Author Topic: Edexcel CHEMISTRY DOUBTS!!!!  (Read 253850 times)

Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1650 on: January 17, 2012, 04:42:04 pm »
I had the same doubt some time ago :P

Check my thread :P
That is not my question :S

I know by which mechanism they react but i can't explain why?
I have read the chemguide website too and I understand it but I can't write it/explain in some easy words  ::)
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Offline MKh

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1651 on: January 17, 2012, 06:26:34 pm »
That is not my question :S

I know by which mechanism they react but i can't explain why?
I have read the chemguide website too and I understand it but I can't write it/explain in some easy words  ::)

Refer to the Edexcel A2 Chemistry Revision Guide. You might find what u r looking for over there.

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1652 on: January 18, 2012, 01:03:41 pm »
Found it (:

EDIT :

There are TWO ways for you to approach this Question

Method ONE :

Primary Haloalkane , group around it is SMALL => hence easy to attack

tertiary Haloalkane , groups around it are Bulky/big => Hence difficult to attack

Method TWO :

primary NOT Stable   [there is an intermidate formed with a C+]

tertiary => Stable , surrounded by electron donating alkyl groups

[don't know what he meant by Alkyl groups though  :-\ ]

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« Last Edit: January 18, 2012, 01:22:12 pm by Golden Girl :) »
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Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1653 on: January 19, 2012, 02:03:44 pm »
Thanks GG :D

Can anyone do this question for me :S

I don't understand why the volume is 12.5 :s

Q20 a(v)

Another doubt:

Why does diluting a weak acid increase its degree of dissociation? :S
« Last Edit: January 19, 2012, 02:41:24 pm by Ang3l »
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Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1654 on: January 19, 2012, 08:21:38 pm »
Plus the above, can any also make trans-esterfication clear for me :S

Trans-esterification is fine for me now
but ofcourse I have another doubt :P

(attached) How is it D :S

Thanks
« Last Edit: January 20, 2012, 11:29:34 am by Ang3l »
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Offline Deadly_king

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1655 on: January 20, 2012, 12:42:34 pm »
Plus the above, can any also make trans-esterfication clear for me :S

Trans-esterification is fine for me now
but ofcourse I have another doubt :P

(attached) How is it D :S

Thanks


Firstly let's look for compounds which have optical isomers, i.e chiral centres!

This leaves us with only option B and D since A and C can in no way show chirality!

Now we look for E-Z isomers, only D is right for B has two methyl groups attached to the carbon containing the double bond! So it cannot show E-Z isomers.

Hope it helps :)

Offline KJ1995

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1656 on: January 20, 2012, 01:08:47 pm »
Thanks GG :D

Can anyone do this question for me :S

I don't understand why the volume is 12.5 :s

Q20 a(v)

Another doubt:

Why does diluting a weak acid increase its degree of dissociation? :S
When we add water the system trys to oppose the change (chattliers principle) by using up the water added which shifts equilibrium to the right, which means more [H+] and [CH3COO-], and less [CH3COOH].. putting those new values in the (Ka) equation will make the value of Ka bigger...and i also dont know why is it 12.5, can you please tell em when you find out?
Thank you

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1657 on: January 20, 2012, 01:15:49 pm »
Plus the above, can any also make trans-esterfication clear for me :S

Trans-esterification is fine for me now
but ofcourse I have another doubt :P

(attached) How is it D :S

Thanks



I'm sorry for interfering but are there any links which helped you out in Trans-esterification  point
Thanks in advance  ???
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Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1658 on: January 20, 2012, 03:40:44 pm »

I'm sorry for interfering but are there any links which helped you out in Trans-esterification  point
Thanks in advance  ???
I used the text-book and the edexcel revision guide.
I dont think we need to know very much about it and the revision guide gave me enough detail needed.

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1659 on: January 20, 2012, 05:18:24 pm »
Q.20 how come it's 12.5 ?!


Okay from the First part of the Question we know that in the Titration 0.25 moldm-3 of NaOH and 0.125 moldm-3 of Ethanoic Acid  !!


so What I did is I found the Ratio as follows =>       Acid : Base
                                                          
                                                                      0.125 : 0.25  divide by 0.125 for both

                                                      I  get a Ratio of => 1 : 2

And from the part (v) we get to know that  25 cm3 of the ACID is USED UP !!!  .. so to find the Volume at the End point (Where the salt/product is given indicating that the ACID is completely KILLED) is

                                                               1  : 2

                                                               25 :  X      

Cross multiply and You'll find that the Volume of the BASE is  =>                   12.5 cm3


I simply used my common sense and it worked :D


if it doesn't work Use the formula from the CIE board (not in the syllabus) ;

(C1V1) acid =  (C2V2)base.... and act smart ;D
« Last Edit: January 20, 2012, 05:31:23 pm by GG »
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Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1660 on: January 20, 2012, 06:10:12 pm »
^ Oh, thank you :D

Another doubt (attached)

Q18 part c (ii)
Why do we use the moles and not the concentration given :s
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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1661 on: January 20, 2012, 06:43:25 pm »
I think most probably that is the case because when you put the Volumes , they cancel out.

I hope I helped (:
« Last Edit: January 20, 2012, 06:50:42 pm by GG »
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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1662 on: January 22, 2012, 08:04:02 pm »
what is the PH of 0.5 moldm-3 ammonia solution given Kb= 1.8 x 10-5 ;)
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Offline Malak

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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1663 on: January 22, 2012, 08:08:15 pm »
what is the PH of 0.5 moldm-3 ammonia solution given Kb= 1.8 x 10-5 ;)
You dont need to know that.
I know the old papers have this but if you check the specification. I doesn't mention Kb anywhere
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Re: Edexcel CHEMISTRY DOUBTS!!!!
« Reply #1664 on: January 23, 2012, 07:01:47 am »
You dont need to know that.
I know the old papers have this but if you check the specification. I doesn't mention Kb anywhere
but I didn't see it  in old pastpapers actually it is in the Edxecel chemistry blue book
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