OCR A Level Chemistry A: Group 2 (H432) – Revision Notes

These notes condense section 3.1.2, Group 2, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover all five learning outcomes, 3.1.2(a) to (e), in Module 3, Periodic table and energy. All of it is full A level content; components 01 and 03 assess Module 3, according to the specification. For full explanations and six worked examples, read the Group 2 study guide first.

Then try the Group 2 practice questions. Other H432 sections can be reached through the OCR chemistry hub. Keep a record of secure outcomes with the printable checklist, and run the free diagnostics to spot gaps elsewhere.

3.1.2(a): s² and the 2+ ion

• Outer configuration: ns². Mg 3s², Ca 4s², Sr 5s², Ba 6s².
• Both s electrons are lost, giving M²⁺ with the previous noble gas configuration.
• Oxidation number of the metal: 0 → +2. The metal is oxidised in every reaction in this section.
• The partner species is reduced: O (0 → −2) with oxygen; H (+1 → 0) with water or acid.

Method in steps: proving a Group 2 reaction is redox

1. Write the oxidation number of the metal on each side (0 and +2).
2. Find the element whose oxidation number falls.
3. Say which is oxidised and which is reduced, each with its numbers.
4. Check electrons lost = electrons gained.

3.1.2(b): The three reactions

Reaction	General equation	Notes
Oxygen	2M(s) + O₂(g) → 2MO(s)	White oxides; Mg burns with a brilliant white light once heated
Cold water (Ca, Sr, Ba)	M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)	Faster down the group; calcium gives a cloudy suspension
Cold water (Mg)	Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)	Very slow
Steam (Mg)	Mg(s) + H₂O(g) → MgO(s) + H₂(g)	Fast; product is the oxide
Dilute acid	M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g)	Salt + hydrogen only (specification limit)

Reactivity order: Mg < Ca < Sr < Ba.

Typical observations for a metal reacting with water or acid: fizzing (effervescence), the metal getting smaller and disappearing, and the mixture getting warmer.

Worked reminder. Show that Ba + 2HCl → BaCl₂ + H₂ is redox:

Ba:  0 → +2     loses 2 e⁻      oxidised
H:  +1 → 0      2 H each gain 1 e⁻ = 2 e⁻      reduced
Cl: −1 → −1     unchanged

Comparing two metals in one answer

When a question asks you to compare two Group 2 metals, use a comparative for every point: “calcium fizzes more vigorously than magnesium”, “barium disappears faster than strontium”. A list of separate observations for each metal, with no comparison, does not answer the question. If the amounts of metal in moles are the same, the final volume of hydrogen is the same; only the rate differs.

3.1.2(c): Ionisation energies and reactivity

Second ionisation energy, as an equation (a definition is not required):

Sr⁺(g) → Sr²⁺(g) + e⁻

Standard values, kJ mol⁻¹:

	Mg	Ca	Sr	Ba
First	738	590	550	503
Second	1451	1145	1064	965
Sum	2189	1735	1614	1468

Method in steps: the reactivity explanation

1. Down the group the atomic radius increases (an extra shell each time).
2. Shielding by inner shells increases.
3. Nuclear attraction on the outer electrons decreases, even though nuclear charge increases.
4. So the first and second ionisation energies decrease.
5. So the two outer electrons are lost more easily, and reactivity increases.

All five links are needed for a full explanation. “Easier to lose electrons” on its own is only the last step.

First ionisation energy and its trends are revised in the Periodicity revision notes.

The M3.1 label on this outcome means you should move freely between a table, a graph and a sentence about the trend.

Reading the data. From Mg to Ca the sum of the first two ionisation energies drops by 454 kJ mol⁻¹. From Sr to Ba it drops by 146 kJ mol⁻¹. The fall continues all the way down, so each metal reacts more readily than the one above it. On a graph of ionisation energy against element, both lines slope downwards from left to right, with the second ionisation energy line above the first for every element: removing an electron from a positive ion always needs more energy than removing one from a neutral atom of the same element.

3.1.2(d): Oxides with water

MO(s) + H₂O(l) → M(OH)₂
M(OH)₂ → M²⁺(aq) + 2OH⁻(aq)

Solution from	Approximate pH
MgO	about 10
CaO	about 12
SrO	about 13
BaO	about 13 or above

• The hydroxides get more soluble down the group, so [OH⁻] rises and the alkalinity increases.
• No oxidation numbers change: this is an acid-base type reaction, not redox.

