OCR A Level Chemistry Module 3

What is periodicity?

The repeating pattern of chemical and physical properties across different periods in the periodic table.

What causes the trend in atomic radius across a period?

Atomic radius decreases due to increasing nuclear charge with similar shielding.

Why does atomic radius increase down a group?

More electron shells → increased distance and more shielding → weaker nuclear attraction.

What is the trend in first ionisation energy across a period?

Increases due to increasing nuclear charge and same shielding.

Why does ionisation energy decrease down a group?

More shells and more shielding → weaker attraction to outer electrons.

Why does magnesium have a higher melting point than sodium?

More delocalised electrons and a higher charge density in Mg²⁺ → stronger metallic bonding.

Why does silicon have a high melting point?

Giant covalent structure with strong covalent bonds throughout → requires lots of energy to break.

Why do phosphorus and sulfur have different melting points?

S₈ has more electrons than P₄ → stronger London forces → higher melting point.

What type of bonding does Na have?

Metallic bonding: attraction between delocalised electrons and positive ions.

What type of bonding does Si have?

Giant covalent bonding between atoms in a lattice.

Why is chlorine a gas at room temperature?

Simple molecular structure with weak London forces → low melting/boiling point.

Why does Na conduct electricity in solid and liquid states?

Delocalised electrons move and carry charge in both states.

Why does Na₂O only conduct when molten?

Ions are fixed in solid but mobile when molten.

What is the structure of CO₂?

Simple molecular with London forces between molecules.

What is the structure of SiO₂?

Giant covalent lattice with strong covalent bonds throughout.

Why does SiO₂ have a much higher melting point than CO₂?

Covalent bonds in SiO₂ are much stronger than London forces in CO₂.

What is the trend in melting point from Na to Ar?

High in metals (strong metallic bonds), peaks at Si (giant covalent), then decreases for P-Ar (simple molecules).

Why is Br₂ a poor conductor?

No delocalised electrons or mobile ions.

Why is Mg a good conductor?

Delocalised electrons can move and carry charge.

What does a dot-and-cross diagram for CaBr₂ show?

Ca²⁺ ion with no outer electrons; Br⁻ with full outer shells (8 electrons), correct charges.

Why does reactivity increase down Group 2?

More shells → more shielding → weaker attraction → electrons are lost more easily.

What causes the jump in successive ionisation energy values?

A new shell is being accessed → closer to the nucleus → higher energy needed.

What does a large increase after the 2nd ionisation suggest?

Element is in Group 2 - third electron is from a new inner shell.

What does a higher ionisation energy suggest about atomic radius?

Smaller atomic radius → stronger attraction between nucleus and electrons.

Which Period 3 element has highest first ionisation energy?

Neon - highest nuclear attraction, smallest atomic radius, full outer shell.

Why is the first IE of aluminium lower than magnesium?

Al's outer electron is in 3p (higher energy) vs Mg's in 3s → easier to remove.

Why does sulfur have a lower IE than phosphorus?

Paired electrons in one 3p orbital in sulfur → increased repulsion → easier to remove.

Which Period 3 elements form ionic chlorides?

NaCl, MgCl₂ - high melting points and conduct when molten (giant ionic lattices).

Which Period 3 elements form molecular chlorides?

SiCl₄, PCl₃, SCl₂ - low melting points, simple molecular structures with London forces.

Why do Group 1 metals increase in reactivity down the group?

More shells and more shielding → easier to lose electron.

What structure does Sr have?

Giant metallic lattice.

What structure does Br₂ have?

Simple molecular.

Why is Sr's melting point higher than Br₂?

Metallic bonds (in Sr) are stronger than London forces (in Br₂).

What causes the difference in conductivity between Sr and Br₂?

Sr has mobile delocalised electrons; Br₂ has no mobile charge carriers.

Why does ionisation energy increase from Li to F?

Nuclear charge increases with similar shielding → electrons held more strongly.

Why does P have a higher boiling point than Cl?

P₄ is a larger molecule than Cl₂ → stronger London forces.

Why does potassium come after argon in the periodic table?

K has one more proton and starts a new period.

How do successive ionisation energies show shell structure?

Large jumps indicate electrons are being removed from a new inner shell.

What is the equation for second ionisation of Sr?

Sr⁺(g) → Sr²⁺(g) + e⁻

What is the equation for third ionisation of Al?

Al²⁺(g) → Al³⁺(g) + e⁻

Why does radius decrease across a period?

Increased nuclear charge pulls electrons closer with same number of shells.

What is the conductivity difference between HgBr₂ and Hg?

Hg conducts in both states due to delocalised electrons; HgBr₂ only conducts when molten due to mobile ions.

What happens to ionic radius as electrons are lost?

Radius decreases due to stronger attraction to unchanged nucleus.

How do you determine group number from ionisation data?

Look for the large jump — it occurs after the number of outer electrons has been removed.

What is the general reactivity trend in Group 2?

Reactivity increases down the group.

Why does Group 2 reactivity increase down the group?

Atomic radius increases, shielding increases, nuclear attraction decreases → electrons are lost more easily.

What is the general equation for a Group 2 metal reacting with water?

M + 2H₂O → M(OH)₂ + H₂

What are the observations when Group 2 metals react with water?

Effervescence, metal dissolves, colourless solution forms (precipitate may form depending on metal).

What is the product of Group 2 metal reacting with oxygen?

A metal oxide: 2M + O₂ → 2MO

What type of reaction is decomposition of Group 2 carbonates?

Thermal decomposition.

What is formed when a Group 2 metal oxide reacts with water?

A soluble hydroxide: MO + H₂O → M(OH)₂

Why is strontium hydroxide more alkaline than calcium hydroxide?

