Investigating Mechanisms Of An Electron Transfer Reaction

Introduction This experiment is aimed at investigating the mechanism of an electron transfer mechanism. In electron transfer reactions, an electron is passed from one metal centre to another metal centre in a neighbouring complex by an inner-sphere or outer-sphere mechanism. This causes a change in oxidation state of the two metals, with one adopting a M+1 charge and the other adopting a N-1 charge (where M and N are the original metal oxidation states before reaction). An inner-sphere mechanism requires breaking/making of metal-ligand bonds, whereas an outer-sphere does not.

Aims & Objectives The aim of this experiment is to obtain a true second rate constant and hence determine which electron transfer mechanism is responsible for the reactions being observed. These reactions are between varying concentrations of [Fe(H2O)4Cl2] and [Co(NH3)5N3]Cl2.

The Reaction

[Co(NH3)5N3]2+ + [Fe(H2O)6]2+ + HCl ? [Co(H2O)6]2+ + [Fe(H2O)6]3+ + 5NH4+ + N3-

Experimental Procedure The following solutions were prepared using the appropriate calculations as set out below:

0.1799g of [Co(NH3)5N3]Cl2 was dissolved in 100cm3 of distilled water. This was labelled solution A. A deep purple solution was observed.

A 0.28M [Fe(H2O)4Cl2] solution was provided (labelled solution B) - a light green solution. Further dilutions of the solution produced 100cm3 volume of the following various concentrations:

Solution Final Concentration (M) Base Stock Used Volume of Base Stock (ml) Volume of HCl (ml) C 0.21 B 75 25 D 0.14 B 50 50 E 0.07 D 50 50

The solutions were allowed to reach room temperature. The spectrophotometer was set to 525nm. 10cm3 of each of solutions B,C,D and E were pipetted into 4 beakers. 10 cm3 of Solution A was added to each solution in turn, the time noted (this is the zero absorbance time) and stirred for a minute. The reaction mixtures were transferred into 4 cuvettes and the absorbance readings measured. This was repeated at 15, 20, 30 and 60 minutes. The reading at infinity was done by placing samples of the four solutions in a water bath set at 60ºC for 20 minutes to send the reaction to completion.

Results

The table below illustrates the spectrophotometer readings for each of the Iron complex solutions made up previously over the space of an hour. An extra reading is included indicating the time at infinity. A graph is plotted.

Time (min) Spectrophotometer Readings B C D E 0 0.525 0.597 0.659 0.770 15 0.247 0.354 0.445 0.633 20 0.203 0.297 0.391 0.596 30 0.156 0.225 0.312 0.535 60 0.111 0.134 0.183 0.396 ? 0.099 0.117 0.104 0.114

Table 1

Analysis

Graph 1

Throughout the reaction Fe2+ ions are in excess and so its concentration assumed to be constant. The expression for the pseudo first-order rate constant k` can be given as: k`=k[Fe2+] Also, k`t= -ln A graph of k`t against time gives the value of k as the gradient of the graph.

Time (min) k`t B C D E 0 0 0 0 0 15 1.057 0.706 0.487 0.234 20 1.410 0.981 0.659 0.308 30 2.011 1.492 0.981 0.444 60 3.570 2.341 1.950 0.844

Table 2

Graph 2

The values for k` are tabulated below against its corresponding concentration and the resulting values plotted in Graph 3

CONCENTRATION OF [Fe(H2O)4Cl2] (M) k` 0.070 0.0139 0.140 0.0325 0.210 0.0387 0.280 0.0587 Table 3

Graph 3: the gradient gives the value of k, the second order rate constant

Inference From the intial graphs, solutions B, C and D show a rapid decrease in absorbance for the first 30 minutes, whereafter the decrease is steady. Solution E showed an almost constant decline in the absorbance. The decrease in absorbance of all four solutions indicate that the molar absorptivity also decreases (proportional relationship with A) because . The ability of the solutions to absorb light decreases then over the course of the reactions.

The suggested end point is reached at t=? where all the solutions have a similar Absorbance reading.

From the final graph, the gradient of the plot is given as 0.2009. This value corresponds to k and its unit is M-1s-1. This value is heavily dependent on the electronic properties of the bridging ligand.

N3- is a good bridging ligand because it can share electron density. The bridging ligand first dissociates from the metal centres (rate-limiting step) therefore this is an inner-sphere electron transfer mechanism. The intermediate formed is the stable [(NH3)5CoN3Fe(H2O)4]4+.