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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...

UltimaOnline SG

some Continual Assessment test

https://www.dropbox.com/s/y6y6ajf0pzvaptv/20160913_204221-1.jpg?dl=0

for what type of reaction is the reaction in III, cher say it's disproportionation/redox

can i say it's disproportionation redox, acid base reaction?

Aldehydes can undergo rxn with NaOH(aq) just like that by itself without heat? do we need to know that aldehydes can ketone can react with strong bases?

https://www.dropbox.com/s/hzuqucmgqglddfi/20160913_204209-1.jpg?dl=0

option 3, fine , it's correct, 9 chiral centres after reaction with LiAlH4

have five sets of two-adjacent chiral carbons in a ring, total of 7 such carbons in this case

which set of two-adjacent chiral carbons can only produce 2 optical isomers?

how many stereo optical isomers are there in this molecule?

U coconaden! That's the Cannizzaro reaction lah! (pronounced machiam hokkien KAN-NI-ZAR-RO). Overall the only acceptable adjective is redox disproportionation (or disproportionation redox, Cambridge accepts either) reaction. It involves a hydride H- transfer, followed by a proton H+ transfer at the end. So to say it's a Bronsted-Lowry acid-base proton transfer, rather than more accurately a redox disproportionation reaction, is misleading and credit unworthy.

Due to the bicyclic rings, 4 chiral C atoms are 'merged' into 2 chiral C atoms, as far as calculation of stereoisomers are concerned.

However, because this is considered beyond A levels, hence in the A level exam, especially if it's MCQ, you may have to consider either possibility as correct (ie. Cambridge may think A level students aren't smart enough to understand or recognize the problematic bicyclic ring circumstance, and so the 'correct' answer may actually be the 'wrong' one; so if both possibilities appear as different options, then u jia lat liao).

If it's a P2 or P3 qn, the exam-smart BedokFunland JC student will give both possible answers, with qualifications and explanations.

Besides, this question (and hopefully Cambridge's questions) only asked for how many chiral C atoms, instead of asking how many stereoisomers are possible.

Flying grenade SG

Both u and YJC teacher made an error here (urs way more coconaden and unforgivable!).

U coconaden, anhydrous doesn't mean no solvent! The reactant is solvated (not solid) and thus can exists in a solution, just not with water (ie. not aqueous solution, a non-aqueous solution lah)!

The solvent has to be inert (ie. no protic OH or NH groups), so that it won't use up the PCl5 or SOCl2 reagent.

The YJC teacher's error : if it's an aprotic solvent which won't react with PCl5 or SOCl2, then neither will [H+] in solution increase, and hence there'll be no change in pH.

But for A level purposes, Cambridge may willingly commit the same 'error' as ur YJC teacher (ie. they may set this same qn), coz they don't expect A level H2 students to think deeply enough to recognize this as an error.

great thanks. now that you wrote what anhydrous means, ppl reading this will finally know

so if the soln is inert, after pcl5 or socl2 reacted with -OH, the HCl will escape the reaction mixture as HCl(g)?

Flying grenade SG

Both u and YJC teacher made an error here (urs way more coconaden and unforgivable!).

U coconaden, anhydrous doesn't mean no solvent! The reactant is solvated (not solid) and thus can exists in a solution, just not with water (ie. not aqueous solution, a non-aqueous solution lah)!

The solvent has to be inert (ie. no protic OH or NH groups), so that it won't use up the PCl5 or SOCl2 reagent.

The YJC teacher's error : if it's an aprotic solvent which won't react with PCl5 or SOCl2, then neither will [H+] in solution increase, and hence there'll be no change in pH.

ok dafuq, means we do tick the option as Correct in the exam, that anhydrous socl2 will result in a decrease in pH of the soln , even though we know that it won't, in a inert soln(e.g. ccl4)? force yourself get confused like this, not healthy

and is it like Since we know socl2 or pcl5 must react in anhydrous condition, if Cambridge present the question similar to above, do u think they expect us to think that the solvent must be inert?

Flying grenade SG

wow, i hope they want us to select the correct Chemistry option, if this unfortunate event happens. i will take my chances and gamble or give or take that 1 mark away and shade the correct Chemistry answer

UltimaOnline SG

great thanks. now that you wrote what anhydrous means, ppl reading this will finally know

so if the soln is inert, after pcl5 or socl2 reacted with -OH, the HCl will escape the reaction mixture as HCl(g)?

