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  • Published on: 15th October 2019
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The production of [Co(NH3)6]Cl3 and [Co(NH3)5Cl]Cl2 was used to observe the link between a metal coordination complex to ligand field strength and how that impacts the compounds color. Both complexes had similar structures, however, due to the unique crystal splitting of each, they each demonstrated important deviations from each other. The cis and trans isomers of[Co(en)2Cl2]Cl were also produced and analyzed in this experiment. They were used as a means of comparison to emphasize the chemistry behind coordination complexes and how it can be used to identify two different isomers from the same complex.

In part one, [Co(NH3)6]Cl3  was synthesized in order to observe how the interaction between a strong field ligand and a metal bring about the chemical properties of it.  CoCl2.6H2O and ammonium chloride were mixed with DI water producing an orange color. Charcoal was used to act as a catalyst and speed up the reaction and when added with H2O2, significantly darkened the color of the solution to a red-brown color. With the addition of these solutions, the balanced equation followed:

2CoCl2•6H2O(s) + 2NH4Cl(s) + 10NH3(aq) +H2O2(aq) →2[Co(NH3)6]Cl3(s) + 14H2O(l)

After the solution heated up and filtered, concentrated HCl was added, yielding an orange precipitate as the product that was then examined using a spectrophotometer. The mass of recovered product [Co(NH3)6]Cl3 was .065 grams, resulting in a 1.12 gram theoretical yield and 5.79% percent yield.  In part two, the mass of recovered product was .401 grams, resulting in a 1.132 gram theoretical yield and 35.4% error. Both percent yields were significantly lower than 100% possibly due to the inability to wash out all the precipitate out of the test tube, resulting in a smaller amount of recovered product. The solution of [Co(NH3)5Cl]Cl2 produced a violet color. The absorption spectra proved this point by showing that the maxima wavelength produced was around 550 nm, indicating the fact that it absorbed a yellow color and appeared as purple. The [Co(NH3)6]Cl3 solution appeared as a yellow-orange color, and the graph’s peak is centered around 450 nm, indicating the absorption of a blue-purple color. The colors have to do with the Δ value as well because of the equation: Δ =E=hv=hc/λ that related Δ to λ: The larger the Δ, the smaller the λ. The idea behind this experiment is that different compounds with different energies were created in order to find the wavelength that will essentially be used to calculate what the Δ value is. By calculating the Δ value, the difference in the properties of having different ligands was immediately noticeable, because certain ligands will produce larger Δ values and other ligands will produce smaller Δ values. The purpose of synthesizing these compounds was to figure out what the Δ values were relative to each compound. The two compounds synthesized in parts one and two were different compounds, with one having one chlorine as a ligand and the other having no chlorine as a ligand, producing different Δ values. The Δ value, the energy difference, is due to ligand field strength because (NH3)6 is a strong field ligand that has higher energy and results in an increase of crystal field splitting compared to that of Cl-, which is a weak field ligand based on the spectrochemical series. The reason behind the production of different colors for each compound could be traced back to the theory of crystal fields, where energy splitting of d orbitals causes variation among the light absorbed due to the different shapes in each orbital and interactions between electrons. The results make sense because the stronger field ligand absorbed a wavelength of 450 nm which is shorter than that absorbed by the weaker field ligand at 580 nm. The larger wavelength results in a lower energy level for crystal field splitting.

In part three of the experiment, the trans-isomer was sytnedized using a solution of en and H2O2. Bubbling occured when the H2O2 was added, until it was aerated. Next, HCl was added dropwise, producing the following equation:

2CoCl2x6H2O + 4H2NCH2CH2NH2 + 4HC; +1/2O2 → 2[Co(en)2Cl2]ClxH2O(HCl)+11H2O.

The mass of recovered product in this step was .654 grams, resulting in a 1.188 gram theoretical yield and 55.1% error. This percent error could be less than 100% due to the fact that these compounds are soluble in water so when they were washed out with water, it is possible that many of them dissolved.  In part four, the trans-isomer was heated and converted into the cis-isomer.

Trans-isomer + H2O → Cis-isomer + H2O

The initial mass of the trans-isomer was .621 grams, resulting in a theoretical yield of .621 grams. However, the final mass of the cis-isomer could not be collected due to water that entered the solution through the ice bath. The water redissolved the crystals, so calculation could not be completed due to no yield. The cis and trans are the same compounds but are isomers of each other, so the color difference between them could not be due to the difference in field strength of the ligands like how parts one and two were, because the same ligand exists in both compounds. Instead, it is the shapes of the compounds that cause a difference in Δ values. The cis isomer is bulkier and therefore has a higher energy than that of the trans-isomer. This relates to the absorption spectrum graph, because the trans-isomer absorbs red light at around 660 nm and appears green to the eye, where as the cis-isomer absorbs yellow light at around 570 nm and appears violet to the eye. The trans-isomer absorbed a longer wavelength (smaller energy), where as the cis-isomer absorbed a shorter wavelength (larger energy). These results match with the geometry of each isomer. The trans-isomer has one peak which indicates the presence and purity of one compound. However, the cis-isomer has two peaks, indicating impurity and presence of two compounds. The second peak indicates that not enough energy was supplied while heating the trans-isomer, and therefore, wasn't fully able to convert to the cis-isomer. However, this could also be due to a fast heating of the solution, which causes an excess of heat, and therefore, supplies too much energy that allows part of the cis to return to the trans isomer. This results in a source of error, because the “perfect” amount of energy was not achieved to force the trans-isomer to fully convert.

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