Lurkers, register with SgForums and ask ur H2 Chem qns here!
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From the number of views H2 Chemistry threads on this forum has garnered, it's pretty clear that hundreds of lurkers (presumably and hopefully the majority of whom are actual JC students) are interested in H2 Chemistry posts.
Well, to all you lurkers out there (especially if you're a JC student), here's a suggestion : Why don't *you* go ahead and post your own H2 Chemistry queries here.
Just register an account with SgForums, and feel free (it's literally free... of course, asking for free online help here... or for that matter anywhere... isn't as effective as actually joining my tuition, but it's still better than having no one reliable to ask for help... or if you disagree with your own private tutor or school teacher and wish to have a 2nd opinion that's reliable and trustworthy) to ask anything relevant to H2 Chemistry here.
For instance, if you wish to discuss a TYS qn or a JC Prelim qn (eg. if you don't understand or disagree with the answer given by the TYS author, or the prelim paper's mark scheme, or your school teacher's explanation. etc), you can just specify the question (eg. "2014 RJC P3 Q2" or "TYS 2014 P3 Q2" etc) and ask your question. If it's a non-TYS qn or non-Prelim qn that involves a diagram, you'll have to use your handphone to take a photo, and upload the image to your Facebook, Twitter, blog, etc, and link to the image in your post.
Terms & Conditions : Only genuine and reasonable questions from genuine and reasonable students asked in a genuinely and reasonably respectful manner will be replied to.
New members of SgForums won't be able to make new threads until you've reached a certain post count, so go ahead and post your H2 Chem qns in this thread till then. -
You called?
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Hi, i am studying Cambridge Internation A level chemistry and have found that the questions asked are easier but very similar to the H2 chemistry papers. And as CIE is increasing the difficulty of papers each year, the questions will be more like the ones in the singapore prelim papers. Hence, i would like to ask questions that i have found tricky and difficult from both CIE and H2(i cant even solve some of the H2 prelim questions).
Q1:)
Please look at Q3(d)(iii) of the following paper
http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w13_qp_23.pdf
How will i know what products will be formed when BrCl reacts with KI....i know its supposed to be displacement reaction but how can i be sure that KBr and KCl will be formed instead of Br2.....
Q2.) Similarly, how would i know the products of the reaction of NaH(sodium hydride) with water if i only that MgH,NaH both react with H2O to give the same colourless guess?
Is there any general way of knowing the products of chemical reactions such as above in keeping the fact that we know all the reactions of the chapter of periodicity(such as chlorides and oxides with water,etc)?
Thank You for answering!
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Hi Light5, welcome to the forum! :)
Q1. Even if you suspect Br2 may be formed as a possible intermediate (in the reduction of BrCl, which may be conceptualized as Br+ and Cl-, with OSes of +1 and -1 respectively), but knowing full well (as required by the A level syllabus) that the redox reaction between Br2 and I- is (of course) thermodynamically feasible under standard conditions, you should always give the final equation with the final product of the completed reaction, ie. in effect acknowledging that any Br2 formed (eg. as possible intermediate) would be further reduced to Br- by I- (which is in turn oxidized only to I2... any further oxidation would require a stronger oxidizing agent, and clues would be given in the question).
Q2. If you (ie. as an A level candidate) correctly understand the nature of metal hydrides, you'll understand that it consists of the metal cation and the hydride anion H-, which being highly unstable (due to the high anionic charge density due to small ionic radius exacerbated by the low electronegativity of H) is thus a strong Lewis base (ie. strong nucleophile and strong Bronsted-Lowry base), and hence readily abstracts a proton from water (which you should be aware is a protic species) to generate H2 gas, leaving behind the metal hydroxide as the by-product (not to be confused with side-product).
Edited to add : The forward reaction is favored both thermodynamically (due to positive entropy change) as well as by Le Chatelier's principle (due to gaseous product leaving the solvated reaction mixture).
If your understanding and familiarity of reaction mechanisms (more commonly employed in organic chemistry, but chemistry is chemistry, and mechanisms are actually every bit as relevant to inorganic chemistry, despite the lack of exposure in this area for all A level students, whether international or Singapore) is strong, you're advised to draw out the mechanism for this (and other inorganic chemistry) reactions, which will enable you to truly understand (rather than just blindly memorize) why and how such reactions occur, and as such also helps you to correctly determine the identity and structure of the products.
