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

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

10 min read 14 parts Lesson 4 of 53 1 quiz

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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.
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Lesson summary

2.4 Groups 1 & 2 — trends, reactions, thermal stability and qualitative tests — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Explain ionisation energy and reactivity trends in Groups 1 and 2, write equations for their reactions, explain solubility and thermal stability trends and carry …

  • With oxygen
  • With chlorine
  • With water
  • Thermal decomposition
  • Carbonate and hydrogencarbonate
  • First ionisation energy

Part 1 of 12Ionisation energy trends

1
Group 1 (alkali metals) atoms have one outer electron and Group 2 (alkaline earth metals) atoms have two.
2
Both groups form ionic compounds by donating these electrons, so they act as reducing agents and are themselves oxidised to M+ or M2+.
3
First and second ionisation energies decrease down the group.
  • The outer electrons are in a shell further from the nucleus and are better shielded.
  • These effects outweigh the larger nuclear charge, so the electrons are removed more easily.
4
Group 2 elements have larger first ionisation energies than the Group 1 element in the same period because of more protons and a smaller radius.

Part 2 of 12Reactivity trend

Reactivity trend
Reactivity increases down both groups.
Atoms lose their outer electron(s) to form ions.
  • A Group 1 ion has the noble-gas configuration because the next shell down is full.
  • Each step down adds a shell, so the outer electron is further away and attracted less.
  • Less energy is needed to remove it, so the metal reacts more readily.
The same reasoning applies to Group 2: magnesium reacts extremely slowly with cold water, while calcium reacts fairly vigorously and exothermically.

Part 3 of 12Reactions of Group 1 metals

Reactions of Group 1 metals
With oxygen
  • With oxygen the metals tarnish as a dull oxide coating forms, more rapidly down the group.
With chlorine
  • With chlorine they burn as in oxygen, forming a white solid chloride, 2M(s) + Cl2(g) → 2MCl(s).
  • Sodium burns with an intense orange flame.
With water
  • With water reactions become more vigorous down the group, giving the hydroxide and hydrogen.

Part 4 of 12Reactions of Group 2 metals

Reactions of Group 2 metals
With oxygen
  • 2M(s) + O2(g) → 2MO(s); Sr and Ba also form peroxides, MO2.
With water
  • M(s) + 2H2O(l) → M(OH)2 + H2(g), except Be, which does not react.
  • Mg reacts extremely slowly with cold water to give a weakly alkaline solution (pH 9–10) because Mg(OH)2 is only slightly soluble.
  • Mg heated in steam reacts vigorouslyMg(s) + H2O(g) → MgO(s) + H2(g).
  • Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g).
With chlorine
  • Mg(s) + Cl2(g) → MgCl2(s).

Part 5 of 12Oxides and hydroxides

1
Metal + oxygen → metal oxide; oxide + water → hydroxide; oxides and hydroxides react with dilute HCl or H2SO4 to form a salt and water.
2
Group 1 oxides react with water to give a colourless alkaline solution, e.g. Na2O + H2O → 2NaOH.
  • O2− + H2O → 2OH−.
  • Group 1 hydroxides neutralise acids: NaOH + HCl → NaCl + H2O.
3
Group 2 oxides are basic, except BeO, which is amphoteric.
  • MgO is only slightly soluble, giving a weakly alkaline solution (pH about 10).
  • CaO reacts vigorously and exothermically to form Ca(OH)2 (limewater is its solution).
  • Alkalinity increases down the group.
4
Group 2 oxides and hydroxides give colourless salt solutions with dilute acid; with H2SO4 insoluble sulfates coat the oxide surface and stop reaction,
  • which a powder and stirring reduce.
5
Uses: Mg(OH)2 is an antacid and Ca(OH)2 neutralises acidic soil.

Part 6 of 12Solubility of Group 2 hydroxides and sulfates

Solubility of Group 2 hydroxides and sulfates
Hydroxides become more soluble down the group.
  • Mg(OH)2 2.0 × 10−5, Ca(OH)2 1.5 × 10−3, Sr(OH)2 3.4 × 10−3, Ba(OH)2 1.5 × 10−2 mol per 100 g water.
  • More OH− in solution gives a higher pH, so the oxide solutions are more alkaline down the group.
Sulfates become less soluble down the group.
  • MgSO4 is soluble and BaSO4 is an insoluble white precipitate.
  • BaSO4 is used in barium meals because toxic Ba2+ is not absorbed.
Group 1 compounds are all soluble, so cations are identified by flame tests, and Ba(OH)2 is still less soluble than KOH.

Part 7 of 12Thermal decomposition of carbonates and nitrates

Thermal decomposition
  • Thermal decomposition breakdown of a compound into two or more substances using heat.
Carbonates
  • Group 2: XCO3 → XO + CO2; more heat is needed down the group.
  • Group 1: only Li2CO3 decomposes at Bunsen temperatures, Li2CO3 → Li2O + CO2; the rest are stable.
Nitrates
  • Group 2 nitrates give the oxide, brown NO2 and O2: 2Mg(NO3)2 → 2MgO + 4NO2 + O2.
  • LiNO3 behaves the same: 4LiNO3 → 2Li2O + 4NO2 + O2.
  • Other Group 1 nitrates give the nitrite and oxygen only: 2XNO3 → 2XNO2 + O2.
  • Do the test in a fume cupboard because NO2 is toxic.
Thermal stability increases down the group.
  • Small, highly charged cations (Li+, Mg2+) polarise the large carbonate or nitrate ion most.
  • The anion bonds are weakened, so decomposition needs less heat.
  • Larger cations at the bottom of the group polarise less.
Group 1 compounds are more stable than Group 2 because the cation charge is lower; lithium behaves like Group 2 because Li+ is very small.

