Pearson International A Level Chemistry Core Practical 9: Follow iodine-propanone and clock reaction rates

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Measure reaction progress with timed sampling or a reproducible clock endpoint and derive rate information from controlled concentration changes.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use reaction mixtures, pipettes, thermostatic bath, timer, thiosulfate titration equipment or clock indicator and data table. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Prepare solutions and equilibrate them at a controlled temperature.
2. Mix at a defined start point and either withdraw timed aliquots for quenching and titration or observe a fixed clock endpoint.
3. Repeat while changing one initial concentration at a time and keeping total volume controlled.
4. Calculate initial-rate proxies or concentration-time data and compare controlled pairs.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: one reactant concentration or temperature.
• Dependent variable: titrated iodine concentration over time or time to the fixed clock endpoint.