Chemistry 9701/23 — October/November 2015

Cambridge AS Level · AS Level Structured Questions · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Chemical Bonding · States of Matter · Atoms, Molecules and Stoichiometry · Chemical Energetics · Equilibria · Nitrogen and Sulfur · +4 more

Q1Chemical BondingStates of MatterAtoms, Molecules and StoichiometryFree sample

Aluminium is a metal in Period 3 and Group III of the Periodic Table.

(a)

Describe the structure of solid aluminium.

DifficultyEasy

Worked solution

Answer

Solid aluminium consists of a regular lattice of aluminium cations ( $\text{Al}^{3+}$ ), with the outer-shell electrons delocalised throughout the structure.

Final answer

Giant metallic lattice of positive ions surrounded by delocalised electrons.

Detailed explanation

Background Concept

Metals bond metallically: atoms lose their outer electrons, which become delocalised — free to move through the whole structure. The result is a lattice of positive ions held together by the electrostatic attraction between these cations and the surrounding 'sea' of delocalised electrons.

Understanding the Question

'Describe the structure' is a recall command word: state the particles present and how they are arranged. Two marks = two distinct points.

Approach

Name the lattice, name the particles (cations/positive ions), and name the electrons and their key feature (delocalised).

Step-by-Step Reasoning

Aluminium has three outer electrons that it readily gives up, so the structure is best described as Al³⁺ ions in a regular (giant) lattice, with the released electrons delocalised. Both elements — the cation lattice and the delocalised electrons — are needed for the two marks.

Key Takeaways

A metallic structure answer must always contain both components: positive ions in a regular arrangement AND delocalised electrons.

Common Mistakes

Writing 'atoms' instead of 'ions' or 'cations'; describing only the lattice without mentioning the delocalised electrons (or vice versa); saying 'molecules' — metals contain no molecules.

Things to Be Careful About

Use the word 'delocalised' precisely — 'free electrons' alone is vaguer and may not always be credited.

Techniques used

describe metallic lattice structureidentify delocalised electrons

(b)

A common use of aluminium is to make the conducting cables in long distance overhead power lines.

(i)

Suggest two properties of aluminium that make it suitable for this use.

DifficultyMedium-Easy

Worked solution

Answer

Any two of:

Good electrical conductor (delocalised electrons carry charge).
Low density (light cables do not sag or need very strong supports).
Corrosion resistant (forms a protective oxide layer).
Ductile (can be drawn out into wires).

Final answer

Electrical conductivity, low density, corrosion resistance, ductility (any two).

Detailed explanation

Background Concept

The properties of metals follow from metallic bonding: delocalised electrons give conductivity; non-directional metallic bonding lets layers of ions slide, giving ductility/malleability; aluminium forms a protective oxide layer, making it corrosion-resistant; aluminium has a low density compared with other metals like copper.

Understanding the Question

'Suggest' means apply your knowledge to the context: cables for long-distance overhead power lines. The properties must be relevant to that use.

Approach

Think about what a power cable needs: to conduct electricity, to be light enough to hang from pylons, to survive weather without corroding, and to be drawn into wire.

Step-by-Step Reasoning

Conductivity is essential for a conductor; low density matters because the cables are long and hang under their own weight (copper is a better conductor but much heavier); corrosion resistance matters outdoors; ductility is needed to make wire. Any two of these score.

Key Takeaways

Application questions reward properties matched to the use, not a random list of metal properties.

Common Mistakes

Listing irrelevant properties (e.g. high melting point, shiny) that do not help a cable; giving 'strong' without justification; naming only one property when two are asked.

Things to Be Careful About

'Suggest' allows sensible alternatives, but each property must be genuinely useful for overhead cables.

Techniques used

relate metallic bonding to propertiesselect properties relevant to an application

(ii)

The cables are attached to pylons by ceramic supports.

Describe the structure of a ceramic material.

DifficultyEasy

Worked solution

Answer

A ceramic has a giant (lattice) structure.

Final answer

Giant lattice structure.

Detailed explanation

Background Concept

Ceramics (such as the aluminium oxide/silicates used in electrical insulators) are hard, high-melting materials made of a giant lattice — a continuous three-dimensional network of atoms or ions, not discrete molecules.

Understanding the Question

One mark only: describe the structure of a ceramic. The mark scheme wants the key word 'giant' or 'lattice'.

Approach

State the structural type; no further detail is required for one mark.

Step-by-Step Reasoning

Ceramic supports are made of materials with extended ionic or covalent networks, so the credited answer is simply 'giant' or 'lattice' structure.

Key Takeaways

For structure questions, the classification word (giant, molecular, metallic) is usually the mark.

