24-25-UNIT 1.7-SIMPLE EQUILIBRIA AND ACID-BASE REACTIONS

Simple Equilibria and Acid-Base Reactions

Overview

  • Equilibrium in chemistry is crucial for understanding industrial processes (e.g., Haber process, contact process).

  • Acid-base reactions are important for hands-on practical work involving chemical calculations.

  • Assumed prior knowledge includes reactions of dilute acids with bases and carbonates.

Mathematical Skills

  • Estimate changes in equilibrium constant (Kc) with temperature changes.

  • Perform calculations related to acid-base titrations and equilibrium constants.

  • Use significant figures and identify uncertainties in titration results.

  • Apply logarithmic functions for pH calculations.

How Science Works

  • Engage in scientific questioning and problem definition.

  • Utilize various methodologies, including ICT, to resolve scientific queries.

  • Conduct experiments with appropriate risk management.

Key Concepts

  • Reversible Reactions: Can proceed in both forward and backward directions.

  • Dynamic Equilibrium: Rates of forward and backward reactions are equal; concentrations are constant, but not necessarily equal.

  • Le Chatelier’s Principle: If equilibrium is disrupted, the system will adjust to counteract the change.

  • Acids and Bases:

    • Acid: Proton (H⁺) donor.

    • Base: Proton (H⁺) acceptor.

    • Strong acid: Fully dissociates in water.

    • Weak acid: Partially dissociates in water.

Chemical Equilibrium

Characteristics

  1. Equilibrium can be approached from either direction.

  2. Equilibrium is dynamic; the reaction continues to occur.

  3. Concentrations remain constant at equilibrium.

  4. Only attainable in a closed system.

Factors affecting Equilibrium

  • Concentration: Changes shift equilibrium to minimize concentration changes.

  • Pressure: Affects equilibrium based on the number of gas molecules on each side of the equation.

  • Temperature: Shifts to absorb or release heat, favoring endothermic or exothermic reactions.

Example Reaction

  • Reaction between chromate (VI) and dichromate (VI):

    • Forward: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O (yellow to orange)

    • Reverse: Adding alkali turns it yellow again.

Acid-Base Reactions

  • Neutralization Reaction: Forms water and a salt from an acid and a base.

  • Common strong acids include HCl, H₂SO₄, and HNO₃.

  • Common weak acids include CH₃COOH (ethanoic acid).

pH Scale

  • pH = -log[H⁺(aq)]; measures acidity/basicity.

  • Strong acids fully dissociate; weak acids do not.

  • pH can be measured by pH meters or indicators like phenolphthalein and methyl orange.

Titration

  • Procedure: Measure precise volumes of acid and base to determine concentration.

    1. Setup includes burette, pipette, conical flask, and indicator.

    2. Record the volume of titrant used to reach endpoint (color change).

    3. Results should be concordant (within 0.2 cm³).

  • Indicators:

    • Phenolphthalein: Colorless in acid; pink in basic solutions (pH 8.2 to 10).

    • Methyl orange: Red in acid; yellow in alkaline solutions (pH 3.1 to 4.4).

Practical Applications of Equilibrium

  • Back Titration: Used for reactions that are slow or difficult to measure, where known excess of a reactant is added and the remainder is determined back

  • Double Titration: Two indicators are used to quantify two different substances in a mixture.

Equilibrium Constant (Kc)

  • Expresses the position of equilibrium in terms of concentrations.

  • Kc = [C]ⁱ[D]ⁱ / [A]ⁱ[B]ⁱ, where [C], [D], [A], and [B] are concentrations of reactants and products.

  • A large Kc indicates a right-shifted equilibrium (more products), a small Kc indicates a left-shifted equilibrium (more reactants).

Summary

  • Understand acid-base reactions, equilibrium dynamics, titration procedures, and the associated calculations to excel in chemistry topics related to equilibria and acid-base reactions.

Questions to Practice

  1. What happens to equilibrium position if temperature, pressure, or concentration is altered?

  2. Describe the theory behind strong vs. weak acids in terms of proton donation.

  3. How to calculate pH and concentrations from standardized solutions?

Practical Titration Questions

  1. Analyze the concentration of lithium hydroxide from titration data with sulfuric acid.

  2. Calculate the mass needed for particular molar concentrations.

  3. Describe techniques employed in preparing a standard solution and performing titrations effectively.