BedokFunland JC's A Level H2 Chemistry Qns (Part 2)
-
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.
-
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?
Originally posted by H4x0ru5: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).
-
Originally posted by glitter58:
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.
Originally posted by glitter58: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".
-
Originally posted by glitter58:
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.
-
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
-
Originally posted by glitter58:
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).
-
Originally posted by ramsey tang:
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
Originally posted by ramsey tang: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.
-
[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.
-
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,
On the chemistry of cements (eg. Portland) used in concretes :
https://en.wikipedia.org/wiki/Cement
-
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
-
Originally posted by hoay:
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.
Originally posted by hoay: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.
-
Originally posted by hoay:
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.
-
^_^
-
Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H2 molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). However, Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 4,950,000 bar or 4,890,000 atm or 71,800,000 psi, hydrogen would display metallic properties: instead of discrete H2 molecules (which consist of two bonding pair electrons bound between two nuclei protons), a bulk metallic phase would form with a solid metallic lattice of protons and the electrons delocalized throughout.
-
See if you can solve this riddle without googling the answer.
You have 2 blue pills and 2 red pills, the 4 pills all mixed together in one hand, and you need to swallow exactly 1 red pill + 1 blue pill at the same time within the next 1 minute, or you will die. If you swallow more than 1 pill of the same color, you will also die. Problem is, you're trapped in a pitch dark room with no light source, so you can't see the colors, you can only feel the pills. How sial?
-
Time yourself. If you can complete this question within 5 mins, you're an A grader. 10 mins, you're a B grader. 15 mins, you're a C grader, and so on.
VJC 2012 P3 Hess Law Qn
ZnO(s) + 3C(s) --> ZnC2(s) + CO(g) ; enthalpy change = + 462 kJ mol-1
ZnO(s) + H2O(l) --> Zn(OH)2(s) ; enthalpy change = - 65 kJ mol-1
ZnC2(s) + 2H2O(l) --> Zn(OH)2(s) + C2H2(g) ; enthalpy change = - 126 kJ mol-1
2C(s) + O2(g) --> 2CO(g) ; enthalpy change = - 221kJ mol-1
2H2O(l) --> 2H2(g) + O2(g) ; enthalpy change = + 572 kJ mol-1
Calculate the formation enthalpy of C2H2(g).
Final Answer = +225.5 kJ mol-1
BedokFunland JC extension :
For each equation, state the type of reaction occurring, and draw the curved-arrow electron-flow reaction mechanism for each reaction.
-
^_^
-
[Chemistry / Medicine] -
Just because you didn't know that the salt used at Macs contains aluminium, doesn't make the salt any less toxic for your body and in particular for your brain :
http://www.dailymail.co.uk/health/article-4047426/Aluminium-DOES-cause-Alzheimer-s-Expert-says-new-findings-confirm-metal-devastating-brain-disease.htmlBeing unaware of a problem doesn't make it go away.
McDonalds (including Singapore Macs) uses an aluminium salt, just read the ingredient list on the small Macs salt packs the next time you're at Macs.
https://www.quora.com/What-kind-of-salt-does-McDonalds-use-on-their-fries
At the very least, always ask for UNSALTED fries at Macs and all fast food restaurants. Of course, quite separate from the aluminium problem, are the French Fries itself (or for that matter, all deep fried foods).
http://time.com/4811136/fried-potatoes-mortality-study/
Also see :
http://www.top10grocerysecrets.com/2015-07-02-top-10-toxic-ingredients-used-mcdonalds.html
-
[Chemistry / Medical ] - Concerns over Aluminium toxicity and Bisphenol A toxicity from soft drinks, including Coca Cola (btw : all artificial-sweeteners sugar-free 'diet' soft drinks are always more toxic than regular-sugar soft drinks).
"Without the invisible layer of BPA-based epoxy coating, an aluminium can of acidic Coca Cola would disintegrate in just three days!"
https://www.wired.com/2015/03/secret-life-aluminum-can-true-modern-marvel/
Also see :
http://www.coca-colacompany.com/contact-us/faqs
http://www.bisphenol-a.org/human/epoxycan.html
https://www.stevespanglerscience.com/lab/experiments/can-ripper/
-
[ Biology / Medical ] - "The Upside of Bad Genes" by Moises Velasquez-Manoff
https://www.nytimes.com/2017/06/17/opinion/sunday/crispr-upside-of-bad-genes.html
Sickle cell was a case in point. The gene is usually found in people who live in, or whose ancestors came from, sub-Saharan Africa, the Arab world and India; in those places, having one copy of the gene can prevent the worst symptoms of malaria. Of every four children our imaginary couple might have, one will probably be afflicted with sickle cell disease, but two would most likely be protected from malarial disease.
Ditto with the gene variants that cause the lung disease cystic fibrosis. In parts of northwest Europe, about 1 in 25 people carries a single copy of the gene. And while two copies cause disease, it has long been hypothesized that having just one protects against tuberculosis — the White Plague that ravaged Europe for a few hundred years.
