Chemistry 9701/21 — October/November 2015

Background Concept

Metallic bonding explains typical metal properties: delocalised electrons conduct electricity; non-directional bonding allows layers of ions to slide, giving malleability/ductility; metals are generally dense, though aluminium is unusually light for a metal.

Understanding the Question

'Suggest two properties ... suitable for this use' — the use is overhead power cables, so the properties must make sense for that application.

Approach

Think about what a long-distance overhead cable needs: it must conduct electricity, be light enough to hang, survive weather, and be drawable into wire.

Step-by-Step Reasoning

• Electrical conductivity: delocalised electrons move when a potential difference is applied — essential for a power cable.
• Low density: aluminium (density ~2.7 g cm⁻³) is much lighter than copper, so cables sag less and pylons can be further apart.
• Corrosion resistance: aluminium forms a thin, adherent Al₂O₃ layer that protects it from further oxidation in outdoor conditions.
• Ductility: the metal can be drawn into wires without breaking.
  Any two of these score the two marks.

Key Takeaways

For 'properties for a use' questions, always connect the property to the application; the mark scheme lists acceptable answers but the property must be a genuine physical/chemical property, not a vague statement.

Common Mistakes

Listing properties irrelevant to the use (e.g. 'shiny', 'high melting point' — melting point is irrelevant for a cable). Vague answers like 'strong' without context may not be credited.

Things to Be Careful About

'Sonducts electricity' is the most essential property here — don't omit it.

Background Concept

Ceramics (like the silicon oxides/aluminates in porcelain) are typically giant covalent (or giant ionic) lattices: every atom or ion is bonded throughout a continuous 3D network, giving hardness and high melting points.

Understanding the Question

One mark only: the mark scheme credits 'giant/lattice'. The key idea is that the structure is not molecular but a giant network.

Approach

State the structure type in one line.

Step-by-Step Reasoning

Ceramic supports are made of materials such as silicon dioxide or alumina, which form giant covalent lattices with strong bonds extending throughout the solid. This is what 'giant/lattice' refers to.

Key Takeaways

Ceramic = giant covalent (or giant ionic) lattice; the word 'giant' is the credited keyword.

Common Mistakes

Describing ceramics as 'molecular' or 'ionic with small molecules' — ceramics are giant lattices, not simple molecules.

Things to Be Careful About

Include the word 'giant' — 'lattice' alone of a molecular solid would not distinguish the structure.

Background Concept

Giant covalent structures like ceramics have all their electrons locked in covalent bonds; there are no delocalised electrons or mobile ions, so they cannot conduct electricity.

Understanding the Question

The cable is live; the support must prevent current reaching the earthed pylon. The property is therefore electrical insulation.

Approach

Ask what the ceramic must do in this circuit context: block current.

Step-by-Step Reasoning

Since the ceramic is a giant covalent lattice with no mobile charge carriers, it does not conduct. This is exactly why it can safely hold a live cable against the steel pylon.

Key Takeaways

Match the property to the function: metal cable conducts; ceramic support insulates.

Common Mistakes

Answering 'hard' or 'strong' — those are true of ceramics but not the property relevant to this use.

Things to Be Careful About

The mark scheme wants '(electrical) insulator' specifically.

Background Concept

Covalent compounds can be giant (diamond, SiO₂) or simple molecular (CO₂, Al₂Cl₆). In simple molecular substances, the covalent bonds within each molecule are strong, but the forces BETWEEN molecules are weak van der Waals (induced dipole–dipole) forces. Physical changes like melting, boiling and sublimation only break the intermolecular forces, not the covalent bonds, so simple molecular substances have low melting/boiling/sublimation points.

Understanding the Question

The clues: a chloride that sublimes at only 180 °C must be simple molecular (a giant structure would need a far higher temperature). You must name the structure/bonding AND explain the low sublimation temperature.

Approach

Mark 1: identify 'simple covalent molecules'. Mark 2: attribute the low sublimation point to weak intermolecular forces being easily overcome.

Step-by-Step Reasoning

Aluminium chloride at around 180–200 °C exists as discrete Al₂Cl₆ molecules. Within each molecule, covalent bonds (including coordinate/dative bonds from chlorine lone pairs to aluminium) are strong. Between molecules there are only weak van der Waals forces. Sublimation separates molecules, so only these weak forces must be overcome — little thermal energy is required, giving the low sublimation temperature of 180 °C.

Key Takeaways

Low melting/sublimation point ⇒ simple molecular; the explanation must reference weak intermolecular forces, never 'weak covalent bonds'.

Common Mistakes

Saying 'weak covalent bonds' — covalent bonds are strong; it is the intermolecular forces that are weak. Saying 'ionic, but with weak ionic bonds' — an ionic lattice would have a very high melting point, contradicting the 180 °C data.

Things to Be Careful About

Use the phrase 'intermolecular forces' or 'van der Waals forces'; 'weak bonds between molecules' is usually accepted but 'intermolecular forces' is the precise term.

