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Chapter 17: Q1TY (page 398)
A monolayer (single layer of atoms) of Cuon the (100) crystal face shown in the margin hasatoms. What current can deposit one layer of Cuatoms onin 1 s?
The current required to form a monolayer of copper atoms is 0.4.90mA.
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For a rotating disk electrode operating at sufficiently great potential, the redox reaction rate is governed by the rate at which analyte diffuses through the diffusion layer to the electrode (Figure 17-15b). The thickness of the diffusion layer is
whereis the diffusion coefficient of reactant is the kinematic viscosity of the liquid
and is the rotation rate (radians/s) of the electrode. There are radians in a circle. The current densityis localid="1655441451764" is
localid="1655441445229"
where nis the number of electrons in the half-reaction, Fis the Faraday constant, and localid="1655441459070" is the concentration of the electroactive species in bulk solution localid="1655441466748" Consider the oxidation oflocalid="1655441474339" in a solution of localid="1655441479067" at +0.90V(versus S.C.E.) at a rotation speed oflocalid="1655441490849" role="math" revolutions per minute. 27The diffusion coefficient oflocalid="1655441497131" and the kinematic viscosity islocalid="1655441503345" Calculate the thickness of the diffusion layer and the current density. If you are careful, the current density should look like the value in Figure 17-16b.
Will(cathode) =0.19 Vreduce at pH 2 by the reaction?
17-17. The experiment in Figure 17 - 9 required 5.32mA for 864s for complete reaction of a5.00 - mLaliquot of unknown cyclohexene solution.
(a) How many moles of electrons passed through the cell?
(b) How many moles of cyclohexene reacted?
(c) What was the molarity of cyclohexene in the unknown?
The cyclic voltammogram of the antibiotic chloramphenicol (abbreviated) is shown here. The first cathodic scan goes from 0 to -1.0 V. The first cathodic wave, , is from the reaction . Peak B in the reverse anodic scan could be assigned to . In the second cathodic scan from +0.9 to -0.4 V, the new peak C appears. Write a reaction for peak C and explain why peak C was not seen in the initial scan.
Cyclic voltammogram of 3.7 ×10-4 chloramphenicol in 0.1 M acetate buffer, pH 4.62. The voltage of the carbon paste working electrode was scanned at a rate of 350 mV/s. [Data from P. T. Kissinger and W. R. Heineman, “Cyclic Voltammetry,” J. Chem. Ed. 1983, 60, 702.]
Coulometric titration of sulfite in wine. Sulfur dioxide is added to many foods as a preservative. In aqueous solution, the following species are in equilibrium:
Bisulfite reacts with aldehydes in food near neutral pH:
Sulfite is released from the adduct in 2MNaOH and can be analyzed by its reaction with to give and sulfate. Excess must be present for quantitative reaction.
Here is a coulometric procedure for analysis of total sulfite in white wine. Total sulfite means all species in Reaction and the adduct in Reaction . We use white wine so that we can see the color of a starch-iodine end point.
1. Mix 9.00 mL of wine plus 0.8gNaOH and dilute to 10.00mL. The releases sulfite from its organic adducts.
2. Generate at the working electrode (the anode) by passing a known current for a known time through the cell in Figure 17 - 10. The cell containsofacetate buffer () plus. In the cathode compartment, is reduced to . The frit retards diffusion of into the main compartment, where it would react with to give.
3. Generate at the anode with a current of for.
4. Inject 2.000mL of the wine/ solution into the cell, where the sulfite reacts with leaving excess.
5. Add of thiosulfate to consume by Reaction and leave excess thiosulfate.
6. Add starch indicator to the cell and generate fresh with a constant current of 10.0mA. A time of 131s was required to consume excess thiosulfate and reach the starch end point.
(a) In what pH range is each form of sulfurous acid predominant?
(b) Write balanced half-reactions for the anode and cathode.
(c) At pH 3.7, the dominant form of sulfurous acid isand the dominant form of sulfuric acid is . Write balanced reactions between andand between and thiosulfate.
(d) Find the concentration of total sulfite in undiluted wine.
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