Yes.

UltimaOnline SG

and is it like Since we know socl2 or pcl5 must react in anhydrous condition, if Cambridge present the question similar to above, do u think they expect us to think that the solvent must be inert?

wow, i hope they want us to select the correct Chemistry option, if this unfortunate event happens. i will take my chances and gamble or give or take that 1 mark away and shade the correct Chemistry answer

Eh don't OCD and worry so much lah. It's a bell-curve afterall. If it's not fair to everyone, it's still fair. Be like me, any ambiguous qn like these, just make a relaxed, stress-free educated guess on what Cambridge wants (based on what Cambridge thinks A level students know) and make ur choice, or (if u're really good enough like me), skip the qn also still can get A grade. *evil laugh*

Flying grenade SG

okay thanks Ultima. you are my inspiration

UltimaOnline SG

Ex-RJC students post their condolences on Facebook RJC Confessions, and hope to guide current RJC students stressed out with life.

https://www.facebook.com/RJConfessions

Flying grenade SG

means the cobalt form covalent bond with both the O atoms?

in some organometallic complex like these, the central transition metal share e- to form covalent bond , instead of donating it as in ionic bonding

Flying grenade SG

https://www.dropbox.com/s/z3toihywdv16i6j/20160914_122128-1.jpg?dl=0

for test using barium nitrate, why need acidify?

ok got it. cos white ppt might be zinc hydroxide . zinc hydroxide dissolves in excess hno3 when OH- not enough such that I.P. > ksp.

why need to test if the sulfate solid salt that is white in color, will decompose under heat or not?

oh cos if it's PbCl2, PbCl2 can decompose, or rather, more likely/easier to decompose compared to PbSO4?

so if test for SO42- using pb(no3)2 , its like both pbcl2 and pbso4 will be precipitated. both are white in color. then u heat it. then after pbcl2 all thermally decomposed alr, then if have a white solid remaining that does not decompose under heat, is pbso4?

UltimaOnline SG

means the cobalt form covalent bond with both the O atoms?

in some organometallic complex like these, the central transition metal share e- to form covalent bond , instead of donating it as in ionic bonding

Covalent with ionic character.

UltimaOnline SG

https://www.dropbox.com/s/z3toihywdv16i6j/20160914_122128-1.jpg?dl=0

for test using barium nitrate, why need acidify?

ok got it. cos white ppt might be zinc hydroxide . zinc hydroxide dissolves in excess hno3 when OH- not enough such that I.P. > ksp.

why need to test if the sulfate solid salt that is white in color, will decompose under heat or not?

oh cos if it's PbCl2, PbCl2 can decompose, or rather, more likely/easier to decompose compared to PbSO4?

Unlike silver(I) halides, lead(II) halides are soluble in hot water.

Flying grenade SG

if Cambridge ask us to write eqn with state symbols including p4o6,

if we write (l) do we get penalised?

from wiki, m.p. p4o6 = 23.8°c , so p4o6 exists as liquid in r.t. liao

cs toh book writes it as p4o6 in solid state

Flying grenade SG

why does most ppt dissolve in higher temp? as temp increase, posn of eqm shift right, favours dissociation?

Flying grenade SG

UltimaOnline SG

if Cambridge ask us to write eqn with state symbols including p4o6,

if we write (l) do we get penalised?

from wiki, m.p. p4o6 = 23.8°c , so p4o6 exists as liquid in r.t. liao

cs toh book writes it as p4o6 in solid state

By convention, use (s). If u wish, u can write (l) with an annotation that "since m.p. p4o6 = 23.8°c". But bear in mind that "room temp" can be either 20°C or 25°C in Chemistry (hence should always specify exactly which).

UltimaOnline SG

why does most ppt dissolve in higher temp? as temp increase, posn of eqm shift right, favours dissociation?

https://www.youtube.com/watch?v=AeX2I8ka0XI

Flying grenade SG

https://www.youtube.com/watch?v=AeX2I8ka0XI

nice

Flying grenade SG

oh wow ok i see, thanks god !

yes, should always indicate. my school trains all of us always write standard conditions, of 298K and 1 atm.

cs toh writes standard cond as 25°c, 1atm.

https://en.m.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

writes

Flying grenade SG

LiAlH4 can reduce everything except alkenes

NaBH4 can only reduce carbonyl compounds

H2, Ni , heat or H2,Pt can reduce everything(except carboxylic acid? idk?) ?

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