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Qualitative Analysis Chemistry 12 AP
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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.
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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).
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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.
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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.
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Those precipitates formed are the hydroxides of the cations,
If the formed hydroxide is basic so it will be insoluble in excess sodium hydroxide, If the formed hydroxide is amphoteric so it will be soluble in excess sodium hydroxide.
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Identifying Cations: NH4+, Na+, Mg2+, Ag+, Fe2+, Fe3+, Cu2+, Al3+, Pb2+, Zn2+, Ba2+
litmus stays red Na+ add 2 drops of dilute NaOH solution. no precipitate NH4+, Na+ add NaOH solution, heat, test gas with red litmus. litmus goes blue NH4+ white precipitate forms Al3+, Zn2+, Pb2+, Mg2+, Ba2+ add excess NaOH solution precipitate disappears Al3+, Zn2+, Pb2+ new sample precipitate remains Mg2+, Ba2+ add 2 drops, then excess NH3 solution new 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+ white precipitate Pb2+ new sample orange precipitate forms Fe3+ add 2 drops KSCN solution dark red solution confirms Fe3+ new sample blue precipitate forms Cu2+ add 2 drops, then excess NH3 solution blue precipitate then deep blue solution Cu2+ new sample add 2 drops, then excess NH3 solution brown precipitate then colourless solution Ag+ brown precipitate forms Ag+
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Identifying Cations I Use the cation flowchart to identify the cation used below: Add NaOH Add KSCN
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Identifying Cations II
Use the cation flowchart to identify the cation used below: Add NaOH Add NH3
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add dilute HCl solution
Identifying Anions: CO32, Cl, I, S2, SO42, NO3, OH bubbles of gas CO32 START: add red litmus litmus goes blue OH, CO32 add dilute HCl solution no bubbles OH Smelly bubbles S2 litmus remains red SO42, Cl, I, NO3 add Ba(NO3)2 solution white precipitate SO42 no precipitate new sample precipitate disappears Cl add AgNO3 solution precipitate Cl, I add dilute NH3 solution precipitate remains I no precipitate NO3
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Identifying Anions I Use the anion flowchart to identify the anion used below: Add red litmus Add Ba(NO3)2 Add AgNO3 Add NH3
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Identifying Anions II Use the anion flowchart to identify the anion used below: Add red litmus Add HCl
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Solution Colors Cations Anions 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
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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
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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+ (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.
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Lab Procedures For Qualitative Analysis
Solution containing ions of all cation groups filtration Precipitates: AgCl, Hg2Cl2, PbCl2 + HCl Solution containing ions of remaining groups filtration + H2S Precipitates: CuS, CdS, HgS, SnS, Bi2S3 Solution containing ions of remaining groups filtration Precipitates: CoS, FeS, MnS, NiS, ZnS, Al(OH)3, Cr(OH)3 + NaOH Solution is now basic, so other S2- will precipitate Solution containing ions of remaining groups filtration Precipitates: BaCO3, CaCO3, SrCO3 + Na2CO3 Solution contains Na+, K+, NH4+ ions
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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
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