The part that water plays in acidity

Read our standard health & safety guidance

Lesson organisation

This lesson needs careful and thorough preparation, planning and organisation. However, the reward is a clear insight into the nature of acids and the role water plays.

Preparation of the solution of hydrogen chloride in methylbenzene must begin two days in advance of the experiment by the teacher or an experienced technician (see note 1). Once prepared it has a limited life, and needs to be kept very well-stoppered in a fume cupboard, from where small volumes can be dispensed to students in stoppered test-tubes. For this reason, the lesson must take place in a laboratory with an adequate fume cupboard – preferably two fume cupboards if available.

The solution in methylbenzene gives off choking corrosive and toxic fumes. For that reason the experiment is suitable as a class experiment only for reliable classes of students age 14 and over, in a well ventilated lab. For other classes the experiment can be done as a demonstration. It may help to organise the practical work on a class-cooperative basis, with different working groups being allocated different tests to perform. This limits exposure to the fumes.

If all groups do all tests, the experiment is likely to take about 40–50 mins. It can be done quicker on a class-cooperative basis.

Take care to collect the methylbenzene solvent residues in a suitable container in a fume cupboard for disposal. Draw the class’s attention to this procedure.

Apparatus and chemicals

Eye protection

Each working group requires:

Oven-dried beakers (100 cm³), 2
Teat pipettes (well dried), 2

Access to:
6 V DC power pack
Connecting wires
Switch
6 V lamp
Steel electrodes, 2 (four pairs should serve up to a total of 16 working groups)
Paper tissues/towels
Universal indicator paper, dried, kept in dessicator or distributed in dry stoppered sample tubes (see note 2)
Forceps

Marble chips (calcium carbonate) (Low hazard), 2
Magnesium ribbon (Low hazard), cut into 1 cm lengths, 2
Dilute hydrochloric acid, 1 mol dm^-3, (Low hazard at this concentration) in a bottle labelled ‘Solution of hydrogen chloride in water’, 100cm³
Purified water

Access to:
Hydrogen chloride (Corrosive, Toxic) solution in methylbenzene (Highly flammable, Harmful) (see notes 1 and 4)

Oven-dried test-tubes (100 x 16 mm), 9 and stoppers to fit, 3
Oven-dried test tubes (150 x 25 mm), 3 and stoppers to fit, 3
Conical flask (100 cm³) to collect solution and stopper to fit,

Technical notes

Calcium carbonate (Low hazard) refer to CLEAPSS Hazcard 19B
Magnesium ribbon (Low hazard) Refer to CLEAPSS Hazcard 59A
Hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47A and Recipe card 31
Hydrogen chloride (Corrosive, Toxic) Refer to CLEAPSS Hazcard 49 and Recipe card 27
Methylbenzene (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 46
Concentrated sulfuric acid (Corrosive) Refer to CLEAPSS Hazcard 98A
Anhydrous calcium chloride (Irritant) Refer to CLEAPSS Hazcard 19A
Sodium chloride (Low hazard) Refer to CLEAPSS Hazcard 47B

1 You need to start preparing the saturated solution of hydrogen chloride in methylbenzene two days before the class experiment. If the technician is not sufficiently experienced the preparation needs to be supervised. It must be done in an efficient fume cupboard.

For one class of 12–16 working groups, place 300 cm³ of methylbenzene in a corked
500 cm³ conical flask, add 10–20 g of good quality anhydrous calcium chloride and leave overnight. Next day, decant into a dry 500 cm³ beaker just before passing dried hydrogen chloride gas into the solvent. The hydrogen chloride generator consists of a large (1 dm³ Quickfit) filter flask fitted with a tap-funnel and side-arm leading to a wash-bottle containing concentrated sulfuric acid to dry the gas, and then to an inverted filter-funnel just dipping into the solvent. The filter flask contains about 50 g of sodium chloride, and concentrated sulfuric acid is added slowly dropwise from the tap-funnel to generate hydrogen chloride. A total addition of about 50 cm³ of concentrated sulfuric acid over about 1 hour should ensure that enough hydrogen chloride is passed to saturate the solvent. Cork the conical flask securely, and if possible store in a dessicator until the start of the lesson.

2 Place small strips of Universal indicator paper in a warm (about 60˚C) oven for a few hours. Transfer into a series of oven-dried screw-top sample tubes (enough for one tube between two working groups, each tube containing about 10 small strips) and store in a dessicator until the start of the lesson. Provide a pair of forceps per tube for picking out the strips.

3 Four conductivity test circuits should be set up around the laboratory. Each consists of a low-voltage DC power pack set at an appropriate voltage connected to a bulb of similar operating voltage. They should have a press switch and two steel electrodes clamped so that they can be easily lowered into a 100 cm³ beaker containing the test solution. Beside each test station, provide a supply of paper towels for cleaning the electrodes after each use.

