Pearson International A Level Chemistry Core Practical 5: Investigate rates of halogenoalkane hydrolysis

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

Compare hydrolysis rates using a common observable endpoint while controlling temperature, mixing and reactant amounts.

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 halogenoalkanes, aqueous ethanol, silver nitrate, water bath, tubes, pipettes and timer. 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. Equilibrate equal reagent volumes and labelled halogenoalkane samples at one controlled temperature.
2. Mix each sample consistently and start timing at the same procedural point.
3. Record time to the agreed silver-halide cloudiness endpoint or use an instrumental turbidity threshold.
4. Repeat and compare reciprocal times only when the same endpoint amount is produced.

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: halogenoalkane structure or temperature.
• Dependent variable: time to a defined precipitate or turbidity endpoint.
• Important controls: