|
Identification
of metallic and non-metallic ions
Francine
Taylor Campbell, Contributor
POINTS
TO NOTE
*
Metallic ions or cations are identified
based on the colour and solubility
of the hydroxides formed from their
reaction with aqueous ammonia and
sodium hydroxide.
*
Non-metallic ions or anions are generally
identified based on the evolution
and identification of gases produced,
when the compounds are heated or reacted
with acids. The colour and solubility
of precipitates formed on reaction
with barium ions and ammonia (for
the halide ions) are also used.
Let
us summarise the reactions of the
metallic ions; Pb2+, Al3+, Ca2+, Zn2+,
Fe3+, Fe2+, NH4+ and Cu2+ with aqueous
ammonia and sodium hydroxide.
Fe2+
produces a dirty green precipitate,
which is insoluble in excess aq NH3
and NaOH.
Fe3+
produces a rusty brown precipitate,
which is insoluble in excess aq NH3
and NaOH
Cu2+
forms a blue precipitate which is
insoluble in excess aq NaOH, but soluble
in excess aq NH3.
These
cations can clearly be identified
based on their colours alone.
Al3+
and Pb2+ both form white precipitates
when reacted with aq. NaOH and NH3.
In aq NaOH the precipitates are soluble
in excess while in aq. NH3 they are
insoluble in excess. As can be seen,
Al3+ and Pb2+ give the same results
so to differentiate between them other
reactions must be taken into consideration.
Pb2+ will form form precipitates with
I-, Cl- and SO42- ions (PbI2 (yellow),
PbCl2 (white), and PbSO4 (white),
while Al3+ will not.
Two
other cations form white precipitates.
Zn2+ forms a white precipitate, which
is soluble both in excess aq. NaOH
and NH3. Ca2+ forms a white precipitate,
which is insoluble in excess NaOH
but does not form a precipitate when
treated with ammonia.
The
NH4+ ion can be identified based on
the evolution of ammonia (NH3) gas
when warmed with aq. NaOH.
The
identification of the anions is a
lot more complicated. The ones we
are concerned with are CO32-, SO42-,
SO32-, NO3-, Br-, Cl- and I-.
One
way to do this is to heat the compound
and identify the gas
produced. CO32- ====== (heat) produces
the gas CO2
NO3-
===== (heat) gives off O2 and/or NO2
SO32-
===== (heat) gives off SO2
These
gases can be identified by their characteristic
tests.
CO2
is weakly acidic and turns lime water
(Ca(OH)2) milky.
O2
is neutral and relights a glowing
splint.
NO2
is a red-brown/yellow-brown acidic
gas.
SO2
is an acidic gas, which turns acidified
potassium manganate (VII) solution
from purple to colourless and changes
acidified potassium dichromate (VI)
from orange to green.
NH3
is an alkaline gas, which forms dense
white fumes with hydrogen chloride
gas.
Some
anions react with dilute and concentrated
acids (if they are added to the solid).
CO32-
reacts with dil and conc acid to release
CO2.
SO32-
forms SO2 gas on reaction with dil
and conc acid.
If
conc. H2SO4 acid is added to a solid
containing the halide ions (I-,
Cl-, and Br-), the following gases
can be formed HI(g) which decomposes
to I2, HCl(g) and HBr which decomposes
to Br2.
The
halide ions, in solution, can be identified
differently based on their reaction
with silver nitrate solution (AgNO3)
and the solubility of the precipitate
formed in ammonia.
Cl-
forms a white precipitate with AgNO3
solution, which is soluble in aq NH3.
Br-
forms a cream precipitate with AgNO3,
which is partially soluble in aq NH3.
I-
forms a light yellow precipitate,
which is insoluble in aq NH3.
Finally
some anions form precipitates with
a solution of Ba2+ ions.
CO32-
forms a white precipitate, which dissolves
on addition of acid to form CO2.
SO32-
forms a white precipitate, which dissolves
on addition of acid to give SO2.
SO42-
forms a white precipitate, which is
insoluble in dilute acid.
In
the next lesson we will discuss some
questions, which require identification
of anions and cations.
Review
the reactions and the observations
given above so that you can follow
in next week's lesson.
*
Francine Taylor-Campbell is
an independent contributor.
|
