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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...

Light5 SG

Lets assume that i know how to assign R and S priorities

To assign these, the lowest priority group has to be oriented away from us..in this case, the lowest priority group is the Hydrogen atom but the problem is that on the Left Hand Carbon atom for each of the structures, the H-atom is shown to lie on the plane rather than pointing out or into the plane...so assuming that we know how to assign R and S, how can we get past this issue?

UltimaOnline SG

Lets assume that i know how to assign R and S priorities

By visualizing the structure vividly in 3D space using your imagination, than slowly rotating (not just the single sigma bond, but move the entire structure) it in your mind, until the lowest priority substituent, is pointing away from yourself. And try using your fingers (orientated in a tetrahedral geometry, eg. left hand 3 fingers in a trigonal pyramidal tripod stand, right hand's forefinger representing lowest priority facing up, before rotating it till it's away from you) to help.

Also see these videos :
https://www.youtube.com/watch?v=auphHXYK8tE
https://www.youtube.com/watch?v=kFD6hzLseVs

Flying grenade SG

What are the causes of amphoterism?

Tried to research online, but to no avail ! The closest relevant info i can find is 'Amphoterism depends on the oxidation states of the oxide' from wiki! But still, this piece of info doesnt help much!

Prior to researching i only know Al2O3 is amphoteric,

but after researching now i know Al(OH)3 , Zn (OH)2, Be(OH)2 are also amphoteric species.

I encountered a qn asking if SrO is amphoteric.

We only learnt and memorised that Al2O3 is amphoteric. So how can we determine(or how can we extrapolate the Only info that we know and memorised Al2O3 is amphoteric) which species is amphoteric when a qn ask?

I know Be2+ and Al3+ have high charge density, but i dk how to link this fact to the 2 ions being able to react with acid and bases

Is the reason for amphoterism high charge density? If it is, then i just memorise which ions have high charge density then amphoteric species?

Thanks for helping.

UltimaOnline SG

What are the causes of amphoterism?

Prior to researching i only know Al2O3 is amphoteric,

but after researching now i know Al(OH)3 , Zn (OH)2, Be(OH)2 are also amphoteric species.

I encountered a qn asking if SrO is amphoteric.

We only learnt and memorised that AL2O3 is amphoteric. So how can we determine which species is amphoteric is a qn asked?

A level shortcut : covalent = acidic, ionic = basic, hence partial covalent partial ionic = amphoteric.

BedokFunland JC (ie University) level : to understand identify and understand why & how a species is amphoteric, and why the A level shortcut works (it's not an explanation, but merely an indirect indicator), you first need to understand why & how an acidic species is acidic, and why & how a basic species is basic, and you *must* draw out the curved-arrow electron-flow reaction mechanisms. It's too much detail to go into on the forums, and my BedokFunland JC students can ask me to teach them these mechanisms, while the rest of you can go ask your school teacher or private tutor if interested.

So do you think strontium oxide qualifies to be amphoteric? Btw, amphiprotic is a subset of amphoteric, but not vice-versa. Don't confuse these terms. Go ask your school teacher or private tutor if interested.

Flying grenade SG

http://papers.gceguide.com/A%20Levels/Chemistry%20(9701)/9701_w13_qp_43.pdf

http://papers.gceguide.com/A%20Levels/Chemistry%20(9701)/9701_w13_ms_43.pdf

http://papers.gceguide.com/A%20Levels/Chemistry%20(9701)/9701_w13_er.pdf

In Q6 part f(i) , i dont understand 2 things.

First of all, i dont understand why G and H are not similar to F...if we rotate both the left and right hand side of the molecule, shouldnt we get F.

Secondly, how does rotating right hand side of J by 60o clockwise make ot identical to F as suggested by the examiner report. And why is Chirality of Left hand carbon of both G and H opposite to that of F.

After all this, how to solve f(iii) ie drawing the fourth optical isomers.

Is this related to R,S configurations or is this something else...as to where can i read more about this?

If you could explain this i detail, i would be grateful..thanks.

Hi Light5! Possible for u to upload a screenshot of the qn in discussion on top of the link of the source of the paper in the future ?

Some of us can't access/view the link

Would be awesome if u could upload a pic of the qn in discussion plus the link source of the content!

Thanks (:

Flying grenade SG

A level shortcut : covalent = acidic, ionic = basic, hence partial covalent partial ionic = amphoteric.

Thanks for helping !

amphoteric compound is a molecule or ion that can React both as an acid or a base

An amphiprotic molecule (or ion) can either donate or accept a proton, thus Acting either as an acid or a base.

