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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

Flying grenade SG

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

Flying grenade SG

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?

Flying grenade SG

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.

Thanks ultima! !! That really alleviates my doubt. Thanks for your elaboration and for enlightening me (and all of us)

Flying grenade SG

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'

Flying grenade SG

Page 308 cs toh advanced study guide , has the phrase "electrophilic aromatic substitution reactions"

Flying grenade SG

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

UltimaOnline SG

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

Generally, just follow trusted sources such as CS Toh, Chan Kim Seng & Jeanne Tan, George Chong. If there are any differences between these trusted authors, or even minor inconsistencies within each author, these differences are almost certainly going to be minor to the point of insignificance, and Cambridge will almost certainly accept all their (slightly differing) answers.

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*

Flying grenade SG

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

Flying grenade SG

Page 311 halogenation of methylbenzene have the phrase electrophilic aromatic substitution

Flying grenade SG

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

UltimaOnline SG

Hi ultima, hope u can help me with this,

Page 303 cstoh guide

Halogenation of alkenes to haloalkanes

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

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.

UltimaOnline SG

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

See, your problem is you (and most Singapore JC students) rely on memorizing from notes. If you understood mechanisms, you could have easily figured out the answer for yourself, even if you've never came across such a reaction in your notes.

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.

Flying grenade SG

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?

Flying grenade SG

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

UltimaOnline SG

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?

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.

UltimaOnline SG

Omg !! I can only think of RCOCl group.

For OH group, need PBr3 present.

For HBr to be liberated, need RCOCl and amines

If you can't figure it out, ask your school teacher or private tutor. I'm not gonna spoonfeed answers on the forum.

Flying grenade SG

For a carboxylate salt to carboxylic acid, we add hcl/h2so4/hno3

What's the reaction called ? Acidification? Redox ah?

Flying grenade SG

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 !

UltimaOnline SG

For a carboxylate salt to carboxylic acid, we add hcl/h2so4/hno3

What's the reaction called ? Acidification? Redox ah?

What's the definition of a redox reaction? Work out the OS of all relevant atoms, and see for yourself if it's a redox reaction. Don't just blindly memorize.

UltimaOnline SG

Page 303 cstoh advanced guide, the aqbr2 with alkene have a product HBr !

Yes, because of the mechanism.

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