Feel free to continue posting your A level Chem qns here all the way till your A levels at the end of the year, Light5! ;) -
Q1.)Thanks alot, couldnt have even thought of bronsted lowry theory for Q2..do you have any advice as to how to study reaction mechanisms for Inorganic chemistry(any book,webisite,etc) that can be of help? Instead of the Period 3 chlorides,oxides and group2,group 7 chapters, should i learn more compounds formed by period 3 elements and their corresponding reactions and if so, which ones should i focus on?
Q2.) I know that HBr is an acid and Br, being more electronegative than H, has a partial negative charge..Also, Br has few lone pairs on it...if this is the case why cant HBr participate in a nucleophilic substitution or nucleophilic addition reaction with halogenoalkanes and aldehydes/ketones respectively?
Q3.) How is Mg + 2HCL = MgCl2 + H2 an acid- reaction? I know that HCl is giving out H+ ions, but it isnt donating these protons to Mg, in fact it is donating Cl- ions to Mg...so how does this reaction stand true with bronsted-lowry acid-base theory?
Q4.) Is there any general strategy for drawing correct structural isomers ensuring that none of them are repeated...how can i do this on a piece of question paper(since we dont get a rough sheet for the MCQ paper 1). Even if you can demonstrate it with an example, that will be fine?
Q5.) What could be the products and the mechanism for the following reaction :
NO2 + H2O = ...?
All i can predict is that nitric acid will be formed as NO2 is an acidic oxide however, that does not balance the equation so what will be the other product?
Again, thank you for answering my questions. I know my queries arent as advanced as those of singapore students but some of these misconceptions have arised while solving CIE past year questions...i will start asking questions related to singapore prelim papers once i start solving some of the difficult P1 and P3s.
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Q1. No worries about focusing on Group vs Period, just do your best to *understand* (with use of mechanisms whenever possible; ie. you can attempt drawing out the mechanisms for most, if not) all of the inorganic reactions required by your own A level syllabus, instead of blind memorization. Make sense of it in your own way (eg. by use of analogies), own it for yourself.
Q2. HX does indeed react in a nucleophilic substitution reaction with alcohols (you can attempt to draw the mechanism for this... fact is, online help via forums is by its nature limited, it's not feasible for me to draw out all the mechanisms you may be interested in; I can and will do this for my own tuition students when they're physically present, but it's not feasible and too time-consuming to do this online... if you're really interested in learning how to draw mechanisms for all A level reactions organic and inorganic, you might want to consider engaging a private tutor, in whichever country you're residing in).
Nucleophilic substitutions of HX with RX can and does occur (of course, this is only meaningful with two different halides are involved), with the position of equilibrium favoring the thermodynamically more stable species; specifically the species with the stronger C-X covalent bond (ie. a more effective head-on or end-on overlap by less diffused electron orbitals to form a stronger sigma bond results in a more exothermic bond formation enthalpy).
However, a twist to this was given in the Singapore-Cambridge 2013 A levels P3 Q1, in which (if you have access to the Singapore A level papers, you might like to check this out) the strongly exothermic (thus thermodynamically favorable) lattice formation enthalpy involved in the ionic precipitation of the lower Ksp sodium halide in propanone (polar aprotic) solvent, together with shifting of position of equilibrium as predicted by Le Chatelier's principle, results in R-I generated at the expense of R-Cl, despite R-Cl being a stronger covalent bond.
However for aldehydes and ketones, any nucleophilic addition of the halide ion will be followed by it's elimination (the mechanism for nucleophilic acyl substitution is addition-elimination) to re-generate the aldehyde or ketone, as the halide ions are good leaving groups due to their low anionic charge densities (as opposed to say, hydride ions from LiAlH4 or alkyl nucleophiles from Grignard reagents).
As for any potential Bronsted-Lowry acid-base reactions between HX and alky halides or aldehyde/ketones, alkyl halides and aldehyde/ketones aren't Bronsted-Lowry basic to begin with (aldehydes and ketones are actually slightly Bronsted-Lowy acidic, as the conjugate bases generated by the deprotonation of their alpha protons, are stabilized by having their negative formal charges delocalized by resonance over both the alpha C atom and the electronegative carbonyl O atom of the enolate ion conjugate base... if you're up for the challenge, draw the mechanism between both resonance contributors and elucidate the resonance hybrid).
