Microscale reactions of chlorine

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Lesson organisation

This is a class experiment in which students, using microscale apparatus, can generate chlorine gas safely themselves in the open laboratory and investigate how it reacts with water and halide ions in solution.

This activity should take about 20 minutes.

Apparatus and chemicals

Goggles

Each pair of students requires:

Student sheet with instructions and diagram of apparatus. Download it here (263 KB).
Clear plastic sheet – e.g. OHP film
Plastic petri dish + lid (9 cm)
Plastic teat pipette (see note 1)
Scissors
Spatula (optional)

Access to solutions contained in plastic pipettes:

Sodium chlorate(I) solution (‘Bleach’), 1.5 mol dm^-3 (10–14% w/v chlorine) (Corrosive) (see note 2)
Dilute hydrochloric acid, 1 mol dm^-3 (Low hazard at this concentration)
Sodium hydroxide solution, 1 mol dm^-3 (Corrosive)
Potassium (or sodium) chloride solution, 0.2 mol dm^-3(Low hazard)
Potassium (or sodium) bromide solution, 0.2 mol dm^-3 (Low hazard)
Potassium (or sodium) iodide solution, 0.2 mol dm^-3 (Low hazard)

Blue Litmus or Universal Indicator paper, about 1 cm

Optional (see Teaching notes):
Silver nitrate solution, 0.1 mol dm-3 (Low hazard but can stain fingers and clothing at this concentration), a few drops

Spatula tip of:
Zinc oxide powder (Dangerous for the environment)
Zinc sulfide powder (Low hazard)

Technical notes

Chlorine (Toxic, Dangerous for the environment) Refer to CLEAPSS Hazcard 22A
Sodium(I) chlorate solution (Corrosive) Refer to CLEAPSS Hazcard 89 and Recipe card 62
Hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47A and Recipe card 31
Sodium hydroxide solution (Corrosive) refer to CLEAPSS Hazcard 91 and Recipe card 65
Potassium chloride solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 51
Sodium chloride solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 63
Potassium (or sodium) bromide solution (Low hazard) Refer to CLEAPSS Hazcard 47B
Potassium iodide solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 55
Sodium iodide solution (Low hazard) Refer to CLEAPSS Hazcard 47B
Silver nitrate solution (Low hazard but can stain fingers and clothing at concentration used) Refer to CLEAPSS Hazcard 87 and Recipe card 58
Zinc oxide (Dangerous for the environment) Refer to CLEAPSS Hazcard 108B

1 The ‘reaction vessel’ for the microscale apparatus is the hemispherical dome cut from the top of a plastic teat pipette. Students may cut this themselves with suitable scissors, or it can be provided.

2 Commercial chlorine-based bleach solutions can be used instead of sodium chlorate(I) solution supplied by laboratory suppliers but they may not be sufficiently concentrated to generate enough chlorine. They are often less than 5% even when fresh. Some commercial bleaches now also contain detergents, which foam when chlorine is generated. They should not be used. Household ‘bleaches’ based on peroxide are becoming more widely available and do not contain chlorine, therefore they should not be used.

3 Chlorine gas is produced in small quantities in this experiment, however, care should be taken if large numbers of students are carrying out this experiment simultaneously. The lab should be well ventilated and students with asthma should be warned not to inhale the gas. Chlorine should be generated for no longer than is necessary to observe the results.

Procedure

HEALTH & SAFETY: Wear eye protection

a Cover the worksheet containing the diagram of the microscale setup with the plastic sheet.

b Place the petri dish directly over the circle on the worksheet.

c Use the microscale reaction vessel provided or make one by cutting the hemispherical top off the teat part of a plastic pipette. Place this in the centre of the petri dish, as shown in the diagram.

d At the corners of the triangle on the worksheet place two to three drops of the test solutions indicated on the diagram. Moisten the small piece of indicator paper and place it in the space between any two of the test solutions, along the side of the triangle.

e Place two drops of bleach solution in the reaction vessel and add three drops of dilute hydrochloric acid. Quickly place the lid on the petri dish to prevent any chlorine escaping.

f Record your observations over the next 10 mins. The greenish-yellow colour of chlorine gas may be visible in the petri dish, especially if viewed from the side. The indicator paper turns red and then becomes bleached. The sodium chloride solution is unaffected. The potassium bromide solution gradually turns pale yellow due to the formation of bromine. The potassium iodide solution turns yellow-brown due to the liberation of iodine by the chlorine.

g When the reactions have finished, add three drops of sodium hydroxide solution to the reaction vessel to stop the generation of chlorine and replace the lid.

Teaching notes

The effect of chlorine gas on the moist indicator paper shows that it dissolves to some extent in water and reacts to produce an acidic and strongly bleaching solution. The reaction is the reverse of the reaction used to generate the gas from bleach. Acidifying bleach solution produces chloric(I) acid, HOCl, which decomposes to produce chlorine:

HOCl(aq) + HCl(aq) → Cl₂(g) + H₂O(l)

When chlorine dissolves in water, it reacts to form the strong acid, HCl, and the weak but strongly oxidising acid, HOCl, which is responsible for the bleaching properties.

The displacement reactions involving chlorine and the solutions containing halide show that chlorine displaces bromine and iodine from solution:

Cl₂(g) + 2KX(aq) → 2KCl(aq) + X₂(aq), where X = Br or I.

or Cl₂(g) + 2X-(aq) → 2Cl^-(aq) + X₂(aq)

This establishes the trend in reactivity of the halogens down Group 7, which could be extended by investigating the reaction of bromine water with halide ions in solution. The reactivity is related to the oxidising power of the halogens, which decreases down the group.

The tendency of halogen atoms to act as oxidising agents by accepting an electron can be related to their atomic radius. The smaller the halogen atom, the stronger the attraction of the nucleus on the electrons in the outer shell. Thus fluorine attracts an extra electron to complete its outer shell, most strongly, and is therefore the most powerful oxidising agent in the Group.

This microscale apparatus can be used in a similar way to investigate other chlorine reactions. The solutions at the three corners of the triangle can be replaced by silver nitrate solution, solid zinc sulfide and zinc oxide respectively. The silver nitrate solution becomes cloudy as the reaction of chlorine with water produces chloride ions in solution (see above), which then forms a silver chloride precipitate.

The zinc sulfide takes on a yellowish tinge due to the formation of elemental sulfur:

Cl₂(g) + ZnS(s) → ZnCl₂(s) + S(s)

Chlorine is reacting as an oxidising agent again. The zinc oxide shows no change although some oxygen gas is probably produced in a similar reaction.

Health and Safety checked, February 2008

Web links

www.chemsoc.org/pdf/LearnNet/rsc/Microscale_select.pdf

http://mattson.creighton.edu/Download_Folder/QuimicaArticleMGC-Oct2005.pdf

http://mattson.creighton.edu/Cl2/index.html

(Websites accessed August 2007)