OxfordAQA A-Level Chemistry: Group 2, the alkaline earth metals (9620) – Revision Notes

These notes cover section 3.2.2.1 Group 2, the alkaline earth metals of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. It is all International AS content, with nothing International A-level only, and sits in Unit 1 (Inorganic 1 and Physical 1); Unit 5 can draw on all content. The reasoning behind each point, with worked examples, is in the Group 2 study guide first.

Then test yourself with the Group 2 practice questions. The OxfordAQA A-Level Chemistry hub links every topic, the printable checklist lists every outcome, and the free 10-minute diagnostics help you decide what to revise first. The full Required practical 3 table, including halide tests, is in the halogens revision notes.

1. Core facts

• Elements in this topic: Mg, Ca, Sr, Ba. Outer configuration ns², so s block.
• They react by losing two electrons: M → M²⁺ + 2e⁻. Oxidation state +2 in compounds.
• Structure of each element: giant metallic lattice.

2. Physical trends (Mg → Ba)

Property	Direction	One-line reason
Atomic radius	increases	an extra occupied shell at each step
First ionisation energy	decreases	outer electron further out and better shielded, so weaker attraction
Melting point	falls from Ca to Ba; Mg is lower than Ca	2+ ions get bigger, so ion–electron attraction weakens

Steps: writing a first ionisation energy explanation

1. Name the shell: outer electron is in a higher shell (e.g. 5s vs 4s).
2. Shielding: more inner shells between it and the nucleus.
3. Compare: weaker attraction, so less energy needed.
4. Deal with nuclear charge: it rises, but steps 1 and 2 outweigh it.

Equation, gaseous on both sides: Ba(g) → Ba⁺(g) + e⁻

Steps: writing a melting point explanation

1. Structure: giant metallic lattice.
2. Bonding: attraction between M²⁺ ions and delocalised electrons.
3. Size: the ion higher in the group is smaller, charge is the same.
4. Result: stronger attraction, more energy to overcome, higher melting point.

Magnesium is the exception; its crystal arrangement differs, so the size argument does not predict it.

3. Reactions with water

Metal	Conditions	Equation	Key observation
Mg	cold water	Mg + 2H₂O → Mg(OH)₂ + H₂	barely any bubbles
Mg	steam	Mg + H₂O → MgO + H₂	bright white flame, white powder left
Ca	cold water	Ca + 2H₂O → Ca(OH)₂ + H₂	fizzing, cloudy white liquid
Sr	cold water	Sr + 2H₂O → Sr(OH)₂ + H₂	faster than Ca
Ba	cold water	Ba + 2H₂O → Ba(OH)₂ + H₂	fastest

• Trend: more vigorous down the group, because the outer electrons are removed more easily.
• Redox: metal 0 → +2 (oxidised); H +1 → 0 (reduced).
• One mole of metal gives one mole of H₂ and one mole of M(OH)₂, which supplies two moles of OH⁻.

4. Titanium extraction

TiCl₄ + 2Mg → Ti + 2MgCl₂
Ti: +4 → 0 (reduced)      Mg: 0 → +2 (oxidised, so Mg is the reducing agent)

Worked reminder. Mass of TiCl₄ that reacts with 1.215 kg of Mg (Mr TiCl₄ 189.9; Aᵣ Mg = 24.3):

n(Mg)       = 1215 ÷ 24.3    = 50.0 mol
n(TiCl₄)    = 50.0 ÷ 2       = 25.0 mol
mass TiCl₄  = 25.0 × 189.9   = 4748 g = 4.75 kg

5. Solubility trends

Compound	Trend down group	Spec wording to use
M(OH)₂	more soluble	Mg(OH)₂ is sparingly soluble
MSO₄	less soluble	BaSO₄ is insoluble

Hook: “hydroxides rise, sulfates sink”. A saturated Ba(OH)₂ solution is far more alkaline than a saturated Mg(OH)₂ solution because it holds more OH⁻.

6. Uses linked to solubility

Compound	Use	Why it suits the job
Mg(OH)₂	antacid (milk of magnesia)	a base that is sparingly soluble, so it neutralises acid without a strongly alkaline solution
Ca(OH)₂	raising soil pH in agriculture	a cheap base that neutralises H⁺ in acidic soil
CaO or CaCO₃	removing SO₂ from flue gases	bases that react with the acidic gas
BaSO₄	barium meal for X-ray imaging	absorbs X-rays; insoluble, so toxic Ba²⁺ is not absorbed

Equations to learn:

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Ca(OH)₂ + 2H⁺  → Ca²⁺ + 2H₂O
CaO + SO₂      → CaSO₃
CaCO₃ + SO₂    → CaSO₃ + CO₂

CaO and CaCO₃ both react 1 : 1 with SO₂; only the carbonate route releases CO₂.

