Edexcel - Chemistry Unit 2 - 4 June 2013

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Is there a particular structure for damp red phosphorous like P4 or is it just 2P +3I2=2PI3 for the mixture of a 3(halogenoalkane) with PI3=alkane(I)+H3PO3?

Red phosphorus is just monatomic P

Reply 621

edex123

can anyone help on june 2011 question 16
The enthalpy change of neutralization of an acid by an alkali is measured by adding 10 cm3 of HCl to 10cm3 of NaOH. 10 cm3 pipettes are used with an accuracy of +/- 0.04cm3.

The overall percentage error in measuring the total volume of the reaction mixture is
A 0.04%
B 0.08%
C 0.4%
D 4.0%

answer in C but why?

Reply 622

posthumus

Original post by edex123

I have an issue with this one too.... apparently your supposed to do this:

0.04 x 2 / 20 x 100 = 0.4 %

I see why you do this, but in physics I'm used to calculating percentage error in each measurement first & then adding those errors together.... don't seem to get the same answer though :confused:

( 0.04/ 10 + 0.04/10 ) x 100 = 0.8%

Reply 623

pineapple78

is it possible that someone could possibly draw out all the diagrams we need to know? pretty please

Reply 624

yousefshah77

Hey guys,

I'm stuck on this question from the May 2011 paper .

10) There would be a major peak in the mass spectrum for butan-1-ol, CH3CH2CH2CH2OH, but not for butan-2-ol, CH3CH2CH(OH)CH3, at m/e value :

A) 15

B) 17

C) 29

D) 43

The answer is D but I don't understand why. All replies are much appreciated.

Reply 625

posthumus

Original post by yousefshah77

Butan-1-ol can have CH3CH2CH2 fragment whereas butan-2-ol can't...

the atomic mass of this fragment is 15+14+14= 43

so the answer is D :smile:

Reply 626

Kurraiyo

Original post by SophieL1996

drying agent for ethanol and how should it be used?

Use any of the common drying agents, like anhydrous magnesium sulphate, calcium oxide, or calcium chloride. Add this anhydrous drying agent to ethanol, let it stand for a while then just filter it out, the water will be gone from ethanol :smile:

Reply 627

Kurraiyo

Anyone knows why tertiary alcohols are not oxidised?

Reply 628

edex123

Original post by posthumus

( 0.04/ 10 + 0.04/10 ) x 100 = 0.8%

i got 0.8% too !
But i guess it makes sense to times the error by 2 since we are calculating total error :smile:

Reply 629

own

Original post by Kurraiyo

Anyone knows why tertiary alcohols are not oxidised?

For an alcohol to be oxidised there has to be at least one hydrogen directly bonded to the carbon atom which is also attached to the hydroxide. You need it as water gets produced and uses one hydrogen from te hydroxyl and one from the carbon.

Posted from TSR Mobile

Reply 630

posthumus

Original post by geor

No C-H bonds to break, there's a good explanation on chemguide :smile:

What about "Steric Hinderance", I thought it was something to do with that too ? :confused:

Reply 631

Goods

Original post by posthumus

What about "Steric Hinderance", I thought it was something to do with that too ? :confused:

High steric hindrance means that the carbon attached to the group is surrounded by bulky alkyl groups, the better explanation in this case is the lack of C-H bonds but steric hindrance is good for comparing SN₁ vs SN₂

Reply 632

charlieejobson

guys what test is the Nh4cl? i've never come across this one!

Reply 633

charlieejobson

also for halogenoalkanes, why do you occasionally have to acidify before they react? does it mean adding an alcohol

Reply 634

Goods

Original post by charlieejobson

guys what test is the Nh4cl? i've never come across this one!

To test for the presence of a hydrogen halide. dip a glass rod in the solution (HCl) and hold in ammonia and dense white smoke of NH₄Cl will form.

Reply 635

Goods

Original post by charlieejobson

also for halogenoalkanes, why do you occasionally have to acidify before they react? does it mean adding an alcohol

I'm not sure but i would guess that adding an acid protonates the haloalkane leading to it being more soluble in water. However i've never heard of it and it doesn't make much sense considering the reagents for haloalkane reactions are all alkali :s-smilie:

Reply 636

F1's Finest

Looking forward to this exam. It seems we all know our stuff inside out.

105/120 and Chemistry is a sealed deal for me.

Reply 637

Goods

Original post by James A

Looking forward to this exam. It seems we all know our stuff inside out.

105/120 and Chemistry is a sealed deal for me.

wanting to apply to cambridge is so soul destroying... i need(ish) 120/120 :'(

Reply 638

Goods

Original post by geor

Same looool. We need about 78/80 for a safe 120. Tricky stuff :s-smilie:

I know the chemistry i just don't like the ''How Science Works'' rubbish. Thats where i always lose marks

Reply 639

posthumus

Original post by Goods

Original post by geor

Not in this case I think! Because even if the carbon was attached to smaller groups but there were still no C-H bonds, the oxidation still would not occur.

Thanks guys :smile:

I actually never knew a C-H bond was required to break from the same carbon as that attached to the halogen. I thought maybe the Hydrogen came from the acidic conditions...

.. I guess I'll just have to remember this :biggrin:

but I still hope it does not come up :redface:

"There is no C-H bond attached to the carbon-halogen bond, therefore oxidation is not possible in tertiary alcohols"
Does that sound like an answer that could get me the 2 marks ? :redface:

Original post by James A