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. 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 _________________________________________.

Cation groups. The initial separation into cation groups is

accomplished in the following order: Cation group I: Ag+, Hg2

2+, 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.

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

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.

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.

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:

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

Acid-base equilibrium NH3(aq) + H+(aq) ↔ NH4

+(aq) Adding acid (H+) shifts equilibrium

to the ______

Complexation Equilibrium Ag+ (aq) + 2 NH3 (aq) → Ag(NH3)2

+

If the amount of ammonia is decreased the equilibrium is shifted to the ______

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 _______

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

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)

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 _____________

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

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++S2O4

2-+2H2O2Cu1++2SO32-+4NH4

+

+4NH4OH 2Cu+1+S2O4

2-+2H2O 2SO32- + 2Cu0 + 4H+

[Zn(NH3)4]2++H++H2OH2ZnO2(stoich?) H2ZnO2+Co(NO3)2 CoZnO2 +2HNO3

lime green

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 _______________________

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

N

N

C C

CH3

CH3

OH

OH

Ni2+ + 2DMG-

NCH3

O-

NCH3

OH

N CH3

OH

N CH3

O-

Ni2+

Another coordination complex The H-bonds help to stabilize this complex This complex is red and will ppt outThe resulting supernatant will be blue (or purple) if Cu is present

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+

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

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

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

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-