* Methods-techniques of preparing and collecting gases - apparatus, chemicals-reagents, and reaction equations at Doc B's
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Doc Brown's Chemistry Gas preparation and collection methods Revision Notes Alphabetical index of gases: ammonia * carbon dioxide * cracking to make alkenes * chlorine * drying gases * hydrogen * hydrogen chloride * nitrogen dioxide * oxygen * sulfur dioxide *
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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 downwards. This is called upward delivery.
e.g. 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. |
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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 downwards. This is called upward delivery.
Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g) |
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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 upwards. This is called downward delivery.
eg 2Pb(NO3)2(s) ==> 2PbO(s) + 4NO2(g) + O2(g) |
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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:
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(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)
Na2S2O5(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + 2SO2(g) or Na2SO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + SO2(g)
NaCl(s) + H2SO4(l) ==> NaHSO4(s) + HCl(g) (iv) 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. |
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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. | |
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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 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:
2H2O2(aq) ==> 2H2O(l) + O2(g) (ii) Hydrogen, (iii) carbon dioxide and (iv) chlorine (moderately soluble, makes 'chlorine water') |
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 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.
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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. | |
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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. |
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  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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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 
e.g.
A mixture of
zinc and hydrochloric
acid makes hydrogen. Hydrogen gives a squeaky pop! with a lit
splint. See also methods 
(i)
Making oxygen from hydrogen peroxide
solution using manganese dioxide catalyst. Oxygen has similar density to
air so must be collected by methods 
U tube
dreschel bottle