Originally posted by UltimaOnline:
See if you're intelligent enough to see how the following Singaporean link explains the answer to this H2 Chemistry question.http://mothership.sg/2016/05/wear-white-movement-to-be-held-on-the-same-day-as-pink-dot-yet-again/
Whether you get it or not, don't post further on this topic on this forum, don't spoil the fun for others! *evil laugh*
Read the article in the link already, but can't draw the link
Nvm, i found the answer that you've written 7 years ago here : http://sgforums.com/forums/2297/topics/320107?page=6
Originally posted by UltimaOnline:
Allow BedokFunland JC to enlighten thee :As far as 2,4-DNPH is concerned, Cambridge doesn't give a flying f**k whether you write "room temperature" or "warm" or "heat" or even if you don't specify the temperature at all.
However, don't mistake Cambridge's kind reasonable tolerance for weak bochup nonchalance : between PCl3 and PCl5, you *must* specify "heat" for 1 of these reagents, or Cambridge will penalize you. If Cambridge asks you to explain why this difference, do you know how to answer?
(let's hope for more such out-of-the-box questions for this year's and all future Singapore H2 Chemistry papers, shall we? That will shut up those armchair critics who take perverse pleasure in criticizing the entire Singapore education system to be all about blind memorization without deeper understanding... such criticism applies more for PSLE, N and O levels, but less so for A levels and certainly not Uni levels; and among A level subjects, such criticism is even less valid for Chemistry than other A level subjects, hopefully Cambridge will continue this trend for Singapore's H2 Chemistry).
Pcl3 requires heat cus pcl3 less reactive compared to pcl5, pcl3 has higher Ea and hence require heat energy supplied to rxn?
Originally posted by UltimaOnline:
Allow BedokFunland JC to enlighten thee :As far as 2,4-DNPH is concerned, Cambridge doesn't give a flying f**k whether you write "room temperature" or "warm" or "heat" or even if you don't specify the temperature at all.
However, don't mistake Cambridge's kind reasonable tolerance for weak bochup nonchalance : between PCl3 and PCl5, you *must* specify "heat" for 1 of these reagents, or Cambridge will penalize you. If Cambridge asks you to explain why this difference, do you know how to answer?
(let's hope for more such out-of-the-box questions for this year's and all future Singapore H2 Chemistry papers, shall we? That will shut up those armchair critics who take perverse pleasure in criticizing the entire Singapore education system to be all about blind memorization without deeper understanding... such criticism applies more for PSLE, N and O levels, but less so for A levels and certainly not Uni levels; and among A level subjects, such criticism is even less valid for Chemistry than other A level subjects, hopefully Cambridge will continue this trend for Singapore's H2 Chemistry).
Thanks ultima! !! That really alleviates my doubt. Thanks for your elaboration and for enlightening me (and all of us)
I've understood the post that you've written in 2009,
My qn is do we need to write LiAlH4 in dry ether, r.t., followed by H2O?
Or can omit the followed by H2O?
Different year(even in the same jc) wrote the followed by H2O,some years they take it out or put it in, different sources(different jc's lecture notes, and books) have, some don't have 'followed by H2O'
Page 308 cs toh advanced study guide , has the phrase "electrophilic aromatic substitution reactions"
Yeah okay, i shall stop asking u about reagents and conditions, too many differing R&C
Cs toh advanced study guide pg 308 indicated halogen of benzene involves halogen carriers and at room temp. But it's summary at pg 315 indicate it needs heat.
Cs toh wrote friedel crafts reactions require heat, but the small hci book written by pang peng cheong wrote r.t.
Nvm, I'll just write one of them in the exams
Originally posted by Flying grenade:Yeah okay, i shall stop asking u about reagents and conditions, too many differing R&C
Cs toh advanced study guide pg 308 indicated halogen of benzene involves halogen carriers and at room temp. But it's summary at pg 315 indicate it needs heat.
