Determining enthalpy changes experimentally allows chemists to quantify energy transfer in reactions using calorimetry. Accurate measurement, controlled conditions, and awareness of experimental limitations are essential for reliable enthalpy values.

Determining Enthalpy Changes Experimentally

Experimental determination of enthalpy changes centres on calorimetry, a technique used to measure temperature changes during chemical or physical processes. These temperature changes allow calculation of the energy transferred to or from the surroundings. Because enthalpy cannot be measured directly, calorimetry provides an indirect yet essential method for analysing thermochemical behaviour under controlled conditions. OCR requires students to understand how to apply q = mcΔT, identify suitable techniques, and evaluate common experimental errors that influence enthalpy values.

Key Principles of Calorimetry

Calorimetry involves measuring the temperature change of a known mass of material, often water or an aqueous solution, when a reaction occurs.

A coffee-cup calorimeter uses nested insulated cups, a lid, a thermometer and a stirrer to measure temperature changes in solution. The reaction mixture is contained inside the cups to reduce heat loss. This arrangement allows enthalpy changes of reactions in aqueous solution to be determined using the measured temperature change. Source

These temperature changes allow calculation of the energy transferred to or from the surroundings.

In calorimetry, the surroundings are assumed to absorb all heat released or supplied by the reaction, though in practice some energy is inevitably lost. Accurate temperature measurement is therefore central to reliable enthalpy calculation.

Types of Calorimetry Used in OCR A-Level Chemistry

Different calorimetric setups are used depending on the reaction and desired measurement. Each method aims to maximise heat transfer to the measured substance while reducing losses to the environment.

Solution Calorimetry

This method is commonly used for reactions occurring in aqueous solution, such as neutralisation, dissolution, or displacement reactions.

Key features include:

• A polystyrene cup or insulated vessel to minimise heat loss

• Measurement of initial and final temperatures using a thermometer or probe

• Stirring to ensure uniform temperature distribution

Because the reaction occurs within the solution, the mass used in the calculation corresponds to the total mass of the solution or water present.

Combustion Calorimetry

This method measures heat released when a fuel undergoes complete combustion.

A combustion calorimeter consists of a spirit burner heating a copper can of water, with a thermometer to record the temperature rise. This setup demonstrates how heat from fuel combustion is transferred to water for enthalpy calculations. The image also highlights potential heat-loss pathways such as around the sides of the can. Source

A known mass of fuel is burned beneath a container of water.

Key aspects:

• Burning takes place in open air or a simple spirit burner

• The mass of fuel burned is recorded before and after heating

• The temperature rise in the water is measured

Due to significant heat losses in open systems, experimental enthalpy values for combustion are typically less exothermic than data-book values.

Practical Techniques for Accurate Measurements

To produce reliable enthalpy data, careful experimental technique is essential. Students must be aware of best practices that reduce uncertainty.

Temperature Measurement and Control

Accurate temperature readings depend on appropriate measurement tools and consistent experimental handling.

• Use digital temperature probes for higher precision when available

• Allow the thermometer to equilibrate before recording readings

• Measure temperatures at regular intervals, especially for reactions with slow heat release

Temperature extrapolation may sometimes be required to account for continued heat exchange after the reaction.

Minimising Heat Loss

Energy loss to the environment is one of the main limitations of school-level calorimetry.

This diagram shows a styrofoam-cup calorimeter equipped with a lid, thermometer and stirring rod to reduce heat loss. The water inside acts as the measured surroundings absorbing energy from the system. The specific application to metal heat capacities extends beyond the OCR syllabus but illustrates the same calorimetric principles. Source

Strategies to reduce heat loss include:

• Using lids or insulation around reaction vessels

• Positioning the thermometer centrally and avoiding contact with vessel walls

• Shielding combustion flames from draughts

Although these precautions improve accuracy, some heat loss is unavoidable.

Sources of Experimental Error

OCR requires discussion of typical errors that influence enthalpy measurements. These errors generally cause enthalpy values to appear less exothermic or less endothermic than expected.

Common sources of error include:

• Heat loss to surroundings: The measured temperature change is smaller than the true value.

• Incomplete combustion: Particularly relevant for fuels with lower volatility, producing soot and releasing less energy than theoretical values.

• Evaporation of volatile substances: Loss of mass leads to inaccurate calculations, especially in combustion experiments.

• Non-standard conditions: Temperature and pressure variations affect enthalpy but are not accounted for during basic calorimetry.

• Assumption that specific heat capacity is constant: In practice, impurities or varying solution concentrations affect heat capacity.

These limitations highlight why experimental values rarely match standard enthalpy changes listed in data books, which are measured under highly controlled conditions.

Improving Reliability and Precision

While perfect calorimetry is not possible in typical laboratory conditions, improvements can significantly enhance accuracy.

Useful improvements include:

• Using calorimeters with improved insulation, such as copper vessels or commercial bomb calorimeters for combustion

• Employing data-logging equipment to produce continuous temperature profiles

• Ensuring complete reaction, particularly in displacement and neutralisation experiments

• Using larger masses of water, reducing relative error in temperature measurement

• Performing repeat trials and taking mean values to reduce random error

These refinements support more reliable enthalpy calculations and strengthen students’ understanding of thermochemical measurements.

Applying Calorimetry to Enthalpy Calculations

Once q = mcΔT is used to calculate the energy transferred, students convert this energy into an enthalpy change per mole of reactant. This requires:

• Calculating moles of the limiting reagent

• Adjusting the sign of ΔH: negative for exothermic, positive for endothermic processes

• Scaling the value to per mole

Although calorimetry provides indirect measurements, it remains a foundational experimental technique for quantifying enthalpy changes in A-Level Chemistry.