Required Practical 10: Temperature Changes in Reacting Solutions

Measuring the effect of a reaction

When chemicals react in solution, the mixture may become warmer or colder. A thermometer measures this change in the surroundings of the reacting particles. A temperature rise indicates an exothermic reaction; a fall indicates an endothermic reaction.

Required practical 10 investigates variables that affect the recorded temperature change. Suitable reactions include acids with metals, acids with carbonates, neutralisation reactions and displacement of metals. The central skill is to change one factor, measure its effect and make a fair comparison. This practical is limited to temperature measurements: calculations of energy changes or $\Delta H$ are not required.

Choosing a fair comparison

One investigation compares different concentrations of hydrochloric acid reacting with sodium hydroxide solution. These substances undergo neutralisation, producing sodium chloride and water.

Use 25 cm³ of hydrochloric acid at each of four concentrations: 0.2, 0.4, 0.6 and 0.8 mol/dm³. Mix each with 25 cm³ of 1.0 mol/dm³ sodium hydroxide. The sodium hydroxide is in excess throughout this range, meaning that enough is available to react with all the acid.

The independent variable is acid concentration. The dependent variable is temperature rise. Keep both solution volumes, the sodium hydroxide concentration, the starting temperature, the cup and lid, and the mixing and temperature-reading method constant. These are the control variables. For example, changing the solution volume as well as concentration would make it harder to decide which change caused the result.

Apparatus that helps retain the temperature change

Use measuring cylinders to measure the solutions, a thermometer to measure temperature, and a polystyrene cup supported inside a beaker. A lid with a thermometer hole helps reduce energy transfer between the mixture and the room. Polystyrene is a thermal insulator, so less energy escapes through the sides than it would through a poorly insulated container.

Keep the thermometer bulb fully immersed in the liquid. Stirring gently distributes thermal energy through the mixture, making the reading more representative of the whole solution.

Carrying out the concentration investigation

1. Wear safety goggles throughout and work under teacher supervision. Acids and alkalis can harm the eyes; use the concentrations provided by your teacher.
2. Support the polystyrene cup in a beaker. Measure 25 cm³ of the first acid solution into the cup and measure 25 cm³ of sodium hydroxide separately.
3. Allow both solutions to reach the same starting temperature. Measure and record that temperature before mixing.
4. Add the sodium hydroxide, replace the lid and stir gently.
5. Measure the temperature at regular short intervals until it begins to fall. Record the highest temperature reached, rather than just the temperature immediately after mixing.
6. Calculate the temperature rise. Rinse and dry the cup, then use fresh solutions to repeat the method at the other acid concentrations.
7. Repeat trials at each concentration and calculate a mean temperature rise from consistent results. Investigate an unusual result by repeating the trial rather than simply discarding it.

Using the same starting temperature matters because otherwise part of the measured change could result from mixing a warmer solution with a colder one, rather than from the reaction.

Turning readings into a conclusion

Calculate the temperature change using:

$$\text{temperature change} = \text{highest or lowest temperature} - \text{starting temperature}$$

For one trial, a starting temperature of 20.0 °C and a maximum of 24.5 °C give a change of $24.5 - 20.0 = +4.5$ °C. The positive change shows that the mixture warmed. If a mixture instead fell from 20.0 °C to 17.0 °C, its change would be $-3.0$ °C: a temperature decrease of 3.0 °C.

Record the concentration, starting temperature, maximum temperature and temperature rise for every trial. Plot mean temperature rise in °C on the vertical axis against acid concentration in mol/dm³ on the horizontal axis. Use the pattern of results to describe how concentration affects the rise.

In this investigation, a more concentrated acid contains more acid in the same volume. With the alkali in excess, more acid can be neutralised, so a larger temperature rise is expected. This prediction applies to the chosen conditions, not to every possible mixture of acid and alkali.

Adapting the method to other reactions

For a solid reacting with a solution, measure the solution volume and starting temperature, then use a balance to measure the solid's mass before adding it. Mix gently and record the highest or lowest temperature reached.

For example, compare equal masses of magnesium, zinc, iron and copper added separately to dilute hydrochloric acid. Change the type of metal, while keeping the acid volume, concentration, starting temperature and measurement method the same. Because metal type is a category, a bar chart is more suitable than a continuous concentration graph.

The same basic temperature-measurement approach can be adapted to acid–carbonate reactions or metal displacement reactions. Choose suitable reactants under teacher supervision and keep all factors except the selected variable constant.

Judging the quality of the results

Energy can still escape through the cup, lid and thermometer. An exothermic reaction's measured maximum may therefore be lower than it would be with perfect insulation. Using a lid, mixing consistently and taking frequent readings helps reduce this problem and the chance of missing the maximum or minimum.

Repeats reveal how consistent the measurements are, but averaging does not remove a repeated heat-loss error. Compare temperature changes under controlled conditions rather than treating them as direct measurements of the energy transferred.

Core method

Measure fixed quantities → record a common starting temperature → mix in a supported polystyrene cup → replace lid and stir → record the maximum or minimum → calculate temperature change → repeat and find a mean.

$$\text{temperature change} = \text{highest or lowest temperature} - \text{starting temperature}$$

Positive change: exothermic. Negative change: endothermic.

Fair investigation

• Change one variable: for example, acid concentration or metal type.
• Measure temperature change, not just final temperature.
• Control reactant quantities, other concentrations, starting temperature, insulation and measurement method.
• For concentration comparisons, plot mean temperature change against concentration. For metal types, use a bar chart.

Apparatus and evaluation

• Polystyrene cup and lid: reduce energy transfer to the room.
• Beaker: supports the cup.
• Stirring: gives a more uniform temperature.
• Fully immersed thermometer bulb and frequent readings: help measure the temperature extreme.
• Repeats: check consistency; they do not eliminate systematic heat loss.
• Wear goggles and follow teacher instructions for chemicals and disposal.

Suitable reactions include acid–metal, acid–carbonate, neutralisation and metal displacement. Energy-change and $\Delta H$ calculations are not required.