Windsor University
School of Medicine
PREPARATION
& COLLECTION OF
NON-METAL COMPOUNDS
UNLESS YOU TRY TO DO SOMETHING BEYOND WHAT YOU HAVE ALREADY
MASTERED, YOU WILL NEVER GROW.
RALPH WALSO EMERSON
Ch19.
J.C. Rowe
Collection of gases
Downward displacement of water
Upward delivery
Downward delivery
Collecting 5 important gases (table)

Downward displacement of water


Downward displacement of water
Suitable for collecting gases which are insoluble in
water.
e.g. hydrogen, oxygen, carbon dioxide.
Not suitable for hydrogen chloride gas and
ammonia gas which are very soluble in water
Collection of a gas by the displacement
of water
Upward delivery



A gas which is less dense than air can be collected
by upwards delivery.
Ex: ammonia & hydrogen
A gas jar (inverted) is connected by a tube to the
flask which is giving off the gas
Downward delivery



A gas which is more dense than air can be
collected by downward delivery.
Ex: carbon dioxide, chlorine, hydrogen chloride &
sulfur dioxide.
A gas jar (upward) is connected by a tube to the
flask which is giving off the gas.
Downward vs. Upward collections






Downward delivery :
Gas jar is upward
Gas is more dense
than air.
Upward delivery:
Gas jar is inverted
(downward)
Gas is less dense than
air.


You need to know how to collect gases produced in
reactions by upward & downward delivery, over
water using a gas syringe.
You need to be able to explain the method of
collection if you are given information about the
density & solubility of the gas concerned.
Collecting 5 important gases
Hydrogen
Oxygen
Carbon
dioxide
Ammonia
less dense
than air?
yes
yes
Upward
delivery
yes
yes
denser than
air?
slightly
Downward
delivery
Chlorine
yes
yes
yes
yes
Solubility in
water?
very low
low
low
over water
yes
yes
yes
very high
moderate
yes
Laboratory preparation of gases
Ammonia, NH3
Carbon dioxide, CO2
Chlorine, Cl2
Hydrogen, H2
Nitrogen, NO2
Oxygen, O2
Sulphur dioxide, SO2

Ammonia, NH3


Ammonia gas can be produced in laboratory by heating
an ammonium salt (ammonium chloride) with a strong
alkali (sodium hydroxide).
NH4Cl (s) + NaOH (aq) …….
NH3 (g) + NaCl (aq)
+ H2O (l)
Identification of gas
Ammonia turns damp red litmus blue; & in the presence
of HCl gas white fumes of ammonium chloride are
formed.
Carbon dioxide, CO2
Reacting any carbonate with any acid forms carbon
dioxide;
For example calcium carbonate & hydrochloric acid :
CaCO3 + 2HCl ……….. CaCl2 + CO2 + H2O

Identification of gas
Carbon dioxide turns lime water “milky”

Chlorine, Cl2

Chlorine is formed by oxidation of concentrated
hydrochloric acid using either manganese dioxide or
potassium permanganate as the oxidising agent.
MnO2 + 4HCl ……
MnCl2 + Cl2 + H20
2KMnO4 + 16HCl …….

2KCl + 5Cl2+2MnCl2
+ 8 H2 O
Identification of the gas.
Chlorine is a pale green gas, which turns moist blue
litmus paper red & then bleaches it.
Hydrogen, H2



Any reactive metal (a meta above hydrogen in the
reactivity series) will displace hydrogen from a
dilute acid except for nitric acid.
Zn (s)+ HCl (aq) …………. ZnCl2 (aq) + H2 (g)
Identification of the gas.
Hydrogen gives a “popping” sound when mixed
with oxygen & ignited.
Nitrogen dioxide, NO2
Nitrogen dioxide is formed during the decomposition of metal nitrates on heating:
2Pb(NO3) 2 (s) …… 2PbO (s)+ 4NO2 (g) + O2 (g)
 Sodium nitrate & potassium nitrate are exceptions
as they produce the corresponding nitrite & oxygen
gas instead.
2NaNO3 (s) ……….
2NaNO2 (s)+ O2 (g)
 Identification of the gas.
Nitrogen dioxide is a brown gas, which turns moist
blue litmus red but does not bleach it.

Oxygen, O2


Oxygen is produced by the decomposition of
hydrogen peroxide (H2O2). Hydrogen peroxide
naturally decomposes slowly to form oxygen &
water on exposure to light.
2 H2O2 (l) ……….. 2 H2O (l) + O2 (g)
Identification of gas.
Oxygen relights a glowing splint.
Sulphur dioxide, SO2
Sulphur dioxide is formed by gently heating a sulphite
salt or hydrogen sulphite with dilute hydrochloric acid or
sulphuric acid:
Na2SO3 (s) + 2HCl (aq) ……. 2NaCl (aq) + SO2 (g)
+ H2O (l)
Na2SO3 + H2SO4(aq) ……. Na2SO4 (aq) + SO2 (g)
+ H2O (l)
 Identification of gas.
When sulphur dioxide is bubbled through acidified
potassium dichromate, the color of the solution changes
from orange to green.

Michael Faraday
A natural law regulates the advance of science. Where
only observation can be made, the growth of knowledge
creeps; where laboratory experiments can be carried
on, knowledge leaps forward.