7. The sulfate test

1. Acidify the sample with dilute HCl (or HNO₃).
2. Add BaCl₂(aq).
3. White precipitate means sulfate:  Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Question	Answer to give
Why BaCl₂?	BaSO₄ is insoluble, so Ba²⁺ precipitates any sulfate present
Why acidify?	to remove CO₃²⁻ (as CO₂), which would otherwise give white BaCO₃
Why not H₂SO₄?	it adds SO₄²⁻, so every sample would test positive

Worked reminder. Excess acidified BaCl₂ is added to 15.0 cm³ of 0.0620 mol dm⁻³ Na₂SO₄ (Mr BaSO₄ = 233.4):

n(SO₄²⁻)   = 0.0150 × 0.0620  = 9.30 × 10⁻⁴ mol
mass BaSO₄ = 9.30 × 10⁻⁴ × 233.4 = 0.217 g

8. Required practical 3: Group 2 cations

Test on a fresh portion	Mg²⁺	Ca²⁺	Sr²⁺	Ba²⁺
add NaOH(aq)	white ppt	slight ppt	very little	none
add dilute H₂SO₄	none	slight ppt	white ppt	heavy white ppt

Read the rows from the trends: a hydroxide precipitate shrinks down the group; a sulfate precipitate grows. Compare solutions of similar concentration.

9. Must-know distinctions

• Atomic radius vs ionisation energy: opposite directions down the group (radius up, IE down).
• Mg with cold water vs steam: hydroxide vs oxide.
• Sparingly soluble vs insoluble: Mg(OH)₂ vs BaSO₄, in the specification’s own words.
• Toxic vs safe barium: soluble barium compounds release Ba²⁺; insoluble BaSO₄ does not.
• Reduced vs reducing agent: titanium is reduced; magnesium is the reducing agent.

10. Linking ideas across the topic

• Radius → IE → reactivity. Bigger atoms hold outer electrons less tightly, so ionisation energy falls and the metals react with water more vigorously. One chain of reasoning explains three trends.
• Solubility → uses. The least soluble hydroxide (Mg) is the gentle antacid; the insoluble sulfate (Ba) is the safe X-ray meal; the insolubility of BaSO₄ is also what makes the sulfate test work.
• Solubility → identification. The same two trends give the NaOH(aq) and dilute H₂SO₄ tests in Required practical 3.
• Redox everywhere. The water reactions and titanium extraction are both redox, with the Group 2 metal oxidised each time.

Quick self-test

1. State the trend in first ionisation energy from Mg to Ba.
2. Name the type of bonding in solid strontium and the particles involved.
3. Which element in Mg–Ba does not fit the melting point trend?
4. Give the products of magnesium with steam.
5. Write an equation for calcium reacting with water.
6. Calculate the amount of hydrogen, in mol, from 2.746 g of barium with excess water. (Aᵣ Ba = 137.3)
7. Which Group 2 hydroxide does the specification call sparingly soluble?
8. Calculate the mass of Ca(OH)₂ that neutralises 0.300 mol of H⁺ in acidic soil. (Mr 74.1)
9. Calculate the mass of SO₂, in kg, removed by 1.683 kg of CaO. (Mr SO₂ = 64.1, CaO = 56.1)
10. Give the oxidation state change of titanium in its extraction.
11. Why is barium chloride solution acidified in the sulfate test?
12. A solution gives a white precipitate with dilute H₂SO₄ but none with NaOH(aq). Which Group 2 ion is most likely present?

Answers

1. It decreases.
2. Metallic bonding: attraction between Sr²⁺ ions and delocalised electrons.
3. Magnesium (its melting point is below calcium’s).
4. Magnesium oxide and hydrogen.
5. Ca + 2H₂O → Ca(OH)₂ + H₂
6. n(Ba) = 2.746 ÷ 137.3 = 0.0200 mol, so 0.0200 mol of H₂.
7. Magnesium hydroxide, Mg(OH)₂.
8. n(Ca(OH)₂) = 0.300 ÷ 2 = 0.150 mol; mass = 0.150 × 74.1 = 11.1 g.
9. n(CaO) = 1683 ÷ 56.1 = 30.0 mol = n(SO₂); mass = 30.0 × 64.1 = 1923 g = 1.92 kg.
10. +4 to 0.
11. To remove carbonate ions, which would also give a white precipitate with barium ions.
12. Ba²⁺.

Where marks are usually lost

• Stating that atoms “get bigger, so IE falls” without naming shielding or the weaker attraction.
• Leaving out state symbols, or writing (s), in a first ionisation energy equation.
• Explaining melting point with “intermolecular forces” or “stronger atoms” instead of metallic bonding.
• Giving the product of magnesium and steam as Mg(OH)₂.
• Swapping the hydroxide and sulfate trends, or calling Mg(OH)₂ “insoluble”.
• Saying BaSO₄ is safe because “barium is not poisonous”; the point is that it does not dissolve.
• Using the wrong TiCl₄ : Mg ratio (it is 1 : 2) in extraction calculations.
• Acidifying the sulfate test with sulfuric acid, or giving no reason for the acid.
• Forgetting that each M(OH)₂ gives two OH⁻ ions in titration or concentration work.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.2.2.1 Group 2, the alkaline earth metals, with the Group 2 part of Required practical 3.