AQA A Level Chemistry: Group 2, the Alkaline Earth Metals

Contents Overview

• Trends in Group 2: The Alkaline Earth Metals
• Solubility of Group 2 Compounds: Hydroxides & Sulfates
• Reactions of Group 2
• Uses of Group 2 Elements

Trends in Group 2: The Alkaline Earth Metals

Chemical Trends

• All Group 2 elements have 2 electrons in their outer shell and form ions with a +2 charge by donating these electrons (reducing agents).
• Reactivity increases down the group due to decreasing ionization energies.
  • First ionisation energy: Energy needed to remove the first outer electron.
  • Second ionisation energy: Energy needed to remove the second outer electron.
• As you go down the group, the outer electrons are more easily removed due to increased shielding and distance from the nucleus.

Reactions

• With dilute hydrochloric acid: increased reactivity and faster hydrogen gas release.
• With oxygen: reactivity increases (e.g., Barium must be stored in oil).

Physical Trends

• Atomic size increases down the group due to the addition of electron shells.
• Melting points decrease because the outer electrons are farther from the nucleus, reducing the bond strength.
• Density shows a non-linear trend affected by atomic radius and metallic structure.

Solubility of Group 2 Compounds: Hydroxides & Sulfates

Solubility of Hydroxides

• Solubility increases down the group, making solutions more alkaline.
• Solubility at 298 K:
  • Mg(OH)₂: 2.0 x 10⁻⁴ mol/100g water
  • Ca(OH)₂: 1.5 x 10⁻⁴ mol/100g water
  • Sr(OH)₂: 3.4 x 10⁻⁴ mol/100g water
  • Ba(OH)₂: 1.5 x 10⁻³ mol/100g water

Solubility of Sulfates

• Solubility decreases down the group.
• Example: Radium sulfate is less soluble than strontium sulfate.

Reactions of Group 2

General Reactions

• Reaction with oxygen: 2M (s) + O₂ (g) → 2MO (s)
• Reaction with water: M (s) + 2H₂O (l) → M(OH)₂ (aq) + H₂ (g)
  • Magnesium reacts slowly with cold water but vigorously with steam.

Reaction with Acids

• Metals react with acids to form salt solutions and hydrogen.
  • Example with HCl: M (s) + 2HCl (aq) → MCl₂ (aq) + H₂ (g)
  • With H₂SO₄, insoluble sulfates may form.

Uses of Group 2 Elements

Calcium

• Limestone (calcium carbonate): Used in agriculture to adjust soil pH.
• Quicklime (calcium oxide): Formed by decomposition of limestone.
• Slaked lime (calcium hydroxide): Formed by hydrating quicklime, used in agriculture and flue gas desulfurization.

Barium

• Barium sulfate: Used in medical imaging (barium meals).
  • Safe due to its insolubility, hence not absorbed into blood.
• Barium salts: Used in sulfate ion tests.

Magnesium

• Magnesium hydroxide: Used as antacid ('milk of magnesia').
• Magnesium in titanium extraction: TiO₂ converted to TiCl₄, reduced by Mg.
  • TiCl₄ (g) + 2Mg (l) → Ti (s) + 2MgCl₂ (l)

This document provides an extensive examination of the properties, reactions, and applications of Group 2 elements, serving as essential revision material for understanding the behavior of alkaline earth metals in AQA A Level Chemistry.