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2.2.2Inorganic Chemistry

Reactions of Group 2 — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 5 of 27

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Trend in Reactivity

2.2.2 Reactions of Group 2
Trend in Reactivity
Increasing reactivity
  • Group 2 metals become more reactive down the group, as shown by faster reactions with water and acids.
Oxidation
  • Each metal loses two electrons to form M²⁺, so it is oxidised from 0 to +2 in all of these reactions.
Ionisation energy
  • Down the group, atomic radius and shielding increase, so the outer electrons are lost more easily despite the larger nuclear charge.
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  • The sum of the first and second ionisation energies falls down the group, which explains the greater reactivity.

Part 2 of 5Reaction with Water

Magnesium and cold water
  • Mg reacts extremely slowly with cold water to form Mg(OH)₂ and hydrogen, with only a few bubbles seen.
Magnesium and steam
  • Mg(s) + H₂O(g) → MgO(s) + H₂(g) occurs vigorously with a bright white flame, giving the oxide rather than the hydroxide.
Calcium
  • Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g) bubbles steadily and the solution turns cloudy because the hydroxide is only sparingly soluble.
Strontium and barium
  • Sr and Ba react more vigorously and form more soluble hydroxides, with the same pattern M + 2H₂O → M(OH)₂ + H₂.
Redox
  • The metal is oxidised from 0 to +2 and hydrogen in water is reduced from +1 to 0.

Part 3 of 5Other Reactions

1
With oxygen, each metal burns to form a white solid oxide: 2Mg(s) + O₂(g) → 2MgO(s), with magnesium burning with a brilliant white flame.
2
With dilute hydrochloric acid, each metal forms a salt and hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
3
Reactions become more vigorous down the group, so calcium fizzes far faster in acid than magnesium does.
4
Group 2 oxides react with water to form hydroxides, for example CaO(s) + H₂O(l) → Ca(OH)₂(aq).
5
The oxides and hydroxides are bases that neutralise acids to form a salt and water.

Part 4 of 5Summary of Observations

Summary of Observations
Mg
  • Slow with cold water; bright white flame in air; steady fizzing with dilute acid.
Ca
  • Steady bubbling with cold water and a cloudy white suspension forming.
Sr, Ba
  • Faster reactions with water, forming hydroxide solutions that are more strongly alkaline.

Part 5 of 5Exam Checklist & Tips

2.2.2 – Key Points for the Exam

  • Reactivity The tendency of a metal to lose electrons and form positive ions.
  • Oxidation Loss of electrons, or an increase in oxidation state.
  • Ionisation energy Energy needed to remove electrons from gaseous atoms.
  • Sparingly soluble Dissolving to only a very small extent, as Mg(OH)₂ does.

Common Mistakes

  • Saying reactivity increases down the group because ionisation energy increases.
  • Writing the product of magnesium with steam as Mg(OH)₂ instead of MgO.
  • Forgetting that Mg reacts only slowly with cold water.
  • Explaining the trend in terms of atoms getting heavier instead of electron loss.

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