Concept 05 / 05The s-Block Elements
Alkaline Earth Metals: Trends, Anomalous Beryllium and Solubility of Compounds
General characteristics of group 2 elements, trends in size, ionisation enthalpy and reducing power, flame colours, reactions with water, air, halogens and acids, anomalous beryllium and its diagonal relationship with aluminium, and trends in solubility and thermal stability of hydroxides, sulfates, carbonates and nitrates.
Group 2: beryllium, magnesium, calcium, strontium, barium, radium. Configuration ; oxidation state .
Trends compared with group 1. Atoms are smaller (higher nuclear charge), so first ionisation enthalpies are higher than group 1; second ionisation enthalpies are higher still, yet the state is favoured because the large hydration and lattice enthalpies of more than compensate. They are harder and denser, with higher melting points (two electrons in metallic bonding). Reducing power increases down the group; values are less negative than group 1 (Be , Mg , Ca , Ba ).
Flame colours: Ca brick red, Sr crimson, Ba apple green. Be and Mg give none (electrons too tightly held).
Chemical properties. Reactivity increases down the group.
Air: Be and Mg are kinetically inert due to an oxide film; powdered Be burns to BeO and ; Mg burns with a brilliant white light to MgO and .
Water: Be does not react; Mg reacts with hot water; Ca, Sr and Ba react with cold water: .
Halogens: all form ; is covalent (chain polymer in the solid, dimer in vapour, monomer above ).
Acids: all liberate from dilute acids; Be is rendered passive by concentrated .
Liquid ammonia: like alkali metals, they give blue-black solutions (except Be).
Anomalous beryllium. Very small size, high charge density and no d orbitals: its compounds are largely covalent; maximum coordination number ; BeO and are amphoteric ().
Diagonal relationship with Al: both are rendered passive by nitric acid; hydroxides are amphoteric; chlorides are covalent, Lewis acidic, bridged and soluble in organic solvents (Friedel-Crafts catalysts); both form fluoro complexes (, ); their carbides give methane with water ().
Solubility trends (see figure). Solubility depends on the balance of lattice enthalpy and hydration enthalpy; both decrease down the group.
Hydroxides and fluorides: lattice enthalpy (small anion) falls faster than hydration enthalpy, so solubility increases: . Basic strength increases the same way.
Sulfates (large anion): lattice enthalpy hardly changes, while hydration enthalpy falls, so solubility decreases: and are soluble, sparingly, and practically insoluble (barium meal).
Carbonates: sparingly soluble, solubility decreasing down the group; thermal stability increases down the group as the cation's polarising power falls ( is unstable, decomposes only above about ).
Nitrates decompose to oxides: .
Key Takeaways:
- Group 2 is harder, denser and less reactive than group 1; state .
- Solubility down the group: hydroxides increase, sulfates decrease.
- Thermal stability of carbonates increases down the group.
- Be resembles Al: amphoteric oxide and hydroxide, covalent bridged chloride, passivity with .
- Pitfall: is insoluble but is fairly soluble; the two trends run in opposite directions.
Brain-Teaser 1: Explain why is soluble but is insoluble in water.
Solution: The sulfate ion is so large that lattice enthalpy changes little from Be to Ba. Hydration enthalpy, however, is very large for the tiny () and much smaller for (). So hydration outweighs lattice energy for but not for .
Brain-Teaser 2: Arrange , , , in order of increasing decomposition temperature and explain.
Solution: . A small, highly charged cation polarises the carbonate ion, weakening a C-O bond and favouring ; it also forms a very stable oxide lattice. Larger cations polarise less, so their carbonates need higher temperatures.
Brain-Teaser 3: What happens when is treated with water and with excess NaOH? Compare with .
Solution: Both hydrolyse (covalent chlorides): . The hydroxide dissolves in excess NaOH as beryllate: , just as dissolves to : amphoteric behaviour, a hallmark of the diagonal relationship.
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