Common Mistakes

Saying 'covalent' alone without 'giant'; describing properties instead of structure.

Things to Be Careful About

The question asks for structure, not properties — properties come in part (iii).

Techniques used

describe giant structure

(iii)

State the property of a ceramic material that makes it suitable for this use.

DifficultyEasy

Worked solution

Answer

It is an electrical insulator (no mobile charge carriers).

Final answer

Electrical insulator.

Detailed explanation

Background Concept

In giant ionic or covalent lattices held rigidly in place (as in ceramics), there are no free-moving charged particles, so no charge can flow — the material is an electrical insulator.

Understanding the Question

Why are ceramic supports used to attach live cables to pylons? Because the ceramic must not conduct electricity from the cable to the pylon (and the ground).

Approach

Name the property directly: electrical insulation.

Step-by-Step Reasoning

The ceramic separates the high-voltage cable from the earthed pylon, so it must not conduct electricity — it is an electrical insulator.

Key Takeaways

Match the property to the function: supports for power lines = insulator.

Common Mistakes

Answering 'strong' or 'hard' — true of ceramics but not the property relevant to electrical safety.

Things to Be Careful About

The word 'insulator' (electrical) is the credited term.

Techniques used

link structure to electrical insulation

(c)

Aluminium reacts with chlorine to form a white, solid chloride that contains $79.7\%$ chlorine and sublimes (changes straight from a solid to a gas) at $180\text{ }^\circ\text{C}$ .

(i)

Describe the structure and bonding in this compound. Suggest how it explains the low sublimation temperature.

DifficultyMedium-Easy

Worked solution

Answer

The chloride is a simple covalent (molecular) compound. The molecules are held together only by weak intermolecular (van der Waals') forces, so little energy is needed to overcome them — hence the low sublimation temperature.

Final answer

Simple covalent molecules with weak van der Waals' intermolecular forces, so little energy is needed to separate them.

Detailed explanation

Background Concept

Simple molecular substances contain strong covalent bonds within each molecule but only weak intermolecular forces (van der Waals') between molecules. Physical changes like melting, boiling and sublimation only break the intermolecular forces, never the covalent bonds, so molecular compounds have low melting/sublimation points.

Understanding the Question

The clue is the low sublimation temperature (180 °C) — a hallmark of a molecular solid. You must name the structure/bonding AND explain how it accounts for the low temperature (two marks).

Approach

Identify: low sublimation point → simple molecular. Then explain: sublimation overcomes intermolecular forces only, and these are weak, so little energy is needed.

Step-by-Step Reasoning

A giant structure (metallic, ionic, giant covalent) would need very high temperatures to break down. A solid that turns straight to gas at only 180 °C must consist of discrete molecules. Between molecules act only weak van der Waals' forces, so only a small amount of energy separates them — the molecules escape to the gas phase easily.

Key Takeaways

For any 'explain the low melting/boiling point' answer: structure → weak intermolecular forces → little energy to overcome them. Never say 'covalent bonds are broken'.

Common Mistakes

Saying 'the covalent bonds are weak' — the covalent bonds are strong; it is the intermolecular forces that are weak. Omitting the explanation (second mark).

Things to Be Careful About

Both parts are needed: identify the bonding/structure, and link the weak intermolecular forces to the low sublimation temperature.

Techniques used

identify covalent molecular structurerelate intermolecular forces to melting/sublimation point

(ii)

Calculate the empirical formula of the chloride. You must show your working.

DifficultyMedium-Easy

Worked solution

Working

Assume 100 g of the compound.

\text{Al: } \frac{20.3}{27.0} = 0.752 \qquad \text{Cl: } \frac{79.7}{35.5} = 2.25

Divide by the smallest:

\text{Al: } \frac{0.752}{0.752} = 1 \qquad \text{Cl: } \frac{2.25}{0.752} = 3

Answer

Empirical formula: $\text{AlCl}_3$

Final answer

AlCl3

Detailed explanation

Background Concept

An empirical formula gives the simplest whole-number ratio of atoms in a compound. Percentage composition is converted to moles by dividing each mass by the relative atomic mass, then the mole values are divided by the smallest to give the ratio.

Understanding the Question

The chloride contains 79.7% chlorine, so it contains 20.3% aluminium. 'Show your working' means the mole calculations must be visible for the method mark.

Approach

Assume a 100 g sample so percentages become grams; convert each mass to moles; divide by the smallest; read off the ratio.

Step-by-Step Reasoning

In 100 g: 20.3 g Al and 79.7 g Cl. Moles of Al = 20.3/27 = 0.752; moles of Cl = 79.7/35.5 = 2.25. Dividing both by 0.752 gives 1 : 3, so the empirical formula is AlCl₃. (Note: this is consistent with the dimeric Al₂Cl₆ found later.)

