Decide whether various reactions are exothermic or endothermic by measuring temperature change in this class practical

Some reactions give out heat and others take in heat. In exothermic reactions the temperature goes up, in endothermic reactions the temperature goes down.

This is a useful class experiment to introduce energy changes in chemical reactions, suitable for 11–14 and 14–16 year olds. Students measure the temperature changes in different reactions taking place in a polystyrene cup, classifying the reactions as exothermic or endothermic.

The experiments can also be used to revise different types of chemical reaction and, with some classes, chemical formulae and equations.

Careful consideration will need to be given as to the most appropriate way to dispense the required chemicals to the class. Special care should be taken with the magnesium ribbon and magnesium powder and, with some classes, teachers may prefer to dispense these materials directly.

The length of time required for carrying out the actual reactions is around 30 minutes, but this will depend on the nature of the class and how the practical is organised.

For 14–16 year old students, the additional class practical and teacher demonstration featured at the bottom of this page provides a further opportunity to practise classifying reactions as exothermic or endothermic, using test tubes instead of polystyrene cups.

Equipment

Apparatus

  • Eye protection
  • Spatula
  • Thermometer, –10 °C to 110 °C
  • Polystyrene cups (expanded polystyrene)
  • Beaker, 250 cm3, in which to stand the polystyrene cup for support (see note 1 below)
  • Measuring cylinder, 10 cm3, x2
  • Absorbent paper

Apparatus notes

  1. Typical expanded polystyrene cups fit snugly into 250 cm3 squat form beakers. This provides a more stable reaction vessel and also prevents spillage if the polystyrene cup splits.

Chemicals

Access to the following solutions (all at approx 0.4 M concentration):

  • Copper(II) sulfate
  • Hydrochloric acid
  • Sodium hydrogencarbonate
  • Sodium hydroxide (IRRITANT)
  • Sulfuric acid

Access to the following solids:

  • Magnesium ribbon, cut into 3 cm lengths
  • Magnesium powder (HIGHLY FLAMMABLE)
  • Citric acid (IRRITANT)

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout.
  • At the suggested concentrations, the solutions (except for sodium hydroxide) represent minimal hazards, although it is probably advisable to label them as HARMFUL. If the concentrations are increased then the solutions must be labelled with the correct hazard warning. The solutions could be provided in small (100 cm3) labelled conical flasks or beakers.
  • Copper(II) sulfate solution, CuSO4(aq) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.
  • Dilute hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043. 
  • Sodium hydrogencarbonate solution, NaHCO3(aq) – see CLEAPSS Hazcard HC095a and CLEAPSS Recipe Book RB084. 
  • Sodium hydroxide, NaOH(aq), (IRRITANT) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085. 
  • Dilute sulfuric acid, H2SO4(aq) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098. 
  • Magnesium ribbon, Mg(s) – see CLEAPSS Hazcard HC059a. The teacher may prefer to keep the magnesium ribbon under their immediate control and to dispense on an individual basis.
  • Magnesium powder, Mg(s), (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC059b. Small amounts of magnesium powder can be provided in plastic weighing boats or similar. The teacher may prefer to keep the magnesium powder under their immediate control and to dispense on an individual basis.
  • Citric acid, HOOCCH2C(OH)(COOH)CH2COOH(s), (IRRITANT) – see CLEAPSS Hazcard HC036c. Small amounts of citric acid can be provided in plastic weighing boats or similar.

Procedure

Reaction of sodium hydroxide solution and dilute hydrochloric acid

  1. Stand the polystyrene cup in the beaker.
  2. Use the measuring cylinder to measure out 10 cm3 of sodium hydroxide solution and pour it into the polystyrene cup.
  3. Measure the initial temperature of the sodium hydroxide solution and record it in a suitable table.
  4. Measure out 10 cm3 of hydrochloric acid and carefully add this to the sodium hydroxide solution in the polystyrene cup. Stir with the thermometer and record the maximum or minimum temperature reached.
  5. Work out the temperature change and decide if the reaction is exothermic or endothermic.
  6. Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of sodium hydrogencarbonate solution and citric acid

  1. Repeat steps 1–3 of the previous experiment, using sodium hydrogencarbonate solution in place of sodium hydroxide solution.
  2. Add 4 small (not heaped) spatula measures of citric acid. Stir with the thermometer and record the maximum or minimum temperature reached.
  3. Work out the temperature change and decide if the reaction is exothermic or endothermic.
  4. Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of copper(II) sulfate solution and magnesium powder

  1. Repeat steps 1–3 of the first experiment, using copper(II) sulfate solution in place of sodium hydroxide solution.
  2. Add 1 small (not heaped) spatula measure of magnesium powder. Stir with the thermometer and record the maximum or minimum temperature reached.
  3. Work out the temperature change and decide if the reaction is exothermic or endothermic.
  4. Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of sulfuric acid and magnesium ribbon

  1. Repeat steps 1–3 of the first experiment, using sulfuric acid in place of sodium hydroxide solution.
  2. Add one 3 cm piece of magnesium ribbon. Stir with the thermometer and record the maximum or minimum temperature reached.
  3. Work out the temperature change and decide if the reaction is exothermic or endothermic.
  4. Once all the magnesium ribbon has reacted, discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Teaching notes

The reactions and types of reaction involved are:

  • Sodium hydroxide + hydrochloric acid → sodium chloride + water (Neutralisation)
    NaOH(aq) + HCl(aq) → NaCl(aq) + H2 O(l)
  • Copper(II) sulfate + magnesium → magnesium sulfate + copper (Displacement, Redox)
    CuSO4 (aq) + Mg(s) → MgSO4 (aq) + Cu(s)
  • Sulfuric acid + magnesium → magnesium sulfate + hydrogen (Displacement, Redox)
    H2 SO4 (aq) + Mg(s) → MgSO4 (aq) + H2 (g)
  • Sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide (Neutralisation)
    NaHCO3 (aq) + H+ (aq) → Na+ (aq) + H2 O(l) + CO2 (g)
    At this level the neutralisation reaction between sodium hydrogen carbonate and citric acid may be a bit complicated – it may be better to just use the word equation. More able students could use H+ (aq) to represent the acid.