Cs toh wrote friedel crafts reactions require heat, but the small hci book written by pang peng cheong wrote r.t.
Nvm, I'll just write one of them in the exams
Don't OCD about it lah. Be like me, lepak lepak, relak relak, anyhow do, anyhow write, anyhow skip, also get my A grade. *GuaiLanz LOL*
Hi ultima, hope u can help me with this,
Page 303 cstoh guide
Halogenation of alkenes to haloalkanes
Bottom of page, i don't understand why cs toh wrote , if bubbled a gas into aqueous Br2, if reddish-brown Br2 soln is rapidly decolorised, (without evolution of fuming HBr gas) indicates presence of c=c bond .
On top, the chemical equation has HBr as a product
Is it because not fuming(means not concentrated) ah?
Why did cs toh bring this up?
Is it because of ethanoyl bromide ?
https://en.m.wikipedia.org/wiki/Acetyl_bromide
Checked(from bp and mp), it's a liquid at r.t.
But it would hydrolyse readily in moist air to produce ethanoic acid and hydrobromic acid (HBr) ?
Why cs toh mentioned *without evolution of fuming HBr gas* ?
Thank you
Page 311 halogenation of methylbenzene have the phrase electrophilic aromatic substitution
Hi ultima, hope u can help out on this !
https://www.dropbox.com/s/fzofr56zwq6qdby/20160525_230655.jpg?dl=0
What product could it be?? Checked reactions, reagents and conditions for methylbenzene and alkenes, don't have that reagent and conditions!
Update : found soln
Electrophilic Additions to Alkenes
Reagents : HBr (gas) or
NaBr/H2SO4/H2
Elements Added :H-Br
Originally posted by Flying grenade:Hi ultima, hope u can help me with this,
Page 303 cstoh guide
Halogenation of alkenes to haloalkanes
Bottom of page, i don't understand why cs toh wrote , if bubbled a gas into aqueous Br2, if reddish-brown Br2 soln is rapidly decolorised, (without evolution of fuming HBr gas) indicates presence of c=c bond .
On top, the chemical equation has HBr as a product
Is it because not fuming(means not concentrated) ah?
Why did cs toh bring this up?
Is it because of ethanoyl bromide ?
https://en.m.wikipedia.org/wiki/Acetyl_bromide
Checked(from bp and mp), it's a liquid at r.t.
But it would hydrolyse readily in moist air to produce ethanoic acid and hydrobromic acid (HBr) ?
Why cs toh mentioned *without evolution of fuming HBr gas* ?
Thank you
Guess what these 2 functional groups are? I don't want to spoil the fun, so I won't spoonfeed you guys the answer on the forum. I'll encourage everyone reading this to figure it out for yourself.
Originally posted by Flying grenade:Hi ultima, hope u can help out on this !
https://www.dropbox.com/s/fzofr56zwq6qdby/20160525_230655.jpg?dl=0
What product could it be?? Checked reactions, reagents and conditions for methylbenzene and alkenes, don't have that reagent and conditions!
Update : found soln
Electrophilic Additions to Alkenes
Reagents : HBr (gas) or
NaBr/H2SO4/H2
Elements Added :H-Br
Sure, you can google out the answer now (ie. non-exam conditions), but what happens when Cambridge thinks up some new reaction that you've never come across before, to be used as an A grade exam question in the A levels?
Well, I wouldn't be surprised if the majority of Singapore JC students wouldn't know the answer as well, in such a case, so I guess you needn't worry too much. It's a bell-curve, afterall.
Originally posted by UltimaOnline:
CS Toh was deliberate and conscientiousness when he wrote that. If Br2 (in CCl4) was used on the analyte, and decolorization of the Br2, together with steamy fumes (indicating HBr) were both observed, that would be indicative of 2 other functional groups (covered in the H2 syllabus), but not the alkene functional group.Guess what these 2 functional groups are? I don't want to spoil the fun, so I won't spoonfeed you guys the answer on the forum. I'll encourage everyone reading this to figure it out for yourself.
