Rate of Isomerism of [Co(en)2Cl2] Essay

In the course of this experiment the rate of isomerism for the coordination involved cisCo(en)2Cl2Cl was determined using 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 meant that it was selected as the wavelength to determine the rate of reincarnation between the isomers. The first order rate never-ending was calculated to be 0.0092 m-1 which is somewhat similar to the literature value of 0.00845 m-1 (Brasted and Hiriyama 1958). The half-life (t 1/2) was found to be 75.34 minutes.IntroductionThe compound Co(en)2Cl2+ is known as a coordination complex, that is, it consists of a Cobalt atom surrounded by an array of molecules to which it is bound, called ligands (Brown 2010). In this case the ligands ar Ethylenediamine which are bidentate, meaning bound in two locations. These ligands are bonded between the two Nitrog en atoms as seen in figure 1 below. This complex occurs in two geometric isomers, cis-Co(en)2Cl2 + and trans-Co(en)2Cl2 +. In this experiment the Chloride salt cook of the complexes were used i.e. cis-Co(en)2Cl2Cl. Figure 1The Trans and Cis isomers of Co(en)2Cl2+Source Shapter ,J. 2014 Experiment 1 Rate of Isomerism cisCo(en)2Cl2Cl, Flinders University, Australia The Cis- isomer form of the complex which is purple in resolve reacts to form the Trans- isomer which is green in solution. This experiment uses spectroscopy to quantify the conversion of the Cis into the Trans isomer due to the fact that the concentration of the Cis isomer is proportional to the loss in absorbance of the two complexes in solution. This isomerism of the Cis-form is a first order reaction and its rate is expressed in equation 1 below. Equation 1By measuring the optical densities at time 0, t and infinity the rate constant k commode be determined with equation 2, a rearranged and substituted form of the f ormula used above. Equation 2a)Which rearranges to giveb)ExperimentalSolutions of 2 10-3 M Cis- and Trans-Co(en)2Cl2Cl were prepared and place in volumetric flasks of 100 ml and 10ml volumes respectively. Samples of these solutions were placed in cuvettes and calculated 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 and bring ind a marking out which was used to identify the correct wavelength for further spectroscopy (see Appendix 1). Using the SP 880 Metertech spectrometer, the absorbance was heedful and recorded as seen in parry 1. Next the Cis isomer solution was warmed in a water bath at 40 degrees Celsius and a sample was taken and cooled in an ice bath. The absorbance of the sample was then measured using the wavelength previously selected. This was repeated at ten minute intervals for an hour. Finally a last sample of the cisCo(en)2Cl2Cl was analysed with the spec trometer 24 hours after to determine the value.ResultsTable 1 Absorption of the Two Isomers at Various Wavelengths Wavelength (nm)CisTrans3500.4680.1443600.2890.0903700.2070.0873800.1800.0913900.1700.0914000.4660.3974100.1230.0704200.0920.0574300.0660.0494400.0490.0474500.0410.0484600.0430.0484700.0490.0454800.0610.0394900.0760.031Wavelength (nm)CisTrans5000.0930.0255100.1100.0205200.1240.0175300.1340.0175400.1370.0205500.1340.0255600.1290.0345700.1220.0445800.1160.0555900.0650.1096000.1020.0716100.0950.0756200.0850.0746300.0730.0676400.0600.0556500.0490.0496600.0350.0386700.0260.0286800.0180.0206900.0130.0137000.0090.008Figure 1 Absorbance against Wavelength of Cis and Trans IsomersTable 2 Absorbance of Cis Isomer at 540 nmTime (min)Cis AbslnDt-Dy = -0.01104 X 2.40936100.117-2.51331-2.519758607200.108-2.63109-2.630158414300.100-2.74887-2.74055822400.093-2.8647-2.850958026500.089-2.93746-2.961357833600.082-3.07911-3.071757639t 0.036Figure 2 Natural Logarithm of the Cis Isomer Absor bance against TimeTable 3 Slope, Y intercept and Regression Error Generated by Linest Function -0.01104-2.409360.000363640.0141620.9956790.015212921.71126840.213292050.000926Calculations dope of Cis Isomer Required to Make 2 x 10-3 M 100mlM = Mass/(Volume Molar Mass)Mass req. = 2 10-3 ((100/1000) 