Similarities and Trends in the Properties of the Group 2 Metals, Magnesium to Barium, and Their Compounds

Specification Reference Inorganic Chemistry: Group 2 10.1

Quick Notes

Group 2 metals react with oxygen, water, and acids to form basic oxides, hydroxides, and salts.
Reactivity increases down the group due to decreasing ionisation energy.
Group 2 carbonates and nitrates decompose on heating
- Thermal stability increases down the group.
Hydroxide solubility increases, sulfate solubility decreases down the group.
Group 2 oxides and hydroxides are basic, Al(OH)₃ is amphoteric.

Full Notes

Reactions of Group 2 Elements

With Oxygen

Group 2 metals burn in oxygen to form white ionic oxides:

General equation: 2M + O₂ → 2MO

These oxides are basic and react with water to form hydroxides (see below).

With Water

Reactions produce a metal hydroxide and hydrogen gas:

General equation: M + 2H₂O → M(OH)₂ + H₂

Magnesium reacts slowly with cold water, faster with steam.
Barium reacts rapidly even with cold water.
Reactivity increases down the group.

With Dilute Acids

Metals react with dilute HCl or H₂SO₄ to form a salt + hydrogen gas (H₂).

For Example Metal (M) with HCl

CIE A-Level Chemistry reaction of Group 2 metals with dilute acids forming salts and hydrogen.

Note: With H₂SO₄, BaSO₄ forms and coats the metal, slowing the reaction due to insolubility.

CIE A-Level Chemistry diagram showing barium sulfate forming on Group 2 metals in sulfuric acid.

Reactions of Oxides, Hydroxides, and Carbonates

With Water

Oxides form hydroxides: MO + H₂O → M(OH)₂
Group 2 hydroxide solubility increases down the group (making the solution more alkaline, higher pH).

With Acids

Group II Oxides react with acids to form salt + water
Group II hydroxides react with acids to form salt + water
Group II carbonates react with acids to salt + water + carbon dioxide

Thermal Decomposition

A thermal decomposition reaction involves breaking down a compound with the use of heat.

Group II Metal Carbonates:

Group II Metal Carbonates decompose to oxide + CO₂ when heated.

CIE A-Level Chemistry thermal decomposition of Group 2 carbonates into oxides and carbon dioxide.

Nitrates:

Group II Metal Nitrates decompose to oxide + NO₂ + O₂ when heated.

Trend: Thermal stability for the carbonates and nitrates increases down the group – it gets harder to break down the carbonate or nitrate with heat.

CIE A-Level Chemistry diagram showing trends in thermal stability of Group 2 carbonates and nitrates.

Larger cations (e.g. Ba²⁺) with a lower charge density cause less polarisation of the anion, making it harder to break a C–O bond (in carbonates) or an N–O bond (in nitrates).

Trends in Properties

Property	Trend down the group
Reactivity	Increases
First ionisation energy	Decreases
Thermal stability of carbonates/nitrates	Increases
Hydroxide solubility	Increases
Sulfate solubility	Decreases

Solubility of Hydroxides and Sulfates

Hydroxides:

Group II Metal Hydroxide solubility increases down the group.

More OH⁻ ions released means the solution has a higher pH.

Sulfates:

Group II Metal Sulfate solubility decreases down the group.

BaSO₄ is insoluble and forms a white precipitate.

Use in medicine: BaSO₄ used in barium meals for X-ray imaging — safe because it is insoluble.

Testing for Sulfate Ions

CIE A-Level Chemistry test for sulfate ions using acidified barium chloride solution to form BaSO4 precipitate.

If SO₄²⁻ present a white precipitate of BaSO₄ forms.

Equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why acidified?
Removes carbonate ions that could give false positives.