Chemistry • Year 11 • Module 1 • Lesson 20
Module 1 Synthesis and Review
Build HSC Band 5–6 extended-response technique by integrating all three Module 1 inquiry questions in multi-criteria evaluation and experimental design tasks.
1. Data + scenario: comparing four materials for an electrode application (Band 5–6)
8 marks Band 5–6
Scenario. A chemical engineer at an Australian electrolysis plant (such as the CSBP ammonia plant in Kwinana, Western Australia) needs to select an electrode material for a chlor-alkali cell. The electrode must: (i) conduct electricity, (ii) withstand temperatures up to 300 °C without melting, (iii) resist chemical attack by chlorine gas and sodium hydroxide, and (iv) be mechanically strong enough to maintain its shape under pressure. The table below summarises data for four candidate materials.
| Material | Melting point (°C) | Electrical conductivity | Chemical resistance (Cl2, NaOH) | Mechanical strength |
|---|---|---|---|---|
| Graphite (C, layered) | >3600 (sublimes) | High (in plane) | Moderate — slowly oxidised by Cl2 | Low tensile strength; brittle |
| Titanium metal (Ti) | 1668 | Moderate | Excellent — forms protective TiO2 layer | High |
| Silicon dioxide (SiO2, quartz) | 1710 | Very poor (insulator) | Good — inert to most acids and bases | High compressive; very brittle |
| Iron (Fe) | 1538 | High | Poor — corrodes rapidly in Cl2 and NaOH | High |
Illustrative data. Real chlor-alkali cells typically use dimensionally stable anodes (DSA) coated with RuO2/TiO2 on a Ti substrate.
Q1. Evaluate the four materials above as candidate electrodes for the chlor-alkali cell. In your response you must:
- For each material, explain whether it satisfies or fails each of the four requirements (i)–(iv), using Module 1 IQ2 bonding and structure reasoning.
- Recommend which single material best satisfies the four requirements overall, justifying your choice with evidence from the data table.
- Explain why SiO2 fails requirement (i), linking to its electron structure using IQ3 reasoning.
- Explain why the metallic bonding model accounts for why titanium satisfies requirements (i), (ii), and (iv).
- State one limitation of your recommended material and suggest one way it could be improved for industrial use.
2. Experimental design — investigating the effect of molecular size on boiling point in alkanes (Band 5–6)
7 marks Band 5–6
Research question. A student claims: “The boiling point of a straight-chain alkane is determined only by whether it is polar or non-polar, not by its molecular size.” Design a scientific investigation to test whether molecular size (number of carbon atoms) affects the boiling point of non-polar covalent molecular substances, using straight-chain alkanes.
Available materials: methane (CH4), butane (C4H10), octane (C8H18), dodecane (C12H26), and hexadecane (C16H34). You have access to standard school laboratory equipment including thermometers (±0.5 °C), heating mantles, condenser set-ups, and data loggers.
Q2. Design the investigation and present it using the format below.
- State a hypothesis (a testable prediction) that addresses the student’s claim, including independent and dependent variables.
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will measure and record boiling points safely.
- Explain what result would support the student’s claim and what would falsify it.
- Link your expected results to Module 1 IQ2 reasoning about dispersion forces and molecular size.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Evaluation of each material against four criteria:
Graphite: (i) Satisfies — high in-plane conductivity due to delocalised π electrons; (ii) Satisfies — very high sublimation point, no melting issue; (iii) Partially fails — slowly oxidised by Cl2, limiting electrode lifetime; (iv) Fails — brittle with low tensile strength (layers slide easily). [1 — correct analysis of at least two criteria for graphite]
Titanium: (i) Satisfies — metallic bonding provides delocalised electrons for conductivity; (ii) Satisfies — MP 1668 °C well above 300 °C; (iii) Satisfies — forms protective TiO2 passivation layer preventing further corrosion; (iv) Satisfies — metallic lattice provides high mechanical strength. [1 — all four criteria correctly applied to Ti]
SiO2: (i) Fails — covalent network insulator; (ii) Satisfies; (iii) Satisfies; (iv) Fails (brittle). [1 — correct classification of at least two]
Iron: (i) Satisfies; (ii) Satisfies; (iii) Fails critically — rapid corrosion; (iv) Satisfies. [1 — identifies critical failure]
Recommendation: Titanium best satisfies all four requirements. It is the only material that simultaneously conducts, withstands 300 °C, resists both Cl2 and NaOH through passivation, and has high mechanical strength. Graphite fails on mechanical strength; SiO2 fails on conductivity; iron fails on chemical resistance. [1 — justified recommendation with comparison to at least two alternatives]
SiO2 failure explained via IQ3: SiO2 is a covalent network solid. Si (Z=14, [Ne]3s²3p²) uses all four valence electrons forming four Si–O covalent bonds throughout the 3D network. O (Z=8, [He]2s²2p&sup4;) uses both available bonding electrons. No electrons are left unbound or delocalised — every electron is localised in a covalent bond. Without mobile charge carriers, SiO2 cannot conduct electricity. [1 — IQ3 electron configuration reasoning correctly applied]
Metallic bonding and titanium requirements (i), (ii), (iv): In Ti, positively charged Ti cations occupy a regular lattice surrounded by a ‘sea’ of delocalised electrons. (i) Conductivity: the mobile electrons carry electrical charge under an applied potential. (ii) High MP: the strong electrostatic attraction between the cation lattice and delocalised electrons requires a large amount of energy to overcome; MP 1668 °C is far above the operating temperature. (iv) Mechanical strength: the non-directional nature of metallic bonding means layers of cations can shift without bond rupture, and the strength of the lattice gives high tensile strength. [1 — metallic bonding model applied to all three criteria]
Limitation and improvement: Ti develops a TiO2 surface oxide layer which, while protective, slightly reduces electrical conductivity over time, increasing cell resistance [1]. Improvement: coat the Ti substrate with a thin layer of ruthenium dioxide (RuO2), which is highly conductive and chemically stable — this is the basis of the dimensionally stable anode (DSA) used industrially [1].
