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  • tokipaqur's Avatar
    4 posts since Nov '17
    • Originally posted by UltimaOnline:


      The A grade boundary is unlikely to go below 70%, no matter what. If P1 is easy, it'll rise back up to nearer 75%. If P1 is tougher than the other 3 Papers, the it'll slide down to nearer 70%.

      Based on your estimates for the Papers completed, as long as you and your friends score near full marks for the upcoming P1, you guys will have a good chance of getting your A grade, no worries.

      Thanks for the reply! :) yeah I think I need a close to perfect, at the least >27 for mcq to scrape an A. No matter how easy/hard p1 is I personally feel the A grade range is not going to be high this year. Heard 2010 is tough but I scored much better for '10 p2 and p3 (>80%) compared to a levels this year, furthermore p4 was a nightmare for us.

       

       

      Edit: just curious, are the % you mentioned your estimates? i do agree it's a reasonable range though, but just curious cos so far i haven't found anyone that knows how exactly the bell curve is applied. Thanks (:

      Edited by tokipaqur 11 Nov `17, 10:28PM
  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by do12453:

      Ultima for q5 on period 3 oxides how many marks will my ans lose out on

       

      I first stated that across the period, oxides become less basic and more acidic.

       

      I gave the equations:

      Na2O + H2O -> 2NaOH (with state symbols), explaining that it dissolved in water to give a highly alkaline ph 13 solution.

       

      Then i used the same example for MgO but with Mg(OH)2 -> Mg2+ + 2OH- as an equilibrium to show lower solubility, thus slightly basic at ph8

       

      Then i said al2o3 was amphoreric and gave these equations:

      Al2O3 + 6H+ -> 2Al3+ + 3H2O

      Al2O3 + 2OH- + 3H2O -> 2AlOH4- 

      Then i said SO3 hydrolyses in water as follows

      SO3 + H2O -> H2SO4 to form a highly acidic solution ph1-2

       

      I am worried because i didnt give equations explicitly showing acid base behaviour (eg Na2O + 2 HCl-> 2NaCl + H2O) and instead used pH of solutions

      So how would my ans fare for the qn

      I know I'm not Ultima but giving my 2-cents worth.As a marker I would give 3-4 marks  at best,more towards 3 marks.

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,890 posts since May '05
    • Originally posted by do12453:

      Ultima for q5 on period 3 oxides how many marks will my ans lose out on

       

      I first stated that across the period, oxides become less basic and more acidic.

       

      I gave the equations:

      Na2O + H2O -> 2NaOH (with state symbols), explaining that it dissolved in water to give a highly alkaline ph 13 solution.

       

      Then i used the same example for MgO but with Mg(OH)2 -> Mg2+ + 2OH- as an equilibrium to show lower solubility, thus slightly basic at ph8

       

      Then i said al2o3 was amphoreric and gave these equations:

      Al2O3 + 6H+ -> 2Al3+ + 3H2O

      Al2O3 + 2OH- + 3H2O -> 2AlOH4- 

      Then i said SO3 hydrolyses in water as follows

      SO3 + H2O -> H2SO4 to form a highly acidic solution ph1-2

       

      I am worried because i didnt give equations explicitly showing acid base behaviour (eg Na2O + 2 HCl-> 2NaCl + H2O) and instead used pH of solutions

      So how would my ans fare for the qn


      Edit : On 2nd thoughts, I've worked out how Cambridge is probably awarding those 6 easy marks. It's all good for most students, and for yourself too, do12453 (you can just estimate you scored 4 marks). No worries.

      Edited by UltimaOnline 12 Nov `17, 10:17AM
    • Originally posted by tokipaqur:

      Thanks for the reply! :) yeah I think I need a close to perfect, at the least >27 for mcq to scrape an A. No matter how easy/hard p1 is I personally feel the A grade range is not going to be high this year. Heard 2010 is tough but I scored much better for '10 p2 and p3 (>80%) compared to a levels this year, furthermore p4 was a nightmare for us.

       

       

      Edit: just curious, are the % you mentioned your estimates? i do agree it's a reasonable range though, but just curious cos so far i haven't found anyone that knows how exactly the bell curve is applied. Thanks (:


      As 'longaf' said on Reddit : "no one can give an exact overall percentage" for the A grade boundary for this year, in fact not even Cambridge or SEAB, simply because the papers haven't been marked by Cambridge, and the raw scores haven't been submitted to SEAB, and no one right now knows how this year's cohort of students have performed (even after P1 is completed, let alone today is before P1), because that's the very idea of a bell-curve, ie. dependent on how every single student in this year's cohort has performed, which right now, neither Cambridge nor SEAB has any idea on this, because the papers aren't marked yet.