Key Takeaways

The 100 g assumption converts percentages directly into masses; always divide by the smallest mole value last.

Common Mistakes

Using the wrong A_r for chlorine (35.5, not 79.7% as a mass); dividing by the largest instead of the smallest; failing to show working when it is explicitly demanded.

Things to Be Careful About

The percentages must sum to 100 — chlorine is 79.7%, so aluminium is 20.3%. Show each division clearly for the method mark.

Techniques used

convert mass percentages to molesdivide by smallest mole ratio

(iii)

At $200\text{ }^\circ\text{C}$ and $100\text{ kPa}$ , a $1.36\text{ g}$ sample of this chloride occupied a volume of $200\text{ cm}^3$ .

Calculate the relative molecular mass, $M_r$ , of the chloride. Give your answer to three significant figures.

DifficultyMedium

Worked solution

Working

pV = nRT \quad \Rightarrow \quad n = \frac{pV}{RT} = \frac{100 \times 10^3 \times 200 \times 10^{-6}}{8.31 \times 473} = 5.09 \times 10^{-3} \text{ mol}

M_r = \frac{m}{n} = \frac{1.36}{5.09 \times 10^{-3}} = 267

Answer

$M_r = 267$ (3 s.f.)

Final answer

267

Detailed explanation

Background Concept

The ideal gas equation $pV = nRT$ links the measurable gas properties to the amount in moles. With moles and mass known, $M_r = m/n$ . R must be used with SI units: p in Pa, V in m³, T in K.

Understanding the Question

Given: m = 1.36 g, V = 200 cm³, p = 100 kPa, T = 200 °C. Find M_r to three significant figures. Unit conversion is the hidden challenge.

Approach

Convert: T = 200 + 273 = 473 K; p = 100 kPa = 100 × 10³ Pa; V = 200 cm³ = 200 × 10⁻⁶ m³. Then n = pV/RT, and M_r = m/n.

Step-by-Step Reasoning

n = (100 × 10³ × 200 × 10⁻⁶)/(8.31 × 473) = 20/3931 = 5.09 × 10⁻³ mol. Then M_r = 1.36/(5.09 × 10⁻³) = 267. This is twice the empirical formula mass of AlCl₃ (133.5), which leads directly to part (iv).

Key Takeaways

Always convert °C to K by adding 273, kPa to Pa (×10³), and cm³ to m³ (×10⁻⁶) before using R = 8.31.

Common Mistakes

Forgetting to convert °C to K (using 200 K); leaving V in cm³; rounding too early so the 3 s.f. answer drifts; quoting the wrong number of significant figures.

Things to Be Careful About

The question explicitly asks for three significant figures — 267 is already 3 s.f.; an answer like 267.4 or 270 would not be credited.

Techniques used

apply the ideal gas equationconvert units of pressure and volumecalculate Mr from mass and moles

(iv)

Deduce the molecular formula of this chloride at $200\text{ }^\circ\text{C}$ .

DifficultyMedium-Easy

Worked solution

Working

Empirical formula mass of $\text{AlCl}_3 = 27 + 3(35.5) = 133.5$ .

\frac{267}{133.5} = 2

Answer

Molecular formula: $\text{Al}_2\text{Cl}_6$

Final answer

Al2Cl6

Detailed explanation

Background Concept

The molecular formula is a whole-number multiple of the empirical formula: molecular formula = (M_r ÷ empirical formula mass) × empirical formula. Aluminium chloride is famous for existing as the dimer Al₂Cl₆ in the gas phase, with two coordinate (dative covalent) bonds from chlorine lone pairs to the electron-deficient aluminium atoms.

Understanding the Question

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

Approach

Divide the M_r by the empirical formula mass to find the multiplier, then multiply the subscripts.

Step-by-Step Reasoning

267 ÷ 133.5 = 2, so the molecular formula is (AlCl₃)₂ = Al₂Cl₆. This is the dimeric form of aluminium chloride, in which two AlCl₃ molecules associate to complete aluminium's octet via coordinate bonds.

Key Takeaways

Always check whether M_r is a multiple of the empirical formula mass — the molecular formula is not always the empirical formula.

Common Mistakes

Answering AlCl₃ (confusing empirical and molecular formula); halving instead of doubling.

Things to Be Careful About

The phrase 'at 200 °C' matters: at lower temperatures Al₂Cl₆ exists as a different associated form; here the gas-phase dimer Al₂Cl₆ is the correct answer.

Techniques used

compare Mr with empirical formula massdeduce molecular formula

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