Transcript Qualitative Analysis Chemistry 12 AP

Qualitative Analysis
Chemistry 12 AP
What is Qualitative Chemical
Analysis?
• It is a branch of chemistry that deals
with the identification of elements or
grouping of elements present in a
sample.
• There are usually two types: qualitative
inorganic analysis and qualitative organic
analysis.
Identifying Ions
• We could use these rules to work out if
hydroxides or chlorides are present in a solution.
(What is one difference between the two ions?)
• To make life easier we could use a flow chart
that shows the order that we need to do tests in
order to eliminate possible ions
• These two flow charts divide all ions into two
groups – anions and cations. Once we have
identified one of the ions in a solution we need
to identify the other. Can also use flame tests
(for soluble metals), or other tests (see next
page).
• Chemical tests are used to confirm that some anions
(negative ions) or cations (positive ions) are present in
an unknown, also to identify unknown gases or liquids.
• If you made a chemical reaction that produces fumes or
gases or something you must have a way to identify this
produced gas or fumes.
Flame Colours of Solutions
If a flame test produces one of the these
colours, that ion is in the solution.
Other colors are possible, but these are
the most common.
Identifying Cations: NH4+,
add 2 drops of dilute
NaOH solution.
Na+, Mg2+, Ag+, Fe2+, Fe3+, Cu2+, Al3+, Pb2+, Zn2+, Ba2+
add NaOH solution,
heat, test gas with red
litmus.
no precipitate
NH4+, Na+
add excess
NaOH solution
white precipitate forms
Al3+, Zn2+, Pb2+, Mg2+,
Ba2+
litmus stays
red
Na+
precipitate disappears
Al3+, Zn2+, Pb2+
new sample
precipitate remains
Mg2+, Ba2+
new
litmus goes
blue NH4+
add 2 drops, then
excess NH3 solution
sample
add dilute H2SO4 solution
white precipitate forms and
disappears, Zn2+
colourless solution
Mg2+
white precipitate
Ba2+
white precipitate forms
Al3+, Pb2+
new sample
add dilute H2SO4 solution
green precipitate forms
Fe2+
colourless solution
Al3+
orange precipitate forms
Fe3+
new
sample
add 2 drops
KSCN solution
white precipitate
Pb2+
dark red solution
confirms Fe3+
new
sample
add 2 drops, then excess
NH3 solution
blue precipitate then
deep blue solution
Cu2+
new
brown precipitate forms
+
sample
Ag
add 2 drops, then excess
NH3 solution
brown precipitate
then colourless
solution Ag+
blue precipitate forms
Cu2+
Identifying Cations I
• Use the cation flowchart to identify the cation
used below:
Add NaOH
Add KSCN
Identifying Cations II
• Use the cation flowchart to identify the cation
used below:
Add NaOH
Add NH3
Identifying Anions:
START: add red
litmus
CO32, Cl, I, S2, SO42, NO3, OH
litmus goes blue
OH, CO32
add dilute HCl
solution
no bubbles
OH
Smelly bubbles
S2
litmus remains red
SO42, Cl, I, NO3
add Ba(NO3)2 solution
bubbles of gas
CO32
white precipitate
SO42
no precipitate
new sample
add AgNO3 solution
precipitate
Cl, I
add dilute NH3
solution
precipitate disappears
Cl
precipitate remains
I
no precipitate
NO3
Identifying Anions I
• Use the anion flowchart to identify the anion
used below:
Add red
litmus
Add Ba(NO3)2
Add AgNO3
Add NH3
Identifying Anions II
• Use the anion flowchart to identify the anion
used below:
Add red
litmus
Add HCl
Solution Colors
Ions
Cations
Anions
Symbol
Colour
Chromium(II)
Copper(II)
Cr2+
Cu2+
Blue
Chromium(III)
Copper(I)
Iron(II)
Nickel(II)
Cr3+
Cu+
Fe2+
Ni2+
Green
Iron(III)
Fe3+
Pale yellow
Cobalt(II)
Manganese(II)
Co2+
Mn2+
Pink
Chromate
CrO42-
Yellow
Dichromate
Cr2O72-
Orange
Permanganate
MnO42-
purple
Some Precipitate Colours
Al3+, Mg2+,Ca2+  white
Cu2+  blue/green
Fe2+  grey/green
Fe3+  red/brown
Precipitated calcium carbonate (CaCO3)
Precipitated copper(II) hydroxide, Cu(OH)2
So How Do I Use Qualitative
Analysis?
• Here is an example of a question:
A sample of a solution of an unknown was treated with dilute
HCl. The white precipitate formed was filtered and washed with
hot water. A few drops of KI solution were added to the hot
water filtrate and a bright yellow precipitate was produced. The
white precipitate remaining on the filter paper was readily
soluble in NH3 solution. What two ions could have been present
in the unknown?
(A) Ag+ and Hg22+ Ask yourself – what happens at each step?
(B) Ag+ and Pb2+
Solution – Ag+ forms a precipitate with
chlorides (AgCl), so what is the other
(C) Ba2+ and Ag+
metal? Think of a metal that would
(D) Ba2+ and Hg22+
likely be insoluble with I-  Pb2+ is a
2+
2+
(E) Ba and Pb
good choice because in our labs, it
formed mostly yellow/orange
precipitates.
Lab Procedures For Qualitative Analysis
Solution containing ions of all cation groups
filtration
+ HCl
Solution containing ions
of remaining groups
Precipitates: AgCl, Hg2Cl2,
PbCl2
filtration
+ H2S
Solution containing ions
of remaining groups
Precipitates: CuS, CdS,
HgS, SnS, Bi2S3
filtration
+ NaOH
Solution is now basic, so other S2- will precipitate
Precipitates: CoS, FeS, MnS,
NiS, ZnS, Al(OH)3, Cr(OH)3
Solution containing ions
of remaining groups
filtration
+ Na2CO3
Solution contains Na+, K+, NH4+ ions
Precipitates: BaCO3, CaCO3,
SrCO3
Questions for Practice
• We will be completing a lab very shortly
with the techniques for qualitative
analysis.
• To assist you in this lab, the following
questions should be completed:
Chang Text: pg. 742 #16.79, 16.81, 16.82
Pg. 158 #4.24