Group 2: The alkali earth metals

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Trends

  • First Ionisation energy decreases
  • Electronegativity decreases
  • Melting point decreases
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Reaction with water

Form metals hydroxides and hydrogen.

Vigour increases down a group.

If the hydroxide is insoluble, it will form a seal on the metal's surface, preventing any further reactions, hence a very slow rate of reaction. (Both Be(OH)2 and Mg(OH)2 are insoluble).

So theoretically, neither magnesium or belium should react with liquid or gaseous water, but magnesium decomposes in high temperatures to form MgO, so magnesium does react with steam but to form MgO as the main product:

Mg(s) + H2O(g) --> MgO(s) + H2(g)

Calcium hydroxide (Ca(OH)2) is not very soluble, so forms a cloudy precipitate in reaction with cold water.

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Solubility

As compounds formed by the alkali earth metals are predominately ionic, their solubility generally depends on the difference in charge between the two ions and to water.

If the attaction between the two ions is stronger than their attraction to water, the compound is insoluble. 

If the attraction between an ion and water is stronger than with the other ion, the compound is soluble. 

Solubility of sulphates decreases down the group.     When sulphuric acid/sodium sulphate is                                                                                                   added...

MgSO4 = soluble                                                                     no precipitate

BeSO4 = soluble                                                                     no precipitate

CaSO4 = sparingly soluble                                                  faint white precipitate

SrSO4 = insoluble                                                                 white precipitate

BaSO4 = insoluble                                                             thick white precipitate

(BaSO4 used as a "barium meal" as it is good at absorbing X-rays so highlights the gut easily after consumption.)

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Testing for sulphate ions

1. Add 1cm^3 aqueous Barium Chloride and 1cm^3 dilute hydrochloric acid to 1cm^3 of the unknown solution. 

2. If a thick white precipitate has formed, and has remained undissolved after adding the dilute hydrochloric acid, either sulphate or hydrogensulphate ions are present in the solution.

**Hydrochloric acid removes any carbonate or hydroxide ions which may affect the result of experiment.**

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Trends in solubility

The solubility of the group II hydroxides increase down the group. 

Mg(OH)2 = insoluble (milk of magnesia.)

Ca(OH)2 = sparingly soluble (slaked lime used to neutralise acidic soil.)

Sr(OH)2 = does not react with dilute sodium hydroxide.

Ba(OH)2 = does not react with dilute sodium hydroxide.

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