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Module 1: Practical chemistry for exams›PAG 3.1 - Determination of the enthalpy change of neutralisation

PAG 3.1 - Determination of the enthalpy change of neutralisation

An overview of PAG 3.1 - Determination of the enthalpy change of neutralisation

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In OCR Chemistry Required Practical 3 (PAG 3.1), the enthalpy of neutralisation, , of hydrochloric acid and sodium hydroxide solution is found through calorimetry.

The temperature is recorded before and after the solution is mixed and the temperature change is obtained through extrapolation of lines of best fit. The is calculated for 1 mole of the acid using the equation .

In OCR PAG 3.1: Determination of the enthalpy change of neutralisation, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

The sodium hydroxide causes severe skin burns and eye damage
Immediately clean up any spills of acid or alkali.

Experimental method for OCR Chemistry Required Practical 3 (PAG 3.1)

A step-by-step guide for conducting an experiment using a calorimeter. The instructions include preparing the calorimeter, measuring hydrochloric acid and sodium hydroxide, recording temperature readings, and mixing the solutions. Illustrations depict the equipment and setup, including a polystyrene cup, measuring cylinders, a thermometer, and a stopwatch.

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.

Plot a graph of temperature against time and extrapolate the cooling line back to the mixing time to obtain a corrected

Calculate the energy transferred using:

Then calculate the enthalpy change per mole using:

(q will be converted into kJ for this equation)

Compare the result with the accepted enthalpy change of neutralisation.