3.1.2(e): Bases in use

Use	Compound	Equation
Neutralising acid soil	Ca(OH)₂	Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O
Antacid	Mg(OH)₂	Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Antacid	CaCO₃	CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

The specification says “for example (but not limited to)”, so expect other Group 2 bases, such as CaO or CaCO₃ on soils, with an equation you build yourself.

Worked reminder. Mass of Mg(OH)₂ (Mr 58.3) that neutralises 25.0 cm³ of 0.400 mol dm⁻³ HCl:

n(HCl)      = 0.0250 × 0.400 = 0.0100 mol
n(Mg(OH)₂)  = 0.0100 ÷ 2     = 0.00500 mol
mass        = 0.00500 × 58.3 = 0.292 g

Method in steps: any neutralisation by a Group 2 base

1. Write the balanced equation.
2. Find moles of the known substance.
3. Use the ratio: one M(OH)₂, MO or MCO₃ reacts with two H⁺.
4. Convert to mass, volume or concentration as asked.

Must-know distinctions

• Second ionisation energy vs forming the 2+ ion. M⁺(g) → M²⁺(g) + e⁻ is the second ionisation. M(g) → M²⁺(g) + 2e⁻ is the sum of the first and second.
• Magnesium with water vs steam. Cold water gives Mg(OH)₂, very slowly. Steam gives MgO, quickly.
• Metal with water vs oxide with water. The metal reaction is redox and releases hydrogen. The oxide reaction is not redox and releases no gas.
• Hydroxide vs carbonate antacid. Both neutralise two H⁺; only the carbonate releases CO₂.
• Nuclear charge vs nuclear attraction. Charge increases down the group; attraction on the outer electrons decreases.

Quick self-test

1. Write the electron configuration of Ca²⁺ in full.
2. Write the equation for the second ionisation energy of magnesium.
3. Name the solid product when heated magnesium reacts with steam.
4. Write the equation, with state symbols, for barium reacting with water.
5. In Ca + 2HCl → CaCl₂ + H₂, give the oxidation number change for the element reduced.
6. Use the table in 3.1.2(c) to find the energy needed to form one mole of Sr²⁺(g) from Sr(g).
7. 2.43 g of magnesium (Ar 24.3) reacts with excess dilute hydrochloric acid. Calculate the volume of hydrogen at RTP, in dm³ (24.0 dm³ mol⁻¹).
8. Calculate the mass of CaCO₃ (Mr 100.1) that neutralises 0.0300 mol of HCl.
9. Calculate the mass of strontium oxide formed when 4.38 g of strontium (Ar 87.6) burns completely. Mr(SrO) = 103.6.
10. Calculate the mass of Ca(OH)₂ (Mr 74.1) needed to neutralise 0.500 mol of nitric acid.
11. Which of MgO and SrO gives the solution with the higher pH, and why?
12. Is the reaction of calcium oxide with water a redox reaction? Give a reason.

Answers

1. 1s² 2s² 2p⁶ 3s² 3p⁶
2. Mg⁺(g) → Mg²⁺(g) + e⁻
3. Magnesium oxide, MgO.
4. Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)
5. Hydrogen, +1 → 0.
6. 550 + 1064 = 1614 kJ mol⁻¹
7. n(Mg) = 2.43 ÷ 24.3 = 0.100 mol = n(H₂); V = 0.100 × 24.0 = 2.40 dm³
8. n(CaCO₃) = 0.0300 ÷ 2 = 0.0150 mol; mass = 0.0150 × 100.1 = 1.50 g
9. n(Sr) = 4.38 ÷ 87.6 = 0.0500 mol = n(SrO); mass = 0.0500 × 103.6 = 5.18 g
10. n(Ca(OH)₂) = 0.500 ÷ 2 = 0.250 mol; mass = 0.250 × 74.1 = 18.5 g
11. SrO. Strontium hydroxide is more soluble than magnesium hydroxide, so its solution has a higher OH⁻ concentration.
12. No. Calcium stays +2, oxygen −2 and hydrogen +1 throughout.

Where marks are usually lost

• Writing the second ionisation as M(g) → M²⁺(g) + 2e⁻.
• Leaving state symbols off ionisation equations.
• Giving Mg(OH)₂ as the product with steam, or MgO with cold water.
• Explaining the trend with “more protons so less attraction”; the decrease is due to distance and shielding.
• Stopping at “loses electrons more easily” without linking to ionisation energy and atomic radius.
• Calling the oxide-water reaction redox.
• Using a 1 : 1 ratio between a Group 2 base and HCl.
• Forgetting CO₂ and H₂O in the carbonate antacid equation.
• Describing barium as “less reactive because it is bigger”; size makes it more reactive.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 3.1.2 Group 2, learning outcomes (a) to (e). Published by OCR.