Energy in or out: classifying reactions

Class 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 and is classified as being exothermic or endothermic.

They are shown a teacher demonstration which illustrates an endothermic dissolving process.  A similar experiment, involving a set of reactions carried out in a insulated cup and using a thermometer to measure temperaturee changes, is described elsewhere in Practical Chemistry: link to to 'Exothermic or Endothermic'.
 

Lesson organisation

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 5 minutes.

Health & Safety and Technial notes

Read our standard health & safety guidance

Wear eye protection. 

Anhydrous copper(II) sulfate (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. 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. 

Sodium hydrogencarbonate, NaHCO₃(s) - see CLEAPSS Hazcard. 

Copper(II) sulfate solution, CuSO₄(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Zinc powder, Zn(s), (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. 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. 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.

1 A large (150 x 25 mm) test-tube 

2 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).

1 A test for water
a Put a spatula measure of white, anhydrous copper(II) sulfate powder into a test-tube.

b Use a dropping pipette to add a few drops of water to the powder.

c Watch what happens and feel the bottom of the tube.

2 The sherbet mixture
a In a dry test tube, mix one spatula measure of citric acid with one spatula measure of sodium hydrogencarbonate.

b Add about 2 cm³ water to the mixture.

c Watch what happens and feel the bottom of the tube.

3 Competition
a Put about 5 cm³ copper(II) sulfate solution in a test-tube.

b Using a spatula, add a small measure of powdered zinc. Stir with a glass rod.

c Watch what happens and feel the bottom of the tube.

4 Teacher demonstration: dissolving ammonium nitrate
a 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.

b Add a large spatula measure of ammonium nitrate.

c Stir and record the temperature after it has dissolved.

Teaching notes

In this activity students meet two exothermic reactions (1 and 3) 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.

Health & Safety checked August 2008