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1

Experiment 18 - Chromate-Dichromate Equilibrium

Observe changes to potassium dichromate in the presence of NaOH, HCl, and BaCl2.
Original color of potassium dichromate = orange

-NaOH changed the system to a yellow color
Caused a shit from right to left in equilibrium

-HCl converted the system back to orange (still different from the control solution)
Caused a shift back to the right, close to equilibrium

-BaCl2 and HCl wells contained yellow, paint-like precipitate.
Caused a shift back to the left, with heavily formed precipitate

2

Potassium Dichromate

K2Cr2O7

3

Lead(II) nitrate

Pb(NO3)2

4

K2Cr2O7

Potassium Dichromate

5

Lead(II) chloride

PbCl2

6

Sodium carbonate

Na2CO3

7

Sodium sulfide

Na2S

8

Nitric acid

HNO3

9

Potassium thiocyanate

KSCN

10

Iron(III)-Thiocyanate

Fe(SCN)3

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Color of chromate ion

yellow

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Color of dichromate ion

orange

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Chromate ion

CrO4^2-

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Dichromate ion

Cr2O7^2-

15

Barium chromate

BaCrO4

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Limestone (calcium carbonate)

CaCO3

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Chromate charge

2-

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Carbonate charge

2-

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Nitrate charge

1-

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Activity of a species

A measure of the effective concentration of an ion in solution

am = y[M]
am = activity of species m
y = activity coefficient
[M] = concentration of the species in moles/liters

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Flame test colors

Calcium salts = red
Barium salts = green
Potassium salts = violet
Sodium salts = yellow

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Silver(I) diammo ion

[Ag(NH3)2]+

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PbCl2 characteristics

dissolves in hot water
white

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AgCl characteristics

reacts with NH3
white

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PbCrO4 characteristics

insoluble in water
yellow

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Lab Group 1

Ag+, Pb2+
Ag+ dissolves in NH4OH and re-precipitates with HNO3; also darkens with exposure to light
Yellow = Pb2+ with acetic acid

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Lab Group 2

Pb2+ (carry over from group 1), Cu2+, Cd2+
Pb2+ precipitates PbSO4 with sulfuric acid
Yellow precipitate confirms cadmium in the presence of sodium dithionite and TA
Blue/black confirms copper in the presence of ammonia to form Cu(NH3)4

28

Lab Group 3

Al3+, Co2+, Fe3+, Ni2+
Green = Co2+
Bright red = Ni2+
Deep red = Fe3+
Pink = Al3+

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Lab Group 4

Ca2+, Ba2+
green flame = Ba2+
red flame = Ca2+

30

Lab Group 5

NH4+, Na+, K+
violet flame = K+
yellow flame = Na+
smell = NH4+

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Acetic acid

CH3COOH

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CH3COOH

Acetic acid

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Sodium dithionite

Na2S2O4

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Copper tetramine

Cu(NH3)4

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Na2S2O4

Sodium dithionite

36

Cu(NH3)4

Copper tetramine

37

[Ag(NH3)2]+

Silver(I) diammo ion

38

CaCO3

Limestone (Calcium carbonate)

39

Pb(NO3)2

Lead(II) nitrate

40

PbCl2

Lead(II) chloride

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Na2CO3

Sodium carbonate

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Na2S

Sodium sulfide

43

HNO3

Nitiric acid

44

KSCN

Potassium thiocyanate

45

Fe(SCN)3

Iron(III) thiocyanate

46

CrO4^2-

Chromate ion

47

Cr2O7^2-

Dichromate ion

48

BaCrO4

Barium chromate