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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[Physics / Chemistry / Biology / Medicine] - Global warming is causing disease epidemics through complex mechanisms humans never suspected possible.

Haley remembered, it had not been very cold that year. There was only one night of hard frost; everyone had talked about it, grateful they did not have to dash out to wrap plants and turn off hose taps while thermometers plunged. The investigators downloaded federal weather data for each year since West Nile first arrived in Texas and plotted the metrics against the case counts. The epidemic year was an outlier on every measure, with the warmest winter, the warmest spring and the heaviest early rainfall in 10 years. As it turns out, the climate changes such as heavier rainfall resulted in greater breeding numbers, allowed more mosquitoes to survive the warmer and shorter winters, causing them to wake sooner, spill out earlier from their winter hiding places, and bite more people than in any other previous year, transmitting so many deadly viruses that killed an unprecedented number of victims.

https://www.nytimes.com/2017/04/20/magazine/why-the-menace-of-mosquitoes-will-only-get-worse.html?_r=0

Global warming is also now melting permafrost and ice that have been frozen for thousands or even millions of years, slowly releasing long-dormant bacteria and viruses that could potentially wipe out humanity.

https://www.google.com.sg/#q=global+warming+viruses+frozen

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do both the statements mean the same thing?

-compound A has intermolecular permanent dipole–permanent dipole attractions

-pd-pd attractions exist between A molecules

or should i write something like "Intermolecular pd-pd attractions exists between A molecules" which of the statements is the most correct?

also do i call pd-pd and id-id as interactions or attractions or forces of attraction?

sorry if my question sounds confusing becuase i am a bit confused about the phrasing of answers.

You're not allowed to use acronyms such as pd-pd in the A level exams. Zero marks.

Interactions include both attractions and repulsions, and depending on context, either or both can be used.

Students are advised to use the following phrasing in the A level exams :

Between polar molecules, the intermolecular interactions are permanent dipole - permanent dipole Keesom van der Waals forces.

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[Chemistry / Biology / Medicine] - Indeed, extremely high dose caffeine can be fatal. Nonetheless, low dose caffeine is helpful, eg. significantly enhances the anti-headache and anti-migraine effects of panadol and/or aspirin.

But caffeine aside, energy drinks are seriously unhealthy, containing dangerously high doses of many questionable ingredients other than caffeine. I recommend you go for isotonic drinks or coconut juice instead of energy drinks, if you're looking for a healthier alternative to soft drinks (which ironically, is still healthier than energy drinks).

Coca Cola (always go for original flavor, 'less sugar' or 'no sugar' = artificial sweeteners, way unhealthier than sugar) is actually healthier than Mountain Dew, which is somewhere between a soft drink and an energy drink.

https://sg.news.yahoo.com/us-teen-dies-caffeine-overdose-004737947.html

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[Medicine] - Could Breast Milk Be the Next Cancer Treatment?

https://sg.yahoo.com/news/breast-milk-latest-cancer-treatment-161252298.html

More on the Biology, Chemistry and Medicine of HAMLET (Human Alpha-lactalbumin Made LEthal to Tumour cells)

Endogenous human alpha-lactalbumin is complexed with a calcium ion and serves as a cofactor in lactose synthesis, but has no tumoricidal properties. The alpha-lactalbumin must be partially unfolded to allow for release of the calcium ion and replacement with an oleic acid molecule. The partially folded conformation is essential to the cytotoxicity of HAMLET, as mutagenesis studies have shown that completely unfolded alpha-lacalbumin does not retain the functional properties of HAMLET. The oleic acid is necessary for stabilizing this molecule in this partially unfolded state.

HAMLET carries out independent attacks on many distinct cell organelles, including mitochondria, proteasomes, and histones, and interferes with cell processes such as macroautophagy. It has been shown that HAMLET binds to the cell surface and rapidly invades cells, with tumor cells taking up far more protein than healthy, differentiated cells. The mechanism of its entry is poorly understood, but recent studies indicate that the oleic acid in the HAMLET complex interacts with phosphatidylserine and o-glycosylated mucin on the plasma membrane, both of which are expressed in greater amounts on the plasma membrane of tumor cells, possibly providing for HAMLET’s specificity.

