Experiment: Determining the heat of neutralisation

Determining the heat of neutralisation is a core Form 5 practical in the Thermochemistry chapter. It shows how a temperature change is used to work out an energy change per mole, and it is one of the clearest examples of an exothermic reaction. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward. Note that the observations here are qualitative; use your own measured readings for any calculation.

Aim

To determine the heat of neutralisation of an acid by an alkali by measuring the temperature change when known volumes of the two solutions are mixed in an insulated container.

Apparatus and materials

• Polystyrene cup (with a lid) as an insulated calorimeter
• Thermometer
• Measuring cylinders
• A dilute strong acid, for example hydrochloric acid of known concentration
• A dilute strong alkali, for example sodium hydroxide of known concentration
• Stirrer and a stopwatch (if temperature is followed over time)

Procedure

1. Measure a known volume of the acid with a measuring cylinder and pour it into the polystyrene cup.
2. Measure the initial temperature of the acid with the thermometer and record it.
3. Measure an equal known volume of the alkali in a second measuring cylinder and record its initial temperature; if the two starting temperatures differ, take their average as the starting temperature.
4. Pour the alkali quickly into the acid in the polystyrene cup and stir the mixture gently.
5. Watch the thermometer and record the highest (maximum) temperature reached by the mixture.
6. Calculate the temperature change from the starting temperature to the highest temperature.
7. Use the temperature change to calculate the heat released, then divide by the number of moles of water formed to obtain the heat of neutralisation per mole.

Expected observations

When the alkali is added to the acid, the temperature of the mixture rises: the thermometer reading climbs to a maximum shortly after mixing and then slowly falls as heat is lost to the surroundings. There is no visible change of colour with these clear solutions, but the warmth of the cup can be felt. The key measurement is the rise in temperature from the starting value to the highest value reached; that rise is what you use in the calculation.

Inference and conclusion

The temperature rises because neutralisation is an exothermic reaction: heat energy is released when hydrogen ions from the acid react with hydroxide ions from the alkali to form water. Using the temperature change, the heat released is calculated with the relationship heat released = mass of solution x specific heat capacity x temperature change (mc-theta), and dividing by the moles of water formed gives the heat of neutralisation per mole. The conclusion is that neutralisation releases heat, and its heat of neutralisation can be found from the measured temperature change.

Science process skills (Paper 3 style)

Identifying variables. If the investigation compared different acids or alkalis, the manipulated variable is the acid or alkali used, the responding variable is the temperature change, and the controlled variables are the volume and concentration of the solutions and the insulated cup used.

Making an inference. From the observation that the temperature rises, infer that the reaction is exothermic and releases heat to the surroundings.

Tabulating and calculating. Record initial temperature, highest temperature and the temperature change in a table, then show the calculation of heat released and of the heat of neutralisation per mole, with units.

Controlling for accuracy. Explain that using a polystyrene cup with a lid reduces heat loss, so the measured temperature change is closer to the true value, improving the accuracy of the result.

Safety precautions

• Wear safety goggles, because the acid and the alkali are both corrosive and can harm the eyes.
• Pour the solutions carefully and avoid splashing, so that corrosive liquid does not reach the skin or eyes.
• Wash any splashes off the skin at once with plenty of water, to limit chemical burns.
• Handle the thermometer gently and do not use it to stir vigorously, so that it does not break.
• Dispose of the neutralised mixture as instructed, rather than pouring concentrated chemicals down the sink.

Common errors

• Reading the temperature too late. If you wait after mixing, heat is lost and the highest temperature is missed; watch the thermometer closely and record the maximum.
• Using a glass beaker. A glass beaker conducts heat away and lowers the measured temperature change; use an insulated polystyrene cup with a lid.
• Not stirring. Without gentle stirring the mixture is uneven and the thermometer may not read the true maximum; stir gently before reading.
• Ignoring the starting temperatures. If the acid and alkali start at different temperatures, use their average; do not use only one.
• Inventing readings. Record the actual temperatures you measure; do not copy numbers from elsewhere, because the calculation must come from your own data.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can set out the calculation clearly, temperature change, heat released with mc-theta, then heat per mole, and can justify each precaution, such as why the polystyrene cup improves accuracy. Heat of neutralisation is a recurring calculation across SPM Chemistry, and the same method of measuring a temperature change and working out energy per mole reappears in the heat of displacement and combustion, and in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).