Carriers of the ApoE4 variant have an up-to-fourfold increased risk of Alzheimer’s disease. In 2010, scientists at Northwestern University found that the gene was more prevalent in tropical and polar populations than in those from mid-latitudes, presumably because it served some function in those regions. Then, strangely, the gene was linked to enhanced cognitive performance in children living in Brazilian slums. And last year, Ben Trumble, an anthropologist at Arizona State University, and colleagues published an intriguing study suggesting that the gene might improve brain function in elderly people living in the Bolivian Amazon. The tribe he studied, called the Tsimane, subsist mostly on what they grow and hunt in the jungle, and about two-thirds have intestinal parasites. Dr. Trumble discovered that if elderly ApoE4 carriers harbored parasites, their cognitive abilities improved relative to noncarriers with parasites. Only carriers without parasites suffered cognitively.
Scientists at Albert Einstein College of Medicine in New York City had recently discovered that ingesting cyanate salts could prevent the “sickling� of red blood cells that leads to the anemia and pain of sickle cell disease. Dr. Jackson knew that a related compound, cyanide, was common in foods across Africa, particularly in the staple crop cassava (which you may know as tapioca). She was also aware that, at the right dose, cyanide could directly protect against the malaria parasite. She realized that regular consumption of cassava — more common in the Southeast than the Northwest — could, by working as an antimalarial drug, affect the prevalence of the sickle cell trait, by making it less advantageous.
Something different was happening in the Northwest, though. Dr. Jackson, who is now at Howard University, thinks that while cassava consumption in the region was insufficient to protect against malaria directly, people who had two copies of the sickle cell gene still ate enough to partly avoid sickling. In that population, diet may have prevented a genetic disease from fully manifesting.
We evolved in environments that are radically different from today’s, and some of our genes may work better in those environments. This complicates the idea of trying to perfect the human genome with technology. Given how much the world has changed in just the past 150 years, and how much it’s likely to change again in the next 150, the question is, “What environment will we optimize our genes for?�
https://www.nytimes.com/2017/06/17/opinion/sunday/crispr-upside-of-bad-genes.html
-
^_^
-
Here's a BedokFunland JC classic question that most JC students (and even many private tutors and JC school teachers) get wrong.
Is Ozone polar or non-polar? By drawing out the molecule, explain your answer.
My BedokFunland JC students can check their answer with me during tuition, all other students can go ask their private tutor or school teacher.
You can check out my compilation of BedokFunland JC H2 Chemistry questions on my website.
-
Originally posted by Claresse216:
Hello! May I ask a question on The Periodic Table?
Qn : When sodium chloride is added to water, a solution is formed which has a pH of 7. When phosphorous pentachloride is added to water, the resulting solution has a pH of 1. Explain these observations, and why these chlorides differ in their reaction with water.
Yo!Due to relatively low ionic charge densities, neither the Na+ ion nor the Cl- ion undergoes significant hydrolysis, hence pH of NaCl(aq) remains neutral.
PCl5, being a covalent chloride, is electrophilic (since each Cl is significantly more electronegative than P), hence readily attacked by water nucleophiles to generate 1 mol of the weak acid H3PO4(aq), as well as 5 mol of the strong acid HCl(aq). Hence a strongly acidic solution results.
-
Originally posted by Claresse216:
1) A 4.50g sample of a carbonate of a Group 2 metal (known to be either calcium or strontium) lost 1.34g in mass when heated strongly. Identify the metal, showing clearly your working.
2) Group 2 hydroxides and nitrates decompose to give the same solid residues but different gaseous products.
A group 2 hydroxide exists as a hydrate, M(OH)2.nH2O. When a 1.575g sample of this hydrte was heated, the mass if the sample decreased by 0.720g. Subsequent hearing to constant mass produced a further mass loss of 0.090g. Calculate the value of n, and deduce the identity of the metal M.
Q1. From 1.34g of CO2(g), find mol of CO2(g). By stoichiometry, this is also the mol of metal carbonate. Giving sample mass of metal carbonate as 4.50g, and now you know the mol of metal carbonate, you can find the molar mass of the metal carbonate, hence can identify the metal.Q2. The first 0.720g of water lost is from the water of hydration, ie. nH2O. The 2nd 0.090g of water lost is from the decomposition of the hydroxide. From 0.090g of water lost, you can find the mol of water lost, which by stoichiometry is also the mol of the entire hydrated metal hydroxide. Hence n = (mol of water lost 0.720g) / (mol of water lost 0.090g).
Now that you know the value of n, you can work out the molar mass of the hydrated metal hydroxide in terms of an algebraic variable, eg. let M be the molar mass of the metal M. Since you know the mol of the hydrated metal hydroxide, and you know its sample mass is 1.575g, and you have its molar mass (in terms of M), hence you can equate the 3 terms together and solve for M, and therefore identify the metal M.
-
Originally posted by JJ_DHS:
Hey UltimaOnline, heres a question from my sch paper
A sample of positively charged chromium ions, Cr^n+, was beamed through an electric field and was deflected by an angle of +8 degrees. A sample of No2^+ was beamed through the same elctric field and was deflected by an angle of +3.01 degrees. With reference to data booklet, calculate the amount of energy required to form 0.2 mol of gaseous Cr^n+ ions from 0.2 mol of gaseous Cr^(n-1)+ ions.
Ans: angle of deflection of Cr^n+=(1/46 divide n/52)x8=3.01
I dont understand why the angle of deflection of Cr is being derived as such. Thanks for the help!
(+1/46)x = +3.01, hence x = 1.3846x10^2
(+n/52)x = +8, hence n = +3Hence energy required to form 0.2 mol of gaseous Cr^3+ ions from 0.2 mol of gaseous Cr^2+ ions = 0.2 mol x 3rd ionization energy (kJ/mol)