Sr(OH)₂ is more soluble, so it produces more OH⁻ ions in solution.

What are the uses of Group 2 compounds in agriculture?

Neutralising acidic soils (e.g. using CaO or Ca(OH)₂).

What is the equation for calcium oxide reacting with water?

CaO + H₂O → Ca(OH)₂

What is the pH of solutions formed from Group 2 oxides?

Above 7 (alkaline), increases down the group due to higher solubility.

What is a typical observation when calcium reacts with water?

Fizzing and white solid disappearing to form a colourless solution.

Why is Ba more reactive than Ca?

Larger atomic radius and more shielding → outer electrons are lost more easily.

What is the formula and charge of calcium nitride?

Ca₃N₂ with Ca²⁺ and N³⁻ ions.

What is the equation for Ca₃N₂ reacting with water?

Ca₃N₂ + 6H₂O → 3Ca(OH)₂ + 2NH₃

What is the dot-and-cross structure of Ca₃N₂?

Three Ca²⁺ ions with no outer electrons; two N³⁻ ions with full outer shells (8 electrons) and correct charges.

What is the structure and bonding in CaO or BaO?

Giant ionic lattice with strong electrostatic attraction between oppositely charged ions.

What is the equation for the neutralisation of Ba(OH)₂ with HCl?

Ba(OH)₂ + 2HCl → BaCl₂ + 2H₂O

What is the general trend in solubility of Group 2 hydroxides?

Increases down the group.

What is the general trend in solubility of Group 2 sulfates?

Decreases down the group.

What is the test for sulfate ions?

Add BaCl₂(aq) → white precipitate of BaSO₄ forms.

Why does BaCl₂ work in the sulfate test?

Ba²⁺ ions form an insoluble white precipitate with SO₄²⁻ ions.

Why is Mg(OH)₂ useful in indigestion tablets?

It is only slightly soluble and neutralises excess stomach acid.

Why does ionisation energy decrease down Group 2?

Atomic radius increases, shielding increases → weaker nuclear attraction.

What is the equation for the second ionisation of Sr?

Sr⁺(g) → Sr²⁺(g) + e⁻

Why is the second ionisation energy of Sr lower than Ca?

Sr has a larger atomic radius and more shielding → weaker nuclear attraction to outer electron.

What is the redox explanation when Mg reacts with CH₃COOH?

Mg is oxidised (0 to +2), H⁺ is reduced (+1 to 0), Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂

What is the equation for Mg reacting with CuSO₄?

Mg + CuSO₄ → Cu + MgSO₄ (Mg is oxidised, Cu²⁺ is reduced)

What is the equation for magnesium reacting with water?

Mg + 2H₂O → Mg(OH)₂ + H₂

What is the equation for Ba₃N₂ reacting with water?

Ba₃N₂ + 6H₂O → 3Ba(OH)₂ + 2NH₃

What does BaO₂ contain in terms of bonding?

Ba²⁺ and a peroxide ion, O₂²⁻ (with a single bond between the O atoms).

Why does BaO₂ not contain two separate oxide ions?

Because it contains a peroxide ion: [O-O]²⁻ with a single covalent bond and two extra electrons.

Why is calcium oxide basic?

It reacts with acids to form salts and water.

What is observed when CaO reacts with water?

Heat released, white suspension forms (Ca(OH)₂).

How can Group 2 compounds be used to analyse unknown solutions?

Use BaCl₂ to test for sulfate ions; presence of white precipitate = sulfate present.

What is the difference in volume of H₂ produced by Ca vs Sr?

More H₂ from Ca (lower Ar = more moles for same mass).

What is a typical use of Ca(OH)₂ in labs or agriculture?

Neutralisation of acidic soils or acidic industrial waste.

What is the balanced equation for 6Ca + P₄?

6Ca + P₄ → 2Ca₃P₂

What happens to magnesium in redox reactions?

Loses 2 electrons → oxidised from 0 to +2

What happens to copper(II) ions in redox with magnesium?

Gain 2 electrons → reduced from +2 to 0

What is the trend in reactivity down Group 17?

Reactivity decreases down the group.

Why does halogen reactivity decrease down the group?

Atoms get larger, more shielding, weaker nuclear attraction → gain electrons less easily.

What is the ionic equation for bromine displacing iodide?

Br₂ + 2I⁻ → I₂ + 2Br⁻

What is the colour of iodine in an organic solvent?

Violet

What is the colour of bromine in an organic solvent?

Orange

Why is iodine less reactive than bromine?

Iodine has more shells, more shielding, and a weaker nuclear attraction for gaining electrons.

What is meant by disproportionation?

Simultaneous oxidation and reduction of the same element in a reaction.

Why is I₂ + H₂O ⇌ HI + HIO a disproportionation?

Iodine is oxidised (0 to +1 in HIO) and reduced (0 to -1 in HI).

What is a benefit of chlorine in water treatment?

Kills bacteria/pathogens/makes water safe to drink.

What is a risk of chlorine in water treatment?

Forms toxic/carcinogenic halogenated compounds.

What is the systematic name of NaBrO₃?

Sodium bromate(V)

What is the oxidation number of Br in NaBrO₃?

+5

In Br₂ + NaOH → NaBr + NaBrO₃ + H₂O, which oxidation states does Br show?

Br is oxidised from 0 to +5 and reduced from 0 to -1 → disproportionation.

What causes the increase in boiling point down Group 17?

More electrons → stronger London forces → more energy needed to overcome intermolecular forces.

What are the boiling points of Cl₂, Br₂, I₂?

Cl₂: -35°C, Br₂: 59°C, I₂: 184°C (increase down group).

What forces exist between halogen molecules?

London (induced dipole-dipole) forces.