The photochemical reactions of chlorine with hydrogen and methane

Read our standard health & safety guidance

Lesson organisation.

HEALTH & SAFETY NOTE: Because of safety considerations, this can only be a demonstration
experiment. The hydrogen-chlorine reaction has been associated with a number of past accidents, so a great deal of thought and care has been put into the Technical Notes and Procedure described. This demonstration should NOT be attempted by inexperienced teachers or non-chemists. A thorough double-check with CLEAPSS is advisable before undertaking it in any case.

The preparation of the gas mixtures requires careful manipulation and is likely to be time consuming. If any of the gases have to be generated chemically, this could be a considerable additional distraction from the purpose of the demonstration. The demonstration ought, if possible, to start from the point where the gases are already available as ready-to-use samples. This preparation should preferably be done within an hour before the demonstration is to be performed, to ensure the chlorine-containing mixtures are still in good condition in the corked bottles.

Carrying out the reaction itself should take only 5 minutes for each of the two reactions.

Apparatus and Chemicals

The teacher will need:

Eye protection
Ear protection (ear plugs)
Safety screens

Access to a fume cupboard
Access to laboratory Natural Gas (methane) supply, with length of flexible delivery tubing

Poly(ethene) (polythene) bottle (60 cm³), 2 - 6 (see note 1)
Corks (not rubber bungs), to fit bottles
Black insulating tape
Plastic water trough (e.g. 2 litre sandwich box or similar)
Measuring cylinder (100 cm³)
Stand and clamp
Slide or data projector with a bulb of rating 300 W or greater (an electronic photographic flash gun is a possible alternative)

Chlorine generator (Chlorine: Toxic, Dangerous for the environment) (see note 2):
Length of delivery tubing, to fit generator
Hydrogen gas (Extremely flammable), cylinder fitted with valve, gas regulator and flexible delivery tube
Ammonia solution, concentrated (‘880’) (Corrosive, Dangerous for the environment), about 50 cm³
Blue litmus paper

Technical Notes

Ammonia solution, concentrated (‘880’) (Corrosive) Refer to CLEAPSS Hazcard 6
Hydrogen gas (Extremely flammable) Refer to CLEAPSS Hazcard 48
Chlorine gas (Toxic, Dangerous for the environment) Refer to CLEAPSS Hazcard 22A and CLEAPSS Laboratory Handbook 13.2.2

1 Prepare the plastic bottles as follows:

• Determine the volume of one of the plastic bottles by filling it with water and pouring the water into a measuring cylinder. Pour half this volume of water back into the bottle and mark the level with a permanent marker pen to give the half-way mark of the bottle.
• Repeat this for each of the remaining plastic bottles – a total of six bottles will provide a reserve in case repeat demonstrations are required.
• Wrap the first three bottles with black insulating tape leaving a ‘window’ about 2 cm x 1 cm centred on the half-way mark. Leave a further three bottles unwrapped for the chlorine-methane reaction.
• Fill each bottle full of water until ready for filling with hydrogen or methane.

2 Generation of chlorine:

Details of two possible methods of generating the chlorine gas required are given in Standard Techniques: Generating, collecting and testing gases. The apparatus should be modified by replacing the thistle funnel with a tap funnel (or separating funnel), as shown below.

The quantities of reagents given there should be doubled to provide enough chlorine to flush the apparatus free of air and for several attempts at the photochemical reaction.

The use of chlorine cylinders is NOT recommended. The teacher and technician will need to decide on an appropriate method for their circumstances. The chlorine generator must be securely clamped and only used in a fume cupboard.

Procedure

Before the demonstration:

HEALTH & SAFETY: The reaction between hydrogen and chlorine can be very violent, and in the wrong circumstances, dangerous. Teachers should NOT vary the procedure significantly from that given without a complete re-assessment of the risks involved. In no circumstances should a glass reaction vessel be used (eg. a gas syringe), nor should significantly larger volumes of gas be used. In the demonstration itself, the polythene bottles should NOT be corked too firmly, to avoid a risk of the bottle bursting instead of the cork being expelled.

a Prepare the hydrogen cylinder and have ready a trough of water and a length of delivery tubing. Purge the delivery tube with hydrogen so that it contains no air, leaving the end under water. Fill the three tape-covered bottles with hydrogen each to the half-way mark on the bottle, and cork securely. If the bottles are left inverted, this will reduce possible diffusion through the cork.

b If the second demonstration, the chlorine-methane reaction, is to be included, a convenient laboratory gas (Natural Gas only) tap should be fitted with a length of flexible delivery tubing, and the three bottles without tapes filled to the mark in similar fashion.

c Arrange the projector and the clamp stand so that the lens of the projector is about 5 cm from where the bottle will be clamped and the beam of the projector points directly at the window on the bottle. Place a safety screen between the bottle and the audience.

d The demonstration room should have subdued lighting, with no sunlight falling on the area where the demonstration will be done and no fluorescent tube lights turned on. The projector should also be left turned off at the wall socket.

e About 15 minutes before the demonstration, prepare and collect the chlorine in the bottles containing hydrogen or methane in a fume cupboard as follows:

• Add the first reagent (solid potassium manganate(VII) or sodium chlorate(I) solution) to the flask, insert the bung carrying the tap funnel, and connect the delivery tube. Place the end of tube in the trough of water.
• Place the hydrochloric acid in the funnel, ensuring the tap is closed at this point. With the fume cupboard extraction turned on, add 5 -10 cm³ of the hydrochloric acid to the flask, allowing the chlorine produced to displace the air in the flask.
• Add more hydrochloric acid as required to produce a steady stream of bubbles in the trough. If there is any chance of bright sunlight impinging on the taped bottles during or after filling, their windows can be covered temporarily with a strip of black tape to eliminate any possibility of the reaction being initiated unexpectedly.
• Uncork one of the polythene bottles under water and hold it over the end of the delivery tube. Fill with chlorine until the remaining water in the bottles has been displaced, then cork securely, and dry the outside.
• Store the filled bottles in the dark in the fume cupboard behind a safety screen until required. Make sure that the cork is pointed away from anyone while the bottle is being transferred.

