Prelaboratory Questions And Problems

*1. Since the oxidation state of oxygen in H2O2 is — 1, oxidizers produce O2 from it, whereas reducers typically produce H20 or OH-. Using a clever approach based on oxygen isotope labeling, it has been shown that all the O2 produced during oxidation of aqueous H202 comes from the peroxide bond and not from water. Thus, the 0-0 bond is not cleaved by oxidation.

On the contrary, reduction of H202 by Fe2+ (as in Fenton's reaction) adds an extra electron that produces the H2O2 radical, which then decomposes into OH" +*OH (see Baird, 1997). This mechanism implies cleavage of the 0-0 bond, promoted by the extra electron that enters a a antibonding orbital.

a) Assuming that the bond order in the 0-0 (i.e., peroxide) bond = 1, what is the bond order of the 0-0 bond in H202"'? (Recall from General Chemistry that the bond order, BO is calculated by subtracting the number of electrons that are located in an antibonding orbital in a bond from those in a bonding orbital; the result is then divided by two).

b) Did you obtain the same bond order for both species? What does your finding imply?

*2. When a small (catalytic) amount of Fe3+ ions is introduced into an electrochemical cell, it can be reduced to Fe2+ at the cathode (reaction 1). Simultaneously, H2O is oxidized to O2 at the anode (reaction 2). This 02 then diffuses to the cathode, where it gets reduced to produce H2O2 (reaction 3). When Fe2+ and H2O2 diffuse away from the electrodes and meet in the solution, Fenton's reaction occurs and *0H radicals are produced (reaction 4). This process is called electro-Fenton (see Oturan, 2001).

a) Write and balance each one of the four reactions described above.

b) Using Hess law, combine these four reactions so as to yield a net reaction (reaction 5) of the form:

where A is an oxidizing gas, B is a highly stable polar liquid, and C is an extremely powerful and unstable oxidizer.

c) Why is Fe3+ not present in the net reaction (reaction 5)? Explain.

d) What percentage of the O2 consumed by the system is produced by the anodic reaction (reaction 2)?

3. Fen ton reaction can occur in natural waters due to the presence of Fe2+ [formed by the photolysis of Fe(III)L complexes], and the presence of H2O2 (formed by the photolysis of humic acids and other substances). Write appropriate equations to depict this phenomenon.

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