4.2.1Physical Chemistry
Reactions of Group 2 Elements — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 99 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4General Reactions
M stands for any Group 2 metal, which forms M²⁺ ions in its compounds.
With oxygen
- With oxygen 2M(s) + O₂(g) → 2MO(s), giving a metal oxide; Mg burns with a bright white flame.
With water
- With water M(s) + 2H₂O(l) → M(OH)₂(aq or s) + H₂(g); beryllium does not react with water.
With dilute HCl
- With dilute HCl M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g), with fizzing as hydrogen forms.
With dilute H₂SO₄
- With dilute H₂SO₄ M(s) + H₂SO₄(aq) → MSO₄ + H₂(g), but SrSO₄ and BaSO₄ are insoluble.
Magnesium and steam
- Magnesium and steam Mg(s) + H₂O(g) → MgO(s) + H₂(g), as magnesium reacts only very slowly with cold water.
Part 2 of 4Trend in Reactivity
Trend in Reactivity
Down the group
- Down the group Reactivity increases from Mg to Ba, so reactions with water and acids become more vigorous.
Reason
- Reason Atoms get larger and the outer electrons are further from the nucleus and more shielded, so they are lost more easily.
Ionisation energy
- Ionisation energy The first and second ionisation energies decrease down the group, so M²⁺ forms more easily.
Observation
- Observation Calcium reacts steadily with cold water, giving bubbles and a cloudy white suspension of calcium hydroxide.
Part 3 of 4Solubility Trends of Compounds
Solubility Trends of Compounds
Hydroxides
- Hydroxides The solubility of Group 2 hydroxides increases down the group, so Mg(OH)₂ is almost insoluble and Ba(OH)₂ is soluble.
Sulfates
- Sulfates The solubility of the sulfates decreases down the group, so BaSO₄ is the least soluble.
Alkalinity
- Alkalinity The more soluble hydroxides give a higher pH, so the water solution of barium hydroxide is more alkaline.
Test use
- Test use These solubility differences are used to identify Group 2 ions, as with barium chloride and sulfate ions.
| Metal | Reaction with water | Hydroxide solubility |
|---|---|---|
| Mg | Very slow with cold water | Very low |
| Ca | Steady fizzing, cloudy solution | Low |
| Sr | More vigorous than Ca | Higher |
| Ba | Vigorous | Highest of the four |
Part 4 of 4Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Group 2 The alkaline earth metals, with two electrons in the outer shell, forming M²⁺ ions.
- Reactivity How readily a metal loses electrons to form positive ions.
- Ionisation energy The energy needed to remove one electron from each atom in a mole of gaseous atoms.
- Insoluble Not dissolving to any appreciable extent in water.
Common Mistakes
- Saying reactivity decreases down Group 2; it increases because the outer electrons are lost more easily.
- Forgetting that beryllium does not react with water.
- Writing M(OH) instead of M(OH)₂, as Group 2 ions have a 2+ charge.
- Saying barium sulfate dissolves; it is insoluble and forms a white precipitate.
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