## International Chemistry Olympiad - Help needed with problems

Lambda-Eyde - 10-1-2011 at 11:15

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:

4Fe<sup>2+</sup> + 4H<sup>+</sup> + O<sub>2</sub> -> 4Fe<sup>3+</sup> + 2H<sub>2</sub>O

What I don't understand is the following:

5 c) The reduction potential for Fe<sup>2+</sup>/Fe<sup>3+</sup> in cytochrome c oxidase is 0,26 V. Calculate the standard reduction potential (E<sup>0</sup> for this reaction at standard conditions.

So what I do is that I look up E<sup>0</sup> for the reactions Fe<sup>3+</sup> + e<sup>-</sup> -> Fe<sup>2+</sup> (0,77 V) and O<sub>2</sub> + 4H<sup>+</sup> + 4e<sup>-</sup> -> 2H<sub>2</sub>O (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<sub>a</sub> = 4,9*10<sup>-10</sup> 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!

Sobrero - 10-1-2011 at 12:48

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)

Magpie - 10-1-2011 at 16:40

 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)....

Do you mean 0.26V vs the 0.77V?

And if so, where did the 0.26V come from? This option wasn't given in the problem statement.

Sobrero - 11-1-2011 at 01:54

D'oh, I mean the 0.26V (which is given in the problem statement:
"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.")

Magpie - 11-1-2011 at 08:25

D'oh My apologies, Sobrero.
Lambda-Eyde - 4-2-2011 at 13:27

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).

Not multiplying by 100 was a sloppy mistake. Which reference do you have for the K<sub>a</sub>?

 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

Magpie - 4-2-2011 at 15:19

Nice work! I have a bit of a feel for how tough the Norwegian standards can be. When in college I had a Ch E classmate from Norway who I thought was bright. I asked why he didn't go to college in Norway. He said he couldn't pass the entrance exams.

He also had an athletic scholarship for ski jumping. Women used to flock around him. Man, I wanted to be that guy!

Lambda-Eyde - 26-3-2011 at 10:41

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]

Fleaker - 27-3-2011 at 10:16

Sounds like you just missed it. Better luck next year!

One of the graduate students in my old research group did IChO as a representative for China. I can only imagine how tough that is. She also passed all of her cums within the first year!

Magpie - 27-3-2011 at 10:34