QCE Chemistry - Unit 2 - Intermolecular forces and gases

Molecular shape, polarity and intermolecular forces

Learn shape, polarity and forces for QCE Chemistry Unit 2 with worked reasoning, KaTeX equations, original diagrams and assessment checks.

Part of the free QCE Chemistry notes library for Unit 2: Intermolecular forces and gases.

Updated 2026-08-10 - 7 min read

Use VSEPR and symmetry to predict shape, bond angle and molecular polarity, then explain property trends from intermolecular forces. This note is designed to be used actively: pause at each prediction, show the particle-level or quantitative reason, and only then compare your reasoning with the worked answer. The aim is not to collect definitions. It is to build a chemical model that remains dependable when the substances, data or experimental context change.

The central chemical model

VSEPR predicts electron-domain arrangements by minimising repulsion. Molecular polarity combines polar bond vectors with three-dimensional symmetry; bond polarity alone is insufficient. Dispersion acts between all particles, while permanent dipoles and hydrogen bonding add stronger interactions where structural requirements are met.

Move between the three levels

• Observable level: Molecular substances differ in boiling point, vapour pressure, viscosity and solubility even when their molar masses are similar.
• Particle level: Electron domains set three-dimensional shape; bond dipoles combine as vectors; the resulting charge distribution determines intermolecular attractions.
• Symbolic level: Lewis structure → domain count → molecular shape and angle → dipole cancellation test → strongest applicable intermolecular force.

The structural sequence explains why CO₂ has polar bonds but no molecular dipole, while bent H₂O is polar and forms hydrogen bonds.

[!MODEL BOUNDARY]

VSEPR predicts approximate geometry for the specified molecules but does not calculate exact bond angles, bond energies or the full electron-density distribution.

The exact relationship

$ \mu_{\mathrm{molecule}}=\sum \vec{\mu}_{\mathrm{bond}} $

The vector sum can be zero even when individual bonds are polar.

Before substituting values, name what each symbol or chemical formula represents in this context. Check units, state symbols and signs. After calculating, test whether the magnitude and direction are chemically plausible. A calculator can execute arithmetic but cannot tell you that an isotope average lies outside the isotope range, a negative absolute temperature was used, or an ionic formula carries a residual charge.

A repeatable reasoning method

1. Draw a complete Lewis structure.
2. Count electron domains around the central atom; multiple bonds count as one domain.
3. Name the molecular shape after ignoring lone-pair positions.
4. Assess bond polarity and whether the vectors cancel by symmetry.

This sequence is a reasoning scaffold, not a sentence template. In a short-response question, compress it to the decisions that earn marks. In a practical or data question, keep the evidence visible: name the observation, quote or process relevant data, and explain how the model supports the conclusion. If the question asks you to analyse, do more than state a trend—use the trend to infer a structure, process or relationship.

Evidence clinic: Explain a boiling-point comparison

Scenario. CH₄ boils at −162 °C, NH₃ at −33 °C and H₂O at 100 °C. Their molar masses are similar enough that mass alone is inadequate.

| Observed or given | Chemical meaning | | --- | --- | | CH₄ is tetrahedral and non-polar | Only London dispersion forces operate between its molecules. | | NH₃ is pyramidal with N–H bonds and a lone pair | It is polar and can hydrogen-bond, raising the energy needed for separation. | | H₂O is bent and each molecule can form an extended hydrogen-bond network | Stronger and more extensive intermolecular association gives the highest boiling point. |

Analysis. Boiling separates molecules rather than breaking their internal covalent bonds. Compare the attractions between molecules and the number of sites available, not just the presence of an H atom.

Defensible conclusion. Intermolecular-force strength and network extent explain CH₄ < NH₃ < H₂O in boiling point.

[!LIMIT OF THE EVIDENCE]

The simple ranking is qualitative; molecular size, shape and polarizability must also be controlled in less carefully matched comparisons.

Worked example

The final answer is only the last line of the reasoning. To learn from the example, cover the steps and reproduce them from the prompt. Then change one feature—an ionic charge, quantity, temperature, molecular shape or measured interval—and predict which steps must change and which chemical principle stays invariant.

Why this matters in unfamiliar questions

Stronger intermolecular forces generally lower vapour pressure and raise boiling point. Solubility depends on whether new solute–solvent attractions can compensate for attractions disrupted during mixing.

QCAA-style questions often provide enough information but distribute it across prose, a diagram and a data table. Start by translating every given item into a chemical role. Mark values that are initial, final, measured or derived. Identify controlled variables before comparing trials. If a conclusion depends on more than one observation, state how the observations work together. Avoid claiming certainty beyond the resolution of the method.

For quantitative work, write the governing relationship before numbers, preserve unrounded intermediate values and round only the final answer to a precision justified by the data. For explanatory work, use a cause chain: structural or experimental change → particle-level consequence → change in collisions, attractions, energy or composition → observed result. That chain is more transferable than a memorised trend.

How to judge practical or data evidence

Use the clinic above as a model: quote the relevant observation, translate it into particle or quantitative meaning, and then state a conclusion no stronger than the method allows. A valid comparison changes one independent variable, defines the dependent measure and controls plausible alternative causes. Replicates reveal random variation; they do not repair a calibration bias, heat loss, contamination or an unsuitable measurement range.

For laboratory work, name hazards that actually arise from the substances and procedure. Reduce risk through concentration, scale, containment, ventilation, temperature control and disposal design before relying on personal protective equipment alone.

Common mistake and repair

The repair is important because Chemistry marking rewards the relationship that justifies an answer. Before finishing, audit four things: particle identity, conserved atoms or charge, direction of energy or matter transfer, and units. If all four remain consistent, the explanation is usually much harder to derail.

Try it yourself

Now answer these without returning to the note:

• What shape is NH₃?
• Why is CO₂ non-polar?
• Which molecules can hydrogen-bond strongly?
• How does stronger IMF affect vapour pressure?

For each response, add a brief verification: charge balance, atom count, a reverse substitution, a limiting case, a particle sketch or a check against the graph. Verification turns a plausible answer into a defensible one.

Assessment transfer checklist

• I can define the relevant model without circular wording.
• I can represent it with the required formula, equation, state symbols or diagram.
• I can show why the observation follows from particles, forces, collisions, energy or amount.
• I can calculate with units and retain sensible precision.
• I can distinguish direct evidence from an inference.
• I can state a limitation without claiming that all evidence is therefore useless.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e
• IUPAC Gold Book

Finished reading? Practise this topic free

Open Chemistry past questions with this Unit 2 topic carried into the question bank, then save your progress for the next review.

Practise this topic free. Free to start. No payment details are required. Exact question coverage depends on the available past-paper syllabus mapping.