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Hexaamminecobalt (III) Chloride

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Title: Preparation of hexaaminecobalt (III) chloride Introduction:

Hexaaminecobalt (III) chloride is a chemical compound with a formula of [Co(NH3)6]Cl3. Hexaaminecobalt (III) chloride is a complex ion as it has a metal cation

which is the cobalt bonded to one or more small molecules or ions which are the ammines. It has an octahedral shape. Hexaaminecobalt (III) chloride is an orange crystal. The colour is exhibited by the [Co(NH3)6]3+ ion. This is because cobalt is a transition metal as it as

incompletely filled d orbitals. When a ligand approaches the d orbitals of the cobalt (III) ion, it splits into two different energy levels. By the absorption of the visible light frequency electron is promoted from lower to higher energy level. When electron returns to the ground state it exhibits the complementary colour of the colour it absorbed.

Hexaaminecobalt (III) chloride is prepared by the recrystallization process.

Recrystallization is a process of purification of solid. Since cobalt is a transition metal, it has various oxidation states. The oxidation state of cobalt in hydrated cobalt (II) chloride is +2. Therefore, hydrogen peroxide, an oxidising agent is added to oxidise the cobalt (II) into cobalt (III) as the oxidation state in hexaaminecobalt (III) chloride is +3. Decolourising charcoal was added as catalyst. When cooling the solution, in ice, the conical flask was scratched with a glass rod to increase the surface area to provide a roughened surface for crystals to be formed.

Objectives:

a) To determine the colour of hexaaminecobalt (III) chloride

b) To determine the percentage yield of hexaaminecobalt (III) chloride

Apparatus: measuring cylinder, 100cm3 conical flask,250cm3 conical flask, spatula, filter papers, Bunsen burner, tripod stand, wire gauze, beakers, thermometer, filter funnel, filter paper, electronic balance, glass rod

Materials: ammonium chloride, hydrated cobalt (II) chloride, concentrated ammonia solution, decolourising charcoal, ice, distilled water, hydrogen peroxide, concentrated HCl, ice

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Procedure:

Preparation of crude solid

1) Approximately 8 g of ammonium chloride and 12 g of hydrated cobalt (II) chloride was added to a 100ml conical flask containing 20 ml of distilled water.

2)2 spatula measures of decolourising charcoal was added and the contents of the conical flask were heated gently bringing the solution to just boiling.

3) The flask was cooled under water from the cold tap water.

4) Working in a fume cupboard, 25 ml of concentrated ammonia solution was added to the flask and then cooled to below 10°C using a beaker of ice and water.

5) 5ml of 20 volume hydrogen peroxide was added to the cooled flask. 6) The contents of the flask was shaken thoroughly for a few minutes

7) This was repeated for further 4 minutes until a total volume of 25ml of hydrogen peroxide had been added

8) When all the hydrogen peroxide had been added, the contents of the flask was transferred to a 250 ml conical flask.

9) A water bath was prepared using a 400ml beaker provided and water in the bath was heated to approximately 60°C. The flask was placed into the water bath and the temperature was maintained for approximately 30 minutes.

10) Then the flask was cooled in ice water to precipitate the crude solid. Purification of the product

1) The crude solid and charcoal was filtered off and then the filter paper containing the solid was placed into a beaker containing 100ml of boiling water which had been acidified with 3ml of concentrated HCl.

2) The solution was then filtered to remove the charcoal and the filtrate was collected in a 100 ml conical flask. The filtrate contains the dissolved cobalt salt.

3)10 ml of concentrated HCl was added to the filtrate and the flask was cooled in ice water. At this stage, crystals of hexaaminecobalt (III) chloride had formed.

4) The crystal was then filtered out by rinsing them with small volume of distilled water. 5) The crystals were then dried by opening out the filter paper and excess water was removed by gently pressing the crystals with more filter paper.

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7) The sample bottle was weighed. The crystals were put and reweighed to obtain the mass of hexaaminecobalt (III) chloride crystal formed.

