Acids and Alkalis Study Notes

CHAPTER 14: INTRODUCTION TO ACIDS AND ALKALIS

A. Notes

Focuses of this chapter:

  • Reactions of typical dilute acids (with metals / bases / carbonates / hydrogencarbonates)

  • Ionization of acids and alkalis in water

  • Basicity of acids

  • Bases and alkalis

  • Reactions of alkalis (with acids / non-metal oxides / ammonium compounds / metal ions)

  • Corrosive nature of concentrated acids and alkalis

1. ACIDS

1.1. Examples of Acids

  • Common laboratory acids include:

    • Hydrochloric acid: HCl(aq)

    • Sulphuric acid: H₂SO₄(aq)

    • Nitric acid: HNO₃(aq)

  • Organic and Inorganic Acids:

    • Organic Acids: Contain carbon and are produced by living organisms.

    • Examples:

      • Ethanoic acid: CH₃COOH

      • Oxalic acid: (COOH)₂

      • Citric acid: C₆H₈O₇

    • Inorganic Acids: Known as mineral acids.

    • Examples:

      • Hydrochloric acid: HCl

      • Sulphuric acid: H₂SO₄

      • Nitric acid: HNO₃

      • Phosphoric acid: H₃PO₄

      • Carbonic acid: H₂CO₃

1.2. Properties of Acids

  • At room temperature, pure acids can be:

    • Colourless gases (e.g., hydrogen chloride).

    • Colourless liquids (e.g., sulphuric acid, nitric acid, ethanoic acid).

    • White crystalline solids (e.g., citric acid, oxalic acid, tartaric acid).

  • Taste: All acids have a sour taste (NEVER taste in laboratory!).

  • Electrolytes: Aqueous solutions are electrolytes and can conduct electricity.

  • pH: All acids have a pH value < 7.0 (at room temperature) and can change the color of acid-base indicators:

    • Litmus: Red (acidic), Purple (neutral), Blue (alkaline)

    • Phenolphthalein: Colourless (acidic & neutral), Purple (alkaline)

    • Methyl Orange: Red (acidic), Yellow (neutral & alkaline)

1.3. Ionization of Acids

1.3.1. Ionization of Acids

  • When hydrogen chloride gas dissolves in water:

    • Reaction: HCl(g) → H⁺(aq) + Cl⁻(aq)

    • Process of forming ions is called ionization.

  • Definition: An acid is a hydrogen-containing molecular compound that gives hydrogen ions as the only cations when dissolved in water.

  • General equation of ionization:
    HA(aq)\rightarrow H^{+}(aq)+A^{-}(aq)
    (where A⁻(aq) is anion specific to the acid).

Reversible reactions

  • ethanoic acid ionization:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

  • The double arrow (⇌) indicates that the reaction is reversible.

  • Forward and backward reactions occur simultaneously.

  • Other examples of reversible reactions:

    • Production of ammonia (Haber Process): \displaylines{N_2(g)+3H_2(g)\rightleftharpoons2NH_3(g)}

    • Reaction between ammonia and water: \displaylines{NH_3(aq)+H_2O(l)\rightleftharpoons NH_4^{+}(aq)+OH^{-}(aq)}

  • Irreversible Reactions

    • Reactions that only proceed in one direction are referred to as irreversible reactions.

    • Single arrows (\rightarrow) are used in their chemical equations

1.3.2. Basicity

  • Definition: The basicity of an acid is the maximum number of hydrogen ions that can be produced by a molecule of the acid when ionized in water.

  • Example of an acid’s basicity:

    • Hydrochloric Acid (HCl): 1 (Monobasic)

    • Sulphuric Acid (H₂SO₄): 2 (Dibasic)

    • Phosphoric Acid (H₃PO₄): 3 (Tribasic)

  • Basicity may not equal total number of hydrogen atoms in the molecule (e.g., Ethanoic Acid: CH₃COOH).

1.4. Acid Reactions

1.4.1. With Metals

  • General equation:
    metal + salt

  • Predicted reactivity: Dilute hydrochloric and sulphuric acids react with metals above copper in the reactivity series.

  • Example 1: Zinc + Dilute hydrochloric acid

    • Chemical equation: Zn(s) + 2HCl(aq)
      ightarrow ZnCl₂(aq) + H₂(g)

    • Ionic equation: Zn(s) + 2H^+(aq)
      ightarrow Zn^{2+}(aq) + H₂(g)

    • Observations:

    • Colourless gas bubbles evolve.

    • Solution becomes warm.

  • Example 2: Lead + Dilute sulphuric acid

    • Chemical equation: Pb(s) + H₂SO₄(aq)
      ightarrow PbSO₄(s) + H₂(g)

    • Observation: Colourless gas bubbles evolved, but reaction stops when lead(II) sulphate forms, blocking further contact.

1.4.2. Neutralization Reactions

  • General equation:
    ext{Acid} + ext{Base}
    ightarrow ext{Salt} + ext{Water}

  • Example: Sodium hydroxide + Dilute hydrochloric acid:

    • Chemical equation: HCl(aq) + NaOH(aq)
      ightarrow NaCl(aq) + H₂O(l)

    • Ionic equation: H^+(aq) + OH^−(aq)
      ightarrow H₂O(l)