Cobalt Lab: Synthetic schemes v2004
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Chemistry 107 Lab
Complex Ions of Cobalt
http://academics.smcvt.edu/chemistry/CHEM_107/107_lab_stuff/Cobalt/cobalt_lab.htm
CH 107 Cobalt lab
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INTRODUCTION -- A large number of cobalt complexes, have been prepared, and you will
make three of them. Typical preparative sequences, like those in this assignment, involve oxidation
of cobaltous [Co2+ or Co(II)] ion in the presence of an appropriate ligand. A ligand can be displaced
by another ligand. In Parts 1 and 2, hexaamminecobalt(III) chloride and
pentaamminechlorocobalt(III) chloride are prepared. In Part 3, preparative procedures are given for
trans-dichloro-bis-(ethylenediamine)cobalt(III) chloride.
Plan to do the first part of Part I and the first part of Part II simultaneously on DAY 1.
Before you come to lab: READ section 19.1 in your text. That is the prelab.
PROCEDURE
Part 1: Preparation of hexaamminecobalt(III) chloride
BEFORE YOU COME TO LAB:
 READ SECTION 19.1 in your text.
 Look up the definition of “LIGAND”.
 Look up the definition of “OXIDATION”.
 Co (II) gets oxidized to Co(III). In the process, O2 gets reduced to what?
 Define “REDUCTION”.
 List the MW of CoCl2 • 6H2O and ammonium chloride.
 Concentrated ammonia is 14.8 mole/L. Calculate the number of moles of each reagent
used in reaction I (except O2!). Don’t worry about ancillary reagents (water, HCl) or catalysts
( C ).
 Calculate how many moles of O2 you’ll need to bubble through to react with all your
CoCl2 • 6H2O. How many liters is this at STP?
 Determine the MW of [Co(NH3)6]Cl3.
 Write the balanced equation for the reaction between ammonia and HCl.
(Hint: Cl- is a spectator ion).
 Draw the structure of [Co(NH3)6]Cl3 (see p 694, 695 in text. Clearly draw the octahedral
geometry)
 Prepare a Table of Reagents as shown in:
http://academics.smcvt.edu/chemistry/CHEM_107/107_lab_stuff/Cobalt/SAMPLE%20FOR
MAT%20FOR%20PRELAB%20COBALT%20LAB.doc
The reaction:
I. 4CoCl2 + 4NH4Cl + 20NH3 + O2  4[Co(NH3)6]Cl3 + 2 H2O
In a 150 mm test tube, mix 2.0 g of cobaltous chloride hexahydrate, CoCl2 • 6H2O, and 1.40 g of
ammonium chloride with 1 mL of water until most of the salts are dissolved. Add 6mL of
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concentrated ammonia and half a spatula of activated charcoal ( C, fresh from the package). Bubble
air through the solution for at least 90 minutes using an aspirator. During the bubbling, the color of
the solution will change from reddish to yellowish brown, as the result of the oxidation of Co(II) to
Co(III) by oxygen. The carbon acts as a catalyst. Filter the mixture and wash with a few mLs of hot
water to remove carbon. Add 2mL of concentrated HCl to the filtrate. The HCl reduces the
solubility of the chloride salt by the common ion effect (Good old LeChatelier’s Principle!).
Transfer the filtrate to a beaker and leave it in your drawer, uncovered, until the next laboratory
period- crystals should form.
You can stop here
Cool your solution in ice water and add more HCl if necessary. Filter the yellow crystals. Wash with
3 mL of cold ethyl alcohol. Ethyl alcohol is volatile, so the crystals will dry quickly. Place the filter
paper with your product in a 250 mL beaker and leave it in your drawer to dry until the next
laboratory period. Then weigh the dried product, and calculate the theoretical yield and
percentage yield that you obtained. In the recent past, yields in excess of 65% have been
obtained. Label and save the product for analysis by the methods described in the
Handout: Analysis
http://academics.smcvt.edu/chemistry/CHEM_107/107_lab_stuff/Cobalt/Analysis%20of%20Co%20Complexes%20%20by%20A
A,%20titration%20grav%20chloride%20&%20UV-Vis.doc
You can stop here
Part 2: Preparation of pentaamminechlorocobalt(III) chloride
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BEFORE YOU COME TO LAB:
READ SECTION 19.1 in your text.
List the MW of Co(NO3)2 • 6H2O and ammonium carbonate.
Concentrated ammonia is 14.8 mole/L. Calculate the number of moles of each reagent
used in reaction II (except O2!). Don’t worry about ancillary reagents (water, HCl) or catalysts (
C ).
