Group 2 Reaction of Group 2 Oxides and Hydroxides

Occurrence Group 2 oxides and hydroxides are not found naturally. Elements usually found as carbonates which decompose on heating to form the oxide. E.g. Calcium carbonate occurs in different forms like limestone, chalk and marble.

Reaction of Oxides with Water Beryllium oxide DOES NOT react with water. Magnesium oxide reacts SLIGHTLY with water. When cold water is added to calcium oxide, the mixture swells, fizzes and lots of energy and water vapour is released: quicklimeSlaked lime A solution of calcium hydroxide in water is what we commonly know as LIMEWATER This exothermic process is sometimes called SLAKING lime. Strontium oxide and barium oxide react in the same way as CaO.

Reaction of Oxides with Hydrochloric/Nitric Acid

Reaction of Hydroxides with Hydrochloric/Nitric Acid

Solubility Determined by two factors: 1.Lattice dissociation enthalpy (energy needed to break up a crystal lattice) 2.Hydration enthalpy (energy released when ions are hydrated) –Ionic size has an effect on these factors.

Group 2 Solubility Trends e.g. What pattern can you see in the solubility of the sulphates and hydroxides down the group?

Group 2 Solubility Trends Solubility of nitrates/chlorides All soluble Solubility of Sulphates Groups 2 salts in which the anion has a -2 charge, e.g. sulphates, are largely insoluble EXCEPTIONS! – magnesium and calcium salts. Anion >> Cation: so lattice enthalpy does not vary much down group as cation size changes. However, hydration enthalpy decreases down the group. Therefore, solubility decreases down the group.

Test for Sulphate Ions The insolubility of barium sulphate can be used as a test for sulphates: unknown sample add dilute HCl to destroy any carbonate. Then add barium chloride or barium nitrate solution. If the unknown sample is a sulphate a white precipitate of barium sulphate is formed

Group 2 Solubility Trends Solubility of Hydroxides There is a decrease in lattice dissociation enthalpy down the group. This outweighs the change in enthalpy of hydration. As a result, there is an INCREASE IN SOLUBILITY down the group.

Insoluble Calcium Hydroxide This can be demonstrated by adding sodium hydroxide solution to calcium nitrate solution. A white precipitate of calcium hydroxide is formed.

Groups 1 & 2 Comparison Group 1 salts are very soluble. –The singly charged cations mean that the lattice dissociation enthalpies are not too high. In comparison, with group 2 compounds likely to have +2/-2 ions, they have a greater dissociation enthalpy. They also have larger hydration enthalpies. However, this cannot compensate for the higher lattice dissociation enthalpy

Solubility of Group 2 Compounds Knowing the solubility of a compound is vital in knowing how to utilize it. In medicine, some areas like soft tissue areas are not always seen using X-rays. Barium compounds, in the form of a ‘barium meal’ or enema, are given in these situations. These allow the shape of that region of the gut to come up more clearly on X-ray. WARNING! – soluble barium compounds are poisonous to people. Fortunately, barium sulphate is insoluble

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