blue).
• Cooling shifts the equilibrium to the left (pink).

Simple explanation: Changing the temperature changes where the equilibrium sits.

Condition 3: Constant Pressure or Volume

The pressure or volume of the system remains constant.

• Pressure affects equilibria involving gases.
• If the pressure changes, the equilibrium shifts to counteract the change.
• Similarly, changing the volume changes the pressure (for gases).

Example: In the Haber process:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• Increasing pressure shifts the equilibrium to the right (toward fewer gas molecules).
• Decreasing pressure shifts the equilibrium to the left (toward more gas molecules).

Simple explanation: Changing pressure or volume shifts the equilibrium.

Summary of Conditions

Intermediate Level

Why These Conditions Matter

• At equilibrium, forward and reverse rates are equal.
• Concentrations remain constant because the rates are equal.
• If concentrations change, the rates change, and equilibrium shifts.

• Temperature affects reaction rates.
• Forward and reverse reactions may have different temperature dependencies.
• Changing temperature shifts the equilibrium position.
• Endothermic reactions are favoured by higher temperatures.
• Exothermic reactions are favoured by lower temperatures.

• Pressure affects equilibria involving gases.
• Increasing pressure shifts equilibrium toward the side with fewer gas molecules.
• Decreasing pressure shifts equilibrium toward the side with more gas molecules.
• Volume changes have the opposite effect (volume and pressure are inversely related).

Simple explanation: Changing any of these conditions disturbs the balance and shifts the equilibrium.

Closed System Requirement

• In an open system, reactants or products can escape.
• This prevents the system from reaching a stable balance.
• A closed system allows the forward and reverse reactions to reach equal rates.

Example: Cobalt Chloride Equilibrium

The book describes an activity with cobalt chloride:

Hydrated cobalt(II) chloride is a pink solid. When heated, it loses water and becomes anhydrous cobalt(II) chloride, a blue solid.

CoCl₂ · 6H₂O (pink solid) ⇌ CoCl₂ (blue solid) + 6H₂O

• Heating: equilibrium shifts right (blue).
• Adding water: equilibrium shifts left (pink).

Simple explanation: Temperature and water concentration affect this equilibrium.

Example: Copper(II) Sulphate Equilibrium

The book also describes:

Hydrated copper(II) sulphate is blue. When heated, it loses water and becomes anhydrous copper(II) sulphate, a white solid.

CuSO₄ · 5H₂O (blue solid) ⇌ CuSO₄ (white solid) + 5H₂O

• Heating: equilibrium shifts right (white).
• Adding water: equilibrium shifts left (blue).

Simple explanation: This is a reversible reaction affected by temperature and water.

Important Points

• - Three conditions for equilibrium:
• 1. Concentration of reactants and products remains unchanged.
• 2. Temperature of the system remains constant.
• 3. Pressure or volume of the system remains constant.
• - Equilibrium requires a closed container.
• - If any of these conditions change, the equilibrium shifts.
• - Temperature affects equilibrium by favouring either the forward or reverse reaction.
• - Pressure affects equilibria involving gases.
• - Concentration changes shift the equilibrium.
• - Examples:
• - Cobalt chloride equilibrium (pink ⇌ blue)
• - Copper(II) sulphate equilibrium (blue ⇌ white)
• - Haber process (N₂ + 3H₂ ⇌ 2NH₃)

Remember

• - Three conditions: constant concentration, constant temperature, constant pressure/volume.
• - Closed system is required.
• - If any condition changes, equilibrium shifts.
• - Temperature affects equilibrium position.
• - Pressure affects gaseous equilibria.
• - Concentration changes disturb equilibrium.
• - Examples:
• - CoCl₂·6H₂O ⇌ CoCl₂ + 6H₂O (pink ⇌ blue)
• - CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O (blue ⇌ white)

Common Mistakes

• 1. Thinking equilibrium can exist in an open system. It requires a closed system.
• 2. Thinking concentrations of reactants and products are equal at equilibrium. They are constant, not necessarily equal.
• 3. Thinking temperature doesn't affect equilibrium. It does — it shifts the equilibrium position.
• 4. Thinking pressure affects all equilibria. It only affects equilibria involving gases.
• 5. Confusing "constant" with "equal." Concentrations are constant but not necessarily equal.
• 6. Forgetting that changing conditions shifts equilibrium. Any change in concentration, temperature, or pressure disturbs the equilibrium.

Quick Revision

• Conditions for equilibrium:
• Equilibrium is achieved when pure reactants, pure products, or a mixture of reactants and products are placed in a closed container.
• Forward and backward reactions occur at the same speed.
• Concentrations of reactants and products remain the same indefinitely as long as the conditions are met.
• Closed system is essential for equilibrium.
• Changing conditions (concentration, temperature, pressure) shifts the equilibrium.
• Examples:
• Cobalt chloride: CoCl₂·6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O
• Copper(II) sulphate: CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O

60-Second Revision

• Three conditions: concentration constant, temperature constant, pressure/volume constant.
• Closed system required.
• Change in any condition → equilibrium shifts.
• Examples: CoCl₂ (pink ⇌ blue); CuSO₄ (blue ⇌ white).
• Warning: at equilibrium, concentrations are constant, not necessarily equal.