ALL CIE CHEMISTRY DOUBTS HERE !!

[Deadly_king]

for nov 2009 ppr 2

The oxidation state of hydrogen in the metal hydrides is always -1 so why in PH₃ the oxidation state is +1 and hence x+3(-1)=0 so x=+3 really need help..Thanks n advance

Phosphorus is not a metal.

Being a non-metal, it cannot give electrons to turn onto +3 oxidation states. It has to accept electrons which is why it has a negative oxidation number 

[moon]

why didn't we find the mole ratio CO₂: H₂O
so that the formula can be C₂H₂...

please i really need help in this question and thank a lot 4ur ppr4 explanation....

[Deadly_king]

why didn't we find the mole ratio CO₂: H₂O
so that the formula can be C₂H₂...

Hmmm........it's not possible to do it using the ratio CO₂ : H₂O

But you must realise that :
1 mole of hydrocarbon will form x moles CO2 and y/2 moles of H2O

Ratio CO₂ : H₂O will hence be x : y/2

These are two unknown values which are not linked in any way. So we cannot solve it.

First you need the ratio hydrocarbon : CO₂ ---> 1 : x
Then use ratio of hydrocarbon to H₂O ---> 1 : y

Only then will you be able to find the values of x and y

[moon]

thank u very much 4ur explanation...

[Deadly_king]

thank u very much 4ur explanation...

You're welcome buddy.

I just hope my explanations were clear enough

[moon]

can somebody plz explain Q5 d,Q8 b(ii),c & Q9a(i) in June 2009 ppr4

I would appreciate ur help, thanx in advance.

[Deadly_king]

can somebody plz explain Q5 d,Q8 b(ii),c & Q9a(i) in June 2009 ppr4

I would appreciate ur help, thanx in advance.

Jun 09 p4 No 5

Hmmm......part (d) will depend on the alcohols you drew in part (a).

If it is a primary alcohol, the latter will get oxidised to a carboxylic acid.

If it is a secondary alcohol, it will get oxidised to a ketone.

If it is a tertiary alcohol, it will not get oxidised. (stays the same)

However you won't be having any tertiary alcohols, since the question asks for unbranched compounds.

[moon]

Jun 09 p4 No 5
If it is a tertiary alcohol, it will not get oxidised. (stays the same)

Do u mean that teertiary alcohol on oxidation will be a tertiary alcohol and not ketone? because the markscheme's answer was CH₃CH₂COCH₂CH₃

[Deadly_king]

Do u mean that teertiary alcohol on oxidation will be a tertiary alcohol and not ketone? because the markscheme's answer was CH₃CH₂COCH₂CH₃

Yeah.........but the compound you drew is not a tertiary alcohol. It is a secondary one. That's why it gets converted into a ketone.

NOTE : A tertiary alcohol is one whose OH group is bonded to a carbon atom which is already bonded to three other carbon atoms. In other words the carbon atom is not attached to any hydrogen atom

P.S : I edited my previous post. Do have a look, it might clear your doubt.

[moon]

Thanks 4 ur explanation but can u explain me why the partition coefficient between fat & water for PCB residues in Q8 b(ii) is greater than 1? Thanks 4 ur cooperation..

[Deadly_king]

Jun 09 p4 No 8

b)(ii) K_PC = Concentartion of PCBs in fats / Concetration of PCBs in water

The above mentioned K_PC should have a high value since from the paragraph written above, we can note that PCBs are highly soluble in fats (the main components of the Inuit diet). Therefore we can conclude that PCBs will be more soluble in fat than in water. Since PCBs is more soluble in fats, it will have a greater concentration in fats rather than water.

c)(i) Four spots were observed after the first solvent has been used. Spots which are just vertically above another spot s counted as one spot only.

(ii) Spot at the bottom.

(iii) Spot at the very top.

If you need more explanation, let me know

[moon]

Thanks a lot.

[Deadly_king]

Jun 09 p4 No 9

a)(i) You just need to draw an exact copy of the kevlar structure just below it. Then you need to show how they are bonded by hyfrogen bonds which occur between on O atom from C=O and one H atom from the opposite N-H.

In other words the bond will be formed from an O atom from the already drawn kevlar structure to a H atom from the structure you'll be drawing.

NOTE : Hydrogen bonds take place only between a H atom and an electronegative atom like oxygen, nitrogen and chlorine.

[cs]

I have a doubt, what is the difference between elimination and reduction in organic chemistry..?

[Deadly_king]

I have a doubt, what is the difference between elimination and reduction in organic chemistry..?

That may depend upon the different reactions. Do you have any example

Reduction -----> Redox reaction where a substance is reduced.

Elimination -----> A molecule is formed as by-product from the reaction. Usually it's water.

Examples :
i) When an aldehyde is reacted with a reducing agent like lithium aluminium hydride dissolved in ether, a primary alcohol is formed.

This is reduction as the aldehyde is reduced to a primary alcohol.


ii) When an alcohol is reacted with concentrated sulfuric acid at a temperature of 180^oC, an alkene is formed with water as by-product.

We say water is being eliminated from the alcohol and the reaction is known as elimination