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2.2.3Physical Chemistry

Reactions of Group 2 — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 58 of 102

📚 AQA AS Chemistry 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 4Reactions of the Metals

Group 2 metals are oxidised in each reaction, and the rate increases down the group.

With oxygen
  • With oxygen Magnesium burns with a bright white flame: 2Mg(s) + O₂(g) → 2MgO(s), forming a white solid oxide.
With cold water
  • With cold water Calcium reacts steadily: Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g), while magnesium reacts very slowly.
With steam
  • With steam Magnesium burns in steam: Mg(s) + H₂O(g) → MgO(s) + H₂(g), giving the oxide and hydrogen.
With dilute acid
  • With dilute acid Magnesium and hydrochloric acid give a salt and hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).

Part 2 of 4Reactions of the Oxides

Reactions of the Oxides
Basic oxides
  • Basic oxides Group 2 oxides are basic because the oxide ion accepts protons from acids to form water.
Hydroxide solutions
  • Hydroxide solutions Calcium oxide and barium oxide react with water to form alkaline hydroxide solutions.
Magnesium oxide
  • Magnesium oxide MgO is almost insoluble, so it forms only a weakly alkaline solution in water.
ReactionEquation
CaO with waterCaO(s) + H₂O(l) → Ca(OH)₂(aq)
MgO with acidMgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
Ca(OH)₂ with acidCa(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)

Part 3 of 4Patterns and Observations

1
When a Group 2 metal reacts, each atom loses two electrons and is oxidised from 0 to +2.
2
Reaction with water becomes faster down the group: magnesium is slow, calcium fizzes steadily and barium reacts more vigorously.
3
Hydrogen gas is released in the reactions with water and acid, which can be seen as bubbles of gas.
4
The hydroxide solution formed from calcium is alkaline, and the solution turns cloudy as less soluble Ca(OH)₂ may precipitate.
5
Ionic equations for the acid reactions simplify to Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g).

Part 4 of 4Exam Checklist & Tips

2.2.3 – Key Points for the Exam

  • Basic oxide An oxide that reacts with an acid to form a salt and water.
  • Oxidation Loss of electrons, which describes what happens to each Group 2 metal atom.
  • Neutralisation The reaction of an acid with a base to form a salt and water.
  • Ionic equation An equation showing only the particles that change, with spectator ions omitted.

Common Mistakes

  • Writing Mg(OH)₂ as the product of magnesium with cold water in every case, when magnesium reacts with steam to give MgO.
  • Missing state symbols, or writing the formula of a Group 2 hydroxide as MOH.
  • Stating that hydrogen is a product of acid–oxide reactions; it forms only with the metal.
  • Forgetting to balance equations, for example 2Mg + O₂ → 2MgO.

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