Calculate how many moles of O2 you’ll need to bubble through to react with all your
Co(NO3)2 • 6H2O. How many liters is this at STP?
Notice that the road to [Co(NH3)5Cl]Cl2, the ultimate product, takes several steps. By
adding up equations II through VI, write the net equation for the conversion of
cobalt(II) nitrate to [Co(NH3)5Cl]Cl2.
Draw the structure of and List the MW of [Co(NH3)5Cl]Cl2. (See page 695 in your text)
Prepare a Table of Reagents as shown in:
http://academics.smcvt.edu/chemistry/CHEM_107/107_lab_stuff/Cobalt/SAMPLE%20FOR
MAT%20FOR%20PRELAB%20COBALT%20LAB.doc
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Place 4.0 g of ammonium carbonate in a 50 mL or 100 mL graduate cylinder and add water to fill to
the 25 mL mark and stir to dissolve the solid. Then add concentrated ammonia to bring the total
volume up to the 35 mL mark and mix. In a 200 mm test tube, dissolve 2.0 g of cobaltous nitrate
hexahydrate, Co(NO3)2 • 6H2O, in 5 mL of water. Now add the first solution to the second solution
and mix. Bubble air through the combined solution for three hours:
II. Co(NO3)2 + (NH4)2CO3 + 3 NH3 + 1/4 O2  [Co(NH3)4CO3](NO3).½ H2O + NH4NO3
(muddy blue-ish purple)
(burgundy red/purple)
The color of the solution will change from muddy blue-ish purple (cobaltous) to burgundy
red/purple (cobaltic). The reaction solution may be LABELLED & stoppered until the next
laboratory period.
You can stop here
Get your LABELLED & STOPPERED SOLUTION OUT. Pour the solution into an evaporating
dish. Evaporate the solution to 10 mL by heating the dish on a beaker of boiling water. Periodically
add a total of 1.2 g of powdered ammonium carbonate in small portions. Let the solution cool to
cause crystallization.
Cool the dish well (15 to 30 minutes) in ice water, and then filter the purple red crystals of
tetraamminecarbonatocobalt(III) nitrate, [Co(NH3)4CO3]NO3 • ½H2O. In the following procedure,
use all of the product you have just obtained. Dissolve the tetraamminecarbonatocobalt(III) nitrate
in 8-10 mL of water in a large test tube. Add concentrated hydrochloric acid until all the carbon
dioxide is expelled (bubbles stop).
III. [Co (NH3)4 CO3 ](NO3) + 3H+ + 2H2O  H2CO3 + [Co(NH3)4(H2O)2]3+ + HNO3
IV.
H2CO3  H2O + CO2 ( g )
About 1 mL should be enough for the complete reaction
Neutralize the solution with concentrated ammonia (check with red litmus) and add 1 mL excess.
Heat the test tube containing the solution in a beaker of boiling water for 20 minutes. The reaction
is:
V. [Co(NH3)4(H2O)2]3+ + NH3  [Co(NH3)5H2O]3+ + H2O
Cool the mixture, then add 10 mL of concentrated hydrochloric acid. Heat the test tube in the
water bath for another half hour. The following reaction occurs:
VI. [Co(NH3)5H2O]3+ + 3Cl- + H+  [Co(NH3)5Cl]Cl2 + H3O+
NET RX: (Adding up II – VI)
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VII. Co(NO3)2 + (NH4)2CO3 + 4NH3 + 1/4O2 + 3HCl  [Co(NH3)5Cl]Cl2 + NH4NO3 +
1.5H2O + CO2 + HNO3
Cool the test tube in ice water. Filter the violet crystals, wash them with a few drops of ice water,
and then with alcohol. Dry as in Part 1, and weigh the dry product, which is
pentaamminechlorocobalt(III) chloride (the modern name), [Co(NH3)5Cl]Cl2.
Calculate the theoretical yield and the percentage yield. Label and save the product for
analysis by the methods described in the Handout: Analysis
You can stop here
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Part 3: trans-Dichloro-bis-(ethylenediamine)cobalt(III) chloride
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BEFORE YOU COME TO LAB:
READ SECTION 19.1 in your text.
List the MW of CoCl2 • 6H2O Calculate the number of moles you will use in this
experiment.
Determine the MW of ethylene diamine (H2N-CH2-CH2-NH2). It is 10% mass/mass
ethylene diamine in water. The density is 1.00 g/mL. Calculate the MOLARITY of the
10% ethylene diamine. Calculate how many Moles of ethylene diamine you will use in
this experiment.
