UNIT CODE
: UDEC1134
UNIT DESCRIPTION: CHEMISTRY LABORATORY I EXPERIMENT NO.
:7
EXPERIMENT TITLE: Synthesis of pentaaminechlorocobalt (III) chloride, [ Co
.
Title: Synthesis of pentaaminechlorocobalt (III) chloride, [ Co
.
Objective: To synthesis of pentaaminechlorocobalt (III) chloride complex. Introduction: Complex compounds comprise of a large group of chemical compounds and is derived from chemical elements, primarily from metals and in particular transition metals. Coordination complexes exists in different forms, such as SO2-, MnO4-, [FeCN6]2-, etc. Many metallic salts exists in the hydrated form, for instance, CoCl2.6H2O and consists of the complex ion such as [Co(H2O6]2+ here six water molecules act as the ligand bonding directly to the central metal ion, Co2+, by coordinate bond through the donor atom from the ligand, that is, the oxygen atom from the H2O ligand. The ligands are arranged in an ordered manner around the central metal atom. Although, there are a number of stereochemistry’s associated with these complexes but the most common ones are those of coordination number 4 (with tetrahedral or square planar geometries) and 6 (with geometries that approximate an octahedron). The two primary types of geometric isomerism that arise in octahedral complexes are cis - trans isomerism and facial-meridional isomerism. The first type arises in complexes
such as [Co(NH3)4Cl2]+ which have two of one type of donor and four of another. The second type arises in complexes such as [Co(NH3)3Cl3] that have three of one type of donor atom and three of another. The objective of this experiment is to synthesize the chloropentaaminecobalt(III) chloride complex. The scheme of the reactions for the preparation is given below:
2CoCl2.6H2O + 2NH4Cl + 8NH3 +H2O2 2[Co (NH3)5(H2O)] Cl3 + 12H2O [Co (NH3)5(H2O)]Cl3 [Co (NH3)5Cl]Cl2 + H2O
Materials and apparatus: Aqueous concentrated ammonia solution, Ammonium chloride, Cobalt (II) chloride 6-hydrate Hydrogen peroxide solution, Concentrated hydrochloric acid, 20ml beaker, 10ml measuring cylinder, Glass rod, Vacuum suction set, Cold water, Hot water bath, Watch glass , Filter paper
Procedure: 1. In 7.5 mL aqueous concentrated ammonia solution, 1.25g ammonium chloride was added. 2. With stirring, 2.5g cobalt (II) chloride 6-hydrate was added in small portion. The next portion was not added until the added portion has dissolved. A yellow precipitate was formed. 3. To the slurry above, with stirring, hydrogen peroxide solution (2.0 mL, 30 %v/v) was added slowly. A strong reaction was occurred. 4. After the reaction has stopped, 7.5 mL concentrated hydrochloric acid solution was added slowly to the red aquapentaamine solution. 5. This product was heated on a hot water bath for 20 min. The product was filtered off by vacuum suction and washed with cold water.
Results: Mass of filter paper + product / g
0.9510
Mass of filter paper / g
0.3357
Mass of product / g
0.6153
Colour of the product
purple
Questions: 1.
Draw the structure of the complex.
2. What is the geometry structure of [ Co(NH3)5Cl ]2+ ?
3. Based on your understanding, why is it that the crystals obtained have to be
washed with cold water? The crystals that obtained have to be washed with cold water is to prevent the loss of the product by redissolving by preserving the solid crystals before vacuum filtration. This is because cold water can reduce the kinetic energy of the crystals molecule effectively instead of warm water. When kinetic energy reduced, the molecules will not collide effectively with other molecules. The crystals formed might be dissolve in the water again if hot water is used and this will affect the result and the result will not be accurate.
4. What are the impurities that may be present in the product? Cobalt oxide ( CoO ), CoCl2.6H2O and NH4Cl 5.
Assuming that the reactions go to completion, how many moles of the product are expected? CoCl2.6H2O as the limiting reagent.
2 moles of
= 2 mol of
No. of mole of
= no. of mole of
Therefor, if the reactions go to completion, 2 moles of the product are expected to be formed.