Marking criteria (8 marks): 1 = correct analysis of at least two criteria for graphite; 1 = all four criteria applied correctly to Ti; 1 = identifies correct failures for SiO2 and Fe; 1 = justified recommendation comparing at least two alternatives; 1 = IQ3 electron config reasoning for SiO2; 1 = metallic bonding model applied to all three Ti criteria; 1 = valid limitation with reasoning; 1 = specific improvement (accept any chemically valid coating/modification that improves conductivity or corrosion resistance).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If molecular size (number of carbon atoms) increases the strength of dispersion forces, then the boiling point of straight-chain alkanes will increase with increasing carbon chain length, contradicting the student’s claim that polarity is the only factor. Independent variable: number of carbon atoms per alkane molecule (5 levels: C1, C4, C8, C12, C16). Dependent variable: boiling point (°C). Controlled variables: (1) straight-chain (unbranched) alkane structure — all molecules must be n-alkanes; (2) atmospheric pressure at 1.0 atm during all measurements; (3) heating rate, measurement method, and apparatus type kept constant. [1 — testable hypothesis with IV, DV, and both controlled variables]
Procedure: (1) Set up a distillation apparatus (round-bottom flask, condenser, thermometer positioned at the side-arm junction). Add a measured volume (5 mL) of the alkane to be tested. (2) Heat gently using a heating mantle. Record the temperature at which the first consistent vapour is produced and condensed (note: methane is a gas at room temperature — its boiling point of −161.5 °C will be taken from the literature as a data point, since direct measurement is unsafe at school). (3) Record the boiling point of each alkane to ±0.5 °C. Allow apparatus to cool completely between runs. (4) Plot boiling point (°C) on y-axis vs number of carbon atoms (C1, C4, C8, C12, C16) on x-axis to reveal the trend. Repeat each measurement three times and take the mean. [1 — four valid steps with boiling point measurement protocol]
What supports the student’s claim: If the boiling points of all five alkanes are approximately equal or show no systematic trend with carbon number, the student’s claim is supported (polarity may be the dominant factor). [1 — correct identification of supporting result]
What falsifies the student’s claim: If boiling point increases systematically with carbon chain length (CH4 < C4H10 < C8H18 < C12H26 < C16H34), the student’s claim is falsified — molecular size, not polarity, explains the trend within non-polar alkanes. [1 — correct falsification criterion]
IQ2 link to expected results: All straight-chain alkanes are non-polar covalent molecular substances. The only IMFs between molecules are dispersion forces. Dispersion force strength increases with molecular size because larger molecules have more electrons, creating stronger and more frequent temporary dipoles. More energy is therefore required to separate larger molecules, giving a higher boiling point. Expected result: boiling point increases linearly (approximately) with carbon number. [1 — dispersion force reasoning correctly linking molecular size to boiling point]
Limitations: (1) Methane (C1) is a gas at room temperature (BP −161.5 °C) and cannot be measured safely in a school laboratory; its boiling point must be taken from literature, introducing a potential reliability issue [1]. (2) Impurities in the alkane samples (even small amounts of branched isomers) could alter the measured boiling point, since branching reduces dispersion force strength; commercial-grade alkanes should be used [1].
Improvement: Repeat each alkane measurement three times, report mean ± standard deviation, and check measured values against published boiling points (literature values) to confirm instrument calibration. [1]
Marking criteria (7 marks): 1 = testable hypothesis naming IV, DV, and both controlled variables; 1 = four clear procedural steps with safe measurement protocol; 1 = correctly identifies the result that supports the student’s claim; 1 = correctly identifies the result that falsifies the claim; 1 = IQ2 dispersion force reasoning linking molecular size to BP; 1 = one valid, explained limitation; 1 = specific improvement with justification (can include second limitation if stated).