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Qual II

Qual II. Separation and Identification of the Group II Hydroxides in an Unknown. Goals. To use the qual scheme provided to ____________________________ the ions Zn +2 , Al +3 , Cu +2 , Ni +2 , Co +2 , Fe +3 , and Mn +2 from each other so their presence can be verified. The standard qual.

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Qual II

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  1. Qual II Separation and Identification of the Group II Hydroxides in an Unknown

  2. Goals • To use the qual scheme provided to ____________________________ the ions Zn+2, Al+3, Cu+2, Ni+2, Co+2, Fe+3, and Mn+2 from each other so their presence can be verified

  3. The standard qual. • The standard qual analysis scheme for cations begins by separating ions into groups (cation groups) based on __________ of their _________, _________, __________, and _______________. • Each group, which consists of a small number of cations, is then analyzed further. • Usually, additional reactions are carried out that _________ ions in the group from each other, and then _______________ tests are performed to _________________________________________.

  4. Cation groups. • The initial separation into cation groups is accomplished in the following order: • Cation group I: Ag+, Hg22+, Pb2+ are ________ as their __________. • Cation group II: Cu2+, Hg2+, Pb2+, Sn4+, Sn2+, Bi3+, Sb3+, Cd2+, As3+ are __________ as ________ from their acidic solution. • All sulfides are black, except CdS (orange), As2S2 (yellow), SnS2 (yellow), Sb2S3 (orange). • !!! Abbreviated scheme: only Bi3+ and Sn4+ and no sulfur containing reagent that has a strong odor.

  5. Cation groups cont’d. • Cation group III: Co2+, Ni2+, Mn2+, Fe2+, Fe3+, Al3+, Cr3+, Zn2+. • All ions in this group, except ______ and ____ are precipitated as s__________ from a slightly basic solution. • Under these conditions ____ and ____ precipitate as ______________. • !!! Abbreviated group: Al3+, Zn2+, Co2+, Ni2+, Fe3+, Mn2+ and Cu2+ (group II) are precipitated as ____________, except zinc and aluminum that form colored amphoteric solids

  6. Groups cont’d. • Cation group IV: Ba2+, Sr2+, Ca2+ are precipitated as _________ from a slightly basic solution (all white precipitates). • Cation group V “soluble group”: Mg2+, NH4+, Na+, K+ • All these separations are based on differences in _________ and formation of complex ions.

  7. Qual approach • Sometimes the _______ of one precipitate obscures the ________ of the other. • For example, PbS is black and Sb2S3 is orange. • So if you have both present, you see ______, but no ____________. • To resolve such difficulties, the qual analysis of a sample is usually approached ___________. • First, reactions are carried out to ________ ions from each other. • Then, the tests are performed to confirm either the presence or absence of each ion.

  8. LeChatelier’s principle in qual. • The separations demonstrate the concept of chemical equilibrium and Le’Chatelier’s principle, which states that if a stress is placed on a system, then the system shifts to minimize the effect of that stress. • The idea of shifting equilibrium one way or another is frequently used in qualitative analysis experiments. • Qualitative analysis separations usually involve one of three types of equilibrium:

  9. Solubility Equilibrium: • AgCl (s) ↔ Ag+ (aq) + Cl- (aq) • The solubility of AgCl will be decreased by addition of Cl- in form of _____. • Cl- from the _____ is the same as Cl- from the _____ • The AgCl solubility is shifted to the ____when ____ is added

  10. Acid-base equilibrium • NH3(aq) + H+(aq) ↔ NH4+(aq) • Adding acid (H+) shifts equilibrium to the ______

  11. Complexation Equilibrium • Ag+ (aq) + 2 NH3 (aq) → Ag(NH3)2+ • If the amount of ammonia is decreased the equilibrium is shifted to the ______

  12. All three equilibria: • All five substances are present: AgCl, Ag+, Cl-, H+, NH3 • A single change effects all three equilibria: • If more acid is added (H+), the reaction NH3(aq) + H+(aq) ↔ NH4+(aq) is shifted to the _____, which decreases the concentration of _______. • This will decrease the amount of _____ available for reaction Ag+ (aq) + 2 NH3 (aq) → Ag(NH3)2+ • Thus, will also increase the concentration of _____ • The increase in -___ will shift the equilibrium in reaction AgCl (s) ↔ Ag+ (aq) + Cl- (aq) to the ____ forming more _______

