Reactions and Solubility

The Group 2 elements undergo redox and neutralisation reactions. Key aspects of Group 2 chemistry include the solubility trends of their hydroxides and sulfates, which are highly tested in AQA exams.

1. Reactions of Group 2 Metals with Water

Reactivity increases down Group 2. The metals react with water to produce a metal hydroxide and hydrogen gas: \[ \text{M(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{M(OH)}_2\text{(aq)} + \text{H}_2\text{(g)} \]

The rate of reaction increases down the group due to the decrease in ionisation energy:

• Beryllium: Does not react with water or steam at all due to a thick, protective oxide layer and high ionisation energy.
• Magnesium: Reacts extremely slowly with cold water, forming a sparingly soluble layer of magnesium hydroxide: \[ \text{Mg(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(aq)} + \text{H}_2\text{(g)} \] However, magnesium reacts rapidly and vigorously with steam. Because of the high temperature, the hydroxide decomposes, yielding solid magnesium oxide (a white ash) and hydrogen gas. The reaction produces a bright white flame: \[ \text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)} \]
• Calcium, Strontium, Barium: React with cold water with increasing vigour. Calcium reacts steadily, producing a cloudy white suspension of calcium hydroxide (slaked lime) as it is sparingly soluble. Strontium and barium react rapidly to produce increasingly clear, strongly alkaline solutions because their hydroxides are more soluble.

2. Reactions of Group 2 Oxides with Water

Group 2 metal oxides are basic. They react with water to form metal hydroxides: \[ \text{MO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{M(OH)}_2\text{(aq/s)} \]

The pH of the resulting solution increases down the group (from pH ~9 to ~14). This is because the solubility of the Group 2 hydroxides increases down the group, allowing more hydroxide ions (\(\text{OH}^-\)) to dissolve and release into the solution:

• \(\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)}\) (pH ~ 9 due to sparingly soluble nature)
• \(\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}\) (pH ~ 12, slightly soluble)
• \(\text{BaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ba(OH)}_2\text{(aq)}\) (pH ~ 13-14, soluble)

3. Reactions of Group 2 Oxides and Hydroxides with Acid

Since Group 2 oxides and hydroxides are basic, they react with dilute acids in neutralisation reactions to form a salt and water:

• Reaction of oxide with hydrochloric acid: \[ \text{MgO(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{O(l)} \]
• Reaction of hydroxide with hydrochloric acid: \[ \text{Mg(OH)}_2\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + 2\text{H}_2\text{O(l)} \]
• Reaction of oxide with nitric acid: \[ \text{BaO(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow \text{Ba(NO}_3)_2\text{(aq)} + \text{H}_2\text{O(l)} \]

4. Solubility Trends of Hydroxides and Sulfates

The trends in solubility down Group 2 for hydroxides and sulfates are opposite to one another. This is a critical concept to memorise:

Hydroxide Solubility (Increases Down Group)

Magnesium hydroxide, \(\text{Mg(OH)}_2\), is sparingly soluble (almost insoluble). Moving down the group, the solubility increases until barium hydroxide, \(\text{Ba(OH)}_2\), which is highly soluble in water.

Sulfate Solubility (Decreases Down Group)

Magnesium sulfate, \(\text{MgSO}_4\), is highly soluble. Moving down the group, the solubility decreases until barium sulfate, \(\text{BaSO}_4\), which is completely insoluble.

5. Practical Uses of Group 2 Compounds

AQA A-Level Chemistry requires you to know several critical, real-world applications of Group 2 compounds based on their solubility and chemical properties:

• Magnesium Hydroxide, \(\text{Mg(OH)}_2\) (Milk of Magnesia): Used as an antacid medicine to treat indigestion. It neutralises excess hydrochloric acid in the stomach: \[ \text{Mg(OH)}_2\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + 2\text{H}_2\text{O(l)} \] Because \(\text{Mg(OH)}_2\) is sparingly soluble, it is safe to swallow as it does not dissolve quickly enough in the body to make the digestive tract highly alkaline and cause tissue damage.
• Calcium Hydroxide, \(\text{Ca(OH)}_2\) (Slaked Lime): Used in agriculture to neutralise acidic soils. Correcting soil pH is essential for plants to absorb nutrients efficiently.
• Barium Sulfate, \(\text{BaSO}_4\) (Barium Meal): Used in medicine as a contrast agent for X-ray imaging of the digestive system. Barium atoms are heavy and absorb X-rays well, making the gut outline visible on the scan. Although barium ions (\(\text{Ba}^{2+}\)) are highly toxic to humans, \(\text{BaSO}_4\) is completely safe to ingest because it is extremely insoluble and cannot dissolve into the bloodstream. It passes harmlessly through the body.

6. Testing for Sulfate Ions (\(\text{SO}_4^{2-}\))

The insolubility of barium sulfate forms the basis of the analytical test for sulfate ions in solution:

1. Acidify with Hydrochloric Acid: Add a small amount of dilute \(\text{HCl}\) to the test sample. This reacts with and removes any carbonate (\(\text{CO}_3^{2-}\)) or sulfite (\(\text{SO}_3^{2-}\)) impurities, preventing them from forming barium carbonate or barium sulfite precipitates. (Do not acidify with sulfuric acid, as this would introduce sulfate ions and give a false positive!).
2. Add Barium Chloride: Add barium chloride solution (\(\text{BaCl}_2\)).
3. Observation: If sulfate ions are present, a dense white precipitate of barium sulfate forms: \[ \text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} \]