A method for preparing a sample of ammonia gas

A 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 downwards. This method of gas preparation 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 method Ex 7. method as an alternative.

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.

A method for preparing a sample of hydrogen gas

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 downwards. This method of gas preparation 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. which can be used.

Zn_(s) + 2HCl_(aq) ==> ZnCl_2(aq) + H_2(g) 

A method for preparing a sample of nitrogen dioxide gas

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

eg Making 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) + 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 upwards.  This is called downward delivery. Examples:

How can you prepare carbon dioxide gas?

A method for preparing a sample of carbon dioxide gas

This method of gas preparation 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.

CaCO_3(s) + 2HCl_(aq) ==> CaCl_2(aq) + H₂O_(l) + CO_2(g) 

How can you prepare sulfur dioxide gas?

A method for preparing a sample of sulphur dioxide gas

(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) 

How can you prepare hydrogen chloride gas?

A method for preparing a sample of hydrogen chloride gas

(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) 

How can you prepare chlorine gas?

A method for preparing a sample of chlorine gas

(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) 

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.

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

A 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:

(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

2H₂O_2(aq) ==> 2H₂O_(l) + O_2(g) 

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

U tube

dreschel bottle

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.

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

Ex 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!

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