How do you do the temperature changes required practical?

What does the temperature changes practical investigate?

It investigates the variables that affect the temperature change when solutions react. The usual version is a neutralisation: sodium hydroxide solution added bit by bit to hydrochloric acid. Other versions react an acid with a metal or a carbonate, or a metal with a salt solution (a displacement reaction, such as zinc powder in copper sulfate solution).

It is a required practical on both GCSE Chemistry and Combined Science: Trilogy.

The idea underneath it: if the mixture gets warmer, the reaction is exothermic and is giving out energy to its surroundings. If it gets colder, the reaction is endothermic and is taking energy in. Neutralisation is exothermic. The reaction of citric acid with sodium hydrogencarbonate is a good endothermic example to know.

What equipment do you need?

The key piece is the insulated cup. A polystyrene cup with a lid keeps the energy from the reaction in the mixture, where the thermometer can measure it, instead of letting it escape into the room.

• Dilute hydrochloric acid and dilute sodium hydroxide solution
• A polystyrene cup, and a lid with a hole for the thermometer
• A 250 cm³ beaker to stand the cup in
• A thermometer or a digital temperature probe
• A 50 cm³ measuring cylinder for the acid and a 10 cm³ one for the alkali
• Eye protection

What is the method?

Measure the acid into the insulated cup and take its temperature, then add the alkali in 5 cm³ portions, recording the highest temperature after each one.

• Measure 30 cm³ of dilute hydrochloric acid into the polystyrene cup, and stand the cup in the beaker so it cannot tip over.
• Take the temperature of the acid and record it.
• Measure 5 cm³ of sodium hydroxide solution and pour it into the cup.
• Fit the lid and stir gently with the thermometer through the hole.
• When the reading stops changing, record the highest temperature it reached.
• Add another 5 cm³ of sodium hydroxide, stir and record the highest temperature again. Keep going until you have added 40 cm³ in total.
• Wash and dry the cup, then repeat the whole experiment and work out a mean temperature for each volume.

What are the variables?

In the neutralisation version, you change the volume of alkali added and measure the temperature. If your class reacted a metal with an acid instead, the independent variable might be the mass of metal or the concentration of the acid, but the dependent variable is still the temperature change.

How do you work out and present the results?

For a single reaction, temperature change = highest temperature − starting temperature. If the acid starts at 20.5 °C and the highest reading is 27.0 °C, the temperature change is 27.0 − 20.5 = 6.5 °C. For an endothermic reaction the temperature falls, so you use the lowest reading instead.

For the neutralisation, plot the mean highest temperature (y-axis) against the volume of sodium hydroxide added (x-axis). The points rise, then fall. Draw two straight lines of best fit, one through the rising points and one through the falling points, and extend them until they cross.

The crossing point gives two answers. Read down to find the volume of alkali that exactly neutralised the acid, and across to find the highest temperature. The biggest temperature change is that temperature minus the starting temperature.

Why does the temperature fall after the peak? Once all the acid has reacted, no more energy is released, and each extra 5 cm³ of cooler alkali cools the mixture, which is also losing energy to the air.

How do you make it accurate and safe?

The main source of error is energy escaping, which makes the temperature change look smaller than it really is. Polystyrene is a good insulator and the lid cuts the energy lost to the air. Standing the cup in a beaker is about stability: it stops the cup tipping over.

Stir after each addition so the whole mixture is at one temperature, and wait until the reading stops changing before you record it. A digital thermometer that reads to 0.1 °C is more precise than a glass one marked in whole degrees. Repeating the experiment and taking a mean makes the results more repeatable.

For safety, wear eye protection. Sodium hydroxide solution is especially harmful to eyes, and the acid irritates skin and eyes. Wipe up spills straight away, and if a glass thermometer breaks, tell your teacher rather than picking up the pieces.

What do exam questions ask about it?

Expect to name the variables and calculate a temperature change. Some questions give you data and ask whether the reaction is exothermic or endothermic. Graph questions ask you to draw the two lines of best fit and read off where they cross, and evaluation questions ask why the measured change is smaller than expected, or why the cup has a lid. For example:

"The temperature of the mixture rises until 25 cm³ of sodium hydroxide solution has been added, then it falls. Explain why the temperature falls after this point. (2 marks)"

Two ideas answer it: all the acid has now reacted, so no more energy is being released, and the extra solution being added is cooler than the mixture, so it brings the temperature down.

Common mistakes

Watch for these, in the lab and on the paper:

• Recording the temperature before it has stopped rising. Watch the thermometer and record the highest reading.
• Joining all the points with one curve. The neutralisation graph needs two straight lines of best fit, and the answer is where they cross.
• Mixing up exothermic and endothermic. If the mixture gets warmer, the reaction is giving out energy, so it is exothermic.
• Leaving off the lid, or using a glass beaker. More energy escapes, so the temperature change comes out too small.
• Giving the highest temperature when the question asks for the change. Subtract the starting temperature.