In SXR205 Activity B, the kinetics of the reaction between the square-planar platinum complex dichloro-(2,2'-bipyridine) platinum (II) (Pt(bipy)Cl2) and 1,2-diaminoethane (en) were investigated. In a different study of this reaction, the data in Table 1 were obtained manually using a spectrometer set at a fixed wavelength of 274nm, the wavelength at which the reactant platinum complex,Pi(bipy)Cl2,absorbs.
The equation for the reaction is: Pt(bipy)Cl2 + en = [Pt(bipy)(en)]+ + 2Cl- (3)
The concentration of Pt(bipy)Cl2 and 1,2-diaminoethane in the reaction mixture in methanol at the beginning of the experiment (after mixing) were 4.0 x 10-5 and 4.0 x 10-2 mol dm-3, respectively.
Table 1 Absorbance values versus time.
1(a) (i) Suggest a reason why the absorbance of the solution does not fall to zero, but becomes constant. You can assume that the reaction has gone to completion and that all of the Pt(bipy)Cl2 has been used up once the absorbance value has become constant.
(ii)Explain why short time intervals between absorbance readings were used for the early part of the kinetic run.
(iii)Beer's law states that, for dilute solutions, the absorbance of a fixed path length (i.e. 'thickness') of a solution of the absorbing species is directly proportional to the concentration of the absorbing species in the solution. Use the data in Table 1 to calculate the absorbance at each time caused by Pt(bipy)Cl2
(iv) Plot your new absorbance data against time. Make sure you give your graph a title and label the axes.
(v)From your graph, obtain three successive 'half-lives', and from these confirm whether or not the reaction is first-order with respect to Pt(bipy)Cl2.
(vi)Explain how you would determine the pseudo rate constant, Kr' ,for the reaction from a suitable graph using the data in Table 1 and part (a) and (iii).
1 (b) In three separate experiments similar to above experiment, the following results (Table 2) were obtained. Note that the concentrations in Table 2 are those in the reaction mixture at the beginning of the reaction. Explain how these results show that the reaction is first order with respect to 1,2-diaminoethane.
Table 2 pseudo rate constant, Kr', for the reaction.
[Pt(bipy)Cl2]0/ mol dm-3 [en]0/mol dm-3 Kr' /s
4.0 x 10-5 0.080 4.76 x 10-3
4.0 x 10-5 0.100 5.95 x 10-3
4.0 x 10-5 0.140 8.33 x 10-3
In this multi-part solution, explanations were provided for how to use absorbance and time data to determine the rate constant and reaction orders for the reaction of Pt(bipy)Cl2 with en (ethylenediamine). After accounting for absorbance of the product, the data was graphed and half lives analyzed to determine that the reaction was first order in Pt(bipy)Cl2. A graph of time versus ln(absorbance) for the first few points resulted in a straight line. Other rate data was used to determine that the reaction was also first order in ethylenediamine.