BedokFunland JC's A Level H2 Chemistry Qns (Part 2)

last SG · 2010-07-26T23:40:35+00:00

With Singapore having the highest incidence of diabetes in the world, every Singapore should take Berberine. Berberine helps to prevent diabetes if you don'...

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Although the new H2 Chemistry syllabus specifies only a qualitative treatment of the Nernst equation is required, but you can bet your ass that most JCs will set Prelim paper questions on a quantitative treatment of the Nernst equation, and so will Cambridge, afterall what better way to illustrate a qualitative understanding than to perform calculations quantitatively?

I have an extensive collection of Nernst equation problems ready for my BedokFunland JC students. All other students can go ask their school teacher or private tutor for practice on Nernst equation problems, and/or go Google out such Nernst equation problems on the internet for yourself to practice on. Have fun! :)

https://en.wikipedia.org/wiki/Nernst_equation

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[H2 Chemistry] - "Why or how do acid attacks cause disfigurement?"

Such a simple Chemistry question with such an obvious answer, but you'll be surprised most JC students don't know the answer. And if you protest, "But acid attacks are not in the H2 syllabus!", my retort is : if you've truly educated yourself in Chemistry, it means to intelligently apply whatever you've learnt within the syllabus, to novel contexts beyond the syllabus. So yeah, certainly expect to have Cambridge do this for Chemistry as well, coz it would be totally unfair and a bloody shame if H2 Chemistry and/or H2 Physics loses to H2 Biology in this didactic, pedagogical, examinational characteristic :

There will be one or more stimulus materials which may be taken or adapted from a source such as a scientific journal or book which may not necessarily relate directly to the content of the syllabus. Questions may require candidates to explain terms used in the passage, analyse data, justify decisions, perform calculations and draw conclusions based on information in the stimulus material.

Acid attack first aid: What you need to do immediately to help victims

https://sg.yahoo.com/news/acid-attack-first-aid-immediately-131700262.html

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To prevent your child growing up with a life-long (and potentially fatal) allergy of tree nuts and peanuts, there is increasing evidence that exposing your young toddler to small amounts of tree nuts and peanuts at a very young age, can help to prevent your child from developing an allergy to tree nuts and peanuts as he/she grows older, machiam vaccinating him to confer immunity. Note that tree nut allergy is a distinct and separate allergy from peanut allergy, and small amounts of both types of nuts should be given to young toddlers at a very young age.

http://mothership.sg/2017/07/child-had-severe-peanut-allergic-reaction-on-sia-flight-when-too-many-people-opened-bags-of-peanuts/

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[Medical] - Startup touts neuro-electrostimulation device as 'medicine for the brain'

https://sg.yahoo.com/news/startup-touts-neuro-stimulation-medicine-brain-182659448.html

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Hi there, I'm revising my A level chem + learning higher level chem to prepare for a chem test so I can exempt from chem mods. I have this question to ask.

Which of the following compounds would react most rapidly with NaCN?

a. 1-bromobutane
b. (R)-2-bromobutane
c. (S)-2-bromobutane
d. 1-bromo-2-methylpropane

My deduction is that either a or d is the answer, since b and c are technically the same compound (except the direction)

Now, how do I know which will react faster? a has 1 R group, compared to d which has 2 R groups. I feel that d is more stable and so d would react most rapidly with NaCN. Am I right?

This is still considered an A level Chem qn, so I'll help you out here. Once you do Uni level Chem proper, you can post your Uni Chem qns at The Chemical Forums, as my professional interest (as Mr BedokFunland JC) is specifically targeted at A level Chem only.

Options A and D are both primary alkyl halides (your thinking that Option D is a secondary alkyl halide is incorrect : only if Option D was 2-bromo-2-methylpropane, then it would be tertiary, and 1-bromo-2-methylpropane is still primary), hence the electronics factor is similar. Therefore, the least sterically hindered Option A would be the fastest.

For Options B and C, apart from your correct deduction that optical isomerism is irrelevant to the rate of reaction, let's consider them for the sake of comparison with Options A & D. Even though electronics would favor the secondary alkyl halides Options B & C for SN1 (over primary alkyl halide Options A & D), but the strong NC- nucleophile would favor SN2 anyway. Hence being SN2, Option A is fastest, followed by Option D, with Options B and C the slowest.

