Group 2Group 2

Reaction of

Group 2 Oxides

and

Hydroxides

OccurrenceOccurrence

• 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 WaterReaction 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:

quicklime Slaked lime

A solution of calcium hydroxide in A solution of calcium hydroxide in water is what we commonly know as water is what we commonly know as LIMEWATERLIMEWATER

This exothermic process is sometimes called SLAKING lime.

Strontium oxide and barium oxide react in the same way as CaO.

Reaction of Oxides with Reaction of Oxides with Hydrochloric/Nitric Acid Hydrochloric/Nitric Acid

Reaction of Hydroxides with Reaction of Hydroxides with Hydrochloric/Nitric AcidHydrochloric/Nitric Acid

SolubilitySolubility

• 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 TrendsGroup 2 Solubility Trends

e.g.e.g.What pattern can you seeWhat pattern can you see

in the solubility of thein the solubility of thesulphates and hydroxidessulphates and hydroxides

down the group?down the group?

Group 2 Solubility TrendsGroup 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 IonsTest for Sulphate Ions

• The insolubility of barium sulphate can be used as a test for sulphates:

unknownsample

add diluteadd diluteHCl toHCl to

destroy anydestroy anycarbonate.carbonate.

Then addThen addbarium chloridebarium chloride

ororbarium nitratebarium nitrate

solution.solution.

If the unknown If the unknown sample is a sulphate sample is a sulphate a white precipitate of a white precipitate of

barium sulphate is barium sulphate is formedformed

Group 2 Solubility TrendsGroup 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 HydroxideInsoluble 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 ComparisonGroups 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 Solubility of Group 2 CompoundsCompounds

• 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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