TeacherExpt:Generating, collecting, and testing gases

From Learn Chemistry Wiki

Gases give rise to particular hazards so great care must be taken when preparing, collecting or testing. How the gas is to be used will differ from experiment to experiment – it is essential to read carefully the specific instructions given or referred to in the experiment details. This is especially important if the gas needs to be dried. Gases can be collected by upward or downward delivery or over water. Refer to specific information on each gas.

Gas preparation (general)

The diagram below shows a typical set of apparatus which can be used to prepare a range of gases.

Gas collection methods

Gas Preparation (specific gases)

Wear appropriate eye protection. The amounts given below are sufficient to generate 1 litre (1 dm³) of each of the named gases:

Carbon dioxide

42 cm³ of 2 mol dm^–3 hydrochloric acid (Irritant) is slowly added to an excess of marble chips. Collect gas by downward delivery or over water (slightly soluble).

Refer to CLEAPSS Recipe Card 26 and CLEAPSS Hazcards 20 and 47A.

Hydrogen

28 cm³ of 3 mol dm^–3 hydrochloric acid (Corrosive) is slowly added to excess zinc granules and 1 g of hydrated copper sulfate (Harmful). Collect gas by upward delivery or over water. Refer to CLEAPSS Recipe Card 26 and CLEAPSS Hazcards 47A and 48.

Hydrogen gas is extremely flammable – ensure there are no naked flames.

Oxygen

50 cm³ of 20 vol hydrogen peroxide (Irritant) is slowly added to manganese(IV) oxide powder (Harmful). Collect gas over water.

Refer to CLEAPSS Recipe Card 27 and CLEAPSS Hazcards 50, 60 and 69.

Oxygen is an oxidising agent.

Chlorine

Work in a fume cupboard (hood). 14 cm³ of concentrated hydrochloric acid (Corrosive) is added to at least 3 g of potassium manganate(VII) (Oxidising, Harmful, Danger to the environment).

Double-check that the acid is hydrochloric and NOT sulfuric. Refer to CLEAPSS Recipe card 26 and CLEAPSS Hazcards 22A, 47A and 81.

Alternatively 5 mol dm^–3 hydrochloric acid (Irritant) is added to 30 cm3³ of recently purchased (10–14% available chlorine) sodium chlorate(I) solution (Corrosive) with plenty of stirring. School samples often react too slowly because old sodium chlorate(I) is used. This will have less than the required 10% available chlorine (as it applies to both methods). Refer to CLEAPSS Recipe Card 26 and CLEAPSS Hazcards 22A, 47A and 89.

Collect gas by downward delivery. Chlorine is classified as Toxic, Irritant and a Danger to the environment.

See also

• Decomposition of hydrogen peroxide (PDF), part of RSC's "Classical Chemical Demonstrations".

Health and safety checked, November 2007