"May you enjoy Chemistry (or any subject) as your very own (ie. make it something you *own for yourself*, and don't regard it as something owned or dictated by anyone else, eg. your school teachers; respect yourself as your own Master of Chemistry). Only with self-ownership, will self-motivation and self-excellence come naturally."

- Mr Heng (BedokFunland JC) aka UltimaOnline
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"He who asks questions may appear to be a fool for that moment;
but he who does not ask, remains a fool forever." - Anonymous
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My tuition students Novabelle Ng (Victoria JC) and Calvin Seah (Nanyang JC) scoring 'A' grade distinctions for their H2 Chemistry & H2 Biology (Tuition subjects) in their recent 2008 'A' level exams.
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See if you can solve these Chemistry Challenge Questions!
Click here for regular updates of new Chemistry questions by me, UltimaOnline!
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The  Ultimate Periodic Table
(An excellent buy; use debit card to purchase from Amazon)

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Click to enlarge the following useful images, and feel free to copy-&-paste the image's URL to use online :

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Stressed by school? Then you will want to come for my Tuition!
 

Parent / Student : "(My son/daughter is) / (I am) already very stressed with all the homework assignments from school. I don't know if he/she/I can tahan any more stress by having tuition!"

Mr Heng : "My Tuition is here to help reduce your stress, not to give you more of it! If you're unable to understand your Science teacher in school, inevitably you're going to be stressed. I'll help you understand your Science, at your own personal pace, something your school teacher cannot do (as he/she has to handle an entire class at once). If you're stressed and overburdened with homework, simply bring your homework assignments to our tuition sessions, and I'll help you out with them (I'm not going to do your homework for you, of course; instead I'll help you to understand the questions, the relevant concepts involved, and I'll guide your problem solving process and check your answers). Remember, my Tuition is here to reduce your stress and help you enjoy your subject!"
 

Greets!

Thank you for visiting my webpage. I'm Mr Heng (mobile phone 8189 4871), a Chemistry & Biology full-time Tuition Teacher offering Tuition services to 'A' level and 'O' level students, held at my place of residence in Lagoon View (between Bedok / Marine Parde). I am a former Ministry of Education (MOE) Teacher, and have over 5 years of experience teaching Chemistry, Biology and Lower Secondary Science at a local Secondary School, before I resigned several years ago to become fully self-employed. I hold a Bachelor's Degree in Science (double major) and a Diploma in Education. And because I love Teaching so much, I've now continued doing this in the form of Tutoring! :)

My Dad, (Mr Heng Hwa Heng), a retired MOE-teacher who taught Mathematics for over 40 years at various Secondary Schools and Junior Colleges, holds a NUS degree in Mathematics. He has authored and co-authored a number of Mathematics books (assessment books, guide books, and a textbook) at the Secondary and JC levels. He also served at MOE HQ for many years. (He's the one offering 'A' level / 'O' level Mathematics Tuition, while I'm offering 'A' level / 'O' level Chemistry & Biology Tuition.)

The Tuition sessions are held at my place of residence, at Blk 5000M #11-56 Lagoon View, located between Bedok (Temasek JC) and Marine Parade (Victoria JC). Group Tuition is significantly less costly but certainly no less effective (note : Individual Tuition is still possible if requested, but will have be more expensive, see my rates below), and there are also other advantages to small-group tuition and is thus recommended. As a new Tuition student, you may prefer form your own new group (ie. with your own friends, 3-4 per group), otherwise you can join an existing Tuition group of students (if available).

All of my tuition students have done very well for their tuition subjects at their local Secondary School Exams, their Sec 4 Prelim Exams, their actual 'O' Level Exams, their JC1 Promo Exams, JC2 Prelim Exams, and (most importantly) at their final 'A' Level Exams; with all my students enjoying noticeable grade point average improvements in their tuition subjects; and for most of my students, actual 'A' grade distinctions for their tuition subjects.
 

On 27 Apr 09, Clifton Goh (Catholic JC) sms'd a day after his 2nd tuition session : "Ty 4 yesterday's help in electrochemistry... I can finally understand the lecture n is even way ahead of the lecturer... my JC lecturer spent 7.3 mins telling us what u can tell me in 1 SMS + ur explanation much clearer."

