Acid-Base Chemistry

This chapter explains the nature of acids, alkalis, and bases based on the ions they produce in aqueous solution. It explores the concepts of pH, acid strength, neutralisation, properties of acids and bases, oxide classification, salt preparation, and ammonia production through the Haber Process.

1. Acids and Bases
  • 🔹 Ion Definitions:

    🔹 Acids produce hydrogen ions (H⁺), alkalis produce hydroxide ions (OH⁻) in aqueous solution.

  • 🔹 pH Scale and Indicators:

    🔹 pH measures the concentration of H⁺ ions. Universal Indicator gives a colour range for different pH levels (0–14).

  • 🔹 Strong vs Weak Acids:

    🔹 Strong acids ionise completely in water (e.g., HCl); weak acids only partially ionise (e.g., CH₃COOH).

  • 🔹 Reactions of Acids:

    🔹 Acids react with metals to give salt + hydrogen gas, with bases to give salt + water, and with carbonates to give salt + water + CO₂.

  • 🔹 Neutralisation:

    🔹 H⁺(aq) + OH⁻(aq) → H₂O(l); this is the reaction between acids and alkalis.

  • 🔹 pH Control in Soils:

    🔹 Calcium hydroxide is added to acidic soils to neutralise excess acidity and improve plant growth.

  • 🔹 Reactions of Bases:

    🔹 Bases neutralise acids and form salts; ammonium salts decompose with alkali to release ammonia gas.

  • 🔹 Oxide Classification:
    • 🔹 Acidic Oxides:

      🔹 Non-metal oxides that react with bases.

    • 🔹 Basic Oxides:

      🔹 Metal oxides that react with acids.

    • 🔹 Amphoteric Oxides:

      🔹 React with both acids and alkalis (e.g., Al₂O₃, ZnO).

    • 🔹 Neutral Oxides:

      🔹 Do not react with either acids or alkalis (e.g., CO, NO).

2. Salts
  • 🔹 Salt Preparation Methods:
    • 🔹 Titration: For soluble salts from acid and alkali.
    • 🔹 Precipitation: For insoluble salts by mixing two salt solutions.
    • 🔹 Excess Solid Method: Add excess metal, oxide, or carbonate to acid, filter, then crystallise the salt.
  • 🔹 Solubility Rules:
    • 🔹 Soluble Salts:

      🔹 All sodium, potassium, ammonium salts; all nitrates; most chlorides and sulfates.

    • 🔹 Insoluble Salts:

      🔹 Silver chloride, lead(II) sulfate, barium sulfate, most carbonates (except Na⁺, K⁺, NH₄⁺).

  • 🔹 Purification Techniques:

    🔹 Filtration, evaporation to crystallisation, drying of purified crystals.

3. Ammonia
  • 🔹 Haber Process:
    • 🔹 Nitrogen and hydrogen react reversibly to form ammonia: N₂ + 3H₂ ⇌ 2NH₃.
    • 🔹 Nitrogen is obtained through fractional distillation of air.
    • 🔹 Hydrogen is obtained through cracking of crude oil.
  • 🔹 Industrial Conditions:

    🔹 450°C, 200 atm pressure, iron catalyst used to optimise yield and rate.

  • 🔹 Reversibility:

    🔹 Forward reaction is exothermic; lower temperature favours yield but slows rate. High pressure favours product side.

  • 🔹 Importance of Ammonia:

    🔹 Used in making fertilisers and nitric acid.

  • 🔹 Data Interpretation:

    🔹 Graphs and tables are used to evaluate optimal temperature, pressure, and catalyst conditions.

  • 🔹 Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
  • 🔹 Production of ammonium sulfate: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄
  • 🔹 Haber Process: N₂ + 3H₂ ⇌ 2NH₃

  • ⚠️ Strong acid means high concentration – it actually refers to full ionisation.
  • ⚠️ All alkalis are bases – only soluble bases are alkalis.
  • ⚠️ All metal oxides are basic – some (e.g., Al₂O₃) are amphoteric.
  • ⚠️ All salts are soluble – some (e.g., AgCl, BaSO₄) are insoluble.

  • 👉 State clearly the definitions of acids, bases, and alkalis based on ion formation.
  • 👉 Use pH values and Universal Indicator colours to determine acidity/alkalinity.
  • 👉 Memorise solubility rules to predict outcomes in salt formation.
  • 👉 Select the correct method (titration, precipitation, excess solid) for salt preparation.
  • 👉 Understand how conditions affect the Haber Process equilibrium and yield.

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