If instead of 1-bromo-2-methylpropane, 2-bromo-2-methylpropane was specified for Option D (which was what you were prolly thinking of), then Option D (tertiary) would be faster than B & C (secondary) only if SN1 was favored (ie. weak nucleophile, and preferably protic solvent). Since SN2 is favored (ie. strong nucleophile, preferably aprotic solvent) in this question (although the solvent isn't specified by the question), then if Option D was 2-bromo-2-methylpropane, Option A (primary) would still be the fastest (even moreso than Option D being 1-bromo-2-methylpropane), followed by Options B & C, with Option D the slowest (due to greatest steric hindrance).

Note that we've only considered nucleophilic aliphatic substitution SN1 and SN2 here, rather than elimination E1 and E2 (since solvent and temperature weren't specified by the question). If E1 and E2 are to be considered, a primary alkyl halide in an aprotic solvent with a strong Bronsted-Lowry base such as the NC- anion would favor E2 over E1, while a tertiary alkyl halide in a protic solvent with a weak Bronsted-Lowry base would favor E1 over E2.

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There are over 200+ visitors to this H2 Chemistry thread every day, and yet almost all are silent lurkers. I know some of you lurkers are Singapore private tutors and Singapore school JC teachers, as well as foreign overseas visitors (students + teachers + tutors). But some of you are Singapore JC students, so to these students, why don't you make use of SgForums to register an account, and post your H2 Chem questions here? It's free to ask for help here, might as well make use of it. (Students who already have their own private tutors should ask your private tutors directly, eg. via WhatsAapp.)

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So it wasn't the lychee fruit per se that caused the acute encephalitis deaths afterall, but the insecticide used.

Insecticide responsible for deaths of 13 children in Bangladesh: study

https://sg.yahoo.com/news/insecticide-responsible-deaths-13-children-bangladesh-study-004100050.html

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My BedokFunland JC H2 Chemistry Challenge Question :

Endosulfan is a derivative of hexachlorocyclopentadiene, and is produced by the Diels-Alder reaction of hexachlorocyclopentadiene with cis-butene-1,4-diol and subsequent reaction of the adduct with thionyl chloride.

Q1. Draw the full electron-flow curved-arrow reaction mechanism for every step of the synthesis pathway :

Cyclopentadiene -----> Octachlorocyclopentane -----> Hexachlorocyclopentadiene -----> Endosulfan alcohol -----> Endosulfan

Technical endosulfan is a 7:3 mixture of stereoisomers, designated α and β. α- and β-Endosulfan are configurational isomers arising from the pyramidal stereochemistry of the teravalent sulfur. α-Endosulfan is the more thermodynamically stable of the two, and β-Endosulfan irreversibly converts to the α isomer, although the conversion is slow.

Q2.
a) Explain why α-Endosulfan is obtained as a racemate.
b) Explain why β-Endosulfan is a meso molecule.
c) Suggest why α-Endosulfan is the more thermodynamically stable of the two isomers.
d) Suggest why the conversion of β-Endosulfan to the α isomer is irreversible.
e) Suggest why the conversion of β-Endosulfan to the α isomer is slow.

https://en.wikipedia.org/wiki/SNi
http://www.masterorganicchemistry.com/2014/02/10/socl2-and-the-sni-mechanism/

Q3.
The chlorination of secondary alcohols with PCl5 results in an inversion of configuration, but when SOCl2 is used, the product could possess either a retention or inversion of configuration, as shown in the diagram above.

a) Explain why addition of pyridine to the SOCl2 reaction results in the inversion of configuration.

b) Use of SOCl2 in either a non-nucleophilic solvent, or in dioxane (a nucleophilic solvent), for reaction with secondary alcohols, both result in the retention of configuration in the final product, but this retention is accomplished by 2 contrasting mechanisms. Explain these 2 contrasting mechanisms and why both result in the retention of configuration in the final product.

