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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

oh -3 because three dative bonds to co, +5 because co connected five more electronegative atoms.

its like when calculating o.s. , treat the lp from N as donated and then (an e-) attracted back to it at the same time, lol. but its great, this is how chemistry works

Good that u understand this (both the calculation and how chemistry works).

Flying grenade SG

Didn't u understand what I wrote? Obviously not (both u and the lone pair).

ok yes i got it alr ! lp on N in pyridine resides in sp2 hybridised orbital. not needed in pyridine's aromaticity ! hence pyridine more basic and nucleophilic than phenylamine !

Thanks, god !

Flying grenade SG

Good that u understand this (both the calculation and how chemistry works).

Thanks, God!

Flying grenade SG

https://www.dropbox.com/s/837vt8arsov7rgr/20160913_084805.jpg?dl=0

https://www.dropbox.com/s/53zyvau1gruxljg/20160913_084812.jpg?dl=0

can this be considered tautomerisation?

in the first step, the pi bond is broken , and there is a intramolecular transfer of a proton

Flying grenade SG

side chain oxidative cleavage of Ethylbenzene yields benzoic acid, CO2 + H2O?

i thought it's benzoic acid and ethanoic acid?

i don't get how ethanedioic acid is produced, which further oxidises to CO2 and H2O

for the oxidative cleavage, isn't only one side of the Ethyl have a new cooh attached, it is both side, to form ethanedioic acid?

Flying grenade SG

when Al lose 3 e- through ionic bonding to form Al3+ , it has tripositive formal charge

Al in Alcl3 is e- deficient, having 6 e- around it , 3 bp, 0lp. Al in group 13, no formal charge

in Al2Cl6, is the formal charge of Al -1 , since it has 4 bp around it? but Al2Cl6 is indeed a neutral molecule

Flying grenade SG

for the acidic hydrolysis of nitrile to form carboxylic acid, i was penalised for using Conc h2so4, HUR, instead of dilute h2so4, heat under reflux

is it liable to be penalised for reagents and conditions that can work, albeit unnecessary?

e.g. a rxn can procede with conc acid, or another rxn can proceed without heat, e.g. for the electrophillic addn of Br2 (aq) to phenol, or E add of hbr to alkene, can happen at room temp, but if a student write heat, will still be penalised becos unsure the rxn need input (heat) energy or not?

Flying grenade SG

interesting that N1 and N4 forms a covalent bond with Zn

https://www.dropbox.com/s/egqpp5rzfk1b4hg/20160913_144056-1.jpg?dl=0

then it's like Zn as a metal didn't really get to donate(as in ionic bonding) it's two 4s electrons, it's merely sharing

also, Zn's electronic configuration in its ground state is alr [Ar] 3d10 4s2

how does Zn still have vacant, energetically accessible orbitals to accept dative bond from N2 and N4?

Flying grenade SG

2011 p3 qn 2 iv

need to calculate pH at end point for the titration curve

cstoh's and sean chua's didn't indicate the pH of the two end points of this diprotic acid

is it look at the marks and evaluate whether cambridge want us to indicate pH at end point on the titration curve?

UltimaOnline SG

https://www.dropbox.com/s/837vt8arsov7rgr/20160913_084805.jpg?dl=0

https://www.dropbox.com/s/53zyvau1gruxljg/20160913_084812.jpg?dl=0

can this be considered tautomerisation?

in the first step, the pi bond is broken , and there is a intramolecular transfer of a proton

-____-"

Obviously not lah! First step is Bronsted-Lowry acid-base deprotonation, followed by nucleophilic addition. The product has 2x the no. of C atoms as the reactant, so how can they be structural isomers (ie. tautomers) of each other?!? Sheeeeesshh!!!

-_________-"

UltimaOnline SG

side chain oxidative cleavage of Ethylbenzene yields benzoic acid, CO2 + H2O?

i thought it's benzoic acid and ethanoic acid?

i don't get how ethanedioic acid is produced, which further oxidises to CO2 and H2O

for the oxidative cleavage, isn't only one side of the Ethyl have a new cooh attached, it is both side, to form ethanedioic acid?

Ask your school teacher or private tutor to explain the mechanism for you, then you'll understand. Move on.

UltimaOnline SG

when Al lose 3 e- through ionic bonding to form Al3+ , it has tripositive formal charge

Al in Alcl3 is e- deficient, having 6 e- around it , 3 bp, 0lp. Al in group 13, no formal charge

in Al2Cl6, is the formal charge of Al -1 , since it has 4 bp around it? but Al2Cl6 is indeed a neutral molecule

2 of the Cl atoms (the dative bond donors) have a unipositive formal charge. Hence overall neutral molecule.

UltimaOnline SG

for the acidic hydrolysis of nitrile to form carboxylic acid, i was penalised for using Conc h2so4, HUR, instead of dilute h2so4, heat under reflux

is it liable to be penalised for reagents and conditions that can work, albeit unnecessary?

Concentration H2SO4 is for dehydration, not hydrolysis. Of course u'll be penalized lah! Sheeesshh!!!

Yes, u'll be penalized if u write heat when rtp will suffice, coz it tells Cambridge that u dunno ur Chem reagents & conditions, and no boss will hire u as a chemist coz u're wasting his electricity and money and resources to do unnecessary nonsense.

UltimaOnline SG

interesting that N1 and N4 forms a covalent bond with Zn

https://www.dropbox.com/s/egqpp5rzfk1b4hg/20160913_144056-1.jpg?dl=0

then it's like Zn as a metal didn't really get to donate(as in ionic bonding) it's two 4s electrons, it's merely sharing

also, Zn's electronic configuration in its ground state is alr [Ar] 3d10 4s2

how does Zn still have vacant, energetically accessible orbitals to accept dative bond from N2 and N4?

Read my reply to Gohby's question at the bottom of the page.

http://sgforums.com/forums/2297/topics/491191?page=10#posts-11184163

UltimaOnline SG

2011 p3 qn 5 iv

need to calculate pH at end point for the titration curve

cstoh's and sean chua's didn't indicate the pH of the two end points of this diprotic acid

is it look at the marks and evaluate whether cambridge want us to indicate pH at end point on the titration curve?

If in doubt whether it's required, just do it.

Flying grenade SG

thanks god for your replies.

Flying grenade SG

Read my reply to Gohby's question at the bottom of the page.

http://sgforums.com/forums/2297/topics/491191?page=10#posts-11184163

in this scenario, is the e config of zn [Ar] 3d10?

Flying grenade SG

If in doubt whether it's required, just do it.

ok got it. for 1st end point, use amphiprotic formula

for 2nd end pt, use kb , find poh then ph

Flying grenade SG

retweet throwback

http://sgforums.com/forums/2297/topics/325645

written 2008

Flying grenade SG

YJC 2016 prelim

https://www.dropbox.com/s/j3ilvq87qk5grym/20160913_172208-1.jpg?dl=0

Answer given is In step 2, PCl5 will be hydrolysed in the presence of water

Heat is required for step 4

consider this qn

https://www.dropbox.com/s/g4mhqly1zxjp0sw/20160913_171845-1.jpg?dl=0

answer : https://www.dropbox.com/s/ftwnj586zg1x5ij/20160913_172024.jpg?dl=0

i dun understand option A

for option B, in anhydrous conditions, i know HCl(g) will be generated when PCl5 is reacted with R-OH.

in this scenario, in aqueous conditions, this rxn is flawed / cannot happen right?

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