Group 2, The Alkaline Earth Metals — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

Section 1 of 11Atomic radius and ionisation energy

Atomic radius

• Atomic radius it increases down the group because the outer two electrons occupy a new, more distant principal shell

Ion formation

• Group 2 metals lose two electrons to form 2+ ions, so they act as reducing agents and are oxidised

Ionisation energies

• Ionisation energies both the first and second decrease down the group
• Nuclear charge increases, but greater shielding and larger distance outweigh it.
• The outer two electrons are easier to remove.

Reactivity

• Reactivity it increases down the group
• With dilute hydrochloric acid, hydrogen bubbles form faster.
• With oxygen, barium is so reactive it must be stored in oil.

Section 2 of 11Melting point trend

Melting point trend

Structure

• Structure giant metallic structures give high melting points

Trend

• Trend melting point generally decreases down the group, with a slight anomaly at magnesium

Reason

• Reason the outer electrons are further from the nucleus, so attraction to the bonding electrons weakens

Section 3 of 11Reactions with water

General equation

• M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)

Redox

• M is oxidised from 0 to +2 and hydrogen is reduced from +1 to 0

Element	Reaction
Mg	Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g) (very slow)
Ca	Ca(s) + 2H2O(l) → Ca(OH)2(s) + H2(g)
Sr	Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)
Ba	Ba(s) + 2H2O(l) → Ba(OH)2(aq) + H2(g)

Magnesium with steam

• Mg(s) + H₂O(g) → MgO(s) + H₂(g), which is vigorous compared with its slow reaction with liquid water

Trend

• Trend reactivity with water increases down the group

Section 4 of 11Solubility of hydroxides

Dissolving

• X(OH)₂(s) → X²⁺(aq) + 2OH^-(aq)

Trend

• Trend solubility increases down the group, so Mg(OH)₂ is sparingly soluble and Ba(OH)₂ is soluble

Hydroxide	Solubility at 298 K / mol per 100 g water
Mg(OH)2	2.0 × 10-5
Ca(OH)2	1.5 × 10-3
Sr(OH)2	3.4 × 10-3
Ba(OH)2	1.5 × 10-2

• Alkalinity
  • higher OH^- concentration raises the pH, so solutions formed from the oxides become more alkaline down the group

Section 5 of 11Solubility of sulfates

Trend

• Trend solubility decreases down the group

Barium sulfate

• Barium sulfate it is virtually insoluble

Test for sulfates

• Test for sulfates add HCl then BaCl₂ (or HNO₃ then barium nitrate), and a white precipitate of BaSO₄ shows sulfate ions

Section 6 of 11Uses of magnesium

Medicine

• Medicine magnesium hydroxide in suspension neutralises stomach acid and treats constipation
  • (milk of magnesia)
• Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l).
• It is safe because it is only sparingly soluble, giving a slightly alkaline solution of pH about 10.

Titanium extraction

• TiO₂ is heated with coke and chlorine
• TiO₂(s) + 2C(s) + 2Cl₂(g) → TiCl₄(g) + 2CO(g).
• TiCl₄ is reduced by magnesium (or sodium): TiCl₄(g) + 2Mg(l) → Ti(s) + 2MgCl₂(l).

Section 7 of 11Uses of calcium: agriculture

Compounds

• Compounds limestone, quicklime (calcium oxide from thermal decomposition of limestone) and slaked lime (calcium hydroxide, made by adding water to quicklime)
  • (impure calcium carbonate)

Purpose

• Purpose they raise soil pH, such as Ca(OH)₂(s) + 2H⁺(aq) → Ca²⁺(aq) + 2H₂O(l)

Optimum pH

• Optimum pH many crops grow best at pH 6–6.5

Choice

• Choice limestone is cheaper and safer to handle, but acts more slowly because it is sparingly soluble

Section 8 of 11Uses of calcium: sulfur scrubbing

• Purpose
  • calcium compounds remove sulfur dioxide from flue gases of coal-fired power stations

1

Waste gases pass into a scrubbing chamber.

2

A wet slurry of calcium oxide and calcium carbonate is sprayed in.

3

CaO(s) + SO₂(g) → CaSO₃(s), or CaCO₃ + SO₂(g) → CaSO₃(s) + CO₂(g).

4

Calcium sulfite is oxidised to calcium sulfate (gypsum): CaSO₃(s) + ½O₂(g) → CaSO₄(s).

• Efficiency
  • it removes 90–95% of the sulfur dioxide

Section 9 of 11Uses of barium

Barium meal

• Barium meal barium sulfate is given before an X-ray of the intestines, because barium absorbs X-rays and the gut shows white

Safety

• Safety barium is toxic, but barium sulfate is insoluble so it is not absorbed into the blood, and only a small amount is swallowed

Other use

• Other use barium chloride or barium nitrate solution tests for sulfate ions

Section 10 of 11Key terms

Term	Meaning
Alkaline earth metals	The Group 2 metals.
Reducing agent	A species that donates electrons and is oxidised.
Sulfur scrubbing	Removing sulfur dioxide from flue gases using a calcium slurry.
Milk of magnesia	A suspension of magnesium hydroxide used as an antacid.
Quicklime	Calcium oxide.
Slaked lime	Calcium hydroxide.

Section 11 of 11Exam technique

1

Saying Group 2 solubility of hydroxides decreases down the group — it increases.

2

Saying Group 2 sulfate solubility increases down the group — it decreases.

3

Saying magnesium reacts vigorously with cold water — it reacts very slowly, but vigorously with steam.

4

Saying barium compounds are always safe — barium sulfate is safe because it is insoluble.