Q3. This is simultaneously a redox reaction as well as a Bronsted-Lowry acid-base reaction. It is a first and foremost a redox reaction because Mg is oxidized to Mg2+, and H+ from the acid is reduced to H2 gas. It is secondarily a Bronsted-Lowry acid-base reaction because HCl (if in aqueous state, more precisely hydroxonium chloride) is being deprotonated to generate it's conjugate base the chloride ion (or more precisely, water and the chloride ion) and thus generating the salt MgCl2(aq). For a P1 MCQ, choose the best option (psychologically profile the Cambridge setter's intentions if you're able to). For a P2 or P3 qn (or whatever equivalent in your syllabus), give *both* descriptive alternative answers with qualifying explanations to secure full credit.
Q4. Every year, many students routinely ask, "is there a shortcut to drawing all possible isomers?". The short answer is : no there isn't. There can be systematic approaches (it's up to you to formulate your own), but there's no magical shortcut to drawing all possible isomers, especially when stereoisomerism (optical and geometrical) is also to be considered.
But there is indeed a method (in fact, this is the *only* correct method, though it's far from being a shortcut) to reliably identifying any repeated structures. That's to name each of your structures. If they've the same name, then they're the same molecule.
No problem Light5. Go ahead and feel free to continue asking your questions from both your CIE A level syllabus, as well as from Singapore Prelim papers. Though when quoting questions from past CIE papers or Singapore Prelim papers, it'll be preferred if you can provide the (eg. http://papers.xtremepapers.com) links to not only the question paper, but to the mark scheme as well (whenever available), for the sake of efficiency and convenience.
Originally posted by Light5:Q1.)Thanks alot, couldnt have even thought of bronsted lowry theory for Q2..do you have any advice as to how to study reaction mechanisms for Inorganic chemistry(any book,webisite,etc) that can be of help? Instead of the Period 3 chlorides,oxides and group2,group 7 chapters, should i learn more compounds formed by period 3 elements and their corresponding reactions and if so, which ones should i focus on?
Q2.) I know that HBr is an acid and Br, being more electronegative than H, has a partial negative charge..Also, Br has few lone pairs on it...if this is the case why cant HBr participate in a nucleophilic substitution or nucleophilic addition reaction with halogenoalkanes and aldehydes/ketones respectively?
Q3.) How is Mg + 2HCL = MgCl2 + H2 an acid- reaction? I know that HCl is giving out H+ ions, but it isnt donating these protons to Mg, in fact it is donating Cl- ions to Mg...so how does this reaction stand true with bronsted-lowry acid-base theory?
Q4.) Is there any general strategy for drawing correct structural isomers ensuring that none of them are repeated...how can i do this on a piece of question paper(since we dont get a rough sheet for the MCQ paper 1). Even if you can demonstrate it with an example, that will be fine?
Again, thank you for answering my questions. I know my queries arent as advanced as those of singapore students but some of these misconceptions have arised while solving CIE past year questions...i will start asking questions related to singapore prelim papers once i start solving some of the difficult P1 and P3s.
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Originally posted by Light5:
Q5.) What could be the products and the mechanism for the following reaction :
NO2 + H2O = ...?
All i can predict is that nitric acid will be formed as NO2 is an acidic oxide however, that does not balance the equation so what will be the other product?
Oxides of nitrogen are a complicated and messy affair, and for A level purposes, Cambridge would likely provide in-context hints if such is asked.
The competent and astute A level candidate would notice that the OS of N in NO2 is +4, but yet the OS of N in the common N containing acids HNO2 and HNO3 (Latin names : nitrous and nitric acid; Stock names : nitric(III) and nitric(V) acids) are +3 and +5, which should lead you to a reasonable deduction that this isn't a straightforward case of hydrolysis of an electrophilic covalent oxide to generate an aqueous acid in which the OS of the heteroatom is conserved, as would usually be the case, eg. CO2 + H2O --> H2CO3, or SO3 + H2O --> H2SO4. At the very least, Cambridge would expect the competent and astute A level candidate to suggest the existence of a disproportionation reaction, and Cambridge more than likely would provide in-context hints within the question as to the exact identities of the products and intermediates (see links below).