285)= 57mgMass of Trans Isomer Required to Make 2 x 10-3 M 10mlM = Mass/(Volume Molar Mass)Mass req. = 2 10-3 ((10/1000) 285)= 5.7mgkk = (lnD0-D -lnDt-D) / tk =0.0092m-1lnDt-DlnD0-D-3.0718-2.5198Half lifet 1/2 =ln2/k=75.3422 minError PropagationMeasured weight of isomersCis 56.7mg Trans 5.7mgResolution of Scales Satorius insure 0.1mgVolumetric glassware100ml 0.1 ml A Class10ml 0.025 ml A ClassDiscussionMethanol was used as a solvent in this experiment to avoid the chemical interactions that may have occurred had water been used in its place (Mahaffy 2011). In the initial solution of the Cis isomer accuracy wasnt as important as it was being measured to find the optimal wavelength for analysis as seen in Figure 1 and Appendix 1. Preparation of the Cis Isomer solution measured to produce Figure 2 required a mass of the compound as close as possible to the calculated amounts (See Results Calculations) in order to produce results that allowed for the accurate determination of the rate constant (k) and half-life (t1/2). The relationship between k and t1/2 is that k is a factor in the formula of t1/2 (See Results Calculations) and determines what unit of time that the half-life is expressed in (in this case minutes-1). Figure 1 plots the absorbance of the two isomers against the various wavelengths from 350nm 700nm. From this chart the wavelength of 540nm was identified as the ideal to be used to determine the reaction rate of the isomers. This was due to the local maxima of the Cis isomer at thispoint which coincides with the local minima of the Trans isomer. This data is confirmed by the printout of the Varian Car 50 UV-Vis spectrometer which also marked the 540nm point on its plot (see Appendix 1).This clear difference in absorption of the two isomers at this wavelength meant that the rate of inter-conversion could be determined. The graph of the natural logarithm of the absorbance of the Cis isomer against time can be seen in figure 2. By observing the points on the plot a straight line of descent confirms that this reaction is first order, a logarithm of the measured absorbance values (Christian 2010). Calculations were carried out on the data in table 2 to determine the k and t1/2 (see Results Calculations). The k value calculated of 0.0092 m-1 roughly matches the figure generated by the linest function 0.01104m-1 (see Table 3, top left) and is also similar to the literature value of 0.00845m-1. With the calculated k value of 0.0092 the t1/2 was determined to be 75.3422 minutes. Slight variations between the k value calculated from the points in figure 2 and that generated by the linest function can be accounted for by the fact t hat the linest function is a calculated line of best fit and so does not fit the data points collected scarce but is an approximation. The slight difference between the literature value of k 0.00845m-1 and the calculated k of 0.0092m-1 can be accounted for by human error in time, volume and mass measurement. The range of error in this measurement is 0.00075 which is the difference between the calculated and literature values.ConclusionIn this experiment the rate constant (k) was determined to be 0.0092m-1 0.00075 and the t1/2 was calculated to be 75.3422 minutes.ReferencesBrasted, R. Hirayama C. 1958 The cis-trans Isomeration of Dichlorbis-(ethylenediamine)-cobalt(III) Chloride and Dichlorbis-(propylenediamine)-cobalt(III) Chloride in Alcohols, Journal of Chemistry, Department of Chemistry, University of Minnesota. Vol. 80 pp 788 794. Brown, T. LeMay H. Bursten, B. Murphy, C. Langford, S. & Sagatys D. 2010. Chemistry The Central Science, Pearson, Australia. pp 1196 1197. Christia n, G. 2010. Analytical Chemistry 6th Ed., privy Wiley & Sons, USA. pp 159, 270 271. Mahaffy, P. Bucat, B . Tasker, R. Kotz, J. Treichel, P. Weaver, G. & McMurry, J. 2011. Chemistry Human Activity, Chemical Reactivity, Nelson Education, Canada. pp 99, 115 116.AppendicesAppendix 1 Figure 3 Absorbance of Cis and Trans Isomers at Various Wavelengths (Varian Car 50 UV-Vis spectrometer)