Lab 6. Prelab Questions.

Questions you NEED to answer in a written form (in your lab notebook) before the Lab. 

• Solve at least three problems from the on-line quiz Rate laws and submit it electronically before the class
• Prove Equations 4, 5 , 8 and 9 from the introduction to this lab
• Assuming that the lifetime without a quencher, t_o, is 500 ns and the k_q's are on the order of :

10⁹ M^-1s^-1 for Fe(H₂O)₆³⁺
10⁶ M^-1s^-1 for Fe(H₂O)₆²⁺
3 x 10¹⁰ M^-1s^-1 for Fe(CN)₆^3-
5 x 10¹⁰ M^-1s^-1 for Fe(CN)₆^4-

calculate the concentrations for each quencher at which t = t_o/2. For your Stern-Volmer plot you will need to have 4 or 5 points spreading more or less evenly over range at which t decreases to as low as ~ 1/4 of t_o. What (maximum) quencher concentration does it correspond to?

• Calculate the diffusion-controlled rate constant, k_diff, using Equation (15):

  k_diff = 4p N_a DR_q

  with N_a being the Avogadro's number 6.022 x 10²³ mol^-1, and D - the sum of diffusion constants:

D = D_O2 + D_Ru(bpy)3²⁺

Do calculations using oxygen as an example, thus D_O2. Similarly for R_q, the quenching radius (separation at which the quenching takes place), use radii of oxygen and Ru(bpy)₃²⁺:

R_q = R_O2 + R_Ru(bpy)3²⁺

Take R_O2 = 0.18 nm and R_Ru(bpy)3²⁺ = 0.50 nm and use Stokes' formula to estimate D_i for each species:

D_i = k_BT/6phR_i

where R_i is the radius of the diffusing particle and h is the viscosity (h(20°C) = 1.009 x 10^-3 kg/m s); k_B is the Boltzmann constant and T is temperature.

What kind of questions you MAY be asked before the Lab.

• What is the difference between fluorescence and phosporescence? which happens faster?
• What is necessary for the energy transfer mechanism of quenching to be efficient? For electron transfer?
• What is a monomolecular, a bimolecular and a pseudo - first order reaction?

Last updated 04/05/07.