      And the astute will note that I'm certainly not giving an "exact overall percentage", I'm merely stating that, just as an atomic orbital is the region of space around a nucleus in which the probability is the highest (but never 100%) of finding an electron associated with that orbital, similarly, I'm merely stating from experience, that the A grade boundary has the highest probability of residing between 70% to 75% (hardly an exact percentage, you see), and of course it's entirely possible (just extremely unlikely), that this year's A grade boundary might go below 70% or above 75%, because nobody knows how easy / tough P1 is, and nobody knows how every single student in this year's 2017 cohort has performed.

      You're probably hoping for me to just directly reveal whether I have insider info from SEAB, or if these values are nothing more than my own personal estimates without evidence, but without revealing too much, let's just say it's 'somewhere in between', ie. I have very good reasons for giving this confident 70% to 75% estimate for the A grade boundary, unlike say, show-off school teachers who delight in sadistically bragging to their students, "I just saw this year's Paper and it's ridiculously easy! The A grade boundary for H2 Chem this year confirm 82.5% liao! Wahahaha!!!"

      Edited by UltimaOnline 12 Nov `17, 12:28AM
    • Originally posted by MapPwner:

      I know I'm not Ultima but giving my 2-cents worth.As a marker I would give 3-4 marks  at best,more towards 3 marks.


      Pls always go ahead to reply other students who might be asking for help these 2 weeks as they prepare for P1, as I won't always be logged onto the forum (my phone has no internet access, so I visit this forum only when I'm at my PC).

  • tokipaqur's Avatar
    4 posts since Nov '17
    • Originally posted by UltimaOnline:


      As 'longaf' said on Reddit : "no one can give an exact overall percentage" for the A grade boundary for this year, in fact not even Cambridge or SEAB, simply because the papers haven't been marked by Cambridge, and the raw scores haven't been submitted to SEAB, and no one right now knows how this year's cohort of students have performed (even after P1 is completed, let alone today is before P1), because that's the very idea of a bell-curve, ie. dependent on how every single student in this year's cohort has performed, which right now, neither Cambridge nor SEAB has any idea on this, because the papers aren't marked yet.

      And the astute will note that I'm certainly not giving an "exact overall percentage", I'm merely stating that, just as an atomic orbital is the region of space around a nucleus in which the probability is the highest (but never 100%) of finding an electron associated with that orbital, similarly, I'm merely stating from experience, that the A grade boundary has the highest probability of residing between 70% to 75% (hardly an exact percentage, you see), and of course it's entirely possible (just extremely unlikely), that this year's A grade boundary might go below 70% or above 75%, because nobody knows how easy / tough P1 is, and nobody knows how every single student in this year's 2017 cohort has performed.

      You're probably hoping for me to just directly reveal whether I have insider info from SEAB, or if these values are nothing more than my own personal estimates without evidence, but without revealing too much, let's just say it's 'somewhere in between', ie. I have very good reasons for giving this confident 70% to 75% estimate for the A grade boundary, unlike say, show-off school teachers who delight in sadistically bragging to their students, "I just saw this year's Paper and it's ridiculously easy! The A grade boundary for H2 Chem this year confirm 82.5% liao! Wahahaha!!!"

      Yep I get your point, was worried cos I'm just scraping an (absolute) A even with fullmarks in p1.

      Btw I have a few questions regarding mark allocation. For 1a, if I didn't explain in terms of atomic orbital/etc, but merely quoted values from the Data Booklet and described the trend, is it likely that I'll get just 1 mark or even 0?

      For the organic questions, would I be penalised for writing maintained at 55 degrees even though it was a deactivating group, and I didn't write limited Cl2(g) to ensure monochlorination? 

       

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,890 posts since May '05
    • Originally posted by tokipaqur:

      Yep I get your point, was worried cos I'm just scraping an (absolute) A even with fullmarks in p1.

      Btw I have a few questions regarding mark allocation. For 1a, if I didn't explain in terms of atomic orbital/etc, but merely quoted values from the Data Booklet and described the trend, is it likely that I'll get just 1 mark or even 0?

      For the organic questions, would I be penalised for writing maintained at 55 degrees even though it was a deactivating group, and I didn't write limited Cl2(g) to ensure monochlorination? 

       


      Hate to be a bearer of bad news here, but you *might* get penalized for all 3 of the questions you asked on, but not for the reasons you think. See below.

      Q1. Similar to the discussion on how I had doubts over how most students chose to answer the acid-base nature of oxides across Period 3 (although on 2nd thoughts, I've recanted that one, Cambridge was probably just being generous with those 6 marks), it seems most students merely described the trend of thermal stabilities down Group 17, rather than the visual observation of their individual decompositions, as Cambridge probably wanted.