(Above) One of my 'A' level students, Novabelle Ng (of Victoria JC), for whom I tutored H2 Chemistry and H2 Biology (in 2007 + 2008), scored distinctions ('A' grades) for both subjects in the 2008 'A' Level Examination. All the best for your future studies and career, Belle! : )

(Above) Another 'A' level student of mine, Calvin Seah Yong Yeh (of Nanyang JC), scored a distinction ('A' grade) in his 2008 'A' Level Exams for H2 Chemistry which I tutored him in (in 2008). All the best for your future studies and career, Calvin! : )
 

(Below) 3SG Peter Ng Chee Yee, one of my ex-Tuition students (now in the Special Investigations Branch of the Singapore Armed Forces)  is also a most talented and passionate pianist - click on his photo below to watch him play his self-arranged beautiful rendition of Pachelbel's Canon.

Peter Ng Chee Yee (TPJC grad 2007), playing "Peter's Canon". Click image above to view video.
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My students during tuition
(above left to right : Peter TPJC; Andrea, Max and YuanWei SRJC, 2007)
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My students with our BedokFunland JC Mascot "BearBear"
(below left & right : JaiLing VJC; WanPing VJC, 2008)
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"I recall times (very often, actually) when the principal gave hour-plus-long speeches (no exaggeration) during daily morning assembly period, and when he was finally done, he always received tremendous applause (sometimes a standing ovation) from all the students (because they were tremendously relieved the speech was over) and indeed (applause) from a number of (us) teachers as well. Incidentally, the principal always believed the tremendous applause was for his (self-believed) brilliant oratorical skills, and always enjoyed the applause."

1) On the problematic Organic Chem question (Paper 3), the selective reduction of only 1 of 2 nitro subtituents on the benzene ring :

There is but 1 way to reduce only one of two nitro groups present (with a high yield), is to use a reagent not taught within the H2 (or even H3) syllabus.
 

This reagent is sodium sulfide - http://en.wikipedia.org/wiki/Reduction_of_nitro_compounds

It's unlikely that the SEAB-Cambridge guys expected you guys to know this reagent, it's more likely that they didn't care about the yield, they just wanted to you to suggest a possible synthesis pathway, no matter how low the yield.

Since this commercial compound exists, there had to be a better (ie. higher yield) pathway for its synthesis. It turns out to be the one involving sodium sulfide. Evidently, SEAB-Cambridge in setting the 2008 H2 Chem paper, thought it would make for an interesting question to use this compound (they didn't care that students would be confused and frustrated by the mysterious selective reduction) and went ahead to ask for an unrealisitc 2-step synthesis pathway, in spite of the fact that sodium sulfide reduction is not taught within it's syllabus, and without such the selective reduction of only the ortho nitro group would result in a pitiably low yield.

I discussed this problematic question on a dedicated Chemistry forum.

kiwi wrote :
>>> from memory sodium sulfide reduction of dinitroarenes stops at mononitroanilines, but you wouldn't be expected to know this for a-levels <<<

macman104  wrote :
>>> I'm in my 4th semester of Organic Chem in the University, and never once have I heard of that reduction (although, some sort of reduction by Na2S does sound vaguely familiar from a lecture, but only briefly).  Apparently it is called the Zinin Reduction, and is pretty old chemistry (with the initial info published in those old German journals, ~1842).  I found an old text relating to it, I've uploaded the PDF for you to see.  At the end of the 3rd page, they mention the selective reduction of di or tri-nitro compounds. <<<

I replied :
>>> Thanks to both of you for taking the time and effort to check out on this. Much appreciated. The Zinin Reduction is obscure, for sure. I couldn't find it in any of my (half-a-dozen) Organic Chem texts, and finally found a mention in pg 1817 of "March's Advanced Organic Chemistry, 6th Edition".
Online search reveals similar obscurity :
http://www.answers.com/Zinin%20Reduction
Well, point proven. MOE-SEAB-Cambridge must think it most amusing to annoy 'A' level (H2 level) students with this question (which is impossible to answer within the context of their limited syllabus). <<<

macman104 replied :
>>> No problem.  And right, ONE part of a page in March.  Not really something you might just browse across (does anyone even "browse" March for fun? lol)...
Anyway, glad the question could be answered, but truly an unfair question I believe. <<<

kiwi replied :
>>> No problem. March - is there anything it doesn't know?... <<<

Click here to view the ancient chemistry journal that describes the Znnin Reduction :
http://infinity.usanethosting.com/Tuition/ZininReduction.pdf
 
 

2) On the ligand question (Paper 2) :

[CuCl2]- or dichlorocopper(I) anion exists, and is linear geometry.

