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6.2.4Additional lessons

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

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📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Section 1 of 6The General Reaction

6.2.4 Acids with Reactive Metals

Metals above hydrogen in the reactivity series can displace it from dilute acids.

Word equation
  • Reactive metal + dilute acid → salt + hydrogen.
Magnesium
  • Magnesium gives magnesium chloride and hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Zinc
  • Zinc with sulfuric acid givesZn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
Iron
  • Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g), forming iron(II) rather than iron(III).

Section 2 of 6Redox Analysis

Redox Analysis
Oxidation
  • The metal loses electrons and its oxidation number increases, for example Mg: 0 → +2.
Reduction
  • H⁺ gains electrons to form H₂, so hydrogen goes from +1 to 0.
Ionic equation
  • Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g); the anion is a spectator.
Reducing agent
  • The metal is the reducing agent and the acid supplies the oxidising agent H⁺.

Section 3 of 6Observations and Tests

1
Effervescence shows that hydrogen gas is being made.
2
The metal slowly dissolves, and the test tube becomes warm because the reaction is exothermic.
3
A lit splint held at the mouth of the tube burns with a squeaky pop.
4
More reactive metals react faster, so rate order is Mg > Zn > Fe.
5
Copper, silver and gold lie below hydrogen and do not react with dilute acids.

Section 4 of 6Linking to Reactivity

Linking to Reactivity
Order of reactivity
  • Rate of hydrogen production increases from iron to zinc to magnesium, reflecting the ease of oxidation.
Safety
  • Hydrogen is flammable, so the gas should be collected away from flames and tested in small quantities.
Evidence of redox
  • The disappearance of the metal and the formation of H₂ show electrons being transferred from metal to H⁺.
Sulfuric acid note
  • Dilute sulfuric acid with lead stops quickly because insoluble lead(II) sulfate coats the metal.

Section 5 of 6Worked Example

Question

  • Calculate the volume of hydrogen released at STP when 0.250 g of magnesium reacts with excess dilute hydrochloric acid.
  • Molar volume of a gas at STP is 22.7 dm³ mol⁻¹.

  1. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), so 1 mol Mg gives 1 mol H₂.
  2. n(Mg) = 0.250 / 24.31 = 0.01028 mol, so n(H₂) = 0.01028 mol.
  3. V = 0.01028 × 22.7 = 0.233 dm³.

Answer 0.233 dm³ (233 cm³) of hydrogen.

Section 6 of 6Exam Checklist & Tips

6.2.4 – Key Points for the Exam

  • Salt The ionic compound formed when metal replaces hydrogen in an acid.
  • Spectator ion An ion present but not changing in the reaction, such as Cl⁻.
  • Reactivity series The order of metals by ease of oxidation.
  • Effervescence The formation of gas bubbles in a liquid.

Common Mistakes

  • Writing iron(III) chloride as the product with dilute HCl; the product is iron(II) chloride.
  • Saying that all metals react with dilute acids, forgetting that copper does not.
  • Using dilute nitric acid as a typical example: nitrate is reduced, so H₂ is not the usual product.
  • Leaving the equation unbalanced, for example Mg + HCl → MgCl₂ + H₂.

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