Background Concept

The empirical formula is the simplest whole-number ratio of atoms in a compound. From percentage composition, assume 100 g of sample so percentages become grams, convert each mass to moles (mass ÷ Ar), then divide all mole values by the smallest to obtain the ratio.

Understanding the Question

The chloride contains 79.7% chlorine, so aluminium is 100 − 79.7 = 20.3%. 'You must show your working' means the moles calculation itself carries a mark.

Approach

Percentage → mass (100 g basis) → moles → divide by smallest → simplest ratio.

Step-by-Step Reasoning

• Al: 20.3 g ÷ 27.0 = 0.752 mol
• Cl: 79.7 g ÷ 35.5 = 2.25 mol
• Ratio: 0.752 : 2.25; dividing by 0.752 gives 1 : 2.99 ≈ 1 : 3
• Empirical formula AlCl₃. Note the ratio is not exactly 3 (2.99) — this is rounding, and 1 : 3 is the correct whole-number ratio.

Key Takeaways

Always convert percentages to moles before finding ratios; the 'divide by smallest' step normalises the ratio to integers.

Common Mistakes

Using 79.7 for Al and 20.3 for Cl (forgetting they must sum to 100%). Rounding 2.99 to 2 or 4 instead of 3. Not showing the division step, losing the working mark.

Things to Be Careful About

Use correct Ar values: Al = 27, Cl = 35.5. Show every step clearly since working is explicitly rewarded.

Background Concept

The ideal gas equation $pV = nRT$ relates pressure (Pa), volume (m³), moles, the gas constant $R = 8.31 \text{ J K}^{-1}\text{ mol}^{-1}$ and temperature in kelvin. Since $n = m/M_r$, rearranging gives $M_r = mRT/(pV)$.

Understanding the Question

Given: mass 1.36 g, volume 200 cm³ at 200 °C and 100 kPa. Find $M_r$ to 3 significant figures. The command 'calculate' demands full working.

Approach

Either use $pV = nRT$ to find moles, then $M_r = m/n$; or use the combined formula $M_r = mRT/pV$ directly. Either way, unit conversion is the critical step.

Step-by-Step Reasoning

• Temperature must be in kelvin: 200 + 273 = 473 K.
• Volume must be in m³ for use with R = 8.31: 200 cm³ = 200 × 10⁻⁶ m³.
• Pressure must be in Pa: 100 kPa = 100 × 10³ Pa.
• Moles: $n = \frac{100 \times 10^3 \times 200 \times 10^{-6}}{8.31 \times 473} = \frac{20}{3930} = 5.09 \times 10^{-3}$ mol.
• $M_r = \frac{1.36}{5.09 \times 10^{-3}} = 267$.
• The mark scheme awards 1 mark for the correct formula/working and 1 mark for the value; the alternative single-formula route scores the same two marks.

Key Takeaways

Memorise the unit conversions for the ideal gas equation: °C → K (+273), cm³ → m³ (×10⁻⁶), kPa → Pa (×10³). The answer to 3 s.f. is 267.

Common Mistakes

Using 200 K instead of 473 K. Using volume in cm³ directly with R = 8.31 (gives a wildly wrong answer). Using R = 8.31 with kPa and dm³ inconsistently. Giving the answer as 267.3 or only 2 s.f.

Things to Be Careful About

The question explicitly asks for three significant figures — write 267. Check that the answer is sensible: it is roughly double the empirical formula mass of AlCl₃ (133.5), which anticipates part (iv).

Background Concept

The molecular formula is a whole-number multiple of the empirical formula: molecular formula = (M_r ÷ empirical formula mass) × empirical formula.

Understanding the Question

From (ii) the empirical formula is AlCl₃ (M_r = 133.5); from (iii) the actual molecular mass is 267. Deduce the molecular formula at 200 °C.

Approach

Divide the experimental M_r by the empirical formula mass; the integer multiplier scales the empirical formula.

Step-by-Step Reasoning

267 ÷ 133.5 = 2, so the molecule contains two empirical units: Al₂Cl₆. This is chemically sensible — aluminium chloride exists as the dimer Al₂Cl₆ in the vapour phase and just below, with two bridging chlorine atoms forming coordinate (dative) bonds; this dimeric structure explains why it is simple molecular with a low sublimation temperature (linking back to part (i)).

Key Takeaways

Molecular formula = n × empirical formula, where n = M_r ÷ empirical formula mass. Aluminium chloride dimerises to Al₂Cl₆ — a classic fact worth knowing.

Common Mistakes

Writing AlCl₃ (the empirical formula) as the molecular formula. Dividing the wrong way (133.5 ÷ 267). Arithmetic slips with 35.5 × 3.

Things to Be Careful About

The subscript 6 on Cl: Al₂Cl₆, not Al₂Cl₃. The dimer is the species present at 200 °C, consistent with the measured M_r of 267.