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Williams syndrome is an educational illustration of how, during intraphysical incarnation into a biological body, your incarnation's intelligence and personality traits are strongly influenced (and indeed to a large extent determined) by your genetics. That is not to say there is no free will (because of course there is), but rather that your soul has chosen the specific genetic conditions of the upcoming physical biological incarnation with which to force into being certain specific personality traits and/or medical conditions for the purpose of working with specific karmic issues and opportunities.

https://en.wikipedia.org/wiki/Williams_syndrome

Social and psychological

Individuals with Williams syndrome report higher anxiety levels as well as phobia development, which may be associated with hyperacusis (high sensitivity to certain frequencies of sound).[31] Compared with other children with delays, those with Williams syndrome display a significantly greater number of fears. 35% of these children met the DSM definition of having a phobia as compared with 1-4.3% for those with other types of developmental delays.[32] Williams syndrome is also strongly associated with attention deficit hyperactivity disorder and related psychological symptoms such as poor concentration, hyperactivity, and social disinhibition.[9]

Furthermore, cognitive abilities (IQ's) of individuals with WMS typically range from mild-to-moderate levels of mental retardation.[33] One study of 306 children with Williams syndrome found IQ scores ranging from 40 to 112 with a mean of 69.32 (an IQ score of 100 is the average in nonaffected populations).[34] IQ scores above this range have been reported in individuals with smaller genetic deletions.[13] In particular, individuals with Williams syndrome experience challenges in visual-motor skills and visuospatial construction. Most affected individuals are unable to spatially orient themselves and many experience difficulty when given a task that requires even the most basic visual problem solving. Many adults with Williams syndrome cannot complete a simple six-piece puzzle designed for young children, for example. These visuospatial deficits may be related to damage to the dorsal cortical pathway for visual processing.[35]

Despite their physical and cognitive deficits, individuals with Williams syndrome exhibit impressive social and verbal abilities. Williams patients can be highly verbal relative to their IQ. When children with Williams syndrome are asked to name an array of animals, they may well list such a wild assortment of creatures as a koala, saber-toothed cat, vulture, unicorn, sea lion, yak, ibex and Brontosaurus, a far greater verbal array than would be expected of children with IQs in the 60s.[36] Some other strengths that have been associated with Williams syndrome are auditory short-term memory and facial recognition skills. The language used by individuals with Williams syndrome differs notably from unaffected populations, including individuals matched for IQ. People with Williams syndrome tend to use speech that is rich in emotional descriptors, high in prosody (exaggerated rhythm and emotional intensity), and features unusual terms and strange idioms.[35]

Among the hallmark traits of individuals with Williams syndrome is an apparent lack of social inhibition. Dykens and Rosner (1999) found that 100% of those with Williams syndrome were kind-spirited, 90% sought the company of others, 87% empathize with others' pain, 84% are caring, 83% are unselfish/forgiving, 75% never go unnoticed in a group, and 75% are happy when others do well.[37] Infants with Williams syndrome make normal and frequent eye contact, and young children with Williams will often approach and hug strangers. Individuals affected by Williams syndrome typically have high empathy, and are rarely observed displaying aggression. In regards to empathy, they show relative strength in reading people's eyes to gauge intentions, emotions, and mental states.[38] The level of friendliness observed in people with Williams is often inappropriate for the social setting, however, and teens and adults with Williams syndrome often experience social isolation, frustration, and loneliness despite their clear desire to connect to other people.[35]

While these children often came off as happy due to their sociable nature, often there are internal drawbacks to the way they act. 76-86% of these children were reported as believing that they either had few friends or problems with their friends. This is possibly due to the fact that although they are very friendly to strangers and love meeting new people, they may have trouble interacting on a deeper level. 73-93% were reported as unreserved with strangers, 67% highly sensitive to rejection, 65% susceptible to teasing, and the statistic for exploitation and sexual abuse was unavailable.[37][39][40][41][42] This last one is a significant problem. People with Williams syndrome are frequently very trusting and want more than anything to make friends, leading them to submit to requests that under normal circumstances would be rejected. There are external problems as well. 91-96% demonstrate inattention, 75% impulsivity, 59-71% hyperactivity 46-74% tantrums, 32-60% disobedience, and 25-37% fighting and aggressive behavior.[37][39][43]