One of the most prominent targets of HAMLET once inside the cell is the mitochondrion. Electron microscopy has revealed physical damage to the mitochondrial membranes and assays have found cytochrome c release and activation of the caspase cascade, the most notable ones being caspases 2, 3, and 9. It is interesting to note that cell death is not prevented by caspase inhibitors, or by BCL-2 or p53 mutagenesis, indicating that the traditional apoptotic caspase cascade is not the ultimate cause of cell death.

Another target of HAMLET is the proteasome. 26S proteasomes are activated in response to large quantities of unfolded HAMLET protein in the cytoplasm, but degradation of HAMLET by the proteasome is unusually slow. Furthermore, in vitro studies have shown that HAMLET is capable of binding the catalytic 20S subunit of the proteasome and disabling its enzymatic activity, an effect that has never before been demonstrated for any protein. However, proteasome inhibition alone does not seem to be responsible for HAMLET-induced cell death, as proteasome inhibitors have been shown to reduce the cytotoxicity of HAMLET.

HAMLET also targets the nucleus, where it interacts with histones to interfere with transcriptional processes. Studies have shown that HAMLET is mostly localized to the nucleus within one hour of invading a tumor cell. Hamlet has been shown to bind with high affinity to individual histone proteins, to be specific H2a, H2b, H3, and H4, as well as entire nucleosomal units. This interaction irreversibly blocks transcription and leads to activation of p53. This process has been demonstrated to be similar to histone hyperacetylation and it was found that histone de-acetylase inhibitors potentiated the effects of HAMLET.

HAMLET cells showed the physiological characteristics of macroautophagy, a process in which cellular components are sequestered in double membrane-bound vesicles that fuse with lysosomes for degradation. Cells also showed decreased levels of mTOR, a known inhibitor of macroautophagy. HAMLET cells and cells under conditions of amino acid starvation (a known initiator of macroautophagy) showed similar expression patterns of autophagocytotic proteins and responded equally well to addition of macroautophagy inhibitors.

https://en.wikipedia.org/wiki/HAMLET_(protein_complex)

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Reminder for JC2 students taking A level exams in 5 months time : Only a few days left before the May 2017 Special Conditional Offer for JC2 Students expires, and the more exacting June 2017 Special Conditional Offer for JC2 Students applies.

A BedokFunland JC Original H2 Chemistry Question

Calculate the value of
i) pKa of water
ii) pKa of hydroxonium ion
iii) pKb of water
iv) pKb of hydroxide ion
Show all relevant working, and assume water solvent and room temperature, obviously.

My BedokFunland JC students can check the answers with me during tuition. All other students can check the answers with your school teacher or private tutor.

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Very unfortunate, poor Ayumi. However, her going deaf is partly her own fault, in 2 cumulative ways : when she had a ENT infection, she chose to be a workaholic and ignored it to continue working ; anyone with an occupational exposure to loud noises / music / sounds should be wearing earplugs (which do not shut out noise completely, just reduces it to safe levels, she would still have been able to perform as a singer with earplugs on). Even at the cinema, let alone at (even noisier) night/dance clubs or pubs (with live bands), and certainly at any (speaker blasting) live concert performance (possibly with the exception of softer classical piano or orchestra type performances at the Esplanade), are hazardously loud activities in which you should consider putting on earplugs. Here's (yet another) excellent BedokFunland JC public service tip : You can buy $2 earplugs at Daiso, and keep it on you at all times (eg. in your wallet or bag, just like you would a small mirror). You never know when you might need them whenever you go out.

http://www.stomp.com.sg/singapore-seen/from-around-the-world/j-pop-star-ayumi-hamasaki-is-going-completely-deaf

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Seeing how you've been asking me A level Chem qns since 5 years back, you're a private tutor or school teacher, not a student, correct?