4 Access to the solution of hydrogen chloride in methylbenzene can be managed
either by students coming to the fume cupboard and using a teat pipette to transfer small volumes to a dry test-tube or by a technician preparing for each group 3 stoppered test-tubes (100 x 16 mm) each containing 2 cm³ of the solution, two stoppered test-tubes (150 x 25 mm) each containing 10 cm³ of the solution, and one stoppered test-tube (150 x 25 mm) containing about 6 cm³ of the solution.

Procedure

HEALTH & SAFETY: Wear eye protection and avoid breathing fumes from the solution of hydrogen chloride in methylbenzene when the corks are removed from containers.

a Take one test-tube each of the solutions of hydrogen chloride in water and of hydrogen chloride in methylbenzene, each containing about 2 cm³. Using forceps (don’t touch the papers with your hands!), drop a small strip of dry Universal indicator paper into each test-tube, and compare the changes in colour of the indicator against the pH colour chart for that indicator.

b Take another test-tube each of the solutions of hydrogen chloride in water and of hydrogen chloride in methylbenzene, each containing about 2 cm³. Drop a small marble chip into each test-tube. Compare the effects of each solution on the marble chip.

c Take another test-tube each of the solutions of hydrogen chloride in water and of hydrogen chloride in methylbenzene, each containing about 2 cm³. Drop a small strip of magnesium ribbon into each test-tube. Compare the effects of each solution on the magnesium.

d In a fume cupboard, pour about 10 cm³ of each of the solutions into two separate beakers. Take the two beakers carefully to a testing station where a conductivity test circuit is provided. Make sure the two steel electrodes are completely dry using a clean paper towel. Test the methylbenzene solution first. Dip the two electrodes into the solution and press the switch. Does the lamp light? Dry the electrodes and repeat with the solution in water. Dry the electrodes again before removing your beakers to the fume cupboard.

e Take a larger test-tube containing about 6 cm³ of the solution in methylbenzene, remove the cork and add about the same volume of purified water. Replace the cork firmly and shake vigorously for about 30 seconds. Pour off the upper layer of methylbenzene.

Repeat tests a-d on the remaining solution. Begin by repeating test d, then use samples of the solution in smaller test-tubes to repeat a-c.

Teaching notes

Be aware of the safety issues associated with students inhaling fumes from the solution of hydrogen chloride in methylbenzene, and of the precautions needed, especially for those with asthma and similar problems. Ensure good ventilation of the lab.

This is an important experiment to develop an understanding of the nature of acid behaviour, and well worth the extra effort involved in planning, preparing and delivering this complex practical session.

In a the indicator paper should show the standard colour change for pH = 1 in aqueous solution. In methylbenzene solution there may well be a colour change (especially if the indicator paper has not been dried thoroughly) but the comparison with the aqueous solution should be marked.

In b there should be the usual fizzing in aqueous acid as carbon dioxide is given off from the calcium carbonate, but no sign of reaction with the methylbenzene solution.

In c there should be the usual evolution of hydrogen (this can be ‘pop’ tested) from aqueous acid, but no reaction with the methylbenzene solution.

In d the aqueous acid should conduct sufficiently for the lamp to light, and gas should b be evolved at each electrode; the smell of chlorine may be detected. In contrast the methylbenzene solution does not conduct or show signs of electrolysis.

In e the water layer suddenly acquires all the properties of the aqueous solution, and observant students may note that the pungent fumes of hydrogen chloride have disappeared from the methylbenzene layer.

The lack of standard acid behaviour of the solution of hydrogen chloride in methylbenzene is in stark contrast to the familiar properties of hydrochloric acid. This is emphasised when on shaking the former with water, the hydrogen chloride clearly transfers to the water and the standard acid properties now appear in the aqueous layer. If students have already been taught ionic theory, the conductivity experiments reveal the presence of ions in the aqueous solution but not in the methylbenzene solution. If the cathode product on electrolysis is identified as hydrogen, then the presence of hydrogen cations can be related to standard acid behaviour.

In the follow-up to the practical work, these conclusions can be drawn and lead to a deeper theory of acid behaviour than that so far taught to students. The latter is the descriptive view of acids being substances that have certain characteristic reactions. Now we have a theory of acid behaviour: it is the presence of hydrogen cations that cause the familiar properties.

Students could be asked to write symbol equations for the reactions with the aqueous solution:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

and

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

If electrode reactions have been studied, these can also be added:

Cathode: 2H⁺(aq) + 2e^- → H₂(g)

and

Anode: 2Cl^-(aq) → Cl₂(g) + 2e^-

Health and Safety checked, February 2008

Web links

Two sites for teachers and interested students with background on the theories of acid-base behaviour

www.visionlearning.com/library/module_viewer.php?mid=58

www.bbc.co.uk/dna/h2g2/alabaster/A708257

(Websites accessed July 2007)