EXPERIMENT 22/ Kinetics of a Diffusion-Controlled Reaction

Objective To determine the rate constant and collision diameter of a diffusion-controlled reaction (anthracene) using fluorescence quenching by carbon tetrabromide.

Errors in lab text: Eq. 15 should replace the term 4pRDN_A/[Q]t₀/{1000/[Q]t₀}by 4pRDN_A[Q]t₀/1000.

Also, in the expressions for a and b in eq. 16 the [Q] should be moved from the denominator to the numerator.

Modified procedure:

• We will use the Turner Model 111 Fluorometer to measure the steady-state fluorescence of anthracene.
• In order to allow deaeration of the solutions, we will use 1-cm stoppered cuvettes. It is important to deaerate each sample the same way (bubbling time).
• Excitation by mercury vapor lamp in the range 340-390 nm requires an excitation narrow-pass filter #7-60. To ensure the fluorescence intensity is not too high, a 1% neutral density filter should also be used, then set the range selector knob to give the highest signal < 100 units.
• On the fluorescence side a #2A sharp cut-off filter is used to absorb all radiation < 415 nm.

Experimental procedure

1. Increase the amount of CBr₄ ”stock” solution from 25 mL to 50 mL.
2. Choose for your 8 dilutions values that make intuitive sense, i.e. 5%, 10, 20, 25, 30, 50, 60, and 80% for example since they have a simple 2x relationship to each other for the most part.
3. Calculate the molecular weights from the formulas: C₁₄H₁₀ and CBr₄, or look up in the Handbook of Chemistry and Physics.
4. Skip step 4 in text, we will not record the fluorescence spectrum. Instead monitor only the AN maximum near 398-400 nm.
5. Deaerate samples in room 112 lab using N₂ gas in a Schlenk line using consistent technique.

Data Analysis

• See the text for #3.
• Follow the text, except for #4, sentence one about the SES equation. Do the second sentence in #4 about tabulating the values of k₁ and error limits.
• Skip #5.
• See page 296 for conversions of h. Conversions: 1 Poise = 1P = 10^-1 kg m^-1 s^-1 and 1cP = 10^-2 P = 10^-3 kg m^-1 s^-1.
• n-Hexane viscosity: h = 0.31 mPa s at 20°C.
• Water viscosity: h = 1.00 mPa s at 20°C or 1.00 cP, since 1,000 Pa s = 1 cP.

Questions and Further Thoughts

• Answer all 8 questions except #5 and #7, but read the description of static quenching in #5.
• To answer question 3 see the section in your textbook (Atkins) on reactions between ions in Ch. 27, the salt effect.