The demonstration:

HEALTH & SAFETY: Wear goggles and ear protection. Place a safety screen between the apparatus and the students.

f Clamp one taped bottle so that the cork points vertically and its ‘window’ is facing the projector and about 5 cm from it (see diagram below). Ensure that the mouth of the bottle is not pointing at any light fittings. Put on ear protection and insist that students put their fingers in their ears.

g Switch the projector on, if possible using the switch at the mains socket in order to be some way away from the explosion. Almost immediately there will be a loud bang and the cork will be fired out of the bottle with some force and hit the ceiling. Should the mixture fail to explode, switch off the projector, then remove bottle to the fume cupboard. Remove the cork and expel the gas mixture in the fume cupboard by inverting the bottle under water in a trough.

h If the reaction is successful, identify the product, hydrogen chloride, by putting moist blue litmus paper in the mouth of the bottle (turns red) and by blowing fumes from the ‘880’ ammonia solution bottle over the mouth of the bottle (white fumes of ammonium chloride are formed).

i After the demonstration get a member of the audience to place their hand in the projector beam to show that there is relatively little heat transmitted, showing that it must be light rather than heat that initiates the reaction.

j The reaction of chlorine with methane can be demonstrated in a similar way. The reaction starts less readily than with hydrogen and it is better to use a bottle with no tape. Otherwise the method is the same except that methane is used instead of hydrogen. A red flash will be seen in the bottle as the reaction takes place and the bottle becomes filled with a sooty deposit. The reaction is much less violent than that with hydrogen. NOTE: a flash gun will not reliably initiate this reaction.

Teaching notes

For intermediate level students, the reaction can be used as a spectacular demonstration of an exothermic reaction, and discussed in terms of the bond breaking and making represented in the overall equation. For post-16 students, this will lead to a discussion of chain reaction mechanisms, and so to the demonstration of the methane-chlorine reaction and its mechanism.

The light initiates a rapid photochemical reaction between hydrogen or methane and chlorine. The mixture explodes with a loud bang, firing the cork into the air. Both reactions are radical chain reactions in which the initiation step is the absorption of a photon of visible light by a chlorine molecule. This causes the Cl-Cl bond to break, forming chlorine atoms, which are free radicals as they have an unpaired electron each:

The Cl-Cl bond energy is 243 kJ mol^–1. This energy corresponds to a wavelength of about 500 nm, in the blue-green region of the visible spectrum, so Cl-Cl bonds may be broken by radiation of wavelengths shorter than this. Advanced students could be asked to calculate the minimum frequency of radiation needed to initiate the reaction, using the bond energy.

In the chlorine-hydrogen reaction, this is followed by chain propagation steps:

Cl•(g) + H₂(g) → HCl(g) + H•(g)
H•(g) + Cl₂(g) → HCl(g) + Cl•(g)

Chain termination takes place via a variety of reactions, such as:

2Cl•(g) → Cl₂(g)
H•(g) + Cl•(g) → HCl(g)
2H•(g) → H₂(g)

which take place on the wall of the vessel to carry away excess energy.

Oxygen molecules can act as inhibitors via reactions such as:

H•(g) + O₂(g) → HO₂•(g)

So the bottle must be carefully filled and corked to prevent oxygen from entering.

Since, in the overall reaction, H₂(g) + Cl₂(g) → 2HCl(g), there is no increase in the number of moles of gas, the explosion must be due to the heat given out (93 kJ per mole of HCl), causing the gas mixture to expand rapidly.

The methane-chlorine reaction starts less readily, and proceeds less rapidly than the hydrogen-chlorine reaction. The first step will be the same – the production of two chlorine free radicals. The first propagation step is likely to be:

Cl•(g) + CH₄(g) → CH₃•(g) + HCl(g)

The next likely step will then be:

CH₃•(g) + Cl₂(g) → CH₃Cl(g) + Cl•(g)

Finally several chain termination steps are possible:

Cl•(g) + Cl•(g) → Cl₂(g)
CH₃•(g) + CH₃•(g) → CH₃CH₃
Cl•(g) + CH₃•(g) → CH₃Cl(g)

Health & Safety checked, August 2008

Web Links

A sequence of still photographs of the explosion in a glass test tube can be seen at: http://www.chem.leeds.ac.uk/delights/texts/expt_28.html

For a simple approach to explaining both these photochemical chain reactions, go to: http://www.avogadro.co.uk/light/fission/bondfission.htm

Another good approach to explaining the methane-chlorine reaction to post-16 students can be found at:
http://www.chemguide.co.uk/mechanisms/freerad/ch4andcl2.html

A microscale version of the experiment, using polythene gas syringes, can be found at: http://mattson.creighton.edu/Cl2/

… and another one at:
www.ase.org.uk/htm/members_area/groups/atse/cbc_05/preparing_gases.pdf

(Websites accessed July 2008)