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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

Hi, may I ask A Level 2012 P3 q5 c?

For this structure elucidation question,I derived that L has no aldehyde/ketone/alcohol/COOH groups.

M has to be an acidic salt, and alkene is present in M.

I have problems deducing M, assuming my L is right.

L is an acid anhydride right? But my L seems to be "cyclic" and I don't know how to add NH3 to it. From the question, it seems that the whole NH3 is added. How can NH3 be added when nothing is removed??

Some of my thinking: L is similar to acyl chloride, so reaction may be similar. But to form salt...?

Correct. Carboxylic acid anhydrides are electrophiles similar to acyl halides.

Draw out (if you're not familiar, you can google it online and familiarize yourself with it, it's compulsory for H3 syllabus, and very useful / good-to-know for H2 syllabus) the addition-elimination mechanism for the nucleophilic acyl substitution, together with a subsequent Bronsted-Lowry acid-base proton transfer.

Only by drawing out the mechanism of this reaction for yourself, will you be able to truly, properly and correctly understand this reaction.

Flying grenade SG

The order of reaction wrt a given reactant is the power to which the reactant's concentration is raised to in the experimentally determined rate equation

the Orders of reactions are experimentally determined quantities

they may or may not be equivalent to the stoichiometric coefficients of the reactants in the balanced chemical equation

what are the significance/implication/meaning when the orders of reactions are the Same as the stoichiometric coefficients of the reactants in the balanced chemical equation?

Flying grenade SG

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

The overall reaction order is the sum x + y + .... For many reactions, the reaction orders are not equal to the stoichiometric coefficients.

Elementary (single-step) reactions do have reaction orders equal to the stoichiometric coefficients for each reactant. The overall reaction order, i.e. the sum of stoichiometric coefficients of reactants, is always equal to the molecularity of the elementary reaction.Complex (multi-step) reactions may or may not have reaction orders equal to their stoichiometric coefficients

UltimaOnline SG

The order of reaction wrt a given reactant is the power to which the reactant's concentration is raised to in the experimentally determined rate equation

the Orders of reactions are experimentally determined quantities

they may or may not be equivalent to the stoichiometric coefficients of the reactants in the balanced chemical equation

what are the significance/implication/meaning when the orders of reactions are the Same as the stoichiometric coefficients of the reactants in the balanced chemical equation?

Hint : draw out the mechanism for such a reaction, then the significance becomes clear.

Flying grenade SG

An elementary reaction is a chemical reactionin which one or more chemical species react directly to form products in a single reaction step and with a single transition state. In practice, a reaction is assumed to be elementary if no reaction intermediates have been detected or need to be postulated to describe the reaction on a molecular scale.^[1]An apparently elementary reaction may be in fact a stepwise reaction, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes.

In a unimolecular elementary reaction, amolecule A dissociates or isomerises to form the products(s)

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

UltimaOnline SG

BedokFunland JC H2 Chemistry Kinetics + Organic Chem Challenge :

For the Singapore 2015 A Levels H2 Chemistry P3 Q4d, draw the full mechanism (including the 2 relevant resonance contributors) to fully explain the kinetics of the reaction.

supercat SG

Correct. Carboxylic acid anhydrides are electrophiles similar to acyl halides.

Only by drawing out the mechanism of this reaction for yourself, will you be able to truly, properly and correctly understand this reaction.

Is it like this? http://imgur.com/mdIfXtV

UltimaOnline SG

Is it like this? http://imgur.com/mdIfXtV

Wrong. You need to reform the acyl group (ie. lone pair on negative formal charged O atom, shifts down to form a pi bond pair) and eliminate the RO leaving group as alkoxide RO-, before proton transfer occurs.

Flying grenade SG

why larger size is a weaker base? less electronegative, lp of e- more readily donated and available , wouldn't it more readily accept proton? larger atomic radius too

i know larger size e- cloud more diffused, less effective overlap when forming covalent bonds, result in weaker covalent bonds

UltimaOnline SG

why larger size is a weaker base? less electronegative, lp of e- more readily donated and available , wouldn't it more readily accept proton?

i know larger size e- cloud more diffused, less effective overlap when forming covalent bonds, result in weaker covalent bonds

I won't spoonfeed you (or anyone) on the forum with the answers. Go figure it out for yourself, and/or go ask your school teacher or private tutor.

What you've described is about nucleophilicity, but Bronsted-Lowry basicity is a different matter altogether. A stronger nucleophile may or may not be a stronger Bronsted-Lowry base, case-by-case basis. Similarly, for ligand strength (which is also a separate matter from ligand field strength).

Flying grenade SG

i think related to charge density and polarisability of e- cloud

a large atomic size affects polarizability by allowing the negative charge on the conjugate base to be distributed better, hence the acid is overall more acidic.

Flying grenade SG

larger ion allows dispersal of charge over a larger area less reactive, less basic , more stable

supercat SG

http://imgur.com/fy4jZXf

Like this??

UltimaOnline SG

http://imgur.com/fy4jZXf

Like this??

supercat SG

Haha thank you :)

Flying grenade SG

https://www.dropbox.com/s/zn8qpqilck3wbvz/20161008_180749-1.jpg?dl=0

NJC 2013 p3 qn 4eiv

Weak base strong acid titration CO32- + HCl

for the calculation of pH when excess HCl is used, do we need to consider common ion effect, where high [H+] from HCl suppresses dissociation of H2CO3

if i consider ntotal from HCl only, i calculate pH = 2.07

if i consider HCl+ 1molH+ from H2CO3 , pH = 1.60

if i consider HCl + 2molH+ from H2CO3 , pH= 1.38

UltimaOnline SG

https://www.dropbox.com/s/zn8qpqilck3wbvz/20161008_180749-1.jpg?dl=0

NJC 2013 p3 qn 4eiv

Weak base strong acid titration CO32- + HCl

for the calculation of pH when excess HCl is used, do we need to consider common ion effect, where high [H+] from HCl suppresses dissociation of H2CO3

if i consider ntotal from HCl only, i calculate pH = 2.07

if i consider HCl+ 1molH+ from H2CO3 , pH = 1.60

if i consider HCl + 2molH+ from H2CO3 , pH= 1.38

Yes, you need to consider common ion effect. In excess HCl, pH is determined by the strongest acid HCl, all the CO3 2- will be protonated and eliminated from solution in the form of CO2.

Flying grenade SG

https://www.dropbox.com/s/yvz3cotvl3r2gt4/20161008_184906-1.jpg?dl=0

https://www.dropbox.com/s/otc8gwvqu6dh0wn/20161008_184909-1.jpg?dl=0

how to distinguish isomer W and X?

why the answer say white solid? phenylamine is colorless-yellow liquid, sodium ethanoate is soluble in water

Flying grenade SG

Yes, you need to consider common ion effect. In excess HCl, pH is determined by the strongest acid HCl, all the CO3 2- will be protonated and eliminated from solution in the form of CO2.

ohh okay wow i see!!

dk why answer write pH= 1.30 though

Flying grenade SG

Yes, you need to consider common ion effect. In excess HCl, pH is determined by the strongest acid HCl, all the CO3 2- will be protonated and eliminated from solution in the form of CO2.

for this qn, conc hcl given is 0.05 M, so i calculate using nHCl = 0.05 ×10/1000

nH+ = nHCl

total vol = 50 + 10 = 60cm3

[H+] = 1/120

pH = -lg [H+] = 2.08 (3sf)

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