11.06 Characteristic Reactions of Acids

Major Reactions of Acids

Acids participate in three characteristic reactions, typically observed with dilute acid solutions:

1. Reaction with Reactive Metals:
   • Metals more reactive than hydrogen displace hydrogen gas from acids.
   • Products: Salt and Hydrogen gas.
   • General equation: metal+acid→salt+hydrogen gas
   • Examples:
     • Magnesium + Nitric acid:
       Mg(s) + 2HNO₃(aq)→Mg(NO₃)₂(aq)+H₂(g)
     • Zinc + Hydrochloric acid:
       Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
   • Note: Highly reactive metals like sodium or calcium react violently and should not be used, while less reactive metals like copper do not react.
2. Reaction with Bases and Alkalis (Neutralisation):
   • Acid reacts with a base/alkali to produce Salt and Water.
   • General equation: acid+base→salt+water
   • Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
   • Examples:
     • Hydrochloric acid + Sodium hydroxide: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
     • Sulfuric acid + Copper oxide:
       H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)
3. Reaction with Carbonates and Hydrogen Carbonates:
   • Acids react with carbonates to produce Salt, Water, and Carbon Dioxide gas.
   • General equation: acid+carbonate→salt+water+carbon dioxide gas
   • Examples:
     • Hydrochloric acid + Calcium carbonate: 2HCl(aq)+CaCO₃(s)→CaCl₂(aq)+H₂O(l)+CO₂(g)
     • Zinc carbonate + Hydrochloric acid: ZnCO₃(s)+2HCl(aq)→ZnCl₂(aq)+H₂O(l)+CO₂(g)

Formation of Salts

• Definition of a Salt:
  • A compound formed when the hydrogen in an acid is replaced by a metal or ammonium ion.
  • The parent acid determines the type of salt:
    • Hydrochloric acid → Chlorides (e.g., NaCl, ZnCl₂).
    • Nitric acid → Nitrates (e.g., KNO₃, Mg(NO₃)₂).
    • Sulfuric acid → Sulfates (e.g., CuSO₄, Na₂SO₄).
• Examples of salts formed with different acids and bases:

Experimental Applications

1. Testing for Carbonates:
   • Add dilute hydrochloric acid to the sample.
   • If bubbles of carbon dioxide are observed, the sample contains a carbonate.
   • Confirm CO₂ gas by passing it through limewater:
     • Limewater turns milky, confirming the presence of CO₂.
2. Preparation of Carbon Dioxide in the Laboratory:
   • Reaction: Marble chips (calcium carbonate) with dilute hydrochloric acid:
     2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Practical Example: Antacid Tablet Experiment

• Purpose:
  • To compare the effectiveness of antacid tablets in neutralising excess stomach acid.
• Materials:
  • Antacid tablets, dilute hydrochloric acid (0.5 mol/dm³), methyl orange indicator, burette, balance, mortar and pestle.
• Method:
  1. Grind the antacid tablet into powder.
  2. Add water and a few drops of methyl orange to the powder in a conical flask.
  3. Perform a titration by adding hydrochloric acid from a burette until the indicator turns red.
  4. Record the volume of acid required for neutralisation.
• Reaction:
  • Active ingredient (e.g., calcium carbonate) reacts with hydrochloric acid:
    CaCO₃(s) + 2HCl(aq) → CaCl₂(aq)+H₂O(l)+CO₂(g)
• Analysis:
  • The tablet requiring the most acid to reach neutralisation is the most effective.

Key Terms

• Salt: A compound formed when hydrogen in an acid is replaced by a metal or ammonium ion.
• Neutralisation: A reaction between an acid and a base/alkali to produce salt and water.
• Effervescence: The fizzing observed when a carbonate reacts with an acid, releasing carbon dioxide.

Example Questions

1. Write a balanced equation for the reaction of magnesium with sulfuric acid.
   Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
2. Explain why antacid tablets are crushed before reaction.
   • Crushing increases the surface area, allowing the reaction to occur more efficiently.
3. Predict the salt formed when potassium hydroxide reacts with nitric acid.
   • Salt: Potassium nitrate (KNO₃).