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Qualitative Analysis

Qualitative Analysis. Chemistry 12/12AP. General Rules. Recap of the general rules: Cations All Group 1 and ammonium (NH 4 + ) ions are soluble Anions All nitrates are soluble All Cl - , Br - , and I - are soluble (except for Ag + , Pb +2 , and Hg 2 +2 )

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Qualitative Analysis

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  1. Qualitative Analysis Chemistry 12/12AP

  2. General Rules • Recap of the general rules: • Cations • All Group 1 and ammonium (NH4+) ions are soluble • Anions • All nitrates are soluble • All Cl-, Br-, and I- are soluble (except for Ag+, Pb+2, and Hg2+2) • All sulfates are soluble (except for Ca+2, Sr+2, Ba+2 and Pb+2) • CO3-2, PO4-3, S2-2, SO3-2, C2O4-2, Cr2O4-2 are insoluble (except for Group 1 and NH4+) • All oxides and hydroxides are insoluble (except for Group 1, NH4+, Ba+2, Ca+2, Sr+2) • We can use these rules in order to predict what ions are present in an ‘unknown’ solution

  3. 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.

  4. 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.

  5. 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.

  6. Identifying Cations: NH4+, Na+, Mg2+, Ag+, Fe2+, Fe3+, Cu2+, Al3+, Pb2+, Zn2+, Ba2+ litmus stays red Na+ add 2 drops of dilute NaOH solution. add NaOH solution, heat, test gas with red litmus. no precipitateNH4+, Na+ litmus goes blue NH4+ add excessNaOH solution precipitate disappearsAl3+, Zn2+, Pb2+ white precipitate formsAl3+, Zn2+, Pb2+, Mg2+, Ba2+ new sample precipitate remainsMg2+, Ba2+ add 2 drops, thenexcess NH3 solution new sample add dilute H2SO4 solution white precipitate forms and disappears, Zn2+ colourless solutionMg2+ white precipitateBa2+ white precipitate formsAl3+, Pb2+ new sample add dilute H2SO4 solution green precipitate formsFe2+ white precipitatePb2+ colourless solutionAl3+ new sample add 2 drops KSCN solution orange precipitate formsFe3+ dark red solution confirms Fe3+ new sample blue precipitate then deep blue solution Cu2+ add 2 drops, then excessNH3 solution blue precipitate formsCu2+ new sample brown precipitate then colourless solution Ag+ add 2 drops, then excessNH3 solution brown precipitate formsAg+

  7. Identifying Cations I • Use the cation flowchart to identify the cation used below: Add NaOH Add KSCN

  8. Identifying Cations II • Use the cation flowchart to identify the cation used below: Add NaOH Add NH3

  9. Identifying Anions: CO32,Cl,I, S2, SO42,NO3,OH bubbles of gasCO32 add dilute HCl solution litmus goes blueOH,CO32 START: add red litmus no bubblesOH Smelly bubblesS2 litmus remains redSO42,Cl,I,NO3 white precipitateSO42 add Ba(NO3)2 solution no precipitate new sample precipitate disappearsCl add dilute NH3 solution add AgNO3 solution precipitateCl,I precipitate remainsI no precipitateNO3

  10. Identifying Anions I • Use the anion flowchart to identify the anion used below: Add red litmus Add Ba(NO3)2 Add AgNO3 Add NH3

  11. Solution Colors

  12. 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

  13. 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.

  14. 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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