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6.2.4Additional lessons

Acids with Reactive Metals — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 135 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5The General Reaction

The metal is oxidised and hydrogen ions are reduced.

Word equation
  • Word equation A reactive metal and an acid react to give a salt and hydrogen gas.
Example
  • Example Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) shows magnesium chloride and hydrogen being formed.
Ionic equation
  • Ionic equation Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g), leaving out the spectator chloride ions.
Observations
  • Observations Bubbles of colourless gas appear, the metal dissolves and the test tube becomes warm.

Part 2 of 5Redox Explanation

Redox Explanation
Oxidation
  • Oxidation Magnesium loses two electrons and its oxidation state rises from 0 to +2.
Reduction
  • Reduction Each hydrogen ion gains one electron, and its oxidation state falls from +1 to 0 in H₂.
Reducing agent
  • Reducing agent The metal is the reducing agent because it donates electrons to the hydrogen ions.
Acid role
  • Acid role The acid is the oxidising agent because the hydrogen ions accept the electrons.

Part 3 of 5Which Metals React

1
Metals above hydrogen in the reactivity series, such as Mg, Zn and Fe, react with dilute hydrochloric and sulfuric acid.
2
The reaction rate falls from Mg to Zn to Fe, which matches the decreasing ease of oxidation.
3
Copper, silver and gold lie below hydrogen and do not release hydrogen from dilute acids.
4
Nitric acid is a special case because the nitrate ion is a strong oxidising agent and it usually gives other products than hydrogen.
5
Weak acids such as ethanoic acid react more slowly because they supply fewer H⁺ ions.

Part 4 of 5Worked Example

Question

  • 0.500 g of magnesium reacts with excess dilute hydrochloric acid.
  • Calculate the volume of hydrogen produced at STP.
  • (Ar of Mg = 24.31;
    • molar gas volume = 22.7 dm³ mol⁻¹)

  1. n(Mg) = 0.500 g ÷ 24.31 g mol⁻¹ = 0.02057 mol.
  2. The equation shows 1 mol Mg gives 1 mol H₂, so n(H₂) = 0.02057 mol.
  3. V(H₂) = 0.02057 mol × 22.7 dm³ mol⁻¹ = 0.467 dm³.

Answer 0.467 dm³ (467 cm³) of hydrogen

Part 5 of 5Exam Checklist & Tips

6.2.4 – Key Points for the Exam

  • Salt An ionic compound formed when the H⁺ of an acid is replaced by a metal ion.
  • Spectator ion An ion that is unchanged and is left out of the ionic equation.
  • Molar gas volume At STP one mole of gas occupies 22.7 dm³.
  • Reactive metal A metal that readily loses electrons and lies above hydrogen in the series.

Common Mistakes

  • Writing H₂O or H as a product instead of hydrogen gas, H₂.
  • Claiming copper reacts with dilute hydrochloric acid to release hydrogen.
  • Leaving the chloride ions in an ionic equation instead of removing spectators.
  • Forgetting that the number of moles of H₂ is not the same as the moles of acid.

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