Why is it that for a Daniel Cell, why do we need a porous pot to keep the solution separated? What is the purpose for the separation of solutions? Like wise for a typical galvanic cell where one beaker contains Zn metal and ZnSO4 while the other contains CuSo4 and Cu metal, connected by a salt bridge. Would it not have been better for the zinc and copper metal to be dipped into the same conducting electrolyte (say H2SO4), instead of two separate solution, to generate electricity?

If the solutions mix, there is no concern about the unbalanced charge built-up?

A Daniell cell is of course, a specific type of Galvanic-Voltaic cell with the following setup :

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

If, as you suggest, "the zinc and copper metal to be dipped into the same conducting electrolyte (say H2SO4), instead of two separate solution", then the electrons would only transfer in the redox reaction that would occur only between the electrolyte H+ reduced and the Zn oxidized, and no electrons would flow in the external wire, in which case the cell would have failed in it's objective to do work (eg. operate a light bulb).

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why doesnt fluorine follow the trend of decreasing bond energy from cl2 to br2 to i2?

This anomaly is one of those interesting tricks (I have an entire BedokFunland JC collection of such fascinating Chemistry anomalies) that I usually only share with and reveal the answer to my own BedokFunland JC students, and I won't spoonfeed these answers here. But you should be able to find the answer for yourself quite easily with Google.

Cambridge is unlikely to ask A level students this question, but if they do, they'll phrase it as, "Suggest a reason for this anomalous trend" to function as a distinction-type question to separate the A graders from the rest. While one can of course easily google out the answer, the point is students don't have access to the internet during the exam paper, which is where exploring beyond the basic syllabus throughout the year will help prepare determined students to get A grade.

when to use the term "delocalised electrons" and "mobile electrons"? what is the difference between these two terms?

As with many other such terminology pairs (eg. heat vs reflux, energy vs enthalpy, shielding vs screening, oxidation state vs oxidation number, resonance vs mesomeric, etc), in most cases they can be used interchangeably and Cambridge will generally accept either term, except in specific contexts where one term is clearly more appropriate than the other.

To explain why molten or aqueous ionic compounds and metals conduct electricity, it's better to use "mobile". To describe the resonance effect, eg. why amides are much less basic than amines, it's compulsory to use "delocalized".

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http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s14_qp_21.pdf

qn 4eiii

why does chloroethane undergo substitution reaction with NaCN(given in qn)? i thought usually KCN is used for nucleophilic sub rxns?

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s14_qp_22.pdf

qn 5b when the qn states , give the structures, can i draw out the structures instead of writing down the structural formula? for example, in general, can i draw https://www.google.com.sg/search?q=propanoic+acid&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi7lYC4rIvUAhXDMI8KHTecC24Q_AUICigB&biw=1047&bih=559#imgrc=Zz9g0od4Nzy2qM:

instead of writing ch3ch2co2h

i feel that drawing out the structure is easier as i tend to get confused when writing down the structural formula in a single line.

In future pls include the links to the mark schemes (on same website), to facilitate convenience.

The counter cation is spectatory hence irrelevant, it's the CN- anion nucleophile which is participatory and reacts. Beyond A levels, there are other considerations why a particular counter cation may be preferred, even if it's only spectatory, but these are not relevant for A levels.

When Cambridge simply asks for "structural formula", they will accept any of the 3 types : displayed or full, skeletal, or condensed. It's only when Cambridge specifies 1 of the 3 types, then only that type of structural formula is accepted. Otherwise, any of the 3, including a hybrid of the 3, will be accepted.

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It's all Chemistry, yo. Different colours reflect different pH values in the different regions of the drink in the cup, different degrees of protonation vs deprotonation, different molarities of conjugate acids vs conjugate bases, different extents of molecular conjugation or overlap of p-orbitals with delocalized electrons, different magnitudes of energy differences between lower and higher molecular orbitals, different wavelengths of visible light absorbed when the electrons transit. It's all Chemistry, yo. The most important subject at A levels.

