QCE Chemistry - Unit 2 - Intermolecular forces and gases

Gas laws, kinetic theory and ideal-gas stoichiometry

Learn gas behaviour 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

Explain P–V–T changes using particle collisions and use molar volume or PV = nRT in reaction calculations. 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

Gas pressure arises from particle collisions with container walls. At constant amount, changing temperature changes average kinetic energy; changing volume changes collision frequency per unit area. The ideal model assumes negligible particle volume and intermolecular attraction.

Move between the three levels

• Observable level: A pressure gauge, thermometer and calibrated container report changes in gas pressure, temperature and volume.
• Particle level: Pressure results from particle collisions with container walls, while kelvin temperature tracks average translational kinetic energy.
• Symbolic level: PV = nRT links the four state variables; a particular gas law follows when specified variables are held constant.

The equation predicts the numerical change and kinetic theory supplies the causal explanation, but both require the constraints to be stated.

[!MODEL BOUNDARY]

The ideal-gas model treats particles as negligible in volume with no attractions; real gases depart most at high pressure and low temperature.

The exact relationship

$ PV=nRT\qquad T(\mathrm K)=T({}^{\circ}\mathrm C)+273.15 $

Never substitute degrees Celsius into the ideal-gas equation.

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. Convert temperature to kelvin.
2. Use consistent pressure, volume and R units.
3. Solve PV = nRT for moles before applying a reaction ratio.
4. Convert the resulting amount to mass or gas volume as requested.

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: Separate a constraint from a gas-law conclusion

Scenario. A rigid sealed cylinder is heated from 300 K to 360 K. Its initial pressure is 100 kPa. Predict the final pressure and explain the particle cause.

| Observed or given | Chemical meaning | | --- | --- | | The cylinder is rigid | Volume is constant. | | The cylinder is sealed | Amount of gas is constant. | | Temperature rises by a factor 360/300 | At fixed n and V, P is proportional to T, so pressure rises by the same factor. |

Analysis. P₂ = 100 × 360/300 = 120 kPa. Faster particles collide with the walls more frequently and with greater momentum change, increasing force per unit area.

Defensible conclusion. The ideal-model prediction is 120 kPa, caused by more forceful and frequent wall collisions at higher average kinetic energy.

[!LIMIT OF THE EVIDENCE]

The prediction assumes the container volume truly remains fixed and the gas remains close to ideal over the temperature range.

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

Real gases depart most from the ideal model at high pressure or low temperature, where particle volume and attractions matter more. The syllabus calculation nevertheless uses the stated ideal relationship.

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 temperature scale is used in PV = nRT?
• At constant T, what happens when volume falls?
• When is ideal behaviour least accurate?
• What must follow a gas mole calculation in reaction work?

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.