3.2.2 Group 2, the alkaline earth metals

Explain the trend in atomic radius of the elements Mg-Ba

Atomic radius increases down group 2 because there are more electron shells

Explain the trend in first ionisation energy of the elements Mg-Ba

1st IE decreases down group 2 because no. of shells increases, giving increased shielding so decreasing the attraction between the nucleus and outer electron

Explain the trend in melting point of the elements Mg-Ba

Melting point generally decreases down group 2 because the number of delocalised electrons and charge of the ion remains the same, while the size of the metal cation increases so charge density of the metal cation decreases, giving a weaker attraction between the metal cations and the delocalised electrons therefore a weaker metallic bond.

• Mg is the exception (its melting point is lower than expected because it has a different crystal structure)

How do the elements Mg-Ba react with water?

• liquid water: M + 2H₂O_(l) → M(OH)₂ + H₂; reactivity increases down group 2 because it is easier to lose 2 electrons (ionisation energy decreases); magnesium reacts very slowly with cold water

• steam: M + H₂O_(g) → MO + H₂; Magnesium burns with a bright white light, producing a white solid (MgO)

How is magnesium used to extract titanium from TiCl₄?

• TiCl₄ + 2Mg → Ti + 2MgCl₂

• 500-1000^oC and inert (Ar) atmosphere

What is the trend in solubility of the group 2 hydroxides?

Solubility increases down the group

• Mg(OH)₂ is sparingly soluble

How are Mg(OH)₂ and Ca(OH)₂ used?

• Mg(OH)₂ is used in medicine to neutralise excess stomach acid

• Ca(OH)₂ is used in agriculture to neutralise acidic soil

How and why are CaO or CaCO₃ used to remove SO₂ from flue gases?

• CaO + SO₂ → CaSO₃ or CaCO₃ + SO₂ → CaSO₃ + CO₂

• SO₂ forms from the combustion of petroleum with sulfur impurities, and contributes to acid rain

• Economic benefit as CaSO₃ is converted to CaSO₄ which is used to make plasterboards

What is the trend in solubility of the group 2 sulfates?

Solubility decreases down the group

• BaSO₄ is insoluble

What is the test for sulfate ions?

• add acidified (with HCl) barium chloride solution

• positive result = white ppt.

Why is BaCl₂ solution used to test for sulfate ions and why is it acidified?

• it forms BaSO₄

• to remove carbonate ions which would otherwise give a false positive of BaCO₃ (also a white ppt.)

• H₂SO₄ should not be used as it releases sulfate ions itself

How is BaSO₄ used?

In medicine as barium meal which is opaque in x rays. It is insoluble therefore harmless.