Our website won't work without javascript. Please enable it or upgrade your browser.

Module 1: Practical chemistry for exams›PAG 3.2 - Determination of an enthalpy change of reaction by Hess’ law

PAG 3.2 - Determination of an enthalpy change of reaction by Hess’ law

An overview of PAG 3.2 - Determination of an enthalpy change of reaction by Hess’ law

3 min

On this page

In OCR Chemistry Required Practical 3 (PAG 3.2), Hess’s Law is used to convert the enthalpy changes of two measurable reactions into the unmeasurable enthalpy change of the thermal decomposition of a solid.

This involves using calorimetry to measure the temperature changes in the reactions of both and with hydrochloric acid and using Hess’s Law to calculate the enthalpy change of the thermal decomposition of .

Chemical reaction diagram showing the transformation of potassium bicarbonate (2KHCO3) into potassium carbonate (K2CO3), carbon dioxide (CO2), and water (H2O), with associated enthalpy changes (ΔrH, ΔH1) and additional products including hydrochloric acid (HCl) and potassium chloride (KCl).

In OCR PAG 3.2: Determination of an enthalpy change of reaction by Hess’ Law, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Potassium carbonate solid causes serious eye irritation
Immediately clean up any spills of acid or alkali.

Experimental method for OCR Chemistry Required Practical 3 (PAG 3.2)

Part 1 Reaction of potassium carbonate and

A step-by-step guide for conducting an experiment using a calorimeter. The image includes numbered instructions for preparing the calorimeter, adding hydrochloric acid, weighing potassium carbonate, recording temperatures, and measuring mass changes during the reaction.

Part 2

Reaction 2 potassium hydrogen carbonate with .

The above steps are repeated using potassium hydrogen carbonate rather than potassium carbonate.

The precision of the balance used is critical for ensuring the accuracy of mass measurements and subsequent calculations.

A digital laboratory scale with a circular weighing platform and a blue control panel displaying a reading of 0.00.

Do

Measure solids and liquids precisely. Ensures accurate mole calculations and enthalpy values. A balance with a resolution of at least two decimal places should be used.

A digital weighing scale with a round platform and a red control panel displaying '0.0' on a green screen.

Don't

Use a balance with a resolution of 1 decimal place or lower.

The use of a lid and polystyrene cup is critical for minimising heat loss to the surroundings and ensuring accurate temperature measurements.

A simple illustration of a disposable cup with a straw, featuring a light blue straw protruding from the top.

Do

Use a lid and a polystyrene cup. Minimises heat loss to surroundings.

A clear beaker filled with blue liquid, with a stirring rod positioned inside.

Don't

Use a glass beaker and leave the calorimeter uncovered.
This will result in excessive heat loss to the surroundings and inaccurate temperature measurements.

The position of the thermometer in the calorimeter is critical for ensuring accuracy of results.

Illustration of a beaker containing a blue liquid with a thermometer inserted, indicating heat exchange with the reaction mixture only.

Do

Position your thermometer so the bulb is fully covered and away from the container walls.

Illustration of a cup with water and a thermometer, demonstrating heat exchange. The thermometer is partially submerged in the blue water, indicating a measurement of temperature.

Don't

Don’t allow the bulb of the thermometer to make contact with the bottom of the container. This will lead to heat transfer with the container by conduction, which will alter the reading and therefore reduce accuracy of your results.

Stirring continuously is critical for ensuring even heat distribution and obtaining precise temperature readings.

A simple illustration of a disposable cup with a straw inserted, showing a gradient fill from light to dark.

Do

Stir continuously.
Distributes heat evenly for consistent temperature readings.

A simple illustration of a disposable cup with a lid, placed next to a light blue straw.

Don't

Stir inconsistently or stop stirring.
Leads to uneven temperature distribution and inaccurate temperature measurements.

Repeating measurements is essential for improving the reliability of results, as it helps to reduce the impact of random errors and identify anomalies.

A table displaying temperature readings over time for four trials. The first three trials show consistent temperatures around 20°C for the first three minutes, with a gap in data for trial 4 at minute 4. From minute 5 to 7, temperatures in trials 1 to 4 vary, with trial 1 reaching a maximum of 30.4°C at minute 5.

Do

Repeat measurements and calculate the mean.
Improves reliability and reduces the impact of random errors and highlights values to exclude.

A table displaying temperature readings over time in a trial. The first three time points (1, 2, and 3 minutes) show a consistent temperature of 20.1°C. The fourth time point is marked with a dash, indicating no data. Subsequent readings at 5, 6, and 7 minutes show temperatures of 30.4°C, 30.3°C, and 30.2°C, respectively.

Don't

Rely on a single measurement. Increases the risk of anomalies affecting the accuracy of results. Results due to a systematic error will not be spotted.

For each reaction, use the measured to calculate:

Then use:

(q will be converted into kJ for this equation)

Place the two experimental values into a Hess cycle and combine them to determine the required enthalpy change.