8.5 Ways to Recognize Equilibrium

This topic explains how to recognize when a chemical system has reached equilibrium. It describes physical and chemical methods for detecting equilibrium.

Definition

Equilibrium: The state of a reversible reaction at which the composition of the reaction mixture does not change.

Physical Method: A method based on measuring the physical properties of the reaction mixture without removing the sample.

Chemical Method: A method in which the amount of a reactant or product is determined by a suitable chemical reaction.

Main Idea

Equilibrium constant expression can be determined by physical as well as chemical methods. The physical method is based on measuring the physical properties of the reaction mixture during the reaction without removing the sample. The chemical method involves determining the amount of a reactant or product by a suitable chemical reaction.

Core Concept

• Physical method: Measures physical properties (e.g., absorbance, colour) during the reaction.
• Chemical method: Withdraws samples and analyzes them chemically (e.g., titration).
• Equilibrium recognition: When the measured property becomes constant, equilibrium is reached.

Detailed Explanation

a) Physical Method (Spectrometric Method)

• This method is based on measuring the physical properties of the reaction mixture.
• This physical property is measured during the reaction without removing the sample from the reaction mixture.
• Let's discuss the spectrometric method.
• This method can be applied when the reactant or product absorbs ultraviolet, visible, or infrared radiation.
• The concentration can be determined by measuring the amount of radiation absorbed.

Example: N₂O_4(g) ⇌ 2NO_2(g) (Colourless) (Reddish brown)

• N₂O₄ is a colourless gas whereas NO₂ is a reddish brown gas.
• The progress of the reaction can be studied by measuring the absorbance at regular intervals.
• Absorbance is proportional to the concentration of NO₂.
• At equilibrium, the spectrometer will show a constant value of absorbance.

b) Chemical Method

• In this method, the amount of a reactant or product is determined by a suitable chemical reaction.
• Consider the reaction between acetic acid and ethanol to form ethyl acetate and water. It is an example of a reversible reaction in solution.

CH₃COOH_(l) + C₂H₅OH_(l) ⇌ CH₃COOC₂H₅(l) + H₂O_(l)

• Suppose this reaction is started by taking 'a' moles of acetic acid and 'b' moles of ethanol in a stoppered flask at room temperature.
• A small amount of mineral acid is added to the mixture to catalyze the reaction.
• The progress of the reaction can be studied by determining the concentration of acetic acid after regular intervals.
• For this purpose, a small portion of the mixture is withdrawn.
• The concentration of acetic acid is determined by titrating it against a standard solution of NaOH using phenolphthalein as an indicator.
• The concentration of acetic acid will decrease until equilibrium is attained.
• At equilibrium, the concentration of acetic acid will become constant, indicating that equilibrium has been reached.

Intermediate Level

Ways to Recognize Equilibrium

Physical Method

• Based on measuring physical properties.
• No sample removal.
• Spectrometric method: Measures absorbance.
• At equilibrium, absorbance is constant.

• N₂O₄: colourless
• NO₂: reddish brown
• Absorbance ∝ [NO₂]
• Constant absorbance → equilibrium

Chemical Method

• Sample is withdrawn and analyzed.
• Titration used to determine concentration.
• Concentration becomes constant at equilibrium.

Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O - Titrate acetic acid against NaOH. - Constant concentration → equilibrium.

Important Points

• 1. Two methods: Physical and Chemical.
• 2. Physical method: Measures physical properties.
• 3. Spectrometric method: Measures absorbance.
• 4. Chemical method: Titration.
• 5. Example: N₂O₄ ⇌ 2NO₂ (physical).
• 6. Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (chemical).
• 7. Constant property → equilibrium.

Remember

• - Physical method: Measure physical property
• - Chemical method: Titration
• - Example: N₂O₄ ⇌ 2NO₂ (physical)
• - Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (chemical)
• - Constant property → equilibrium

Common Mistakes

• 1. Thinking equilibrium means reaction stops: It continues at the molecular level.
• 2. Confusing physical and chemical methods: Physical = measure property; Chemical = titration.
• 3. Forgetting that absorbance is proportional to concentration: It is.
• 4. Not waiting for constant value: Equilibrium is reached when property becomes constant.

Quick Revision

• Two methods: Physical and Chemical.
• Physical method: Measures physical properties.
• Spectrometric method: Measures absorbance.
• Chemical method: Titration.
• Example: N₂O₄ ⇌ 2NO₂ (physical).
• Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (chemical).
• Constant property → equilibrium.

60-Second Revision

• Physical method: Measure physical property
• Chemical method: Titration
• Example: N₂O₄ ⇌ 2NO₂ (physical)
• Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (chemical)
• Constant property → equilibrium

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• Topic: 8.5 Ways to Recognize Equilibrium
• Chapter: 8 (Chemical Equilibrium)

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