More resources

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Explore this topic further

Try this student activity on temperature changes in exothermic and endothermic reactions, featuring teacher notes and a downloadable worksheet. The activity is designed to accompany the experiments that use polystyrene cups.

Test tube reactions (not suitable for 11–14 years)

In this practical, students carry out three test tube reactions and use their hands on the base of the test tube to detect whether the process gives out or takes in energy, classifying them as exothermic or endothermic.

Students are also shown a teacher demonstration, which illustrates an endothermic dissolving process with ammonium nitrate crystals.

This works very well as a class experiment with students working in small groups of two or three. The three student experiments together with the teacher demonstration should take no more than 30–40 minutes. The teacher demonstration using ammonium nitrate should take no more than five minutes.

Apparatus

  • Eye protection
  • Spatula
  • Test tubes, x3
  • Dropping pipette
  • Measuring cylinder, 10 or 25 cm3
  • Glass stirring rod

Additional apparatus for the teacher demonstration

  • Boiling tube (a large test tube, 150 x 25 mm)
  • Spatula
  • Glass stirring rod
  • Thermometer (see note 9 below)

Chemicals

  • Anhydrous copper(II) sulfate (HARMFUL), about 1 g
  • Citric acid (IRRITANT), about 1 g
  • Sodium hydrogencarbonate, about 1 g
  • Copper(II) sulfate solution, 0.5M, 5 cm3
  • Zinc powder (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT), about 1 g

For the teacher demonstration

  • Ammonium nitrate crystals (OXIDISING), about 5 g

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout.
  • Anhydrous copper(II) sulfate (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC027c. An ‘old’ sample of anhydrous copper(II) sulfate may already have been partly hydrated on exposure to the air. The anhydrous salt can be regenerated by heating in a hot oven.
  • Citric acid, HOOCCH2C(OH)(COOH)CH2COOH(s), (IRRITANT) – see CLEAPSS Hazcard HC036C. 
  • Sodium hydrogencarbonate, NaHCO3(s) – see CLEAPSS Hazcard HC095A. 
  • Copper(II) sulfate solution, CuSO4(aq) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.
  • Zinc powder, Zn(s), (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC107. Under no circumstances must the zinc powder be allowed to come into contact with ammonium nitrate. The two solids should be kept far apart at all times.
  • Ammonium nitrate, NH4NO3(s) (OXIDISING) – see CLEAPSS Hazcard HC008. Under no circumstances must the zinc powder be allowed to come into contact with ammonium nitrate. The two solids should be kept far apart at all times. It is recommended that ammonium nitrate is used only by post-16 students, or by teachers as part of a demonstration. If students are to experience endothermic dissolving, they can use KCl.
  • Consider using a digital thermometer with a clear display for the demonstration.

Procedure

Carry out the following reactions. Find out whether the reaction:

  • Gives out energy (exothermic), or
  • Takes in energy (endothermic)

A test for water: the reaction of water and anhydrous copper(II) sulfate

  1. Put a spatula measure of white, anhydrous copper(II) sulfate powder into a test tube.
  2. Use a dropping pipette to add a few drops of water to the powder.
  3. Watch what happens and feel the bottom of the tube.

The sherbet mixture: the reaction of sodium hydrogencarbonate and citric acid

  1. In a dry test tube, mix one spatula measure of citric acid with one spatula measure of sodium hydrogencarbonate.
  2. Add about 2 cm3 water to the mixture.
  3. Watch what happens and feel the bottom of the tube.

A competition reaction: the reaction of copper(II) sulfate solution and zinc

  1. Put about 5 cm3 copper(II) sulfate solution in a test tube.
  2. Using a spatula, add a small measure of powdered zinc. Stir with a glass rod.
  3. Watch what happens and feel the bottom of the tube.

Teacher demonstration: dissolving ammonium nitrate

  1. Take about 10 cm3 of water in a test tube. Ask a student to note its temperature or display the temperature with a digital thermometer.
  2. Add a large spatula measure of ammonium nitrate.
  3. Stir and record the temperature after it has dissolved.

Teaching notes

In this activity students meet two exothermic reactions (1 and 2) and two endothermic reactions (2 and 4).

More able students should be encouraged to appreciate that although these experiments demonstrate gain or loss of energy to or from the surroundings, chemists are more interested in the loss or gain of energy by the chemicals themselves.

Thus, for an exothermic process, the surroundings gain energy whereas the chemicals lose an equivalent amount.

In endothermic reactions the surroundings lose energy, which is gained by the chemicals themselves.

Students can be asked to draw simple energy diagrams for each type of reaction. It may be necessary to provide an introduction to explain the conventions of energy-level diagrams.

Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology. 

The experiment is also part of the Royal Society of Chemistry’s Continuing Professional Development course: Chemistry for non-specialists. 

© Nuffield Foundation and the Royal Society of Chemistry