GCSE OCR Gateway
C4:2b Carbonate and sulphate, halide and nitrate tests.
Learning
Objectives:
C4.2b To be able to describe tests to identify various aqueous anions:
carbonates using hydrochloric acid and limewater; sulfates using hydrochloric
acid followed by aqueous barium chloride solution; chloride, bromide and iodide
using nitric acid followed by silver nitrate solution.
Anions are negatively charged ions so
named because they are attracted to and discharged from the positive anode during electrolysis.
There are
several common anions in school and college chemistry and each one comes from a
simple acid.
Acid
|
Hydrochloric
(HCl)
|
Sulphuric
(H2SO4)
|
Nitric
(HNO3)
|
Carbonic
(H2CO3)
|
Anion
|
Chloride
Cl–
|
Sulphate SO42–
|
Nitrate
NO3–
|
Carbonate
CO32–
|
How to test for halide ions
Halide
ion refers to the anions of chlorine, bromine and iodine. The halides: chlorides, bromides and iodides
require just one simple test to identify them.
First
to the solution you suspect contains halide ions add a few drops of nitric
acid. The nitric acid is there to
remove any other ions particularly carbonate ions that would also form a
precipitate with silver nitrate since silver carbonate (Ag2CO3)
is a white insoluble solid.
Second,
add several drops of 0.05M silver nitrate solution (AgNO3(aq)) to the solution of halide
ion. The colour of the precipitate will tell
you which halide ion is present.
The
silver ions in the silver nitrate solution form insoluble coloured precipitates
with chlorides, bromides and iodides.
With Chlorides:
Silver
nitrate forms a white precipitate with chlorides.
AgNO3
(aq) +
NaCl(aq) ⟶
AgCl(s) + NaNO3
(aq)
White
ppt
The
ionic equation:
Ag+
(aq) + Cl—(aq)
⟶
AgCl(s)
With Bromides:
Silver
nitrate forms a cream precipitate with bromides.
AgNO3
(aq) + NaBr(aq) ⟶
AgBr(s) + NaNO3
(aq)
Cream
ppt
The
ionic equation:
Ag+
(aq) + Br—(aq)
⟶
AgBr(s)
With Iodides:
Silver
nitrate forms a yellow precipitate with iodides.
AgNO3(aq)
+ NaI(aq) ⟶
AgI(s) + NaNO3(aq)
Yellow ppt
The
ionic equation:
Ag+
(aq) + I—(aq)
⟶
AgI(s)
Further confirmatory tests:
If dilute
ammonia solution is added to the white precipitate of silver chloride, the
precipitate should dissolve.
If concentrated
ammonia solution is added to the cream precipitate of silver bromide, the
precipitate should dissolve.
The
yellow precipitate of silver iodide is insoluble in ammonia solution.
Effect of bright sunlight
Lastly,
put the three precipitates in bright light or sunlight and after a few minutes
the white precipitate of silver chloride turns dark purple, the bromide turns
dark green but the iodide remains yellow.
Summary
How to test for a sulphate SO42–
First
to the solution you suspect contains sulfate ions add a few drops of
hydrochloric acid. The hydrochloric acid is
there to remove any other ions particularly carbonate ions that would also form
a precipitate with barium chloride since barium carbonate (BaCO3) is
a white insoluble solid.
Second,
add several drops of 0.1M barium chloride solution (BaCl2(aq)) to the solution of sulfate ion.
The
barium ions in the barium chloride solution form an insoluble white precipitate
with sulphate ions.
BaCl2
(aq) + Na2SO4(aq)
⟶
2NaCl(aq)
+ BaSO4(s)
White ppt
The
ionic equation:
Ba2+(aq)
+ SO42—(aq)
⟶
BaSO4(s)
How to test for a carbonate CO32–
First, carbonates
are usually solids, only Group 1 carbonates are soluble in water. So to the solid (or solution) you suspect
contains carbonate ions add a few drops of hydrochloric acid. The hydrochloric acid reacts to form carbon dioxide
and fizzing occurs instantly.
Second, pass
the gas produced into limewater and if it is carbon dioxide you will see
cloudiness or chalkiness in the limewater.
Hydrochloric
acid neutralizes the carbonate ions to from carbon dioxide.
CaCO3
(s) + 2HCl(aq) ⟶ CaCl2(aq)
+ CO2(g)
+ H2O(l)
The
ionic equation:
CO32-(aq)
+ 2H+(aq) ⟶ CO2(g) + H2O(l)
And when
the carbon dioxide gas enters limewater the following reaction takes place.
CO2(g) +
Ca(OH)2(aq) ⟶ CaCO3(s) + H2O(l)
chalkiness
How to test for the nitrate ion NO3–
Now this
test isn’t always included in school and college chemistry but so what it’s
different and a great test with great chemistry.
Of course
the reason for the absence of nitrate tests in school and college chemistry is
that they are not always reliable.
I think I
can agree with Wiki when it says:
Testing
for the presence of nitrate via wet chemistry is generally difficult compared
with testing for other anions, as almost all nitrates are soluble in water. In
contrast, many common ions give insoluble salts, e.g. halides precipitate with
silver, and sulphate precipitate with barium.
The
nitrate anion is an oxidant, and many tests for the nitrate anion are based on
this property. Unfortunately, other oxidants present may interfere and give
erroneous results.
A test
that is reliable uses an alloy of copper,
aluminium and zinc called Devarda’s Alloy.
If you
suspect a solution contains a nitrate ion then add a few drops sodium hydroxide
solution and a piece of Devarda’s Alloy to the solution.
Heat the
solution gently in a fume cupboard and test the gas given off with a piece of
damp red litmus paper. If the paper turns
blue the gas is likely to be ammonia and the ion in the solution is a nitrate!!
3 NO3− + 8Al
+ 5OH− + 18H2O → 3NH3 +
8[Al(OH) 4]−
It is
unusual for any alkaline gas to evolve in any test in wet chemistry other than
ammonia so you can be pretty sure that the evolution of an alkaline gas
confirms the presence of the nitrate anion.
In my
next posts, I’m going to discuss titration technique and develop and update
some earlier work in this blog.
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