Exothermic or endothermic?

Read our standard health & safety guidance

Lesson organisation

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.

Apparatus and chemicals

Eye protection

Each group of students will need:

Polystyrene cup (expanded polystyrene)
Beaker (250 cm³) in which to stand the polystyrene cup for support (see note 1)
Thermometer (–10°C to 110°C)
Measuring cylinder (10 cm³), 2
Spatula
Absorbent paper

Access to the following solutions:
(all at approx 0.4 mol dm^–3 concentration); (see note 2)

Copper(II) sulfate (Low hazard)
Hydrochloric acid (Low hazard)
Sodium hydrogencarbonate (Low hazard)
Sodium hydroxide (Irritant)
Sulfuric acid (Low hazard)

Access to the following solids (see note 3):

Magnesium ribbon (Highly flammable), cut into 3 cm lengths.
Magnesium powder (Highly flammable).
Citric acid (Irritant).

Technical notes

Copper(II) sulfate (Low hazard) Refer to CLEAPSS Hazcard 27C and CLEAPSS Recipe Card 19
Hydrochloric acid (Low hazard) Refer to CLEAPSS Hazcard 47A and CLEAPSS Recipe Card 31
Sodium hydrogencarbonate (Low hazard) Refer to CLEAPSS Hazcard 95C and CLEAPSS Recipe Card 64
Sodium hydroxide (Irritant) Refer to CLEAPSS Hazcard 91 and CLEAPSS Recipe Card 65
Sulfuric acid (Low hazard) Refer to CLEAPSS Hazcard 98A and CLEAPSS Recipe Card 69
Magnesium ribbon (Highly flammable) Refer to CLEAPSS Hazcard 59A
Magnesium powder (Highly flammable) Refer to CLEAPSS Hazcard 59A
Citric acid (Irritant) Refer to CLEAPSS Hazcard 36C

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.

2 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.

3 Small amounts of the solids can be provided in plastic weighing boats or similar. The teacher may prefer to keep the magnesium ribbon and powder under their immediate control and to dispense on an individual basis.

Procedure

SAFETY: Wear eye protection throughout.

Reaction of sodium hydroxide solution and dilute hydrochloric acid

a Stand the polystyrene cup in the beaker.

b Use the measuring cylinder to measure out 10 cm³ of sodium hydroxide solution and pour it into the polystyrene cup.

c Measure the initial temperature of the sodium hydroxide solution and record it in a suitable table.

d 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.

e Work out the temperature change and decide if the reaction is exothermic or endothermic.

f 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

a Repeat steps a – c of the previous experiment, using sodium hydrogencarbonate solution in place of sodium hydroxide solution.

b Add 4 small (not heaped) spatula measures of citric acid. Stir with the thermometer and record the maximum or minimum temperature reached.

c Work out the temperature change and decide if the reaction is exothermic or endothermic.

d 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

a Repeat steps a – c of the first experiment, using copper(II) sulfate solution in place of sodium hydroxide solution.

b Add 1 small (not heaped) spatula measure of magnesium powder. Stir with the thermometer and record the maximum or minimum temperature reached.

c Work out the temperature change and decide if the reaction is exothermic or endothermic.

d Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of sulfuric acid and magnesium ribbon

a Repeat steps a – c of the first experiment, using sulfuric acid in place of sodium hydroxide solution.

b Add one 3 cm piece of magnesium ribbon. Stir with the thermometer and record the maximum or minimum temperature reached.

c Work out the temperature change and decide if the reaction is exothermic or endothermic.

d 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)

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.

Sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide

NaHCO₃(aq) + H⁺(aq) → Na⁺(aq) + H₂O(l) + CO₂(g)

Health & Safety checked, June 2007