Tuk Tuk Cha now selling colour-changing gradient drinks too

http://mothership.sg/2017/05/tuk-tuk-cha-now-selling-colour-changing-butterfly-pea-gradient-drink-too/

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http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s14_qp_41.pdf

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_s14_ms_41.pdf

qn 3biii) what is the significance of the word excess in the qn? will the product be different if kmno4 is not used in excess?

4a) when asked to indetify functional group, is benzene a functional group?

4bii) for drawing the structure, in the mark scheme, isnt Br in the 3rd position with regards to one of the OH groups? (altho the br in the 2nd position with regards to the other oh group) but i thought OH group is 2 and 4 directing so why is the bromine substitued there?

Q3) Excess is just to play safe that the required answer will be the completely oxidized product (because there are several partially oxidized intermediates possible, since there are 3 functional groups to be oxidized per molecule here), to ensure complete oxidation of benzene side chain *and* the 2 alkene groups, hence *excess* KMnO4 (ie. mole ratio >> 1:1, in fact, the actual stoichiometry required is > 3:1, because to fully oxidize *each* of the 3 functional groups will require more than 1 mole of KMnO4, but almost all Singapore A level students will die if Cambridge asks *exactly* how many moles required in total) is required to generate the fully oxidized product. Otherwise, if 'excess' wasn't specified by Cambridge, some students may be confused and imagine that KMnO4 might be limiting, and these students will have no idea which of the 3 functional groups gets priority to be oxidized, and oxidized to which extents.

Q4a) Yes it is.

Q4b) *Each* phenolic OH group is activating *and* ortho-para directing. Since there are 2 phenolic OH groups adjacent to each other, hence with excess Br2(aq) (instead of Br2 in CCl4), all 3 available positions may be brominated (note the Cambridge Mark Scheme is reasonable, unlike Singapore JC mark schemes which are anal, and Cambridge accepts either 2 or 3 Br atoms substituted), even after considering the deactivating effects of the aldehyde group (which is why after considering both activating and deactivating effects, Cambridge accepts either 2 or 3 Br atoms substituted in, although the better answer is still 3 Br atoms substituted in, all factors considered).

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Hi! This is a relatively general question but how do I predict the products of inorganic reactions given the reactants and I am asked to balance such a reaction equation? Is there a thought process to deduce the products? E.g a reaction between Fe2O3 and CO. Thanks!

There is no shortcut to predicting products. You've got to apply all your chemistry knowledge from across different topics to determine which (in tougher questions, more than one type of reaction is possible, hence a mixture of different products may be obtained).

For Fe2O3 and CO, unless otherwise specified by the (eg. data based) question, otherwise at O and A levels, the only possible reaction (covered by the syllabuses) is redox, where iron is reduced and carbon is oxidized.

Fe2O3 + 3CO --> 2Fe + 3CO2

Sorry to clarify, assuming I know it is a redox reaction, is there any way of knowing which oxidation state an element would be oxidised/reduced to in order to help predict a product?

For A levels, use the Data Booklet's redox potentials to determine which is the most thermodynamically feasible (ie. change in Gibbs free energy most negative), PLUS use familiarity of expected knowledge within syllabus PLUS look for clues in the question.

For O levels, use familiarity of expected knowledge within syllabus, ie. memorize the most stable OSes and expected redox products for commonly tested compounds.

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[H2 Biology] - Man experiences the laws of segregation, independent assortment and dominance, first-hand (hands-on-learning, geddit?).

Powell only started to notice something amiss, when he realized the woman's breast sizes were quite different from his wife's.

http://stomp.straitstimes.com/singapore-seen/from-around-the-world/drunk-guy-spends-hot-night-with-girl-who-turns-out-to-be-his

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The smell of an *approaching* thunderstorm with lightning & thunder.. that's ozone.

The smell *during* a thunderstorm... that's petrichor and geosmin.

