11.07 Strong and Weak Acids and Alkalis

Key Concepts

1. Acid Strength:
   • Strong Acid: Completely ionizes in water, producing the maximum possible concentration of hydrogen ions (H⁺).
     Examples: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), Nitric acid (HNO₃).
     Dissociation: HCl → H₂OH⁺(aq)+ Cl⁻(aq)
   • Weak Acid: Partially ionizes in water, producing a lower concentration of H⁺ ions.
     Examples: Ethanoic acid (CH₃COOH), Carbonic acid (H2CO3), Citric acid.
     Dissociation (reversible):
     CH₃COOH ↔ H⁺(aq) + CH₃COO⁻(aq)
2. pH and Acidity:
   • pH Scale: Ranges from 1 to 14.
     • Lower pH = Higher H+ ion concentration = Stronger acid.
   • A change of 1 pH unit equals a 10-fold difference in H+ ion concentration.
3. Ionization and Conductivity:
   • Strong acids produce more H+ ions, leading to higher electrical conductivity.
   • Weak acids produce fewer ions, resulting in lower conductivity.
   • Example: Hydrochloric acid conducts electricity better than ethanoic acid of the same concentration.
4. Neutralization:
   • Acids react with alkalis to form salt and water.
     General equation: acid+alkali→salt+water
   • Ionic equation for neutralization:
     H⁺(aq) + OH⁻(aq) → H₂O(l)
   • Example:
     Hydrochloric acid + Sodium hydroxide:
     HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
   • Spectator Ions (e.g., Na⁺, Cl⁻) do not participate in the reaction.
5. Proton Transfer:
   • Acids are proton donors (release H⁺).
   • Bases are proton acceptors (accept H⁺).

Strong Acids vs. Weak Acids

Reactions of Acids

1. Reaction with Bases (Neutralization):
   • General equation: acid+base→salt+water
   • Example:
     Hydrochloric acid + Sodium hydroxide:
     HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
2. Reaction with Metals:
   • General equation: acid+metal→salt+hydrogen gas
   • Example:
     Zinc + Hydrochloric acid:
     Zn(s) + 2HCl(aq)→ZnCl₂(aq) + H₂(g)
3. Reaction with Carbonates:
   • General equation: acid+carbonate→salt+water+carbon dioxide
   • Example:
     Calcium carbonate + Hydrochloric acid:
     CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) +H₂O(l) + CO₂(g)

Neutralization Reaction and Ionic Equation

• Hydrogen ions (H⁺) from the acid react with hydroxide ions (OH−) from the alkali to form water:
  H⁺(aq) + OH⁻(aq) → H₂O(l)
• Example: Hydrochloric acid and sodium hydroxide:
  HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Proton Exchange in Acids and Bases

1. Acids:
   • Proton donors: Release H+ ions.
   • Example: Hydrochloric acid: HCl → H⁺+ Cl⁻
2. Bases:
   • Proton acceptors: Accept H⁺ ions.
   • Example: Sodium hydroxide: NaOH → Na⁺+ OH⁻

Key Equations and Reactions

1. Hydrogen Chloride Dissolving in Water:
   HCl(g) → H₂OH⁺(aq) + Cl⁻(aq)
2. Ethanoic Acid Dissociation (Reversible):
   CH₃COOH ↔ H⁺(aq) + CH₃COO⁻(aq)
3. Neutralization (General Ionic Equation):
   H⁺(aq) + OH⁻(aq)→H₂O(l)

Questions

1. Define acid and base using proton transfer.
   • Acid: Proton donor.
   • Base: Proton acceptor.
2. Why does ethanoic acid have lower conductivity than hydrochloric acid of the same concentration?
   • Ethanoic acid is a weak acid, partially ionized, producing fewer ions in solution.
3. Write the equation for the reaction between zinc and hydrochloric acid.
   Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
   This is a redox reaction where:
   • Zinc is oxidized (Zn→Zn²⁺ + 2e⁻ ).
   • Hydrogen is reduced (2H⁺+ 2e⁻→ H₂ ​).