Opposite solubility trends for group 2 hydroxides and sulfates

The solubility of group 2 metal hydroxides and sulfates follows opposite trends down the group:

• Group 2 metal hydroxides become more soluble down the group.
• Group 2 metal sulfates become less soluble down the group.

This is summarised in the table below:

Reactions of group 1 and 2 oxides with water

Group 1 and group 2 metal oxides are basic oxides and readily react with water to alkaline solutions.

For group 1 oxides:

• M₂O_(s) + H₂O_(l) ➔ 2MOH_(aq)

For group 2 oxides

• MO_(s) + H₂O_(l) ➔ M(OH)_2(aq)

These reactions release hydroxide ions (OH^-) making the solutions strongly alkaline. The ionic equation is:

• O^2-_(aq) + H₂O_(l) ➔ 2OH^-_(aq)

Reactions of group 1 and 2 oxides and hydroxides with acid

The oxides and hydroxides of group 1 and group 2 react with dilute acids in neutralisation reactions to form salts and water.

Group 1 examples:

Lithium oxide and lithium hydroxide react with hydrochloric acid to form lithium chloride and water:

• Li₂O_(s) + 2HCl_(aq) ➔ 2LiCl_(aq) + H₂O_(l)
• LiOH_(s) + HCl_(aq) ➔ LiCl_(aq) + H₂O_(l)

Group 2 examples:

Calcium oxide and calcium hydroxide react with sulfuric acid acid to form calcium sulfate and water:

• CaO_(s) + H₂SO_4(aq) ➔ CaSO_4(aq) + H₂O_(l)
• Ca(OH)_2(s) + H₂SO_4(aq) ➔ CaSO_4(aq) + 2H₂O_(l)

Thermal decomposition of carbonates and nitrates

Group 1 compounds

Group 1 carbonates are very stable and don't decompose with Bunsen burner heating.An exception is lithium carbonate which decomposes to form lithium oxide and carbon dioxide:

• Li₂CO_3(s) ➔ Li₂O_(s) + CO_2(g)

Group 1 nitrates (MNO₃) decompose to nitrites and oxygen, for example:

• 2NaNO_3(s) ➔ 2NaNO_2(s) + O_2(g) An exception is lithium nitrate which decomposes to form lithium oxide, nitrogen dioxide and oxygen:

• 2LiNO_3(s) ➔ Li₂O_(s) + 2NO_2(g) + 1⁄2O_2(g)

Group 2 compounds

Group 2 carbonates (MCO₃) decompose to oxides and carbon dioxide, for example:

• CaCO_3(s) ➔ CaO_(s) + CO_2(g)

Group 2 nitrates, (M(NO₃)₂) decompose to oxides, nitrogen dioxide and oxygen, for example:

• 2Ca(NO₃)_2(s) ➔ 2CaO_(s) + 4NO_2(g) + O_2(g)

Thermal stability of carbonates and nitrates

The thermal stability of carbonates and nitrates increases as we move down both group 1 and group 2. To understand this trend, we first need to introduce two key concepts:

• Ion polarisation - The distortion of an anion's electron cloud by a nearby cation.
• Polarising power - A cation's ability to distort an anion's electron cloud.

The trend in thermal stability can be explained by considering two main factors:

1. Cation size:

• As we move down the group, cations become larger.
• Larger cations have lower charge density (charge spread over a larger area).
• This results in lower polarising power, causing less anion distortion.
• Less distortion leads to stronger C-O and N-O bonds, increasing stability.

1. Cation charge:

• Group 2 cations (+2) have higher charge than group 1 cations (+1).
• Higher charge results in greater polarising power.
• This makes group 2 compounds generally less stable than their group 1 counterparts.

Testing the thermal stability of carbonates and nitrates

• Group 2 carbonates - Measure the time taken to produce enough CO₂ gas to turn limewater cloudy.
• Group 1 nitrates - Measure the time taken to produce enough O₂ gas to relight a glowing splint.
• Group 2 nitrates - In a fume cupboard, measure the time taken to produce brown NO₂ gas.

Identifying carbonate and hydrogencarbonate ions

To confirm the presence of carbonate (CO₃^2-) or hydrogencarbonate (HCO₃^-) ions:

1. Add dilute nitric acid (HNO₃) dropwise to a sample containing an unknown ion.
2. Observe carefully for effervescence caused by bubbles of carbon dioxide (CO₂) gas being evolved: CO₃^2-_(aq) + 2H⁺_(aq) ➔ CO_2(g) + H₂O_(l)

HCO₃^-_(aq) + H⁺_(aq) ➔ CO_2(g) + H₂O_(l)

Confirm the presence of CO₂ by passing bubbles of the gas through limewater (calcium hydroxide solution), in a connecting test tube. If CO₂ is present, the limewater will turn cloudy due to the formation of a white calcium carbonate precipitate, confirming the original presence of carbonate ions in the sample:

CO_2(g) + Ca(OH)_2(aq) ➔ CaCO_3(s) + H₂O_(l)

Identifying sulfate ions

To confirm the presence of sulfate (SO₄^2-) ions:

1. Add dilute nitric acid (HNO₃) dropwise to a sample containing an unknown ion.
2. Add a few drops of a solution containing aqueous Ba²⁺ ions, such as barium chloride (BaCl₂) or barium nitrate (Ba(NO₃)₂), to the sample.
3. Observe carefully for the formation of a white precipitate of barium sulfate (BaSO₄): Ba²⁺_(aq) + SO₄^2-_(aq) ➔ BaSO_4(s)