In one experiment, a group of children with Williams syndrome showed no signs of racial bias, unlike children without the syndrome. They did show gender bias, however, suggesting separate mechanisms for these biases.[44]

Genetic Cause

Williams syndrome genes[45][46]

ASL· BAZ1B· BCL7B· CLDN3· CLDN4· CLIP2· EIF4H· ELN· FZD9· FKBP6· GTF2I· GTF2IRD1· HIP1· KCTD7· LAT2· LIMK1· MDH2· NCF1· NSUN5· POR· RFC2· STX1A· TBL2· TRIM50· TRIM73· TRIM74· WBSCR14· WBSCR18· WBSCR21· WBSCR22· WBSCR23· WBSCR24· WBSCR27· WBSCR28

Williams syndrome is a microdeletion syndrome caused by the spontaneous deletion of genetic material from the region q11.23 of a chromosome 7, so that the person is hemizygous for those genes.[47][48] The deleted region includes more than 25 genes, and researchers believe that being hemizygous for these genes probably contributes to the characteristic features of this disorder. CLIP2, ELN, GTF2I, GTF2IRD1, and LIMK1 are among the genes that are typically deleted from one chromosome in people with Williams syndrome. Researchers have found this hemizygosity for the ELN gene, which codes for the protein elastin, is associated with the connective-tissue abnormalities and cardiovascular disease (specifically supravalvular aortic stenosis and supravalvular pulmonary stenosis) found in many people with this syndrome. The insufficient supply of elastin may also be the cause of full cheeks, coarse voice, hernias and bladder diverticula often found in those with Williams syndrome. Studies suggest that hemizygosity in LIMK1, GTF2I, GTF2IRD1, and perhaps other genes may help explain the characteristic difficulties with visual–spatial tasks. Additionally, there is evidence that the hemizygosity in several of these genes, including CLIP2, may contribute to the unique behavioral characteristics, learning disabilities, and other cognitive difficulties seen in Williams syndrome.[49]

https://en.wikipedia.org/wiki/Williams_syndrome

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Can I ask something about H2 chemistry question from VJC Prelim 2011/P3/Q4(b) on organic deductive on C10H8O2,which is a neutral compound.

In determing the structure of D,my answer was C6H6-C(carbon 1 of benzene ring)H=C(CH3)COO-(bond bonded back to carbon 2 of benzene ring),resulting in structure having 1 benzene ring and 1 6 membered cyclic ring suiting the formula and subsequent reactions.But the answer given was 1 benzene ring and 1 5 membered cyclic ring whereby the C=C bond was placed outwards of the 5 membered cyclic ring.

Is my answer considered wrong as my 6 membered cyclic ring structure will lead to bond-angle strain making the structure unstable?Or is it acceptable?

Also,just asking is this question considered difficult?

Your fatal error (ie. why your alternative answer will not be accepted by Cambridge), is that your E (unlike VJC's answer) is not capable of geometric isomerism, as your double-bond is within a 6 membered-ring.

This question is of average difficulty for an upper tier JC such as VJC (for upper tier JC prelim papers, there have been many deductive elucidation questions much tougher than this), so this question would be considered slightly difficult for mid tier JC, and rather difficult for lower tier JCs and MI. Cambridge A level questions should be of this difficulty or slightly easier, but anything is possible.

For yourself, I expect you didn't have any difficulty solving this question, so you prolly found this easy, except that your answer is wrong, and Cambridge would award you with some credit, exactly how much depends on the question (whether deductions are required or only final elucidations), and how you presented your answer.

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For me,comparing to all prelim electrolysis questions and TYS,the hardest(and only) electrolysis question I have come across so far is the 2012 A level H2 Chem P3 electrolysis question about drawing of the diagram of an electrochemical cell to measure the relative oxidising powers of chlorine/iodine.The question itself felt ambiguous as I only merely thought of putting SHE at 1 end and pumping the halogen at 1 mol dm^-3,1 atm,25 degree into the solution of the halide ions at another end only,never expecting iodine to be one end,chlorine the other end as i cant imagine I2(s) being in solution.