But [CuCl4]3- or tetrachlorocopper(I) ion exists too, and can exist in either tetrahedral geometry, or octahedral & square planar geometry if it's [Cu(Cl4)(H2O)]3-. 6 ligands form an octahedral geometry, the Cl- ligands occupy square planar while the H2O ligands occupy top and bottom.

[CuCl4]2- or tetrachlorocopper(II) ion exists too, and can exist in either tetrahedral geometry, or octahedral & square planar geometry if it's [Cu(Cl4)(H2O)2]2-). 6 ligands form an octahedral geometry, the Cl- ligands occupy square planar while the H2O ligands occupy top and bottom.

Since all 5 of the above ions exist, arguably by right, SEAB/Cambridge should accept more than one possible answer for at least some of the questions.

The more (dative aka coordinate) bonds a central metal ion can form with ligands (electron donors), the happier it will be. Simply because more bonds formed results in a more exothermic process, which as far as Gibbs Free Energy is concerned, is certainly a good, feasible thing. True, there might be a slight decrease in entropy as a result (which is not a good, feasible thing), but usually the favourable enthalpy change outweighs the unfavourable entropy change. There is also electronic stability to consider, of course.

You should be aware that Be2+ ion, being in period 2, does not have empty 3d orbitals required to expand its octet. Consequently, any complex ions formed with Be2+ has to be tetravalent or 4 bonds, ie. tetrahedral geometry.

In that case, why would Cu+ or Cu2+ want to just form CuCl or CuCl2 and stop there? Wll there's obviously the matter of molarity and availability to consider. If there's enough Cl- ions available, it makes sense that you would want to get [CuCl2]- ion and (adding even even more Cl- ions) [CuCl4]3- ion, and for Cu2+ ion, you would obtain [CuCl4]2- ion.

But if the more bonds formed the merrier, then why stop at 4 Cl- ion ligands? Why not 6 Cl- ligands? Because Cl being in period 3, is too huge to have 6 Cl- ligands crowding around the central metal ion. There would be too much electronic and steric repulsion.

So we only get 4 Cl- ligands? Then obviously, we would get a tetrahedral geometry, wouldn't we?

But compare the H2O molecule to Cl- anion. H2O is much smaller (for 2 reasons - Cl- is in period 3 and it is an anion too). Hence, the [Cu(H2O)6]2+ and [Cu(H2O)6]+ ions exist (though the process is not as exothermic as if Cl- ligands were used, hence ligand displacement or ligand exchange takes place).

Which means that even in tetrachlorocopper(I) and tetrachlorocopper(II) ions, it is possible, under different conditions, that water molecules might squeeze in between the Cl- ligands, and you would get the octahedral geometry. 6 ligands form an octahedral geometry, the Cl- ligands occupy square planar while the H2O ligands occupy top and bottom.

The reason why Cl- ligands occupy square planar, is because they're much larger, bring greater electronic and steric repulsion as compared to the smaller H2O ligands, and hence square planar for the 4 Cl- ligands would be furthest apart and the most stable, while the smaller H2O ligands occupy top and bottom.

That being said, the tetrahedral [Cu(Cl)4]2- and [Cu(Cl)4]3- complex ions also exist, just as the 'square planar' (octahedral, really) [Cu(Cl4)(H2O)2]2-) and [Cu(Cl4)(H2O)2]3-) complex ions also exist.
 
 

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(Above)  Wee Wen Shih

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Animation of DNA Replication

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Recommended 'A' Level Chemistry Books
(Available from Popular Bookstore, authored by CS Toh, Step-by-Step publications)

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The Periodic Table

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TheUltimate Periodic Table
(An excellent buy; use debit card to purchase from Amazon)

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Standard Redox Potentials

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Greek Alphabet Symbols

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Visit my main website page here

Visit my Pachelbel's Canon webpage here