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.

new sample

precipitate disappearsAl3+, Zn2+, Pb2+

precipitate remainsMg2+, Ba2+

add 2 drops, thenexcess NH3

solution

white precipitate formsAl3+, Zn2+, Pb2+, Mg2+,

Ba2+

add excessNaOH solution

add 2 drops of dilute NaOH

solution.

no precipitateNH4

+, Na+

add NaOH solution, heat, test gas with

red litmus.

litmus stays red Na+

litmus goes blue NH4

+

blue precipitate then deep blue solution

Cu2+

blue precipitate formsCu2+

brown precipitate formsAg+

brown precipitate then colourless solution Ag+

add 2 drops, then excess

NH3 solution

add 2 drops, then excessNH3 solution

orange precipitate formsFe3+

green precipitate formsFe2+

white precipitate formsAl3+, Pb2+

white precipitate forms and disappears, Zn2+

add dilute H2SO4 solution

colourless solutionAl3+

white precipitatePb2+

add dilute H2SO4 solution

colourless solutionMg2+

white precipitateBa2+

add 2 drops KSCN solution

dark red solution confirms Fe3+

Identifying Cations: NH4+, Na+, Mg2+, Ag+, Fe2+, Fe3+, Cu2+, Al3+, Pb2+, Zn2+, Ba2+

new sample

newsample

newsample

newsample

new sample

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: CO32, Cl, I, S2, SO4

2, NO3, OH

litmus remains redSO4

2, Cl, I, NO3

add Ba(NO3)2 solution white precipitateSO4

2

add AgNO3 solution

no precipitate

precipitateCl, I

add dilute NH3 solution

precipitate disappearsCl

precipitate remainsI

no precipitateNO3

new sample

START: add red litmus

litmus goes blueOH, CO3

2

add dilute HCl solution

bubbles of gasCO3

2

no bubblesOH

Smelly bubblesS2

Identifying Anions I

• Use the anion flowchart to identify the anion used below:

Add Ba(NO3)2 Add AgNO3 Add NH3Add red litmus

Identifying Anions II

• Use the anion flowchart to identify the anion used below:

Add HClAdd red litmus

Solution Colors

Cations

Ions 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

Anions

Chromate CrO42- Yellow

Dichromate Cr2O72- Orange

Permanganate MnO42- purple

Some Precipitate Colours

Al3+, Mg2+,Ca2+ white

Cu2+ blue/greenFe2+ grey/greenFe3+ 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 Hg2

2+

(B) Ag+ and Pb2+

(C) Ba2+ and Ag+

(D) Ba2+ and Hg22+

(E) Ba2+ and Pb2+

Ask yourself – what happens at each step?

Solution – Ag+ forms a precipitate with chlorides (AgCl), so what is the other metal? Think of a metal that would likely be insoluble with I- Pb2+ is a good choice because in our labs, it formed mostly yellow/orange precipitates.

Lab Procedures For Qualitative Analysis

Solution containing ions of all cation groups

Solution containing ions of remaining groups

Solution containing ions of remaining groups

Solution containing ions of remaining groups

Solution contains Na+, K+, NH4+ ions

+ HCl Precipitates: AgCl, Hg2Cl2, PbCl2

Precipitates: CuS, CdS, HgS, SnS, Bi2S3

Precipitates: CoS, FeS, MnS, NiS, ZnS, Al(OH)3, Cr(OH)3

+ H2S

+ NaOH

+ Na2CO3

filtration

filtration

filtration

filtration

Solution is now basic, so other S2- will precipitate

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