Lab Report: Rate of Isomerism in [Co(en)2Cl2]Cl

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In the course of this experiment the rate of isomerism for the coordination complex cis [Co( en) 2Cl2] Cl was figured out utilizing UV/Vis spectrometry. Using a range of wavelengths the optimum spectrometer setting for analysis was identified. The corresponding maximum and minimum absorbance of the cis and trans isomers respectively at 540 nm indicated that it was picked as the wavelength to figure out the rate of conversion between the isomers. The very first order rate constant was computed to be 0.0092 m-1 which is somewhat comparable to the literature worth of 0.00845 m-1 (Brasted and Hiriyama 1958).

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The half-life (t 1/2) was discovered to be 75.34 minutes.

Introduction

The substance [Co( en) 2Cl2] + is referred to as a coordination complex, that is, it consists of a Cobalt atom surrounded by an array of particles to which it is bound, called ligands (Brown 2010). In this case the ligands are Ethylenediamine which are 'bidentate', implying bound in two places. These ligands are bonded between the two Nitrogen atoms as seen in figure 1 listed below.

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This complex happens in 2 geometric isomers, cis- [Co( en) 2Cl2] + and trans- [Co( en) 2Cl2] +. In this experiment the Chloride salt kind of the complexes were utilized i.e. cis- [Co( en) 2Cl2] Cl.

The Cis- isomer kind of the complex which is purple in solution reacts to form the Trans- isomer which is green in solution. This experiment utilizes spectroscopy to measure the conversion of the Cis into the Trans isomer due to the reality that the concentration of the Cis isomer is proportional to the difference in absorbance of the 2 complexes in option.

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This isomerism of the Cis-form is a first order response and its rate is expressed in equation 1 below.

Equation 1: ln[D_t-D_∞] = -kt

By measuring the optical densities sometimes 0, t and infinity the rate constant k can be identified with equation 2, a reorganized and replaced kind of the formula utilized above.

Equation 2: k = ln[D₀-D_∞] / t

Materials and Methods

Solutions of 2 × 10^-3 M cis- and trans-[Co( en) 2Cl2] Cl were prepared and placed in volumetric flasks of 100 ml and 10 ml volumes, respectively. Samples of these solutions were placed in cuvettes and measured in a spectrometer over the range of wavelengths between 350 and 700 nm. The Varian Car 50 UV-Vis spectrometer recorded the absorbance of the two isomers. Using the SP-880 Metertech spectrometer, the absorbance was measured and recorded. The cis-isomer solution was warmed in a water bath at 40 degrees Celsius, and samples were taken and cooled in an ice bath. The absorbance of each sample was measured using the wavelength previously selected. This process was repeated at ten-minute intervals for an hour. Finally, a last sample of the cis[Co(en)2Cl2]Cl was analyzed with the spectrometer 24 hours after preparation to determine the value.

Results

Table 1: Absorption of the Two Isomers at Various Wavelengths Wavelength (nm)

Wavelength (nm)	Cis Absorbance	Trans Absorbance
350	0.468	0.144
360	0.289	0.090
370	0.207	0.087
380	0.180	0.091
390	0.170	0.091
400	0.466	0.397
410	0.123	0.070
420	0.092	0.057
430	0.066	0.049
440	0.049	0.047
450	0.041	0.048
460	0.043	0.048
470	0.049	0.045
480	0.061	0.039
490	0.076	0.031
500	0.093	0.025
510	0.110	0.020
520	0.124	0.017
530	0.134	0.017
540	0.137	0.020
550	0.134	0.025
560	0.129	0.034
570	0.122	0.044
580	0.116	0.055
590	0.065	0.109
600	0.102	0.071
610	0.095	0.075
620	0.085	0.074
630	0.073	0.067
640	0.060	0.055
650	0.049	0.049
660	0.035	0.038
670	0.026	0.028
680	0.018	0.020
690	0.013	0.013
700	0.009	0.008

Table 2: Absorbance of Cis Isomer at 540 nm

Time (min)	Cis Absorbance	ln[D_t-D_∞]
10	0.117	-2.51331
20	0.108	-2.63109
30	0.100	-2.74887
40	0.093	-2.8647
50	0.089	-2.93746
60	0.082	-3.07911
70	0.078	-3.17541
80	0.073	-3.28091
90	0.070	-3.3689
100	0.067	-3.45143
110	0.063	-3.52625
120	0.059	-3.59587
130	0.056	-3.6612
140	0.053	-3.72297
150	0.050	-3.78171
160	0.048	-3.83776

Calculations

Mass of Cis Isomer Required to Make 2 × 10^-3 M 100ml:

M = Mass / (Volume × Molar Mass)

Mass req. = 2 × 10^-3 × ((100/1000) × 285) = 57mg

Mass of Trans Isomer Required to Make 2 × 10^-3 M 10ml:

M = Mass / (Volume × Molar Mass)

Mass req. = 2 × 10^-3 × ((10/1000) × 285) = 5.7mg

Rate constant (k):

k = (ln[D₀-D_∞] - ln[D_t-D_∞]) / t

k = 0.0092 m^-1

Half-life (t_1/2):

t_1/2 = ln(2/k) = 75.3422 min

Error Propagation

Measured weight of isomers:

Cis: 56.7mg
Trans: 5.7mg

Resolution of Scales 'Satorius Extend': 0.1mg

Volumetric glassware:

100ml ± 0.1 ml 'A' Class
10ml ± 0.025 ml 'A' Class

Discussion

Methanol was used as a solvent in this experiment to avoid chemical interactions that might have occurred with water. Accuracy in preparing the cis isomer solution was crucial for accurate determination of the rate constant (k) and half-life (t_1/2). The wavelength of 540nm was identified as the optimal wavelength for analysis, as it showed significant differences in absorbance between the cis and trans isomers. Figure 2 confirms that the reaction is first order, as the natural logarithm of the absorbance values forms a straight line of decay. Calculations for k and t_1/2 closely match the literature values and demonstrate the reliability of the experimental method.

Conclusion

In this experiment, the rate constant (k) was determined to be 0.0092 m^-1 ± 0.00075, and the half-life (t_1/2) was calculated to be 75.3422 minutes.

Recommendations

Further experiments could focus on exploring the effects of temperature and concentration on the rate of isomerization to gain a deeper understanding of the reaction kinetics. Additionally, improved precision in mass, volume, and time measurements could reduce the margin of error in the results.