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Chemistry: Post your doubts here!

iKhaled

iKhaled

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freezingfires said:
Can anyone please please tell how to calculate the enthalpy change for this reaction:
This is from October november 2008 paper 4 q4b
C40H82 : C16H34+2C12H24(it says considering the bonds broken and bonds formed use the data booklet to calculate the enthalpy change for the reaction)
Click to expand...
i explained this in details in the previous page of this thread go check it out :p
 
iKhaled

iKhaled

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Daisy_94 said:
Guys can u xplain me q1(ii) in m/j 08 ....I dont understand isnt the E cell = (more +ve)-(less +ve) .....so why isnt it +1.36-0.77 instead it is 0.77-+1.36 ..pls help !
Click to expand...

post a link to the question so we can help uuuuu
 
Ashique

Ashique

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Daisy_94 said:
Guys can u xplain me q1(ii) in m/j 08 ....I dont understand isnt the E cell = (more +ve)-(less +ve) .....so why isnt it +1.36-0.77 instead it is 0.77-+1.36 ..pls help !
Click to expand...

The concept you should keep in mind is not the more positive minus the less positive, but rather, Er-Eo- Which means the species being reduced minus the species being oxidized. The question probably refers to the feasibility of the reactions.
 
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freezingfires

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iKhaled said:
hey...
C40H82 -------> C16H34 + C24H48

a good way of explaining this question is to see how many bonds u have before the reaction and how many bonds u have after the reaction for example...

before the reaction we have 39 C-C bonds and 82 C-H bonds...after the reaction we have 15+22 = 37 C-C bonds and one C=C bond and 48+34 = 82 C-H bonds

so we have broken 39-37 = 2 C-C bonds and formed 1 C=C bond
bond broken = 2 x 350 = +700
bond formed = -610 = -610
enthalpy = + 90 kjmol^-1 !!

thats it..the mark scheme got the answer as +180 because he has used a different equation in cracking instead of forming C24H48 as our alkene he formed 2 moles of C12H24 and in that way we will have 2 C=C bonds instead of one and 4 C-C bonds instead of 2 because one mole of C12H24 contains 1 C=C bond and 10 C-C bonds so the 2 moles will contain 20 c-c bond and 2 C=C bonds 20+15= 35 so 39-35 = 4 C-C bonds so we had broken 4 C-C bonds and formed 2 C=C bond..in that way u will get 180 kj per mole instead of 90..both are correct though depending on the equation u made ;)

i hope u got it!!
Click to expand...

Thanks man got it!! :)
 
Wanderer

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Does structural formula mean we have to draw the skeletal formula....for eg can't we write CNCH2CH2CH2CN instead of drawing it in a skeletal form...please please reply
In ms they usually give the skeletal..but isnt the formula i wrote about also structural?
 
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samir2012

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Need URGENT help with chemistry P4 applications......i already have the booklet but i found it rather incomplete. Anything else out there to give me a COMPLETE summary of the application chapters?? plz reply and help me quickly :)
 
iKhaled

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Silent Hunter said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s10_qp_42.pdf

its question 4 (c) ? how to do it ....... by the way any tips for this paper..... its just so confusing at times :\ :\


iKhaled

or any one?
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hey..

first of all quote the values of the half cells from the data booklet u have Fe^3+/Fe^2+ = 0.77.... O2+H^+/H20 = 1.23 we have put this value because this is when it is oxidized under acidified conditions and for the oxidation in the alkaline conditions u have Fe(OH)3/Fe(OH)2 + OH- = -0.56 and O2+H2O/OH- = 0.40 (this value is when it is oxidized under alkaline conditions so lets calculate the E cell of both reactions

E(acid conditions) = 1.23 = 0.77 = 0.46
E(alkaline conditions) = 0.4-(-0.56) = 0.96

note that the higher the value of the E cell the more likely the reaction will occur..values below 0.3 v means a reaction may not happen or it is very slow and any value less than zero means the reaction will not happen at all..so as u can see when its in alkaline conditions it is more likely to happen..so yeah thats all

hope u got it!
 
Student12

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Can anyone explain the effect of concentration when doing the prefrentional discharge? HELPPP!
 
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Taha.Sajid

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sagar65265

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iKhaled said:
yes this is very weird :O i want the explanation of this too hmm...
Click to expand...

Think of the reaction in this way; replace all the Silicon with Carbon atoms and Chlorine atoms with Hydrogen atoms (only to visualize the reaction, not to do it in reality!) so that you get a structure that is equivalent to ethane, with 1 "C-C" bond and 6 "C-H" bonds. They are basically asking you for the enthalpy change when the "ethane" molecule splits to give two "methane" molecules, right?

The simplest way to do this is to remove the "C-C" bond and add a "Hydrogen" atom in between to each "methyl" radical to give two "methane" molecules, so if you do just that, you'll find that the only bonds needed to be broken are a "C-C" bond and a "H-H" bond.

In reality, this translates to breaking a Silicon-Silicon bond and a Chlorine-Chlorine bond, and forming two Silicon-Chlorine bonds to give the final structure!

Hope this helped!
Good Luck for all your exams!
 
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raamish

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Can anybody show me the diagram i must draw of tertiary structure of protein asked in qs 8a j08 chemistry ppr(A2). Can somebody post the picture of the diagram they drew. And in qs 8b) can somebdy explain me why we didnt use all the tripeptides given. bcuz i used all. N terminal end and C terminal end peptides i drew correct. Please help thanku :)
 
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