  13. To obtain correct results • ___________________________ to insure that your sample is not contaminated with other ions • _________________________ to insure that one substance does not mask the test for another • For ppt.s, __________ are a critical step • Use proper ________________________________ • Insure that the reagents are _______________ so that the reaction occurs throughout the test tube • Do not __________________ until you are finished!!!! • Remember: you have 2 weeks for this one, if you need it

  14. S.II • Use half of unknown • Mix it up and get some of the _______, if any appears to be present • Add 3M NH3 (NH4OH) until you do not notice more solid (precipitate) forming • This step separates the ions that you are interested in from other _______ that may be present • Mn+ + n OH- M(OH)n ppt (Mn represents any metal ion)

  15. S.II.A Isolation of Zn & Al • S.II + 6d 3M NaOH • Zn(OH)2 + 2OH- [Zn(OH)4]2- • Al(OH)3 + OH- [Al(OH)4]1- • Both products are ions that allow these metals to be dissolved due to the _____________

  16. T.II.A.1-3 Testing for Al • S.II.A. supernate + 5d 6M HCl • [Zn(OH)4]2- + 4H+  Zn2+ + 4H2O • [Al(OH)4]1- + 4H+ Al3+ + 4H2O • T.II.A.1 + 1mL 6M NH3 (NH4OH) • Al3+ + 3NH4OH  Al(OH)3 + 3NH4+ gelatinous

  17. T.II.A. 4&5 Testing for Zn • T.II.A.3 supernatant + 1d 6M HNO3 + 2d 0.05M Co(NO3)2 then E • Only add Na2S2O4 if there is a blue tint present • Blue is from ____________ • 2[Cu(NH3)4]2++S2O42-+2H2O2Cu1++2SO32-+4NH4++4NH4OH • 2Cu+1+S2O42-+2H2O  2SO32- + 2Cu0 + 4H+ • [Zn(NH3)4]2++H++H2OH2ZnO2(stoich?) • H2ZnO2+Co(NO3)2 CoZnO2 +2HNO3 lime green

  18. S.II.B. Isolation of Cu & Ni • S.II.A + 1mL 6M NH3 • Cu(OH)2 + 4H2O  [Cu(H2O)4]2+ + 2OH- • Ni(OH)2 + 6H2O  [Ni(H2O)6]2+ + 2OH- • [Cu(H2O)4]2++4NH3 [Cu(NH3)4]2+ deep blue • [Ni(H2O)6]2++ 6NH3 [Ni(NH3)6]2+ blue • Both products are ions that allow these metals to be ___________ due to the formation of the _______________________

  19. T.II.B Testing for Ni &Cu • S.II.B.3 supernatant + 30d DMG • DMG is

  20. Ni2+ + 2DMG- • Another coordination complex • The H-bonds help to stabilize this complex • This complex is red and will ppt out • The resulting supernatant will be blue (or purple) if Cu is present

  21. S.II.C Isolation of Cobalt • S.II.B.3 ppt + 5d 6M HCl • Co(OH)2 + 6H2O  [Co(H2O)6]2++ 2OH- • [Co(H2O)6]2++ 2OH- Co(OH)2(s)+6H2O • Add H+to get rid of this possibility (react with OH-) • Co(OH)2 + 4Cl- CoCl42- +2OH- • Co2+ + 6NH3  [Co(NH3)6]2+

  22. T.II.C • S.II.C.3 supernatant + 10d NH4SCN • Co2+ + 4SCN- [Co(SCN)4]2- excess unstable in H2O in methanol stable in alcohol

  23. S.II.D Separation of Mn from Fe • S.II.C.3 ppt + 4d H2O + 3d H2O2 +  • Mn(OH)2+ in air  MnO2 + 2H2O • Redox: Mn2+  Mn4+ • MnO2 + H2O2  MnO(s) + H2O + O2 • Redox: Mn+4  Mn2+ • 2Fe(OH)3 + 3H2SO4  Fe2(SO4)3 + 12H2O • Fe2(SO4)3 is water soluble

  24. T.II.D Testing for Iron • S.II.D.3 supernatant + 10d KSCN • Fe3+ + 6SCN- [Fe(SCN)6]3-

  25. T.II.E Testing for Manganese • S.II.D.3 +20d 2M H2SO4 + 3d H2O2 • MnO2 + H2O2  MnO(s) + H2O + O2 • MnO + 2H+  Mn2+ + H2O • + 10d H2O + NaBiO3 + 2d 3M HNO3 • 2e-+ 6H+ + BiO3-  Bi3+ + 3H2O • Do this so we have a non-interfering species to accept e-s • 4H2O + Mn2+ MnO4-+ 8H++5e-

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