Chemistry: Post your doubts here!

[smzimran]

...ths is rather like a political statement.......i mean i am doing a whole lot of practice at school.......

Then dont worry too much about the practical.
Its weightage is also quite low i know people with b or c grade in the practical exam and a in MCQ and theory and they get a final 'a' grade..

 

[smzimran]

oh great ! =)
hehe lol ya ok my bad ! good luck in your a2

Thanks...

 

[tom ed]

Thanks...

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w09_qp_12.pdf can u help me solve q 20......sorry fr bothering u..

 

[smzimran]

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w09_qp_12.pdf can u help me solve q 20......sorry fr bothering u..

Its A.
First check for cis isomers...B and D are eliminated
The answer could be A or C.
Now you have to draw the structure using the info provided to check between the two answers, and you will find out that A is correct (5 double bonds)

 

[Silent Hunter]

AOA

wanted a general understanding of oxidation of alcohols? I am confused over "reflux" and "distill over " terms?

Thank You

 

[ismailmz]

Well oxidation is when alcohol is burnt in oxygen..
But here there is something more we have to know that is , with primary, secondary and tertiary...
The oxidising agent is always Potassium Dichromate with dilute sulfuric acid.( ACIDIFIED POTASSIUM DICHROMATE)
To distinguish between primary, secondary and tertiary is very easy and this how u do it...
1. With tertiary alcohol the dichromate does not change colour ie it remains orange.
2.Secondary alcohol will form Ketone ie the colour changes from orange to green
3. Primary Alcohol forms aldehyde. And when further heated under reflux with acidified potassium dichromate it becomes ACID..

Well about reflux and distill over, I dont know how to explain it... would be glad if some other member helped...

AOA

wanted a general understanding of oxidation of alcohols? I am confused over "reflux" and "distill over " terms?

Thank You

 

[Nibz]

Primary alcohols are oxidised to aldehydes and then to Carboxylic acids i.e the half-way product of the oxidation of primary alcohol is an aldehyde.
You heat the reaction with a dichromate under reflux in both conditions but to get an aldehyde, you distill it off immediately once it's formed.

 

[omg]

Please post the link to this paper, it would be helpful and time saving..

help??? i postd the link too

 

[smzimran]

help??? i postd the link too

Actualy i have not started paper 5 yet thats why.... am realy sory, hope smebdy else wl help

 

[tom ed]

Its A.
First check for cis isomers...B and D are eliminated
The answer could be A or C.
Now you have to draw the structure using the info provided to check between the two answers, and you will find out that A is correct (5 double bonds)

..can u help me draw the structure....

 

[omg]

Actualy i have not started paper 5 yet thats why.... am realy sory, hope smebdy else wl help

 

[Student12]

Any tips for AS- practical paper 3 ? what all do we have to know ? Any website for practicals ?? Specific topics or something ??

 

[leosco1995]

[url=http://www.xtremepapers.com/CIE/index.php?dir=International A And AS Level/9701 - Chemistry/&file=9701_s09_qp_32.pdf]Summer 2009, practical paper variant 2[/url]

How do we figure out the electrical charges? And how do we do the part after it?

 

[tom ed]

Summer 2009, practical paper variant 2

How do we figure out the electrical charges? And how do we do the part after it?

first equation ...left side:
Mn+4(-2)=-1
Mn-8=-1
Mn=+7
right side:
Mn=2+..or +2...this means that the oxidation number of Mn decreased from +7 to +2....by accepting 5 electrons....so first blank=5 electrons

second equation...left side:
2C +4(-2)=-2
2C=+6
C=+3
right side:
2C+4O=0
2C+4(-2)=0
2C=+8
C=+4
if u look at the second equation one carbon atom's oxidation is changing from +3 to +4...this means that one carbon atom loses one electron to increase it's oxidation state from +3 to +4....since there are two carbon atoms changing their oxidation states from 3 to 4......second blank=2 electrons.....also in the above second equation ...see the coloured part...for 2 Carbons there is a difference of 2 in oxidation state so 2 electrons.......hope u get it....

 

[omg]

can smbdy help me wid p5??

 

[abcde]

Need help with the following 2 MCQs:
35. Since BaNO3 is the oxidising agent, shouldn't it be reduced? Why is BaO formed? Why can't Mg(NO3)2 be formed? (The correct option is B.)
30. The er mentions that in this exceptional circumstance, options A and B were both credited. Why do you think B was credited? (I can come up with just one case cis-trans isomerism.)

 

[asexamskillme111]

heyy everyone
http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_s11_qp_12.pdf
questions 17, 22 and 29.
for question 29 i get how 2 new chiral carbon atoms are added on creating the diol but how is one chiral atom removed with hot, acidified KMnO4?

 

[smzimran]

Need help with the following 2 MCQs:
35. Since BaNO3 is the oxidising agent, shouldn't it be reduced? Why is BaO formed? Why can't Mg(NO3)2 be formed? (The correct option is B.)
30. The er mentions that in this exceptional circumstance, options A and B were both credited. Why do you think B was credited? (I can come up with just one case cis-trans isomerism.)

35.
Barium is reduced check it either by o.s or just by the fact that there is loss of oxygen from BaNO3 to form BaO. (Removal of oxygen is reduction)
As the fuel is 'lit up' it means heat is provided and when heat is given to nitrates, they decompose to metal oxide, NO2 and O2....
Mg(NO3)2 cannot be formed due to the same reason: The nitrate decomposes due to heat so there cannot be a displacement reaction of Mg to form magnesium nitrate...

 

[smzimran]

Need help with the following 2 MCQs:
35. Since BaNO3 is the oxidising agent, shouldn't it be reduced? Why is BaO formed? Why can't Mg(NO3)2 be formed? (The correct option is B.)
30. The er mentions that in this exceptional circumstance, options A and B were both credited. Why do you think B was credited? (I can come up with just one case cis-trans isomerism.)

30.
I cant figure that out myself, what i think is that it has something to do with ethers and their reactions with conc. H2SO4

 

[Wonder girl]

how can i prepare for chemistry paper 5.....i need some guidelines and some tips for doing calculation