Topic 6 of 20Cambridge A Levels

Group 2 Chemistry

Exploring the periodic trends and reactions of the alkaline earth metals, from magnesium to barium.

Group 2 of the Periodic Table contains the alkaline earth metals. For A Level Chemistry, we focus on magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements are characterised by having two electrons in their outermost s-orbital, leading to a valence shell electron configuration of ns². When they react, they lose these two electrons to form a dipositive ion (M²⁺) with a stable, noble gas configuration.

### Atomic and Physical Properties

As you descend Group 2, predictable trends emerge:

Atomic Radius: The atomic radius increases down the group. This is because each successive element has an additional electron shell, placing the valence electrons further from the nucleus.

Ionisation Energy: The first ionisation energy (energy required to remove one mole of electrons from one mole of gaseous atoms) decreases down the group. This is due to the combined effect of increased atomic radius and increased electron shielding from the inner shells. The outermost electrons are held less tightly by the nucleus and are therefore easier to remove. The second ionisation energy is always higher than the first but follows the same decreasing trend.

Melting Point: The melting points generally decrease down the group, though there is an anomaly with magnesium having a lower melting point than calcium. This trend is related to the strength of the metallic bonding. As the atomic size increases, the distance between the positive nuclei and the delocalised electrons increases, weakening the metallic bond.

### Chemical Reactivity

Group 2 elements are powerful reducing agents because they are readily oxidised by losing two electrons: M(s) → M²⁺(aq) + 2e⁻. The reactivity of the metals increases down the group, consistent with the decreasing ionisation energy.

Reaction with Oxygen:

The metals burn in oxygen to form a solid white metal oxide. The reaction becomes more vigorous down the group.

2Mg(s) + O₂(g) → 2MgO(s)

Reaction with Water:

The elements react with water to produce the metal hydroxide and hydrogen gas. The reactivity increases significantly down the group.

* Magnesium reacts very slowly with cold water, but reacts rapidly with steam to form magnesium oxide and hydrogen: Mg(s) + H₂O(g) → MgO(s) + H₂(g).

* Calcium reacts steadily with cold water: Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g).

* Barium reacts vigorously with cold water.

### Properties of Group 2 Compounds

Solubility Trends:

Solubility of Hydroxides (M(OH)₂): The solubility of Group 2 hydroxides increases down the group. Magnesium hydroxide, Mg(OH)₂, is sparingly soluble (often described as 'insoluble'), whereas barium hydroxide, Ba(OH)₂, is much more soluble. Consequently, the pH of their saturated solutions increases down the group, as more OH⁻ ions are present in solution.

Solubility of Sulfates (MSO₄): In contrast, the solubility of Group 2 sulfates decreases down the group. Magnesium sulfate, MgSO₄, is very soluble, while barium sulfate, BaSO₄, is famously insoluble. This insolubility makes the addition of an acidified solution of barium chloride (or nitrate) a standard test for sulfate ions, as a dense white precipitate of BaSO₄ is formed.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Thermal Stability:

Thermal stability refers to the resistance of a compound to break down upon heating. For Group 2 carbonates and nitrates, stability increases down the group.

Thermal Decomposition of Carbonates: Group 2 carbonates decompose upon heating to form the metal oxide and carbon dioxide gas.

MCO₃(s) → MO(s) + CO₂(g) (e.g., CaCO₃(s) → CaO(s) + CO₂(g))

The temperature required for decomposition increases down the group, indicating an increase in thermal stability. This is explained by the polarising power of the M²⁺ cation. The smaller Mg²⁺ ion has a high charge density and strongly polarises (distorts) the electron cloud of the large carbonate ion (CO₃²⁻), weakening the C-O bonds and making it easier to decompose. The larger Ba²⁺ ion has a lower charge density and causes less polarisation, so the carbonate is more stable.

Thermal Decomposition of Nitrates: Group 2 nitrates decompose on heating to yield the metal oxide, brown nitrogen dioxide gas, and oxygen.

2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g)

Similar to the carbonates, the thermal stability of the nitrates increases down the group due to the decreasing polarising power of the M²⁺ cation.

Key Points to Remember

1Reactivity increases down the group as ionisation energy decreases.
2Group 2 elements are reducing agents, readily forming M²⁺ ions.
3Solubility of Group 2 hydroxides, M(OH)₂, increases down the group.
4Solubility of Group 2 sulfates, MSO₄, decreases down the group; BaSO₄ is insoluble.
5Thermal stability of carbonates (MCO₃) increases down the group.
6Thermal stability of nitrates (M(NO₃)₂) increases down the group.
7The trend in thermal stability is explained by the decreasing polarising power of the M²⁺ cation as it gets larger.

Pakistan Example

Limestone and the Cement Industry in Pakistan

Pakistan has vast deposits of limestone (calcium carbonate, CaCO₃), especially in the Salt Range in Punjab and in Khyber Pakhtunkhwa. This natural resource is the primary raw material for Pakistan's large cement industry. The first major step in cement production is heating limestone to over 900°C in a kiln. This process is a direct industrial application of the thermal decomposition of a Group 2 carbonate, a key reaction studied in this topic: CaCO₃(s) → CaO(s) + CO₂(g). The resulting calcium oxide (quicklime) is a critical component of cement, making this chemical concept economically vital for Pakistan's infrastructure and development.

Quick Revision Infographic

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