No, your method of answering the question is not acceptable. The clue in the question is that you're tasked to measure the *relative* oxidizing powers of the *two* different halogens in a single setup (implied by the question).

If you used a SHE setup, you need to carry out two separate setups instead of one, in which case you'll be heavily penalized and Cambridge will at most give you half the number of marks available for this question.

Also, the new syllabus requires 1 bar, not 1 atm for standard pressure.

Lastly, I2(s) is not part of the required setup. What is required is 1M of I-(aq) and 1M of I3-(aq), ie. the triiodide anion.

In this setup, the Cl2 will be reduced to Cl- at the cathode, and the I- anion is oxidized to the I3- anion at the anode. The reading from the (high resistance) voltmeter gives the required answer.

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;Þ

A BedokFunland JC Original H2 Chemistry Challenge

Draw the full curved-arrow electron-flow reaction mechanism for every step of the full methamphetamine synthesis (starting from benzene!) via the Leuckart reaction pathway as illustrated below.

https://en.wikipedia.org/wiki/Methamphetamine

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[Chemistry / Medicine] - Trisha the Chemist's explanation missed out explicitly specifying the redox aspect of the 'copper dissolving' reaction (don't be misled thinking the redox reaction isn't thermodynamically feasible based on the standard redox potentials on the data booklet, because you'll be poisoned to death way before the [Cu2+] ever reaches 1M or standard molarity, tis a cunning trick question I hope Cambridge unleashes on H2 Chem students). In addition, there are several copper metabolic genetic medical conditions that make even a low [Cu2+] deadly for some people.

Chemist Debunks That Nasty Rumor About Moscow Mule Mugs Being Poisonous

https://sg.news.yahoo.com/chemist-debunks-nasty-rumor-moscow-100024945.html

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[Medical] - The MTHFR gene provides instructions for making an enzyme called methylenetetrahydrofolate reductase, and mutations in this gene have been implicated in a staggering number of medical conditions. Educate thyself.

https://www.google.com.sg/search?site=&source=hp&q=MTHFR+gene+mutation

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A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from the Singapore-Cambridge 2008 H2 Chemistry A Level Exam

When C6H5COCHO is reacted with OH-, it is converted into C6H5CH(OH)COO-.

i) Draw out the full curved-arrow, electron-flow mechanism for the reaction.

ii) Identify (with reasons) the rate determining step, and concordantly state the rate law for the reaction.

iii) State the specific type of reaction (ie. not the type of mechanism) that has occurred, and explain why this reaction is considered unusual in organic chemistry.

iv) Justify your answer using either OSes or FGLs.

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A 2017 BedokFunland JC Original H2 Chemistry Challenge, adapted from a Singapore-Cambridge A Level Exam Qn.

Explain why, despite the Br-Br bond being weaker than the Cl-Cl bond, the rate of reaction between chlorine and phosphorus is faster than the rate of reaction between bromine and phosphorus (assuming the same allotrope of phosphorus is used, and assuming the same state or phase for both halogens).

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JJC P2 Q1(e) why is it that even though reaction is independant of [I2], when [I2] is halved, time is halved? wouldn't that be first order?

Q5(c)(iii) How does this mechanism look like? Is it possible for the Br- from step 1 to attack instead?

P3 Q1(b)(ii) Why does the answer state that when the reaction just becomes feasible G=0? Isn't the reaction feasible when G<0? When G=0 isn't it just at equilibrium?

P2 Q1e) If I2 was 1st order, then the half-life would be constant, ie. no change. Since I2 is a 0th order reactant, half-life is halved when molarity is halved.

P2 Q5ciii) SN1. If the leaving group Br- attacked back, then you would end up with the original reactant, ie. no net reaction occurred.

P3 Q1(b)(ii) Yes you're right, but what exact value are you going to substitute in for delta G? -1? -0.1? -0.01? -0.001? So just use delta G = 0 first, then state that for the reaction to be thermodynamically feasible, the temperature needs to be higher than 958K (ie. take initiative to improve on the given mark scheme answer, when answering such questions in the A levels).

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