For jc students : examples of the aforementioned can be found in the wikipedia page

SrO cannot be amphoteric. Charge density of Sr2+ not large enough to polarise Oatom's e- cloud to cause the SrO bond to become partially ionic partially covalent

Flying grenade SG

Oops

Extracted this from my notes which in turn was extracted from kim seng chan's book

Cause for amphoteric

High charge density high polarising power, polarises water molecules and weakens O-H bonds in water molecules, enabling water to protonate forming H3O+, causing the solution to be acidic, enabling it to react with strong bases

Explain why BeO is amphoteric whereas the other group 2 oxides are basic

Due to high charge density of Be ion, there is covalent character in Beryllium oxide. Be atom is highly electron deficient, giving it acidic characteristic, hence enabling it to react with strong bases such as NaOH. Presence of O2- allows BeO to react with acid to form salt and water

UltimaOnline SG

Hi Light5! Possible for u to upload a screenshot of the qn in discussion on top of the link of the source of the paper in the future ?

Some of us can't access/view the link

Would be awesome if u could upload a pic of the qn in discussion plus the link source of the content!

Thanks (:

With the filename given, you can look up the same Cambridge paper yourself from other similar websites, eg. XtremePapers.com

Flying grenade SG

N doesn't withdraw electron density from O, but it just donates less to O than either H or R would. Can you see how that results in a less polar O-H bond? Or do you need me to explain further?

Hmm i try,

O is more electronegative than N. O withdraws some e- density from N. Magnitude of delta -ve on O decreases. The O-H bond in hydroyxlamine is less polar compared to O-H bond in water. Hence strength of H bond between hydroxylamine molecules is weaker than strength of Hbond between water molecules.

Hence water has a higher B.P. than hydroxylamine even though hydroxylamine can from more Hbonds per molecule than water.

Flying grenade SG

Can help check or improve?

In any case, Thanks for your help and explanations in helping me solve chem qns !

UltimaOnline SG

Hmm i try,

Hence water has a higher B.P. than hydroxylamine even though hydroxylamine can from more Hbonds per molecule than water.

Indeed, the O-H bond in hydroxylamine is less polar, compared to in H2O or in alcohols, due to the electronegative N atom. So far so good. But here's where your school cocked up : the boiling point of hydroxylamine should actually be higher, not lower, than H2O, as number of H bonds formed is more important than strength of H bonds. Simply look at their melting points, and you'll see I'm right. However, in practice, hydroxylamine never gets to reach its theoretical boiling point (ie. above 100 deg C), because it readily decomposes (at 58 deg C) into water, ammonia, and either dinitrogen monoxide or nitrogen gas (depending on mechanism pathway and reaction conditions).

Flying grenade SG

Page 160 cs toh advanced guide

'Maximum buffer capacity' section

Is it that for an acidic buffer or alkaline buffer, theoretically, the buffer can have the pH of the solution remains(almost) unchanged, when acid or base of concentration of up to 10 times the concentration of the weak acid / weak base, added to the buffer solution?

UltimaOnline SG

'Maximum buffer capacity' section

Yes, mathematically, a solution can be considered a valid buffer as long as the ratio of molarities of conjugate acid-base pairs lies within 1:10 and 10:1, ie. pH = pKa (+1 or - 1)

Flying grenade SG

Wow!!! Okay damn cool!!! Thanks for these info, really appreciate it (:

Flying grenade SG

Yes, mathematically, a solution can be considered a valid buffer as long as the ratio of molarities of conjugate acid-base pairs lies within 1:10 and 10:1, ie. pH = pKa (+1 or - 1)

Woah damn cool add 5moldm-3 of acid/base to a buffer solution comprising of 0.5moldm-3 weakacid/base , ph still remains unchanged

UltimaOnline SG

Woah damn cool add 5moldm-3 of acid/base to a buffer solution comprising of 0.5moldm-3 weakacid/base , ph still remains unchanged

A buffer solution's pH changes *slightly* when a *small* amount of strong acid or base is added to it. Note that highlighted words. And Cambridge will require you to write equations to explain exactly how a buffer works. The exact equations will of course, depend on exactly what is the conjugate acid-base pair of the buffer solution.

Qn : Why did the acid go to the gym?
Ans : http://www.infinity.usanethosting.com/Tuition/#BedokFunlandJC_Chemistry_Qns

Flying grenade SG

Regarding " As a chemist, ...Edited by UltimaOnline" post , schools (or at least my current jc) still uses mercury thermometers ! Darn it

Thanks for the toxicity of mercury tip and advice!

Flying grenade SG

Qn : Why did the acid go to the gym?
Ans : http://www.infinity.usanethosting.com/Tuition/#BedokFunlandJC_Chemistry_Qns

Thank you for elaborating!!!

Yep sorry, i meant to write pH remains almost unchanged. I'm just marvelled by the fact that buffer solution can buffer up to 10x its concentration

Flying grenade SG

http://www.infinity.usanethosting.com/Tuition/H2Chem_What_is_this_Functional_Group.jpg

For qn 1

Either ester or tertiary amide is it?

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