Required Practical: Temperature Changes in Reactions

Purpose of the Practical

This practical is designed to measure temperature changes during chemical reactions to identify whether they are exothermic or endothermic. It helps you understand how energy is transferred between the reacting chemicals and their surroundings.

• Measure temperature changes when reactants are mixed.
• Investigate whether the reaction releases energy (exothermic) or absorbs energy (endothermic).
• Understand energy transfer in chemical reactions.

Apparatus and Materials

• Thermometer or temperature probe (digital thermometer preferred for accuracy)
• Reactants such as an acid (e.g. hydrochloric acid) and an alkali (e.g. sodium hydroxide)
• Calorimeter or an insulated container (e.g. polystyrene cup) to reduce heat loss
• Measuring cylinders or pipettes for accurate volume measurement
• Stirring rod to mix reactants thoroughly

Method Steps

1. Measure a known volume of the first reactant (e.g. 25 cm³ of hydrochloric acid) and pour it into the calorimeter or insulated container.
2. Record the initial temperature of this reactant using the thermometer or temperature probe.
3. Measure the same volume of the second reactant (e.g. 25 cm³ of sodium hydroxide).
4. Quickly add the second reactant to the first in the calorimeter and stir gently but continuously.
5. Record the temperature at regular intervals (e.g. every 30 seconds) until the temperature stops changing.
6. Calculate the temperature change by subtracting the initial temperature from the highest or lowest temperature recorded.
7. Repeat the experiment to improve reliability and calculate an average temperature change.

For example, if the initial temperature of the acid is 20.06C and the highest temperature recorded after mixing is 28.56C, the temperature change is:

$\Delta T = 28.5^\circ C - 20.0^\circ C = 8.5^\circ C$

Safety and Accuracy

• Wear safety goggles and gloves to protect eyes and skin from acids and alkalis.
• Use appropriate volumes and concentrations to avoid excessive heat or splashing.
• Use a calorimeter or insulated container to minimise heat loss to the surroundings, improving accuracy.
• Stir the mixture gently to ensure even temperature distribution.
• Repeat the experiment several times to check for consistent results and improve reliability.
• Ensure the thermometer or temperature probe is calibrated and read at eye level to avoid parallax errors.

Understanding Temperature Changes in Reactions

When a reaction is exothermic, energy is released to the surroundings, causing the temperature to rise. Examples include neutralisation reactions between acids and alkalis.

When a reaction is endothermic, energy is absorbed from the surroundings, causing the temperature to fall. Examples include dissolving some salts in water.

The temperature change measured in this practical is a direct indication of the energy transferred during the reaction.

Example: Calculating Temperature Change in an Exothermic Reaction

Hydrochloric acid at 25.06C is mixed with sodium hydroxide. The highest temperature recorded is 33.06C. Calculate the temperature change.

$\Delta T = 33.0^\circ C - 25.0^\circ C = 8.0^\circ C$

The positive temperature change shows the reaction is exothermic, releasing energy to the surroundings.

Improving Accuracy and Reliability

• Use a digital temperature probe for more precise readings.
• Insulate the reaction container with cotton wool or a polystyrene cup to reduce heat loss.
• Stir the mixture gently and consistently to ensure uniform temperature.
• Repeat the experiment and calculate an average temperature change to reduce random errors.
• Use the same volumes and concentrations for each repeat to maintain consistency.

For instance, if you mix 20 cm9 of 1.0 mol/dm3 hydrochloric acid at 21.06C with 20 cm9 of 1.0 mol/dm3 sodium hydroxide and the highest temperature recorded is 28.06C, the temperature change is $\Delta T = 28.0^\circ C - 21.0^\circ C = 7.0^\circ C$, indicating an exothermic reaction.