Concentrated HCl is 12 M. Calculate how many moles of HCl you will use in today’s
experiment.
Draw the structure of and List the MW of [Co(en)2Cl2]Cl• HCl (See p 698 in your text)
Prepare a Table of Reagents as shown in:
http://academics.smcvt.edu/chemistry/CHEM_107/107_lab_stuff/Cobalt/SAMPLE%20FOR
MAT%20FOR%20PRELAB%20COBALT%20LAB.doc
Dissolve 1.5 g of cobaltous chloride hexahydrate (CoCl2 • 6H2O) in 10 mL of distilled water in a
200 mm test tube. To the solution, add 5 mL of a 10% ethylenediamine solution and mix well. Try
not to get the ethylenediamine solution on your skin or clothing because it is toxic. If you do get
some on you, wash the area well with water immediately. Fit the test tube with glass tubing and a
stopper for bubbling air through the solution. Clamp the assembly in a beaker of water on a ring
stand so that it can be heated during the aeration. Bring the water to boiling, and draw air through
the test tube with an aspirator for one hour. The oxidation proceeds faster at the higher
temperature, and since ethylenediamine is not very volatile, the ethylenediamine will not be lost
during aeration. Ammonia, as in Parts 1 and 2, would be lost readily. Because evaporation of the
reaction solution will occur during aeration, add distilled water from time to time to keep the volume
of solution between 3 and 5 mL. After one hour of aeration, remove the test tube from the water
bath, and immediately add 5mL of concentrated hydrochloric acid. The dark red solution should
turn dark green. Cool the test tube in ice water. If the dark red solution does not turn green as the
hydrochloric acid is added, concentrate the solution in an evaporating dish on a beaker of boiling
water (as in Part 2), and add 1 mL of concentrated hydrochloric acid before cooling. The dark green
crystals are the hydrogen chloride salt of trans-dichloro-bis-(ethylenediamine)cobalt(III) chloride.
This can be placed in your drawer uncovered until next week for crystal formation.
H2O
VIII. 2 CoCl2(aq) + 4 H2N-CH2-CH2-NH2 + 4 HCl + ½ O2  2 [Co(en)2Cl2]Cl • HCl +
IX. [Co(en)2Cl2]Cl • HCl + heat  [Co(en)2Cl2]Cl
+ HCl(g)
Filter the crystals. If the yield is small (less than half a gram), it may be advisable to concentrate the
solution from which the crystals were obtained. Another crop will probably result. If the mixture is
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left in your drawer for several days, beautiful large diamond-shaped, emerald-green crystals may be
obtained. Place the green plates of trans-[Co(en)2Cl2]Cl • HCl in a DRY 150 mm test tube with 5
mL of anhydrous methyl alcohol, crush the crystals, and stir to form a slurry. Heat the test tube for
15 minutes in boiling water. No more hydrogen chloride should be given off. This can be tested by
holding a piece of moistened blue litmus paper over the mouth of the test tube. Weigh the dried
product, and calculate the maximum yield theoretically possible from 1.5 g of cobaltous chloride and
calculated number of moles of ethylenediamine (remember there are two ethylenediamine ligands in
each molecule of [Co(en)2Cl2]Cl. Calculate the theoretical yield and the percentage yield.
Label and save the product for analysis by the methods described in the Handout: Analysis
You can stop here
NOW THAT YOU HAVE MADE THREE COBALT COMPLEXES, ANALYZE THEM
ACCORDING TO THE PROTOCOLS IN THE ANALYSIS HANDOUT.
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-Adapted by Dr. K. Olgiati from the original handout by Dr. J. van Houten.
We gratefully acknowlege the assistance of Melissa Gaudette, Paul Frail, Nicholas James and Dr.
Sarah Locknar. Some material was adapted from an article in the Journal of Chemical Education by
R.D. Foust and P.C. Ford [JCE 47:165 (1970). See also Coord. Chem. Rev. 12: 151 (1974)].
DO NOT REFERENCE THE JCE, COORD CHEM REV or ANY OTHE SOURCE IN YOUR
PAPER UNLESS YOU YOURSELF HAVE OBTAINED THEM THROUGH INTERLIBRARY
LOAN, READ THEM AND USED THEM. CITING SOURCES OTHER THAN ONES YOU
YOURSELF HAVE ACTUALLY USED WILL RESULT IN 500 LAB POINTS DEDUCTED.
ARE WE CLEAR?
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