If you're interested in a more in-depth look into this reaction (it's entirely possible Cambridge might provide some of the following info in a data based question, and query the A level candidate accordingly across several related topics in the A level syllabus, possibly even asking for the reaction mechanism to be drawn out, ala Singapore-Cambridge A level 2012 P3 Q3), you can check out :
http://pubs.rsc.org/en/content/articlelanding/2003/cp/b208564j#!divAbstract
http://www.researchgate.net/publication/234036456_The_Heterogeneous_Hydrolysis_of_NO2_in_Laboratory_Systems_and_in_Outdoor_and_Indoor_Atmospheres_An_Integrated_Mechanism -
Originally posted by Light5:
Do you have any advice as to how to study reaction mechanisms for Inorganic chemistry (any book,webisite,etc) that can be of help?
Books to recommend for A Level Chemistry (in particular Inorganic Chemistry) :
http://www.infinity.usanethosting.com/Tuition/#Books_for_H2_Chemistry
Inorganic Chemistry Made Easy by George Chong KL
Understanding Advanced Chemistry Through Problem Solving : Physical and Inorganic Chemistry by Jeanne Tan and Kim Seng Chan
Understanding Advanced Physical Inorganic Chemistry : The Learner's Approach by Jeanne Tan and Kim Seng Chan -
Hi, this thread seems empty but I would like to ask qns if possible. Hope someone would reply. question iz from Organic Chem Made Easy by George Chong KL. From page 163, qns part d. Why the reaction is with ClCH2CO2H instead of CH3CH2Br for rxn number III. And if with ClCH2CO2H, how is the reaction mechanism. Why the COOH suddenly disappear in the product? Thanks
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Also, any tips for finding isomers at the most efficient way as possible and tips for finding chiral carbon (esp at MCQ qns)? Thanks
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Originally posted by Mrworry:
Hi, this thread seems empty but I would like to ask qns if possible. Hope someone would reply. question iz from Organic Chem Made Easy by George Chong KL. From page 163, qns part d. Why the reaction is with ClCH2CO2H instead of CH3CH2Br for rxn number III. And if with ClCH2CO2H, how is the reaction mechanism. Why the COOH suddenly disappear in the product? Thanks
George Chong made a typo error in the structure of the product. He's a one-man show, without the vast proof-reading resources/networking of say, Chan Kim Seng & Jeanne Tan, and hence there are more typo errors in George Chong's books compared to Chan Kim Seng & Jeanne Tan's books ; but all of these books (from all 3 of these authors) are still excellent and highly recommended for all A level H2 Chem students, together with CS Toh's A level Study Guide.
The correct structure of the product should be :
No, there are no shortcuts to qns involving isomers and chiral carbons, just be as systematic as you can, and naming can help to eliminate repeated structures. -
Ah... But why the nucleo sub mechanism (in part e, the qns is missing, but I think it's about mechanism) is different? the arrows looks like elimination there instead. Xie xie :p
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Originally posted by Mrworry:
Ah... But why the nucleo sub mechanism (in part e, the qns is missing, but I think it's about mechanism) is different? the arrows looks like elimination there instead. Xie xie :p
Nothing wrong with George Chong's SN2 nucleophilic aliphatic substitution mechanism (just that the transition state isn't shown). It's SN2 because you're dealing with a primary alkyl halide (whose primary carbocation in the unfeasible event of an SN1 would be further destabilized by the electron-withdrawing by induction COOH group). Elimination goes by E1 and E2 mechanisms. -
Yeah I get it that it is SN2. I guess i have to read again about elimination vs substitution
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wow. will watch it now.
Qn page 159-160 on isomers. Is the way to do it via trial and error? Any tips? xie xie~
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Originally posted by Mrworry:
wow. will watch it now.
Qn page 159-160 on isomers. Is the way to do it via trial and error? Any tips? xie xie~
This is a TYS qn. Yes, but if you know what you're doing, it's a systematic trial-and-error, not a haphazard one. Students are advised to give full detailed working (exactly as George Chong has done) to arrive at the correct structures.
