Qual II

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 Zn2, Al3,
  Cu2, Ni2, Co2, Fe3, and Mn2 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 contd.

• 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 contd.

• 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
LeChateliers principle in qual.

• The separations demonstrate the concept of
  chemical equilibrium and LeChateliers
  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)42-
• Al(OH)3 OH- ? Al(OH)41-
• 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)42- 4H ? Zn2 4H2O
• Al(OH)41- 4H ? Al3 4H2O
• T.II.A.1 1mL 6M NH3 (NH4OH)
• Al3 3NH4OH ? Al(OH)3 3NH4
• gelatinous

17
T.II.A. 45 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 ____________
• 2Cu(NH3)42S2O42-2H2O?2Cu12SO32-4NH44NH4O
  H
• 2Cu1S2O42-2H2O ? 2SO32- 2Cu0 4H
• Zn(NH3)42HH2O?H2ZnO2(stoich?)
• H2ZnO2Co(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)42 2OH-
• Ni(OH)2 6H2O ? Ni(H2O)62 2OH-
• Cu(H2O)424NH3 ? Cu(NH3)42 deep blue
• Ni(H2O)62 6NH3 ? Ni(NH3)62 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)62 2OH-
• Co(H2O)62 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)62

22
T.II.C

• S.II.C.3 supernatant 10d NH4SCN
• Co2 4SCN- ? Co(SCN)42-
• 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 Mn4 ? 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)63-

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- 8H5e-