Gas Preparation and Collection Methods at Doc Brown's Chemistry Clinic for IGCSE-GCSE-GCE-AS-A2-BTEC revising teaching learning on-line self-assessment test revision docbgaspreparation

	Ex 1. Method for preparing and collecting a gas less dense (lighter) than air by heating solid reactants. The less dense gas rises into, and displaces, the more dense air. This is called upward delivery. e.g. Heating a mixture of ammonium chloride and calcium hydroxide solids gives ammonia which has a very pungent odour! and turns red litmus blue. See also Ex 7. method. 2NH₄Cl_(s) + Ca(OH)_2(s) ==> CaCl_2(s) + 2H₂O_(l) + 2NH_3(g) To make dry ammonia you need a U tube packed with granules of calcium oxide between the horizontal pyrex tube and the vertical inverted collection test tube.
OR	Ex 2. Method for preparing and collecting a gas less dense (lighter) than air by reacting a liquid and a solid. The less dense gas rises into, and displaces, the more dense air. This is called upward delivery. e.g. A mixture of zinc and hydrochloric acid makes hydrogen. Hydrogen gives a squeaky pop! with a lit splint. See also methods Ex 5., Ex 6. and Ex 7. Zn_(s) + 2HCl_(aq) ==> ZnCl_2(aq) + H_2(g)
	Ex 3. Method for preparing and collecting a gas more dense (heavier) than air by heating the reactants. The more dense gas sinks down into, and displaces, the less dense air. This is called downward delivery. eg Making nasty brown nitrogen dioxide by heating lead(II) nitrate crystals (thermal decomposition). The solid 'deflacrates', it crackles as the gas formed splits the crystals apart. See also method Ex 7. 2Pb(NO₃)_2(s) ==> 2PbO_(s) + 4NO_2(g) + O_2(g)
Ex 4. Method for preparing and collecting a gas more dense (heavier) than air by reacting a solid and a liquid. The more dense gas sinks down into, and displaces, the less dense air. This is called downward delivery. Examples: OR collected in boiling tube	(i) Calcium carbonate (limestone/marble chips) with hydrochloric acid makes carbon dioxide. Can also be done via Ex 6. but carbon dioxide is moderately soluble and does make 'carbonated water. See also Ex 8. for carbonate test. CaCO_3(s) + 2HCl_(aq) ==> CaCl_2(aq) + H₂O_(l) + CO_2(g) (ii) Sulphur dioxide from solid sodium metabisulphite or sodium sulphite and excess dilute hydrochloric acid. The nasty choking gas turns potassium dichromate(VI) paper from orange to green. Should be done in fume cupboard. Na₂S₂O_5(s) + 2HCl_(aq) ==> 2NaCl_(aq) + H₂O_(l) + 2SO_2(g) or Na₂SO_3(s) + 2HCl_(aq) ==> 2NaCl_(aq) + H₂O_(l) + SO_2(g) (iii) Nasty acrid Hydrogen chloride is formed when conc. sulphuric acid is mixed with solid sodium chloride. Should be done in fume cupboard. NaCl_(s) + H₂SO_4(l) ==> NaHSO_4(s) + HCl_(g) (iv) Chlorine from conc. sodium chlorate(I) and conc. hydrochloric acid. Very dangerous and should be done in a fume cupboard. NaClO_(aq) + 2HCl_(aq) ==> NaCl_(aq) + H₂O_(l) + Cl_2(g) All of these can be done via Ex. 5 or Ex 7. below.
	Ex 5. Method for preparing and collecting a gas of any density by reacting a solid and a liquid at room temperature. e.g. making carbon dioxide Ex 4., chlorine Ex 4., hydrogen Ex 2., hydrogen chloride Ex 4., oxygen Ex 6., sulfur dioxide Ex 4.
This is called collecting over water, or displacement of water or pneumatic trough collection.	Ex 6. Method for preparing and collecting a gas of any density by reacting a solid-liquid, as long as the gas is not too soluble in water! (dissolving or reacting). All gases are less dense than air and will displace the water downwards. No good for soluble gases like ammonia, hydrogen chloride, nitrogen dioxide or sulphur dioxide. You can collect in inverted gas jar if bigger sample required. You have to watch for 'sucking back' effects. Examples: (i) Making oxygen from hydrogen peroxide solution using manganese dioxide catalyst. Oxygen has similar density to air so must be collected by methods Ex 5., Ex 6. or Ex 7. 2H₂O_2(aq) ==> 2H₂O_(l) + O_2(g) (ii) Hydrogen, (iii) carbon dioxide and (iv) chlorine (moderately soluble, makes 'chlorine water')
Ex 7. Method for preparing and collecting a gas of any density by reacting a solid-liquid or heating solids. The angled boiling tube minimises the risk of contaminating the gas syringe with solids or liquids eg making ammonia or nitrogen dioxide. Its a smaller scale alternative to Ex 5. and using a Pyrex tube suitable for small scale heated experiments.
	Ex 8. A simple way to test for a carbonate. Add acid to the suspected carbonate. Collect a sample of the gas in a teat pipette from just above the reaction mixture. Bubble the gas sample into calcium hydroxide solution (limewater) and a milky white confirms the gas is carbon dioxide formed from the original carbonate.
U tube dreschel bottle	Ex 9. Use of a U tube. This is useful if the dry gas is needed. It is inserted in the apparatus set-up between the reaction container and the gas collection system. It is packed with a solid water absorbing drying agent e.g. anhydrous calcium chloride (not for ammonia), calcium oxide (not acidic gases like sulphur dioxide, carbon dioxide and chlorine), anhydrous sodium sulphate. A dreschel bottle can also be used e.g. the gas is bubbled through concentrated sulphuric acid which will dry the gas. It cannot be used to dry alkaline gases like ammonia, with which it will react exothermically to form the solid salt ammonium sulphate.
Ex 10. A cracking experiment. This diagram outlines a way of demonstrating cracking larger hydrocarbons into smaller molecules. This is a thermal decomposition reaction catalysed by aluminium oxide (or broken porous pot). The gases produced can be tested with (i) a match! and (ii) bromine water, if it is decolorized from orange to colourless, then unsaturated alkenes were formed. This is another example of over water, or displacement of water collection. The dreschel bottle is to collect any sucked back water if the hot gasses cool and contract. Cold water on the hot Pyrex tube has very nasty effect! plus the risk of fire!


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The sites resources include revision notes, quizzes and worksheets which provide support for home study or tuition for homework and coursework help e.g. science investigations for any of the key stage courses indicated, but I do not supply lesson plans. Dr W P Brown I 29-11-2007 * KS3 SCIENCE * KS4 SCIENCE GCSE IGCSE GCE AS A2 IB CHEMISTRY Online free help resources for Key Stages 3 SATs (S.A.T.s), 4 & 5AQA, Edexcel, OCR, CIE GCSE IGCSE BTEC Science, GCE, AS, A2 Advanced subsidiary Chemistry A levels, IB Diploma and US K12 (K-12 grades) courses and examinations and revising for the various syllabuses and specifications. Exploring the site for lessons, plans, ideas for projects and coursework, professional development. Through hard work the site has been built up over the course of many years with no need of special pc software except FrontPage and Hot Potatoes (uvic) for quizzes and worksheets. 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