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As chemistry oct 2009 mcq

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kaipai:
Hello all!
First simple query::how do you all preapare for chemistry AS level???What is most imporatant:notes??Book??Tutor???Tell me plz...

now paper question:This quest is 25 of the oct 2009 mcq paper. Plz answer which is crect answer?Explain as well plzz in simple terms. NOw plz tell me how to add pictures/ diagram in this post??e.g i could not add q.23 of the same paper. plz tell me how to add a question from pdf. file without typing.

25 Which equation represents a valid propagation step in the free radical reaction between ethane
and chlorine?
A C2H6 + Cl• ? C2H5Cl + H•
B C2H5Cl + Cl• ? C2H4Cl• + HCl
C C2H6 + H• ? •C2H5  + HCl
D C2H5• + Cl • ? C2H5Cl
good by take care,
regards,
K.pai

guitarbuddy:
The answer to the question is B..
The explanation is easy. The answer cannot be D since it is a step of termination step.. A is producing an H radical and in C C2H6 is reacting with an H radical.. Propagation step is only possible when the reactant reacts with a halogen, mostly Chlorine and produces a Cl radical or a CxHy radical... Therefore B is the correct answer.

kaipai:
When no one comes to rescue......
KPai is sad.....
But from the darkness and a question....
comes out the one and only guitar buddy.....
Wonderful my buddy....
You are GREAT!!!!

very many thanksss for your reply....You are corrrect...
I am sorry...but if you can plz... plz...answer the new following questin of may 2010::
Q21. When heated with chlorine, the hydrocarbon 2,2 dimethybutane unddergoes free radical substituion.
IN a propagation step the free radical X• is formed.

            CH3
            |
CH3CH2 C-CH3  + Cl• = X• + HCl
            |
            CH3
How many different forms of X• are possible???
a. 1   B.2 C.3   D.4

Please explain...

Good bye and do take care...

Yours truly,,,,
KaiP

guitarbuddy:
I dont know the answer exactly but i think it is C (3) because there are 3 carbon atoms in the straight chain and Cl can only attach to  straight chain carbon atoms

Nid:

--- Quote from: kaipai on June 18, 2012, 07:12:05 am ---When no one comes to rescue......
KPai is sad.....
But from the darkness and a question....
comes out the one and only guitar buddy.....
Wonderful my buddy....
You are GREAT!!!!

very many thanksss for your reply....You are corrrect...
I am sorry...but if you can plz... plz...answer the new following questin of may 2010::
Q21. When heated with chlorine, the hydrocarbon 2,2 dimethybutane unddergoes free radical substituion.
IN a propagation step the free radical X• is formed.

            CH3
            |
CH3CH2 C-CH3  + Cl• = X• + HCl
            |
            CH3
How many different forms of X• are possible???
a. 1   B.2 C.3   D.4

Please explain...

Good bye and do take care...

Yours truly,,,,
KaiP

--- End quote ---

The halogen radical pulls a hydrogen atom off the alkane to form an alkyl radical.

(CH3)3-C-CH2-CH3

So one distinct radical can form when a hydrogen atom is taken off the methyl group(or Carbon 1)
CH2(CH3)2-C-CH2-CH3*

2nd one can form when a hydrogen atom is taken off the alkyl group(CH2)
(CH3)3-C-CH-CH3*

And third one can form when a hydrogen atom is taken off the 4th carbon
(CH3)3-C-CH2-CH2*

Should be C

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