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

https://www.scientificamerican.com/article/storm-scents-smell-rain/

Also read the 500 comments below the article.

http://www.nstperfume.com/2010/11/15/smell-fetishes/

On a related note,

https://www.ar15.com/forums/t_1_5/907443_Why_does_concrete_have_that_smell_when_itandapos_s_raining_wet_.html

On the chemistry of cements (eg. Portland) used in concretes :

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

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

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

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The chemistry of one of the most cruel cases of malicious poisoning in modern history, and the perpetrator was identified yet escaped justice due to her elite rich family connections... :

https://www.google.com.sg/#q=tsinghua+university+zhu+ling+thallium+poisoning

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In an experiment, a sample of a pure gas is put into a syringe at a temperature of 300Kand pressure of 16kPa. The gas is compressed until the volume occupied by the gas is halved.

After compression the temperature of the gas in the syringe is 373K and the pressure is 40kPa.

Which statement is correct?

A Intermolecular forces between the gas molecules are significant.

B It is possible to calculate the number of moels of gas present using these data alone.

C the gas is behaving ideally.

D the pressure used are too high for ideal has behaviour.

I did not understanf this behaviour by the gas. The pressure is increasing from 16 kPa to 40kPa, this means that molecules are going far apart it means ideal behaviour. But then Temperature also increases from 300K to 373K....What is happeneing?

The formation of significant intermolecular attractions (eg. van der Waals forces or hydrogen bonds) between the gas molecules when the gas is compressed (ie. molecules brought closer together) is exothermic, resulting in an increase in temperature. Hence, option A is the answer.

Before compression : PV = nRT, nR = (16000)(2 vol) / (300)

After compression : PV = nRT, nR = (40000)(1 vol) / (373)

If gas behaves ideally, then nR before compression, divided by nR after compression, will be 1.00, which is not the case if you do the division. Hence, the gas is not behaving ideally.

Sure this is the link for Question paper, MS.

http://pastpapers.papacambridge.com/view.php?id=Cambridge%20International%20Examinations%20%28CIE%29/AS%20and%20A%20Level/Chemistry%20%289701%29/2016%20Nov/9701_w16_qp_12.pdf

http://pastpapers.papacambridge.com/view.php?id=Cambridge%20International%20Examinations%20%28CIE%29/AS%20and%20A%20Level/Chemistry%20%289701%29/2016%20Nov/9701_w16_ms_12.pdf

The examiners' report for this question states:

29% of candidates chose the correct answer C. The mst commonly chosen answer was D chosen by 29% of candidates. If a fixed mass of gas is behaving ideally the sum PV/RT gives a constant value. for both the conditions given in the question

PV/RT =6.418 (if volume of 1dm3 and 0.5 dm3 are assumed. therefore gas is behaving ideally.

the paper is NOV 2016/12/Q.8

Cambridge and I are both correct. The error is yours, hoay. You gave the wrong temperature after compression.

Using Cambridge values, nR (before compression) / nR (after compression) = 1.00, hence the gas is behaving ideally.

In future to avoid typos, you can directly post the online link to the question and mark scheme, instead of typing out the question.

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Yes i come to know the compression factor.

Sometimes the gas Dimerizes this reduces the no of gas molecules...When the gas dimerizes ? is it related to temperature? I know of ethanoic dimerizing but that is due to Extensive H-bonding...is this anything to do with Ideal beahviour?

Gases which dimerize and monomerize are a select few, for A level purposes, most notably NO2 monomer which exists in equilibrium with N2O4 dimer, via a free radical mechanism. (Of courser, there's AlCl3 and Al2Cl6, but that's (s), (l), (g), (g) equilibria, and involves polymeric lattice structure in solid state as well, dimerization mechanism via dative covalent bonding).

Yes, it's related to temperature, enthalpy and entropy, all 3 variables incorporated in Gibbs free energy. The NO2 - N2O4 equilibria is a perfect illustration of how all 4 variables are inter-connected, as well as how these 4 variables are connected to the shifting of position of equilibrium.

http://www.rsc.org/learn-chemistry/resource/res00001739/le-chateliers-principle-the-equilibrium-between-nitrogen-dioxide-and-dinitrogen-tetroxide

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