Edexcel International A Level Chemistry: The elements of Groups 1 and 2 (YCH11) – Revision Notes

These notes condense Topic 8B, The elements of Groups 1 and 2, from the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017, namely outcomes 8.11 to 8.23, with Core Practicals 3 and 4. Topic 8B falls within Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols (IAS). Longer explanations, with fully worked examples, are in the study guide.

Once these notes feel familiar, move on to the practice questions. Other Edexcel IAL Chemistry topics are listed on the course hub and printable checklist, and a free 10-minute diagnostic points you to weak spots. For mole basics, see the amount of substance revision notes.

Trends down the groups (8.11, 8.12)

Ionisation energy falls down Group 1 and down Group 2. Give all three reasons:

• an extra shell each time, which places the outer electron further out;
• more inner-shell shielding;
• these outweigh the larger nuclear charge.

Reactivity rises (Li → K; Mg → Ba) because the outer electron or electrons are lost more easily. The metals act as reducing agents: Na → Na⁺ + e⁻; Ca → Ca²⁺ + 2e⁻.

Equations to know (8.13, 8.14)

With	Group 1	Group 2
O₂	4Li + O₂ → 2Li₂O; 2Na + O₂ → Na₂O₂ (excess O₂); K + O₂ → KO₂	2Ca + O₂ → 2CaO (same pattern for Mg, Sr, Ba)
Cl₂	2K + Cl₂ → 2KCl	Sr + Cl₂ → SrCl₂
H₂O	2Li + 2H₂O → 2LiOH + H₂	Ba + 2H₂O → Ba(OH)₂ + H₂; Mg + H₂O(g) → MgO + H₂
Oxide + water	K₂O + H₂O → 2KOH	SrO + H₂O → Sr(OH)₂
Oxide + acid	Li₂O + H₂SO₄ → Li₂SO₄ + H₂O	MgO + 2HNO₃ → Mg(NO₃)₂ + H₂O
Hydroxide + acid	NaOH + HCl → NaCl + H₂O	Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Observations worth a mark: sodium melts into a silvery ball; potassium burns with a lilac flame on water; magnesium burns with a dazzling white light; calcium in water gives steady fizzing and a white suspension.

Oxidation number of oxygen: oxide −2, peroxide (O₂²⁻) −1, superoxide (O₂⁻) −½.

Solubility (8.15)

Down Group 2	Hydroxides	Sulfates
Trend	more soluble	less soluble
Least soluble	Mg(OH)₂	BaSO₄
Most soluble	Ba(OH)₂	MgSO₄

Memory hook: the two trends run in opposite directions, and barium sits at the extreme of each.

Thermal stability (8.16)

Method in steps for an “explain” answer:

1. Name the cation’s charge and size (ionic radius).
2. Say the cation polarises (distorts) the electron cloud of the large anion.
3. Say this weakens the C–O or N–O bonds in the anion.
4. Conclude: smaller or more highly charged cation → more polarising → decomposes more easily.

Decomposition products at a glance

Compound	Products on heating
Group 2 carbonate	oxide + CO₂
Li₂CO₃	Li₂O + CO₂
Na₂CO₃, K₂CO₃	no change at Bunsen temperatures
Group 2 nitrate	oxide + NO₂ + O₂
LiNO₃	Li₂O + NO₂ + O₂
NaNO₃, KNO₃	nitrite (MNO₂) + O₂ only

Order of stability: MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃, and the nitrates follow the same order. Group 1 salts beat Group 2 salts because the cation has half the charge and is larger.

Flame colours (8.17, 8.18)

Li⁺	Na⁺	K⁺	Ca²⁺	Sr²⁺	Ba²⁺	Mg²⁺
crimson	yellow-orange	lilac	brick red	crimson red	pale green	none visible

Why colours form: electrons absorb heat energy and jump to higher levels, then drop back, emitting light. The energy gaps are fixed for each element, so the frequencies (and colour) are characteristic.

Procedure in steps: clean nichrome wire in concentrated HCl until it gives no colour → moisten with acid → pick up solid → hold in the hottest part of a non-luminous flame → record the colour.

Gas tests and decomposition practical (8.18)

Gas	Test or identifying feature
CO₂	turns limewater milky
O₂	relights a glowing splint
NO₂	brown gas; turns damp blue litmus red (acidic)

Fair comparison: same amount in moles, same tube, same flame and height, same volume of limewater. Time to cloudiness or to the first brown fumes measures how easily each salt decomposes.

Ion tests (8.19)

• Carbonate / hydrogencarbonate: dilute acid → fizzing → limewater milky. CO₃²⁻ + 2H⁺ → CO₂ + H₂O; HCO₃⁻ + H⁺ → CO₂ + H₂O.
• Sulfate: dilute HCl then BaCl₂(aq) → white precipitate. Ba²⁺ + SO₄²⁻ → BaSO₄. The acid removes carbonate, which would also give a white precipitate.
• Ammonium: NaOH(aq) and warm → ammonia, which turns damp red litmus blue and gives white smoke with HCl fumes. NH₄⁺ + OH⁻ → NH₃ + H₂O; NH₃ + HCl → NH₄Cl.

