2.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Groups 1 & 2 — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

5 min read 13 parts Lesson 4 of 12 2 quizzes

📚 IAL Edexcel Modular (Chemistry) Hub / Topical Revision

0%read

Lesson summary

2.4 Groups 1 & 2 — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Explain trends in ionisation energy and reactivity in Groups 1 and 2, describe reactions of the elements, oxides and hydroxides, solubility and thermal stability trends, and flame tests and qualitative tests for ions.Outer…

Outer electrons
Across a period
Evidence in Group 2
With oxygen
With chlorine
With water

Section 1 of 13Ionisation energy trends in Groups 1 and 2

Ionisation energy trends in Groups 1 and 2

Outer electrons

Group 1 has one electron in the outer shell and Group 2 has two
Group 1 metals form 1+ ions and Group 2 metals form 2+ ions by losing these electrons, so they are reducing agents and are themselves oxidised.
Group 1 and 2 ions have noble gas configurations.

First and second ionisation energies decrease down each group

Atomic radius and shielding increase down the group.
These outweigh the increase in nuclear charge, so the outer electrons are lost more easily.

Across a period

Group 2 elements have larger first ionisation energies than the Group 1 element in the same period,
because they have more protons and a smaller radius

Section 2 of 13Reactivity trend

Reactivity trend

Reactivity increases down both groups

The outer electron is further from the nucleus and more shielded.
Less energy is needed to remove it, so the metals lose electrons more readily.

Evidence in Group 2

Evidence in Group 2 magnesium reacts extremely slowly with cold water but calcium reacts fairly vigorously in an exothermic reaction

Section 3 of 13Reactions of Group 1 metals

Reactions of Group 1 metals

With oxygen

With oxygen form metal oxides, which is why the metals tarnish in air
The dull oxide coating forms more rapidly down the group.

With chlorine

With chlorine burn to form a white ionic chloride
2M + Cl₂ → 2MCl, for example 2K + Cl₂ → 2KCl.
Sodium burns with an intense orange flame in both chlorine and oxygen.

With water

With water form the hydroxide and hydrogen, and the reaction becomes more vigorous down the group
2M + 2H₂O → 2MOH + H₂.

Section 4 of 13Reactions of Group 2 metals

Reactions of Group 2 metals

With oxygen

With oxygen form oxides: 2M + O₂ → 2MO
Strontium and barium can also form peroxides containing O₂²⁻, as in Ba + O₂ → BaO₂.

With chlorine

With chlorine form chlorides: Mg + Cl₂ → MgCl₂

With water

With water form the hydroxide and hydrogen: M + 2H₂O → M(OH)₂ + H₂
Beryllium does not react with water.
Magnesium reacts only very slowly with cold water, giving a weakly alkaline solution of about pH 9–10 because Mg(OH)₂ is sparingly soluble.
Heated magnesium reacts vigorously with steamMg + H₂O → MgO + H₂.

Section 5 of 13Oxides and hydroxides

Group 1 oxides with water

Group 1 oxides with water give colourless alkaline solutions
Na₂O + H₂O → 2NaOH.
The oxide ion takes protons from water: O²⁻ + H₂O → 2OH⁻.

Group 2 oxides with water

Group 2 oxides with water give alkaline solutions that become more alkaline down the group
MgO is only slightly soluble, giving about pH 10.
CaO reacts vigorously and releases much energy, forming Ca(OH)₂ (limewater is the solution).

Oxides and hydroxides with acids

Oxides and hydroxides with acids form a salt and water
Metal oxide + dilute HCl gives a chloride and water, and metal oxide + dilute H₂SO₄ gives a sulfate and water.
CaO + 2HCl → CaCl₂ + H₂O.

Insoluble sulfate layer

Insoluble sulfate layer when CaO, SrO or BaO react with dilute H₂SO₄, a layer of sparingly soluble sulfate coats the solid and stops the reaction
Using powdered oxide and stirring reduces this problem.

Uses

Mg(OH)₂ is an antacid and Ca(OH)₂ neutralises acidic soil

Section 6 of 13Solubility trends

Solubility trends

Group 2 hydroxides

Group 2 hydroxides solubility increases down the group
Mg(OH)₂ is 2.0 × 10^-5 mol per 100 g water and Ba(OH)₂ is 1.5 × 10^-2.
More OH⁻ ions form, so the solutions are more alkaline.

Group 2 sulfates

Group 2 sulfates solubility decreases down the group
MgSO₄ is soluble and BaSO₄ is insoluble.
BaSO₄ is used for barium meals because toxic Ba²⁺ is not absorbed.

