Pearson International A Level Chemistry Core Practical 2: Determine an enthalpy change using Hess's Law

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 two or more temperature changes that form a Hess cycle and combine the derived enthalpy changes with correct signs.

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 insulated cups with lids, thermometer or temperature probe, measured solutions or solids, balance, pipette or cylinder and stirrer. 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. Measure reactant amounts and initial temperatures using an insulated calorimeter.
2. Mix promptly, stir consistently and record temperature frequently through the maximum or minimum.
3. Calculate solution heat change using mass, specific heat capacity and corrected temperature change where justified.
4. Convert to molar enthalpy changes and combine the cycle with explicit arrow directions.

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: reaction route or reactant combination.
• Dependent variable: temperature change and derived molar enthalpy change.