Chem cations

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Chem cations

  1. 1. Qualitative Analysis of Cations
  2. 2. Classic SeparationScheme forQualitative Analysis of21 Common CationsZumdahl 7e, Figure 15.12. Copyright © Houghton Mifflin 15–2 Company. All rights reserved.
  3. 3. Group I Cations Ag+, Pb2+ Add dilute HClAgCl(s) white precipitatePbCl2(s) white precipitateAg+ + Cl  AgCl(s)Pb2+ + 2Cl  PbCl2(s)
  4. 4. Group II Cations Cu(II), Cd(II), As(III), Sb(III), Bi(III), Sn(IV) Add H2S (acidic)CuS(s), CdS(s), As2S3(s), Sb2S3(s), Bi2S3(s), SnS2(s) Cu2+(aq) + H2S(aq)  CuS(s) + 2H+
  5. 5. Precipitation of Bismuth SulfideCopyright © Houghton Mifflin 15–5Company. All rights reserved.
  6. 6. From Left to Right: Cadmium Sulfide, Chromium(III)Hydroxide, Aluminum Hydroxide, and Nickel(II) HydroxideCopyright © Houghton Mifflin 15–6Company. All rights reserved.
  7. 7. Figure 15.13 TheSeparation of theGroup I Ions inthe ClassicScheme ofQualitativeAnalysisCopyright © Houghton Mifflin 15–7Company. All rights reserved.
  8. 8. Aqueous Ammonia is Added to Silver Chloride (white). Silver Chloride, Insoluble in Water, Dissolves to Form Ag(NH3)2+ (aq) and Cl-(aq) AgCl(s) + 2NH3(aq) Ý Ag(NH3)2+(aq) + Cl (aq)Copyright © Houghton Mifflin 15–8Company. All rights reserved.
  9. 9. Flame Testfor Potassium Copyright © Houghton Mifflin 15–9 Company. All rights reserved.
  10. 10. Flame Testfor SodiumCopyright © Houghton Mifflin 15–10Company. All rights reserved.
  11. 11. Preliminary Tests1. Add H2O 4. Add HCl2. Add NaOH 5. Add sulfide a. Dilute NaOH a. Acidic b. Excess NaOH b. Basic3. Add NH3 6. Add Na2CO3 a. Dilute NH3 7. Add H2SO4 b. Excess NH3 8. Flame tests
  12. 12. Preliminary Tests• Test 1. Add water – Insoluble oxychlorides may be observed as a faint white cloudiness (turbidity) • Sb3+ + H2O + Cl Ý SbOCl(s) + 2H+ • Bi3+ + H2O + Cl Ý BiOCl(s) + 2H+ – Increased acid concentration favors soluble form All species are in aqueous solution unless indicated otherwise
  13. 13. Preliminary Tests• Test 2. Reactions with NaOH – Test 2a. Dilute NaOH. • Most cations form insoluble hydroxides – Mg2+ + 2OH  Mg(OH)2(s) – Al3+ + 3OH  Al(OH)3(s) – Test 2b. Excess NaOH • Most metal hydroxides remain insoluble – Mg(OH)2(s) • Amphoteric hydroxides dissolve – Al(OH)3(s) + OH  Al(OH)4 All species are in aqueous solution unless indicated otherwise
  14. 14. Reactions with NaOH Amphoteric metal hydroxides
  15. 15. Preliminary Tests• Test 3. Reactions with Ammonia – Test 3a. Dilute NH3. Weak base raises pH. • Mg2+ + 2NH3 + 2H2O  Mg(OH)2(s) + 2NH4+ • Ni2+ + 2NH3 + 2H2O  Ni(OH)2(s) + 2NH4+ – Test 3b. Excess NH3. • Most metal hydroxides remain insoluble. – Mg(OH)2(s) • Soluble ammine complexes form in some cases. – Ni(OH)2(s) + 6NH3  Ni(NH3)62+ + 2OH All species are in aqueous solution unless indicated otherwise
  16. 16. Reactions with NH3 Soluble ammine complexes
  17. 17. Preliminary Tests• Test 4. Add HCl – Solubility Rule: All chlorides are soluble, except those of Ag+, Pb2+ and Hg22+ – Reaction with HCl • Ag+ + Cl  AgCl(s) All species are in aqueous solution unless indicated otherwise
  18. 18. Preliminary Tests• Test 5. Add sulfide – Heating thioacetamide in water produces H2S – Test 5a. Acid sulfide. The Group II cations precipitate in acidic (0.3 M HCl) sulfide solution. • Cu2+ + H2S  CuS(s) + 2H+ – Test 5b. Basic sulfide. The Group III cations precipitate when the solution is made basic • Zn2+ + H2S  ZnS(s) + 2H+ All species are in aqueous solution unless indicated otherwise
  19. 19. Groups II and III Cations
  20. 20. Preliminary Tests• Test 6. Reactions with Na2CO3 – Most metal carbonates are insoluble • Ni2+ + CO32  NiCO3(s) • Ca2+ + CO32  CaCO3(s) • Exceptions: K+, Na+, NH4+ – In sulfide separation scheme • most cations are removed in Groups I, II, and III as insoluble chlorides or sulfides • the remaining insoluble carbonates precipitate as the Group IV cations. – Ca2+, Mg2+, Ba2+ All species are in aqueous solution unless indicated otherwise
  21. 21. Preliminary Tests• Test 7. Add H2SO4 – Solubility rule • Most sulfates are soluble, except those of Ba2+, Sr2+, and Pb2+ – The insolubility of BaSO4 provides a basis to distinguish it from the other Group IV insoluble carbonates. • Addition of acid causes carbonates to dissolve – BaCO3(s) + 2H+  Ba2+ + H2O + CO2 • If H2SO4 is used as the acid, BaSO4 is formed. – BaCO3(s) + 2H+ + SO42  BaSO4(s) + H2O + CO2 All species are in aqueous solution unless indicated otherwise
  22. 22. Preliminary Tests• Test 8. Flame test – Used primarily to help identify soluble cations • Solubility rule: – All Group IA salts (Li+, Na+, K+, Rb+, Cs+) and NH4+ salts are soluble. • Na+, K+ yield positive flame tests. • NH4+ is also a soluble cation, but yields no useful result in the flame test. It is identified from its properties of volatility and basicity – Add base, heat, and test with moist litmus – NH4+ + OH  NH3(g) + H2O(l) All species are in aqueous solution unless indicated otherwise
  23. 23. Preliminary Tests and Sulfide Separation Scheme • Reactions in the preliminary tests can be used in conjunction with the sulfide scheme to identify all 21 cations. • Especially useful are – reactions with NaOH – reactions with NH3

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