Endothermic and Exothermic
reactions
1.
Endothermic and Exothermic reaction;
Sodium thiosulfate
Sodium thiosulfate can be used to demonstrate the absorption and release of heat in endothermic and
exothermic reactions.
Instructions
 Place approximately 10 cm depth of sodium thiosulfate crustals into a clean test tube. Do not
add any water.
 Place the test tube into a beaker of very hot water until the crystals absorb sufficient heat (endothermic process) to dissolve in their own water of crystallisation.
 Place a small wad of cotton wool in the neck of the test tube and then transfer the test tube to a
beaker of cold water until the liquid is at room temperature. If the liquid solidifies, repeat the
heating and cooling.
 Remove the wad of cotton wool and place a small crystal of sodium thiosulfate into the liquid
and observe the rapid crystallisation of the liquid and the release of heat (exothermic process).
2. Freezing




mixture Barium hydroxide and
ammonium thiocyanate (Teacher demonstration)
Place a few drops of water onto a small square of plywood and then place a small beaker on top
of the water drops.
Place a generous desert spoon of ammonium thiocyanate crystals into the beaker and then add a
similar quantity of barium hydroxide.
Stir the solid mixture and observe that it becomes very slushy and eventually a solution.
The temperature of the solution drops below 0oC and the students should be able to observe the
formation of ice on the side of the beaker and the drops of water between the beaker and the
plywood square will freeze, gluing the two together, allowing you to lift the beaker to
demonstrate what has occurred.
3.
Freezing mixture Citric acid and sodium
bicarbonate (Student practical)
A freezing mixture similar to the one demonstrated above can be prepared using chemicals readily
available at the super market.
 Place 50 mLs of water into a 250 mL beaker and add approximately 19.2 g (0.1 mol) citric
acid. There is a small but detectable decrease in temperature.
 Slowly add with stirring, 25.2 g (0.3 mol) of baking soda.
 There is considerable effervescence and the temperature will eventually drop to below 0oC.
4. Exothermic process Aluminium foil (Student
practical)
Mercury(II) chloride forms an amalgam with the freshly exposed surface. The exposed aluminium
reacts with oxygen from the air to form aluminium oxide.
4Al(s) + 3O2(g) → 2Al2O3(s)





Have students place a 10 cm x 10 cm square of aluminium foil on the palm of their hand.
Using a pair of tongs, moisten a cotton wool ball with mercuric chloride solution.
Carefully rub a small area at the centre of the aluminium foil with the moistened cotton wool.
After a few moments, a very exothermic reaction occurs between the exposed aluminium and
oxygen from the air.
Have the students fold the aluminium foil into a small parcel so that they do not touch the
mercuric chloride solution.
5.
Exothermic process Copper sulphate
(Student practical)
As well as demonstrating an exo-thermic process, this practical also demonstrates one of the
factors (the formation of a complex ion) contributing to the colour of transition metal compounds




Place 1 cm depth of hydrated copper sulphate crystals (CuSO4.5H2O) into a dry test tube.
Cautiously heat over a small blue flame until white anhydrous copper sulphate is formed.
Allow to cool to room temperature.
Add drops of water and monitor the temperature of the test tube as the copper sulphate is
rehydrated.
6.
Exothermic process Hydrogen balloons
(Teacher demonstration)
This procedure has been demonstrated in the Year 9 – 11 practical work shop.
The aluminium oxide layer is removed by the NaOH
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
Once the oxide has gone, effervescence starts due to:
2Al(s) + 2NaOH(aq) + 6H2O(l) → 2NaAl(OH)4(aq) + 3H2(g)