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ALL CIE CHEMISTRY DOUBTS HERE !!
Deadly_king:
Nov 08 P4 No7(b)
Condensation reaction occurs between the NH2 group of one amino acid and the CO2H group of another amino acid to form a peptide linkage as described in the first picture.
However the questions ask about the formation about a tripeptide. Hence it will be a reaction involving 3 amino acids. The displayed formula of a tripeptide is shown in the second picture.
NOTE : The R groups of atoms may be replaced by almost any atom. However it will be much easier and simpler to replace it by H atom. Your answer should not consist of R.
moon:
Thank u very much for ur reply as well as ur explanation. It is really helpful, but can u explain Q4(b)(iii), of the same year Nov. 2008? You have explained Q3 not Q4 and what is the formula of the tripeptide as shown in the markscheme?....I really do appreciate ur co-operation.
Deadly_king:
--- Quote from: moon on September 27, 2010, 09:20:46 pm ---Thank u very much for ur reply as well as ur explanation. It is really helpful, but can u explain Q4(b)(iii), of the same year Nov. 2008? You have explained Q3 not Q4 and what is the formula of the tripeptide as shown in the markscheme?....I really do appreciate ur co-operation.
--- End quote ---
Oops ....... sorry for the confusion dude.
4(b)(iii)
That will depend on the equation you wrote for part (b)(i).
Let's take it to be : C8H18 -----> 2C2H4 + C4H10
Now you need to find the enthalpy change for this reaction.
Number of bonds to be broken in the reactant : 18C-H bonds and 7C-C bonds.
Total number of bonds to be formed in the products : 2C=C bonds + 8C-H bonds and 3C-C bonds + 10C-H bonds.
Use the values from the Data Booklet to find :
(i) Energy required for bond-breaking : 18(410) + 7(350) = 9830 KJ/mol
(ii) Energy given out for bond formation : 2(610) + 8(410) + 3(350) + 10(410) = 9650 KJ/mol
Standard enthalpy change of reaction : 9830 - 9650 = +180 KJ/mol
NOTE : Bond breaking is endothermic(+ve) while bond forming is exothermic(-ve).
Same method is applied whatever the equation you wrote in (b)(i).
(iv)
The conditions include a temperature of about 600oC. Hence this suggests that the reaction is endothermic since heat must be supplied for the reaction to occur and this is confirmed by the entahlpy change obtained in (b)(iii).
7(b)
The formula of the tripeptide can vary. This is why the marking scheme only mentioned 'correct tripeptide'. The product in the second picture I uploaded earlier is the right answer. Except that it is asking for displayed formula, so you need to elaborate on the bonds and replace all the R groups by H for the simplest tripeptide. :)
moon:
Thank you very much for ur help. I got it... :)
Deadly_king:
--- Quote from: moon on September 28, 2010, 12:27:33 pm ---Thank you very much for ur help. I got it... :)
--- End quote ---
You are welcome pal :)
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