methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc b

Doc Brown's Chemistry Revision Notes

Gas preparation and collection methods revision notes

 

This page describes a variety of methods and techniques for preparing and collecting gases - summarised details including diagrams of the apparatus, the chemicals-reagents needed, and the gas preparation reaction equations

Alphabetical index of gas preparations

ammonia * carbon dioxide * cracking to make alkenes * chlorine

drying gases * hydrogen * hydrogen chloride * nitrogen dioxide * oxygen * sulfur dioxide

  

HAZARD WARNING SYMBOLS

 

methods of gas preparation - apparatus, chemicals and equation (c) doc b

Ex 1. How can you prepare ammonia gas?

Apparatus and method for preparing a sample of ammonia gas

Procedure 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 downwards. This method of gas preparation is called upward delivery.

e.g. methods of gas preparation - apparatus, chemicals and equation (c) doc bHeating a mixture of ammonium chloride and calcium hydroxide solids gives ammonia which has a very pungent odour! and turns red litmus blue. See also method Ex 7. method as an alternative.

2NH4Cl(s) + Ca(OH)2(s) ==> CaCl2(s) + 2H2O(l) + 2NH3(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.

methods of gas preparation - apparatus, chemicals and equation (c) doc b

Ex 2. How can you prepare hydrogen gas?

Apparatus and method for preparing a sample of hydrogen gas

Procedure 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 downwards. This method of gas preparation is called upward delivery.

methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc be.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. which can be used.

Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g) Advanced Chemistry Page Index and Links

 methods of gas preparation - apparatus, chemicals and equation (c) doc b

Ex 3. How can you prepare nitrogen dioxide gas?

Apparatus and method for preparing a sample of nitrogen dioxide gas

Procedure 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 upwards. This method of gas preparation is called downward delivery.

eg methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc bMaking nasty brown nitrogen dioxide (nitrogen(IV) oxide) 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. as an alternative.

2Pb(NO3)2(s) ==> 2PbO(s) + 4NO2(g) + O2(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 upwards.  This is called downward delivery. Examples:

methods of gas preparation - apparatus, chemicals and equation (c) doc b 

Procedure - how can you prepare carbon dioxide gas?

Apparatus and method for preparing a sample of carbon dioxide gas

This method of gas collection is called downward delivery.

(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.

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

 

Procedure - how can you prepare sulfur dioxide gas?

Apparatus and method for preparing a sample of sulphur dioxide gas

methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc b(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.

Na2S2O5(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + 2SO2(g) 

or Na2SO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + SO2(g) 

 

Procedure - how can you prepare hydrogen chloride gas?

Apparatus and method for preparing a sample of hydrogen chloride gas

methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc b(iii) Nasty acrid Hydrogen chloride is formed when conc. sulphuric acid is mixed with solid sodium chloride. Should be done in fume cupboard.

Advanced Chemistry Page Index and LinksNaCl(s) + H2SO4(l) ==> NaHSO4(s) + HCl(g) 

 

Procedure - how can you prepare chlorine gas?

Apparatus and method for preparing a sample of chlorine gas

(iv) methods of gas preparation - apparatus, chemicals and equation (c) doc bmethods of gas preparation - apparatus, chemicals and equation (c) doc bChlorine 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) + Cl2(g) 

or from the chemical reaction

conc. hydrochloric acid + damp potassium manganate(VII) crystals ==> chlorine gas + other products. The potassium manganate(VII), old name potassium permanganate, acts as an oxidising agent - it oxidises the chloride ion (Cl-) to chlorine molecules (Cl2).

All of these can be done via methods Ex. 5, Ex. 6 (its not too soluble and a way of making small amounts of 'chlorine water') or Ex 7.  described below.

methods of gas preparation - apparatus, chemicals and equation (c) doc b

Ex 5. Apparatus and method for preparing and collecting a gas of any density by reacting a solid and a liquid at room temperature. e.g. for making carbon dioxide Ex 4., chlorine Ex 4., hydrogen Ex 2., hydrogen chloride Ex 4., oxygen Ex 6., sulfur dioxide Ex 4.Advanced Chemistry Page Index and Links

 methods of gas preparation - apparatus, chemicals and equation (c) doc b

This is called collecting over water, or displacement of water or pneumatic trough collection.

Ex 6. How can you prepare oxygen gas?

Apparatus and method for preparing a sample of oxygen gas

This method of gas preparation is by water displacement

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 liquid water 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:

methods of gas preparation - apparatus, chemicals and equation (c) doc b(i) Making oxygen from hydrogen peroxide solution using manganese dioxide catalyst.

Oxygen has similar density to air so must be collected by method Ex 5 as the only alternative

2H2O2(aq) ==> 2H2O(l) + O2(g) 

(ii) Hydrogen, (iii) carbon dioxide and (iv) chlorine (moderately soluble, makes 'chlorine water')

methods of gas preparation - apparatus, chemicals and equation (c) doc bEx 7. Apparatus and 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. Advanced Chemistry Page Index and Links
 methods of gas preparation - apparatus, chemicals and equation (c) doc b 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.

methods of gas preparation - apparatus, chemicals and equation (c) doc bU tube

methods of gas preparation - apparatus, chemicals and equation (c) doc bdreschel 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.

methods of gas preparation - apparatus, chemicals and equation (c) doc b

How can you demonstrate the process of cracking hydrocarbons in the laboratory?

methods of gas preparation - apparatus, chemicals and equation (c) doc bEx 10. A cracking experiment. This diagram outlines a way of demonstrating high temperature thermal cracking of larger hydrocarbon alkanes like paraffin oil/wax into smaller molecules i.e. smaller/lower alkanes and alkenes. 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! Advanced Chemistry Page Index and Links

   
   
 

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