No prob. Welcome to the forum, and keep your questions coming all the way till the last day of the A levels. -
Can you elaborate on this systematic trial and error? Thx
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(i.e maybe with reference to this point)
Xie xie
How to edit my post lol
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There are only 2 possible alkyl halides for the structure based on the description given, and only 3 different beta protons available in each case (which upon dehydrohalogenation, generates 3 structurally isomeric alkenes, only some of which possess geometrical isomerism).
Hence, the so-called systematic trial-and-error, means to list out all possible solutions, while intelligently bearing in mind the conditions set out by the questions. Concordantly, you will be able to readily eliminate one of the alkyl halides, leaving the remaining alkyl halide to be the required answer. -
Can I ask questions from my university Chemistry modules here too?
I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.
Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?
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Originally posted by White Dust:
Can I ask questions from my university Chemistry modules here too?
I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.
Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?
Sorry, all Chemistry qns beyond the Singapore A level H2 Chemistry syllabus should be asked on ChemicalForums.com instead :
http://www.chemicalforums.com/
Just as some people will find Physics naturally easier than Biology, some others will find Biology naturally easier than Physics. Every soul is unique, with unique interests, strengths and (if you're free-thinking enough to appreciate this) unique past life backgrounds and soul karma. Infinite Diversity in Infinite Creation, as the Vulcans (from Star Trek) would say.
Of course, it's entirely possible that you're exactly equally excellent (or sucky) at both Physical and Organic Chemistry, but that's pretty rare (fact is, almost everyone has a personal preference between dogs and cats; very few people love or hate both dogs and cats exactly equally).
To make a prudent and judicious decision, you should explore the syllabus details of both alternative modules before making a decision. But I leave you with a (rather obvious) tip : if you're more Physics and Mathematics inclined, you'll enjoy Physical Chemistry better ; If you're more Biology and Medicine inclined, you'll enjoy Organic Chemistry better.
Personally, at the Uni level, I much preferred (and still do prefer) Organic Chemistry over Physical Chemistry. -
Originally posted by White Dust:
Can I ask questions from my university Chemistry modules here too?
I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.
Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?
I personally agree that CM1121 is more difficult. It is based on some understanding of thermodynamics and kinetics (CM1131) and even greater understanding of Atomic Orbitals, HOMO-LUMO Theory, HSAB Theory etc (CM1111 Inorganic I). However there are other factors to consider as well, like workload, bell-curve, lesson plan, grading and exam formats etc.... If you are really interested I can tell you more about these on CM1131 and CM1121 here.
I'm a Chem Major and I think majority of my peers are more of the memory-workers, and this observation probably extends to batches after and before mine. So in this case, CM1131 is more scorable. Most people would feel that CM1131's contents is boring though.
p.s.I like Inorg and Org Chem much more but I suck much more at both.
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http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_11.pdf
^ How to solve Q28 from this paper, the one about ladenberg structure ....i cant find a way to name the isomers drawn ..... and how to know whether the isomers are distinct from each other or not.?
http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_12.pdf
^ Q30...there is an ester linkage that will break to form CO2Na but what about the CON linkage.....i havent done this linkage in AS chemistry so how can i know whether it will hydrolyse or not?
^Q18 of the same paper: Dosent CO oxidise to CO2 in the atmosphere naturally, i cant find it written in any book that CO dosent oxidise spontaneously in the atmosphere?
^Q37 of the same paper: To me, it seems none of the C atoms in all 3 structures lie in the same plane...so how to solve this?
Following are the mark schemes for both papers :
http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_11.pdf
http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_12.pdf
Thanks a lot !
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Q28. bottomleft + topleft ; bottomleft + bottomright ; bottomleft + topright.
Q30. Google "hydrolysis of amide".
Q18. CO is *eventually* oxidized to CO2 in the atmosphere, not *spontaneously*. Spontaneity implies both kinetic feasibility as well as thermodynamic feasibility. Here, while it's thermodynamically feasible to oxidize CO to CO2, the activation energy is fairly high. Nonetheless, I agree that this is one of those lousy Cambridge MCQs where more than 1 option might be arguably acceptable.
Q37. Options 1 and 2 include at least two sp2 C atoms (with 2 sp3 C atoms). Option 3 has only one sp2 C atom (with three sp3 C atoms) ; sp2 C atoms are planar, while sp3 C atoms are tetrahedral, ie. non-planar.
No problem, good that you provide links to both Qns and Mark Schemes for ease of discussion.