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: $Zn(s) + 2HCl(aq) <br>ightarrow ZnCl₂(aq) + H₂(g)$

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

  • Observations:

  • Colourless gas bubbles evolve.

  • Solution becomes warm.

• Example 2: Lead + Dilute sulphuric acid

  • Chemical equation: $Pb(s) + H₂SO₄(aq) <br>ightarrow PbSO₄(s) + H₂(g)$

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

1.4.2. Neutralization Reactions

• General equation:
  $ext{Acid} + ext{Base} <br>ightarrow ext{Salt} + ext{Water}$

• Example: Sodium hydroxide + Dilute hydrochloric acid:

  • Chemical equation: $HCl(aq) + NaOH(aq) <br>ightarrow NaCl(aq) + H₂O(l)$

  • Ionic equation: $H^+(aq) + OH^−(aq) <br>ightarrow H₂O(l)$