International Chemistry Olympiad - Help needed with problems

Hello! In october I took the first qualification test for the Norwegian IChO finals, scoring 88 out of 100 points and becoming number 9 (out of 1007 ) in the country. I'm taking the second qualification test on wednesday. 183 students qualified for the test, but I have to be among the top 16 to get into the Norwegian finals! So I need some help with some of last year's problems (which I'm using for practice). Please bear with me and explain what I'm doing wrong!

This is from a problem related to the enzyme cytochrome c oxidase where the following reaction occurs:

4Fe²⁺ + 4H⁺ + O₂ -> 4Fe³⁺ + 2H₂O

What I don't understand is the following:

5 c) The reduction potential for Fe²⁺/Fe³⁺ in cytochrome c oxidase is 0,26 V. Calculate the standard reduction potential (E⁰ for this reaction at standard conditions.

So what I do is that I look up E⁰ for the reactions Fe³⁺ + e^- -> Fe²⁺ (0,77 V) and O₂ + 4H⁺ + 4e^- -> 2H₂O (1,23 V).
I reverse the sign of the one with the lowest reduction potential and add them together to get the reduction potential and get 1,23 V - 0,77 V = 0,46 V. But that's wrong! The answer is 1,23 V - 0,26 V = 0,97 V. The paper with the solution doesn't provide any reasoning or explanation, and my teacher (and my textbook) couldn't give me a satisfying explanation either. I know this should be easy as hell, but I don't understand the reasoning behind the solution.


Another one:

1, 3) In a 0,010 M HCN solution, what is the percentage of protolysis? (K_a = 4,9*10^-10 M)

This is what I do:




But that's also wrong, as the alternatives are 0,0025 %, 0,025 %, 0,25 % and 2,5 %. What went wrong here?



I'd appreciate any help you can give me. These tests go beyond the curriculum and are at a higher level than what's expected from a student my age, so the books are of little use. Two days left now, wish me luck!

1) Just think about this: the reaction is taking place in the presence of an enzyme (on the enzyme surface so to speak), and these conditions differ from ordinary aqueous solutions. The enzyme forms bonds with the iron ions, and thus has a significant effect on the Fe2+/Fe3+ reduction potential (that's why you should use the given value, 0.77V, of Fe2+/Fe3+ in cytochrome c oxidase), and it shouldn't have a significant effect on the O2/H2O-reduction potential, so you can use its tabellated value.

2) If you divide x by the total HCN concentration, you get the FRACTION of protolysis, not the percentage . Multiply by 100, and if you want 'exactly' 0.025%, use the correct Ka of HCN (which is more like 6.2E-10 and not 4.9E-10).

Some years back I also participated in these IChO in my country, passed the first qualification but failed to continue to the international round.

Lykke til! (Jeg har bodd i Norge)

Quote: Originally posted by Sobrero

1) Just think about this: the reaction is taking place in the presence of an enzyme (on the enzyme surface so to speak), and these conditions differ from ordinary aqueous solutions. The enzyme forms bonds with the iron ions, and thus has a significant effect on the Fe2+/Fe3+ reduction potential (that's why you should use the given value, 0.77V, of Fe2+/Fe3+ in cytochrome c oxidase)....

D'oh My apologies, Sobrero.

Just wanted to say thanks for your help, although I'm a little late! I got the results yesterday, and I have qualified for the Norwegian IChO finals in March!

Quote: Originally posted by Sobrero

2) If you divide x by the total HCN concentration, you get the FRACTION of protolysis, not the percentage . Multiply by 100, and if you want 'exactly' 0.025%, use the correct Ka of HCN (which is more like 6.2E-10 and not 4.9E-10).

Quote: Originally posted by Sobrero

Some years back I also participated in these IChO in my country, passed the first qualification but failed to continue to the international round. Lykke til! (Jeg har bodd i Norge)

Takk

Update: The finals are over, but I didn't qualify for the IChO.

I'm a little disappointed, not in the fact that I didn't qualify, but because I didn't work all that hard during the training week. The three hour crash course in physical chemistry was intense, as was quantum mechanics... During the lectures, I aced organic synthesis and thought it was simple as hell, but the finals were worse. I was the best of the sixteen with 4/14 points from the synthesis task... (Btw, it was a synthesis of pentobarbitol and thiopental starting from diethyl malonate)

Yesterday, after the results from the finals were out, we also got the results from the second qualification (The top 16 for the finals aren't internally ranked until the final test is done). I was number one out of 181 with 89/100 points, and the following three had 71/100 points!

That was both pretty cool and a huge bummer: I absolutely destroyed the rest of the finalists in the second qualification, but in the finals I placed somewhere around 12-13 out of the 16...

[Edited on 26-3-2011 by Lambda-Eyde]

[Edited on 26-3-2011 by Lambda-Eyde]