Group 1 compounds

Group 1 compounds all common ones are soluble
Group 1 cations give no precipitates and are identified by flame tests.
Even Ba(OH)₂ is far less soluble than NaOH or KOH.

Section 7 of 13Thermal decomposition of carbonates and nitrates

Thermal decomposition of carbonates and nitrates

Group 2 carbonates

XCO₃ → XO + CO₂
More heat is needed down the group.

Group 1 carbonates

Group 1 carbonates stable at Bunsen temperatures, except Li₂CO₃ → Li₂O + CO₂
Decomposition temperatures rise down the group.

Group 2 nitrates

Group 2 nitrates give the oxide, nitrogen dioxide and oxygen
2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂.

Group 1 nitrates

Group 1 nitrates give the nitrite and oxygen: 2XNO₃ → 2XNO₂ + O₂
Lithium nitrate behaves like Group 2: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂.

Stability comparison

Stability comparison can be made by timing how long until brown gas appears or oxygen relights a glowing splint,
- or until limewater turns milky for carbonates

Section 8 of 13Explaining thermal stability

The cation radius increases down the group.

Charge density and polarising power of the cation decrease.

The nitrate or carbonate ion is polarised less, so its bonds are weakened less.

More heat is needed to decompose the compound, so thermal stability increases.

Explaining thermal stability

Group 1 and Group 2

Group 1 compounds are more stable because their cations have lower charge

Lithium

Lithium behaves like Group 2 because Li⁺ is very small and highly polarising

Section 9 of 13Flame tests

Flame tests

Clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid and heating it in a blue Bunsen flame until it gives no colour.

Dip the wire in concentrated acid, then into the solid sample.

Hold the sample at the edge of the blue flame and record the colour, avoiding a wire that glows red.

Explanation
- heat promotes electrons to higher energy levels
- When they fall back, they emit light of specific wavelengths.

Ion	Flame colour
Li⁺	Red (crimson)
Na⁺	Yellow-orange
K⁺	Lilac
Ca²⁺	Brick red
Sr²⁺	Crimson red
Ba²⁺	Pale green

Magnesium

Magnesium gives no flame colour because the energy emitted is outside the visible spectrum

Limitation

Limitation only one type of ion should be present, because mixed colours make identification unreliable

Section 10 of 13Tests for gases and ions

1Carbon dioxide

Carbon dioxide turns limewater milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

2Oxygen

Oxygen relights a glowing splint

3Nitrogen dioxide

Nitrogen dioxide toxic brown gas that dissolves to form an acidic solution
Nitrates are therefore heated in a fume cupboard.

4Carbonate or hydrogencarbonate

Carbonate or hydrogencarbonate add dilute acid and test the gas with limewater
2H⁺ + CO₃²⁻ → CO₂ + H₂O and H⁺ + HCO₃⁻ → CO₂ + H₂O.

5Ammonium

Ammonium warm with sodium hydroxide and test the gas with damp red litmus paper
NH₄⁺ + OH⁻ → NH₃ + H₂O.
Ammonia turns damp red litmus blue and forms white smoke of NH₄Cl with HCl fumes.

6Sulfate

Sulfate acidify with dilute hydrochloric acid, then add aqueous barium chloride
A white precipitate of barium sulfate forms: Ba²⁺ + SO₄²⁻ → BaSO₄.
HCl is added first to destroy carbonates, which would form a white BaCO₃ precipitate.
Sulfuric acid must not be used because it adds sulfate ions.

Section 11 of 13More from the specification

Section 12 of 13Key terms

Term	Meaning
First ionisation energy	The energy to remove one mole of electrons from one mole of gaseous atoms to form 1+ ions.
Shielding	Reduction of the nuclear attraction on outer electrons by inner electron shells.
Thermal decomposition	Breaking down of a compound using heat.
Polarising power	The ability of a cation to distort the electron cloud of an anion.
Thermal stability	Resistance of a compound to decomposition on heating.
Limewater	A solution of calcium hydroxide that turns milky with carbon dioxide.
Flame test	A test that identifies metal ions by the colour they give to a flame.

Section 13 of 13Exam technique

Saying nuclear charge decreases down the group — it increases, but shielding and distance outweigh it

Reversing the solubility trends — hydroxides become more soluble and sulfates less soluble down Group 2

Saying thermal stability decreases down the group — larger, less polarising cations make it increase

Adding sulfuric acid first in the sulfate test — acidify with hydrochloric acid

Using a wire that glows red in the flame test — this is confused with a flame colour

Last updated: 15 October 2026