I see. No HBr added when using Br2 in ccl4. But there is HBr formed when using aq Br2. How? Is it for aqbr2 rxn with alkene, the HBr less concentrated or something?
Originally posted by UltimaOnline:
CS Toh was deliberate and conscientiousness when he wrote that. If Br2 (in CCl4) was used on the analyte, and decolorization of the Br2, together with steamy fumes (indicating HBr) were both observed, that would be indicative of 2 other functional groups (covered in the H2 syllabus), but not the alkene functional group.Guess what these 2 functional groups are? I don't want to spoil the fun, so I won't spoonfeed you guys the answer on the forum. I'll encourage everyone reading this to figure it out for yourself.
Omg !! I can only think of RCOCl group.
For OH group, need PBr3 present.
For HBr to be liberated, need RCOCl and amines
Originally posted by Flying grenade:I see. No HBr added when using Br2 in ccl4. But there is HBr formed when using aq Br2. How? Is it for aqbr2 rxn with alkene, the HBr less concentrated or something?
Originally posted by Flying grenade:Omg !! I can only think of RCOCl group.
For OH group, need PBr3 present.
For HBr to be liberated, need RCOCl and amines
For a carboxylate salt to carboxylic acid, we add hcl/h2so4/hno3
What's the reaction called ? Acidification? Redox ah?
Originally posted by UltimaOnline:
No you're wrong. And it's always because of the mechanisms. Draw them out, instead of blindly memorizing. If you need guidance drawing the mechanisms, go ask your school teacher or private tutor.
Page 303 cstoh advanced guide, the aqbr2 with alkene have a product HBr !
Originally posted by Flying grenade:For a carboxylate salt to carboxylic acid, we add hcl/h2so4/hno3
What's the reaction called ? Acidification? Redox ah?
Originally posted by Flying grenade:Page 303 cstoh advanced guide, the aqbr2 with alkene have a product HBr !
I am so dead. This i also dk how do
Ultima help me please
https://www.dropbox.com/s/52xg56a9dwrn9zp/20160526_125914.jpg?dl=0
Why is left C oxidation state of -3?
Is it because from
https://en.m.wikipedia.org/wiki/Electronegativities_of_the_elements_(data_page), carbon more e-ve than hydrogen?
In between C-C bond no electron withdrawing or donating, cus e-ve same between 2 carbon atoms, assume both share e- equally, hence C connected to C do not affect oxidation state right?
For ethanoate ion, i can't find my mistake! ! Help me please T-T. The O with mononegative charge has oxidation state of -1 right?
For acidification, is it right to write proton transfer acidification reaction?
I'm asking this because, for example, there's no meaning in stating 'esterification' when forming ester, we write condensation+NU Sub , etc
So just wondering if there's a more precise way to describe acidification
Originally posted by Flying grenade:I am so dead. This i also dk how do
Ultima help me please
https://www.dropbox.com/s/52xg56a9dwrn9zp/20160526_125914.jpg?dl=0
Why is left C oxidation state of -3?
Is it because from
https://en.m.wikipedia.org/wiki/Electronegativities_of_the_elements_(data_page), carbon more e-ve than hydrogen?
In between C-C bond no electron withdrawing or donating, cus e-ve same between 2 carbon atoms, assume both share e- equally, hence C connected to C do not affect oxidation state right?
For ethanoate ion, i can't find my mistake! ! Help me please T-T. The O with mononegative charge has oxidation state of -1 right?
Originally posted by Flying grenade:For acidification, is it right to write proton transfer acidification reaction?
I'm asking this because, for example, there's no meaning in stating 'esterification' when forming ester, we write condensation+NU Sub , etc
So just wondering if there's a more precise way to describe acidification
Originally posted by UltimaOnline:
Obviously. OS = -1 + -1 = -2
I found my mistake le. Lmao!!! O.s. of O is -2.
Thanks ultima !!!