The Kinetics of a Diffusion-Controlled Reaction - Lab Report Example

Lab Report 18 March The Kinetics of a Diffusion-Controlled Reaction Objective: To determine the rate constant and diffusion diameter of a diffusion-controlled reaction using the technique of fluorescence quenching.IntroductionThis experiment involved the study of a bimolecular reaction between two different species in solution (anthracene and carbon tetrabromide). A bimolecular reaction is a reaction that occurs mechanistically as A + B products, with a mechanistic rate = k [A][B] (Wright 184).

For reactions occurring in solutions, the solvent exerts an effect as a consequence of it being a condensed phase, with its molecules very close together even in the case of a chemically inert medium (Arnaut, Formosinho, and Burrows 225).In fluorescence quenching, a reaction takes place between an electronically excited molecule and a species that removes the electronic excitation (quencher). In this experiment, anthracene was the electronically excited species while carbon tetrabromide was the quencher.

ProcedureA 250 ml solution of 0.1 mM anthracene (AN) was prepared in spectrometric quality n-hexane and used as a “solvent” to prepare 25 ml of 15 mM “stock” solution of CBr4. Eight dilutions of the AN/CBr4 were made using AN as the solvent starting with 0% (10 ml of AN) and ending with 100% (10 ml of CBr4). The solutions were then introduced into a fluorescence cell and deaerated with dry N2 for two minutes one after the other. The full fluorescence spectrum was then recorded using the instrumental conditions set by the TA.

The fluorescent intensities of all the dilute solutions were recorded.CalculationsFrom the graph of I0/I vs [Q], the slope is 0.139 Lmol-1But slope = k τ0Therefore k = slope/ τ0= 90.139 Lmol-1)/(1.656 × 10^-5 s)= 8393.7 Lmol-1s-1From the SES equation, k = 8RT/3000hn-Hexane viscosity: h = 0.31 mPa s= (8 × 0.082 × 298)/ (3000 × 3.1× 10^-9) = 2.1× 10^7 Lmol-1s-1From the S-V plot, at [Q] = 0, the value of I0/I = -1.386ResultsTable 1: Numerical values of If and [Q]If (AN fluorescence intensity) (nm)[Q] (CBr4 concentration)(%)IO/IfIO/If -1148700-1107951.40.41058201.40.4533.2352.81.8524.6502.81.8355.2654.23.2202.3807.46.4100.59514.813.896.610015.414.4Figure 1:A graph of Io/If vs [Q]Table 2: k valuesS-V plotSES equationDiffusion-controlled rate constant k (Lmol-1s-1)8393.72.1× 10^7DiscussionThe Stern-Volmer plot in this experiment is curved upward because the system is considered to be suddenly exposed to a steady-state excitation source.

A graph of I0/I vs [Q] gives Kτ0 as the slope. The rate constant value obtained from the Stern-Volmer plot was 8393.7 Lmol-1s-1 while the theoretical value is estimated at 5.37 × 10^10 Lmol-1s-1. The rate constant obtained in this experiment is lower than the theoretical value due to factors such as ground-state complex formation between A and Q, competition between A and Q for the incident light, and polarization effects. From equation 10 a graph of (I0/I) versus [Q] gives Kqτ0 as the slope, and knowing the numerical value of τ0 then the value of Kq can be calculated.

Questions1. The reaction between anthracene and CBr4 is much faster than their separation and, therefore, the kobs ≈ k1.2. Fluorescence titration technique3. Direct observation of fluorescence decay can be used as evidence for the existence of charge transfer in the quenching process.4. K1 4πRD/{1000[Q]}….(10)(I0/I) - 1= Kq[Q]τ0 …..(14)Therefore, Kq= {(I0/I) – 1}/[Q]τ0For I0/I = [1+ 4πRDNA/{1000[Q]τ0}]/Y ….(15) if Y=1 then equation (15) changes toI0/I = 1+ 4πRDNA/{1000[Q]τ0}(I0/I) - 1= 4πRDNA/{1000[Q]τ0, using equation 15 then (I0/I) - 1= K1NA/τ0K1 = {(I0/I) – 1} τ0/ NA, If Y= 1, then Kq K15.

Time-resolved and temperature-dependent stationary measurements of fluorescence will help determine the equilibrium constant.6. Increasing the solvent polarity lowers the solvent-relaxed energy level and increases solvent re-orientation with respect to the fluorophore dipole in the excited state.7. Solvents with low viscosity are good in obtaining transient effect in diffusion processes since lowest primary recombination yields are observed. Long fluorescence lifetime will be good to use.8. De-aeration using nitrogen works by reducing the partial pressure of oxygen whereby the equilibrium is shifted from dissolved oxygen towards oxygen in the gas phase.