Results:

Reaction Changes in the colour of the solution NH4Cl + CoCl2·6H2O + H2O Purplish red

NH4Cl + CoCl2·6H2O + H2O + charcoal + heat Purplish black

After the addition of ammonia Yellowish black After the addition of hydrogen peroxide Purplish black After heating to approximately 60°C for 30 minutes Reddish brown After the addition of 100cm3 boiling water and

3cm3 concentrated HCl

Black

After filtration Orange solution

After the addition of 10cm3 of concentrated HCl and cooling in ice water

Orange solution with orange precipitate After filtration and drying of crystal Orange crystal

Table 8.1: Colour changes in solution

Mass of petri dish 28.72 g

Mass of petri dish and[Co(NH3)6]Cl3 31.36 g

Mass of [Co(NH3)6]Cl3 2.64 g

Table 8.2: Mass of [Co(NH3)6]Cl3

Discussion:

The initial colour of cobalt (II) in hydrated cobalt (II) chloride was purplish red and the final colour of cobalt (III) in hexaaminecobalt (III) chloride is orange. This clearly shows that cobalt is a transition metal with an incompletely filled d orbital as it forms coloured compound and has different oxidation states.

The percentage yield obtained is less than 100% due to loss of solute when the

solution was transferred from the 100cm3 conical flask to 250 cm3 conical flask. Besides that, hexaaminecobalt (III) chloride might not have fully crystallised when the crystals were filtered out and some of the crystals might have been loss due to evaporation.

Conclusion:

The colour of hexaaminecobalt (III) chloride is orange. The percentage yield obtained from this experiment is 19.75%.

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Questions: 1)

2) The crystal obtained had to be washed in cold water to prevent the risk of dissolving the crystal which may lead to an inaccurate percentage yield.

3) CoCl2·6H2O + ½H2O2 + NH4Cl + 5 NH3 [Co(NH3)6]Cl3 + 7 H2O No. of moles of NH4Cl = ( ) = 0.15 moles No. of moles of CoCl2·6H2O

=

( ) ( ) ( ) = 0.05 moles

Therefore, number of moles of [Co(NH3)6]Cl3 is 0.05 moles.

Theoretical mass of [Co(NH3)6]Cl3

No. of moles x molar mass = 0.05 moles x [58.9+6(14)+18(1)+3(35.5)] = 13.37 g Percentage yield x 100 % = x 100% = 19.75%

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Reference:

1. Bjerrum, J. and McReynolds, J. (1946) Inorganic Synthesis. p.216-222.

2. G. Pass, H. Sutcliffe (1974), Practical Inorganic Chemistry, Chapman and Hall, Ltd: 3. Helmenstine, A. (n.d.) How to Perform a Recrystallization. [online] Available at:

http://chemistry.about.com/od/chemistryhowtoguide/ss/recrystallize.htm [Accessed: 8 Jun 2012].

4. Herrmann, P. (2010) Recrystallisation. [online] Available at:

http://fog.ccsf.cc.ca.us/pherrman/documents/Recryst208.pdf [Accessed: 8 Jun 2012]. 5. Hill, G. and Hollman, J. (2012) Chemistry in Context. United Kingdom: Thomas Nelson

& Sons Ltd, p.27.

6. J. Tanaka, S.L. Suib (1999), Experimental Methods in Inorganic Chemistry, Prentice Hall: Upper Saddle River, NJ. 272-74

7. James E, H. (1983) Inorganic Chemistry. 3rd ed. p.360. 8. Macalester (n.d.) Recrystallisation. [online] Available at:

http://www.macalester.edu/~kuwata/Classes/2004-05/chem%20111/111l%20experiment%206%20-%20co%20synthesis.pdf [Accessed: 8 Jun 2012].

New York. 80-81.

9. Wired Chemist (2012) Recrystallization. [online] Available at:

http://www.wiredchemist.com/chemistry/instructional/laboratory_tutorials/recrystallizatio n [Accessed: 8 Jun 2012].

References

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