Group 2: The Alkaline Earth Metals

1. Trends Down Group 2 (The Alkaline Earth Metals)

The elements of Group 2 (Magnesium to Barium) possess two valence electrons in their outermost \(s\) orbital (\(ns^2\)).

• Atomic radius increases down the group as extra electron shells are added.
• First and second ionisation energies decrease down the group due to increased shielding and greater distance from the nucleus.
• Reactivity increases down the group because less energy is required to remove the two outer electrons.

The first ionisation energy falls even though the nuclear charge increases. The extra shells put the outer electrons further from the nucleus and shield them more. See the ionisation energy notes.

2. Reactions with Water and Steam

Reaction with Cold Water

Group 2 metals react with liquid water to produce metal hydroxides and hydrogen gas:

\[ \text{M}(s) + 2\text{H}_2\text{O}(l) \rightarrow \text{M(OH)}_2(aq) + \text{H}_2(g) \]

Mg(OH)₂ and Ca(OH)₂ are only sparingly soluble, so they form as a solid or a cloudy suspension rather than a solution.

• Magnesium: Reacts extremely slowly with cold water over days, forming a weakly alkaline suspension of Mg(OH)₂.
  Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)
• Calcium: Reacts steadily with cold water (steady fizzing). The liquid turns cloudy as slightly soluble Ca(OH)₂ precipitates.
  Ca + 2H₂O → Ca(OH)₂ + H₂
• Strontium: Reacts in the same way as calcium, more vigorously.
• Barium: Reacts vigorously and exothermically, forming a clear alkaline solution of soluble Ba(OH)₂.

Magnesium Reaction with Steam

3. Opposing Solubility Trends

A classic OxfordAQA exam requirement is mastering the opposing solubility patterns of Group 2 hydroxides versus sulfates:

4. Industrial and Medical Applications

(a) Barium Sulfate (BaSO₄) as a Barium Meal

Barium sulfate is administered to patients as a radio-opaque contrast agent for X-ray imaging of the digestive system. Although free Ba²⁺ ions are highly poisonous, BaSO₄ is insoluble in water, so it passes harmlessly through the gut without entering the bloodstream.

(b) Magnesium Hydroxide (Mg(OH)₂) in Medicine

Used as an antacid (Milk of Magnesia) to treat indigestion and heartburn. It neutralises excess stomach hydrochloric acid (Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O). Because it is only sparingly soluble, it does not create a hazardous caustic environment in the gut.

(c) Calcium Hydroxide (Ca(OH)₂) in Agriculture

Known as slaked lime, it is spread on acidic fields to raise soil pH and optimise crop yields.

(d) Titanium Extraction Using Magnesium (Kroll Process)

Titanium cannot be extracted by reducing titanium dioxide with carbon because brittle titanium carbide (TiC) would form. Instead, titanium is extracted in a multi-step process using magnesium:

1. TiO₂ is converted to titanium(IV) chloride: \[ \text{TiO}_2(s) + 2\text{Cl}_2(g) + 2\text{C}(s) \rightarrow \text{TiCl}_4(l) + 2\text{CO}(g) \]
2. TiCl₄ is purified by fractional distillation.
3. TiCl₄ is reduced by molten magnesium in an inert argon atmosphere at approximately 1000°C: \[ \text{TiCl}_4(g) + 2\text{Mg}(l) \rightarrow \text{Ti}(s) + 2\text{MgCl}_2(l) \]

(e) Flue Gas Desulfurisation (FGD)

Waste gases from coal-fired power stations contain acidic sulfur dioxide (SO₂). It is removed by spraying with an alkaline slurry of calcium oxide or calcium carbonate:

\[ \text{CaO}(s) + \text{SO}_2(g) \rightarrow \text{CaSO}_3(s) \]

\[ \text{CaCO}_3(s) + \text{SO}_2(g) \rightarrow \text{CaSO}_3(s) + \text{CO}_2(g) \]

5. Practice Questions