Titration and uncertainty (8.20 to 8.23)

Quantity	Formula
Amount from solution	n = c × V (V in dm³)
Concentration	c = n ÷ V
Mass concentration	g dm⁻³ = c × M
Percentage uncertainty	(uncertainty ÷ measured value) × 100
Titre uncertainty	2 × uncertainty of one burette reading
Overall uncertainty	add percentage uncertainties

Indicators: methyl orange red → orange as alkali goes in (yellow in alkali); phenolphthalein colourless → pink as alkali goes in.

Method in steps for a titration calculation:

1. Pick concordant titres and take their mean.
2. Moles of the known solution = c × V.
3. Use the equation ratio to get moles of the unknown.
4. Scale up if only a portion of a solution was titrated.
5. Divide by volume in dm³; multiply by M for g dm⁻³.

Core Practical 3: titrate a pipetted portion of hydrochloric acid with sodium hydroxide of known concentration. Core Practical 4: weigh a solid acid by difference, dissolve, transfer with rinsings to a volumetric flask, make up to the mark, invert to mix, then titrate the sodium hydroxide against it.

Cutting uncertainty: bigger masses and titres; rinse burette and pipette with their own solution; read the bottom of the meniscus at eye level; add dropwise near the end point; swirl constantly.

Quick self-test

1. Give two reasons why the first ionisation energy of potassium is lower than that of sodium.
2. Write an equation for potassium burning in oxygen and give the oxidation number of oxygen in the product.
3. Write an equation for magnesium reacting with steam.
4. Which Group 2 hydroxide is least soluble, and which Group 2 sulfate is least soluble?
5. Write an equation for the thermal decomposition of lithium nitrate.
6. Explain in one sentence why barium carbonate is more thermally stable than calcium carbonate.
7. Calculate the volume of CO₂ (in cm³, at room temperature and pressure) from complete decomposition of 0.843 g of MgCO₃ (Mr 84.3). Use 24.0 dm³ mol⁻¹.
8. Which ion gives a crimson red flame in Group 2?
9. A solution of KOH has concentration 0.0850 mol dm⁻³. Give its concentration in g dm⁻³ (Mr 56.1).
10. 20.0 cm³ of 0.0500 mol dm⁻³ H₂SO₄ is neutralised by 16.00 cm³ of NaOH. Find c(NaOH).
11. A titre is 18.30 cm³ and each burette reading is ±0.05 cm³. Find the percentage uncertainty.
12. Describe the test for ammonium ions, with the result.

Answers

1. Potassium has one more occupied shell, so its outer electron is more distant from the nucleus, and inner electrons shield it more.
2. K + O₂ → KO₂; oxygen is −½.
3. Mg + H₂O → MgO + H₂
4. Mg(OH)₂; BaSO₄.
5. 4LiNO₃ → 2Li₂O + 4NO₂ + O₂
6. Ba²⁺ is larger than Ca²⁺ with the same charge, so it polarises the carbonate ion less.
7. n = 0.843 ÷ 84.3 = 0.0100 mol; V = 0.0100 × 24 000 = 240 cm³.
8. Sr²⁺.
9. 0.0850 × 56.1 = 4.77 g dm⁻³.
10. n(H₂SO₄) = 1.00 × 10⁻³ mol; n(NaOH) = 2.00 × 10⁻³ mol; c = 2.00 × 10⁻³ ÷ 0.01600 = 0.125 mol dm⁻³.
11. (0.10 ÷ 18.30) × 100 = 0.55%.
12. Warm with NaOH(aq); the gas turns damp red litmus blue (and forms white smoke with HCl fumes).

Where marks are usually lost

• Stopping at “more shells” for the ionisation energy trend without linking it to a weaker attraction for the outer electron.
• Writing MO₂ or M₂O for Group 2 oxides, or an unbalanced 2Na + O₂ → Na₂O.
• Giving H₂ and Mg(OH)₂ for magnesium with steam; with steam the product is MgO.
• Describing “a precipitate” in the sulfate test without the colour, or leaving out the acid.
• Using dry litmus for ammonia; the paper must be damp, because ammonia acts as an alkali only in solution.
• Forgetting that NO₂ is identified by its brown colour and acidic reaction, not by a splint test.
• Mixing up the Group 1 and Group 2 nitrate equations, or giving NO₂ from sodium nitrate.
• Writing “the atom polarises the anion”; it is the cation that does it.
• Including the rough titre in the mean, or quoting the mean to fewer decimal places than the readings.
• Forgetting to scale up when only 25.0 cm³ of a 250 cm³ solution was titrated.
• Adding absolute uncertainties of different quantities instead of percentage uncertainties.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited. Scope: Topic 8B, outcomes 8.11 to 8.23.