Part 8 of 12Gas tests

CO2
  • CO2 turns limewater milky as CaCO3 forms: Ca(OH)2 + CO2 → CaCO3 + H2O.
O2
  • O2 relights a glowing splint.
NO2
  • NO2 toxic brown-orange gas that gives an acidic solution in water: 2NO2 + H2O → HNO3 + HNO2.
Stability can be compared by timing how long until brown gas appears, a glowing splint relights or limewater turns milky.

Part 9 of 12Flame tests

1
Clean a nichrome or platinum wire loop by dipping in concentrated acid and holding it in a blue Bunsen flame until no colour shows.
2
Dip the loop in the solid sample.
3
Hold it at the edge of the blue flame without letting the wire glow red.
4
Record the colour.
Flame tests
Only one type of ion must be present, because mixed ions give mixed colours.
Heat promotes electrons to higher energy levels; they fall back and emit visible light of a specific wavelength.
IonFlame colour
Li+Red (crimson)
Na+Yellow-orange
K+Lilac
Ca2+Brick red
Sr2+Crimson red
Ba2+Pale green
  • Mg2+ gives no colour because the energy emitted is outside the visible spectrum.

Part 10 of 12Tests for ions in solution

1Ammonium
  • Ammonium warm with NaOH(aq); NH4+ + OH− → NH3 + H2O.
  • NH3 turns damp red litmus blue.
  • NH3 gives white smoke of NH4Cl with HCl fumes.
2Carbonate and hydrogencarbonate
  • Carbonate and hydrogencarbonate add dilute HCl; 2H+ + CO32− → CO2 + H2O and H+ + HCO3− → CO2 + H2O.
  • The gas turns limewater milky.
3Sulfate
  • Sulfate acidify with dilute HCl, then add BaCl2(aq); Ba2+ + SO42− → BaSO4(s), a white precipitate.
  • HCl is added first to remove carbonates, which would also give a precipitate.
4Halides
  • Halides acidify with HNO3, add AgNO3(aq);
    • AgCl is white, AgBr cream and AgI yellow, and ammonia solution distinguishes them.

Part 11 of 12More from the specification

More from the specification
Outermost Electron and Group Reactivity
  • Going down Group 1 and Group 2, atomic radius and shielding increase, weakening the electrostatic attraction for the outermost electron and lowering first ionization energy.
Thermal Decomposition of Carbonates
  • Group 2 carbonates decompose upon heating to produce the metal oxide and carbon dioxide gas, with thermal stability increasing going down the group as cation polarising power decreases.
Thermal Decomposition of Nitrates
  • Group 2 nitrates thermally decompose to form metal oxide, brown nitrogen dioxide gas,
  • and oxygen gas, following the general equation 2 M(NO3)2 → 2 MO + 4 NO2 + O2.
Reactions with Dilute Hydrochloric Acid
  • Group 2 metals and basic oxides react readily with dilute hydrochloric acid to produce aqueous metal chloride salts and hydrogen gas or water.
Outermost Electron and Group Reactivity
  • Going down Group 1 and Group 2, atomic radius and shielding increase, weakening the electrostatic attraction for the outermost electron and lowering first ionization energy.
Thermal Decomposition of Carbonates
  • Group 2 carbonates decompose upon heating to produce the metal oxide and carbon dioxide gas, with thermal stability increasing going down the group as cation polarising power decreases.
Thermal Decomposition of Nitrates
  • Group 2 nitrates thermally decompose to form metal oxide, brown nitrogen dioxide gas,
  • and oxygen gas, following the general equation 2 M(NO3)2 → 2 MO + 4 NO2 + O2.
Reactions with Dilute Hydrochloric Acid
  • Group 2 metals and basic oxides react readily with dilute hydrochloric acid to produce aqueous metal chloride salts and hydrogen gas or water.
Carbon dioxide
  • A gas that turns limewater cloudy.

Part 12 of 12More on this topic

  • CaCO3 is used in cement and to make lime;
    • Ca(OH)2 (slaked lime) neutralises acidic soil and limewater tests for CO2.
TermMeaning
Thermal decompositionBreaking down a compound by heat, e.g. CaCO3 → CaO + CO2.
Polarising powerThe ability of a cation to distort the electron cloud of an anion.
LimewaterAqueous Ca(OH)2, used to test for CO2.
Slaked limeCa(OH)2, which neutralises acidic soil.
NucleophileAn electron-pair donor that attacks a δ+ carbon.
EliminationRemoval of a small molecule from adjacent carbons to form a C=C.

Key terms

TermMeaning
First ionisation energyThe energy to remove one electron from each atom in one mole of gaseous atoms.
ShieldingThe reduction in nuclear attraction on outer electrons because of inner electron shells.
Thermal decompositionBreaking down a compound by heating it.
Polarising powerThe ability of a cation to distort the electron cloud of an anion.
Thermal stabilityResistance of a compound to decomposition on heating.
Flame testA test that identifies metal ions from the colour they give a flame.
AmphotericAble to react as both an acid and a base.

Avoid these mistakes

1
Saying Group 2 hydroxide solubility decreases down the group — it increases, while sulfate solubility decreases.
2
Ignoring shielding when explaining the trend in ionisation energy — shielding and distance outweigh nuclear charge.
3
Claiming Group 1 carbonates all decompose at Bunsen temperature — only lithium carbonate does.
4
Adding BaCl2 to a sulfate test without acid first — carbonates would also give a precipitate.
5
Explaining thermal stability using only cation size — link it to charge density and polarisation of the anion.

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