6. Calculate the percentage yield of the product?
Molar mass of CoCl2.6H2O =58.73 + 2(35.45) + 6[2(1.01) + 16.00] =237.95 g mol-1 Number of mol of CoCl2.6H2O
=
= 0.01 mole of CoCl2.6H2O (theoretical yield) Molar mass of [Co(NH3)5Cl]Cl2. =58.93 + 5[14.00 + 3(1.01)] + 3(35.45) =250.43 g mol-1 Mole of [Co(NH3)5Cl]Cl2. = = 2.47 x 10-3 mole of [Co(NH3)5Cl]Cl2. (actual yield) Percentage yield =
x 100%
= 24.7% 7. Suggest how the percentage yield can be improved in the experiment above ?
The percentage yield can be improved if the experiment is repeated twice. Furthermore, increase in the temperature of water bath. Add more HCl to give out all the CO2, rinse the compound with 2 to 3 ml of ethanol repeatedly while filtering. Set the substance into an oven at 120 degrees Fahrenheit to dry and let the moisture out for accurate weighing. Apart from that, the product need to wait until it is completely dry before it was weighed to improve the percentage yield. The cobalt (II) chloride 6-hydrate should be added slowly, after the previous added portion has dissolved in the mixture so that the reaction is complete and all the cobalt (II) chloride 6-hydrate is dissolved in the solution to improve the percentage yield. After heating the product and wait for the product to be cooled, a longer time can be spent so that more precipitate can be formed to improve the percentage yield.
Discussions:
In this experiment, synthesis of pentaaminechlorocobalt (III) chloride, [ Co
complex has studied. The mass of ammonium chloride used in this
experiment is 1.2601g while the mass of cobalt(II) chloride 6-hydrate used is 2.5060g. The synthesis of the complex started with the presence of ammonia, cobalt (II) chloride or often called as the cobalt (II) chloride 6-hydrate is readily oxidised by atmospheric oxygen to give a variety of cobalt(III) complexes. For example, the presence of ammonia triggers the oxidation of cobalt (II) chloride to hexamminecobalt(III) chloride. The ammonia and chloride ligands of these complexes use lone pairs of electrons to form bonds with the cobalt cation. The cobalt ion ends up having a charge of 3+; each chloride ion is 1-, and the ammonia ligands are neutral, so [Co(NH3)6] has a net 3+ charge and [Co(NH3)5Cl] has a net 2+ charge. These complex cations crystallize from solution as chloride salts. The reaction can be explained as follows:
2CoCl2.6H2O + 2NH4Cl + 8NH3 + H2O2 → 2[Co(NH3)5(H2O)]Cl3 + 12H2O When ammonia solution, ammonium chloride and cobalt (II) chloride was mix, CoCl 2.6H2O will undergoes oxidation to become [Co(NH3)5(H2O)]Cl3.
→
Then the hydrochloric acid was added into the solution and reduces the Co3+ in [Co(NH3)5(H2O)]Cl3 to C2+ with substitute the H2O with Cl-
[Co(NH3)5(H2O)]Cl3 → [Co(NH3)5Cl]Cl2 + H2O A coordination compound. The bonds in coordination compounds are two-electron bonds (like covalent bonds), but both electrons come from the ligand (a Lewis base with an unpaired electron pair). The metal ion serves as a Lewis acid (an electron-deficient species which can use its empty orbitals for the bond). The precautions that we had taken are use gloves when handling it, wear goggles and handle the reagents in the fume hood. These are because hydrochloric acid is a concentrated acidic reagent with a choking odor and ammonia solution is a concentrated basic reagent with a strong odor.
Conclusion: The purple coloured complex compound of pentaaminechlorocobalt (III) chloride, [Co(NH3)5Cl]Cl2 was synthesized and the percentage yield is 24.7 %.
References:
http://www.uncp.edu/home/mcclurem/courses/chm226/Isomerism_Coordination_Chemistr y.pdf
http://www.macalester.edu/~kuwata/Classes/200405/chem%20111/111l%20experiment%206%20-%20co%20synthesis.pdf
