Some reactions give out heat and others take in heat. In exothermic reactions the temperature goes up, in endothermic reactions the temperature goes down. In this experiment, various reactions are examined. Temperatures are measured to decide whether a particular reaction is exothermic or endothermic

This is a useful class practical to introduce energy changes in chemical reactions. The students measure the temperature changes in four reactions, and classify 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.

There are five solutions and three solids involved. Careful consideration will need to be given as to the most appropriate way to dispense these 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.

Equipment

Apparatus

  • Eye protection
  • Polystyrene cup (expanded polystyrene)
  • Beaker (250 cm3) in which to stand the polystyrene cup for support (note 1)
  • Thermometer (–10°C to 110°C)
  • Measuring cylinder (10 cm3) x2
  • Spatula
  • 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 HC47a 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 a–c 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 a–c 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 a–c 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) + H2O(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)
    H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g)
  • Sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide (Neutralisation)
    NaHCO3(aq) + H+(aq) → Na+(aq) + H2O(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.