      As such, most students see 3 marks allocated and try to fill in more points relating to the trend (such as effectiveness of orbital overlap), but still missing out the individual thermal decomposition descriptions. Quoting Data Booklet bond dissociation enthalpy values for this question gets no marks, unfortunately for you (and many students).

      Based on my interpretation of what Cambridge probably wanted (as opposed to most students' interpretation), Cambridge could or should have fairly allocated 4 marks for this question (in contrast, Cambridge was inconsistently generous to allocate 6 marks for the acid-base nature of oxides across Period 3) ; that they stingily allocated only 3 marks here (and as such misleading many students) is likely thus : 1 mark for describing the trend down the Group (including a generic decomposition equation and stating that HCl is thermally stable and does not decompose), 1 mark for explaining the trend (just relationship between bond length and ease of bond dissociation will suffice, no need effectiveness of orbital overlap, no need quote Data Booklet values), and 1 mark for describing the different visual observations for HI and HBr upon heating.

      Organic Questions :

      "Limited" is actually indeed required to ensure monochlorination, and should have been included for a good answer. Cambridge has in some past papers let it slide, but in some other past papers penalized it. So you may or may not get away with this one.

      For nitration of a benzene (whether activated or deactivated), what Cambridge required was "heat under reflux" ideally, although they *may* accept "maintained at 55 degrees Celsius", so you *may* get away with this one.

      The irony is that while many (but certainly not all) Singapore students managed to correctly recognize that the aryl ketone substituent was deactivating, they erroneously thought that Cambridge was being sneaky-cunning and had required "more than 55°C" for the answer, when ironically, retired A level examiner-marker Jim Clark warns that :

      "Benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric (sic) acid at a temperature not exceeding 50°C. At higher temperatures there is a greater chance of getting more than one nitro group substituted onto the ring. You will get a certain amount of 1,3-dinitrobenzene formed even at 50°C."

      That's for benzene, so for deactivated benzene, 55°C is actually (and ironically for you) reasonable. But then again, that's Jim Clark's opinion (different A level syllabuses in the UK used slightly different temperatures) ; in the pre-2017 H2 syllabus, Cambridge actually wanted "50°C < T < 60°C" as the ideal student answer for nitration of unsubstituted benzene.

      But for the 2017 P3 Qn, Cambridge actually only required (and wanted) "heat under reflux" (more correct than just "heat", since this is a synthesis question, but fortunately for many students, Cambridge will probably accept just "heat", but seriously you students should have known better), since this new syllabus no longer requires (or even wants, ie. Cambridge will ignore) specific temperature values submitted by the student.

  • gabs1234's Avatar
    2 posts since Jan '16
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,890 posts since May '05
    • Originally posted by gabs1234:

      Why do you say that you recant your opinion on the 6m oxide qn? 


      Cambridge's fault lah.

      If they expected what most of you students wrote, then it doesn't seem worth 6 marks to me.

      If they expected what I would write if I took the A level paper this year (ie. beyond what JCs teach, ie. why ionic oxides are basic, why covalent oxides are acidic, why ionic oxides with covalent character are amphoteric), then that's worth more than 6 marks (because those points would be in addition to, not in exchange of, what you guys have written).

      So neither here nor there, just take it (ie. the more likely case is) that the standard answers most of you wrote can get the 6 marks lor.

      But Cambridge really nonsense this year, some questions so time-consuming only so few marks allocated, then some questions only got short answers yet so many marks allocated. Sheesh. *clap*clap* good job, Cambridge.

      But on an objective note, like MapPwner said, it's not that the paper's questions are super difficult, it's mostly that the entire paper (all the questions collectively) is time consuming, which results in a paper on the difficult side.

      So overall this year, the consensus amongst the general student population is that this year's Papers (assume P1 is of similar difficulty to all the other Papers) are significantly tougher than previous years, so yeah I expect the A grade boundary to slide down closer towards 70%.

      Then of course, like over at Reddit, you have students, or more accurately, friends of students, and school teachers, who brag that this is a ridiculously easy paper, it's a walk in the park, etc.

      Hey, the questions are easy for me as well, but you don't see me bragging (oh, I see what I did there) that this is an easy paper, because I think this is a time-consuming hence difficult paper, relative to previous years. So I agree with MapPwner that the A grade boundary should slide down towards 70%, but (let's be realistic), SEAB will resist lowering the A grade boundary to anything less than 70% (or the Universities will complain to MOE of grade inflation making it harder to select students for elite Uni courses).

      Thing is, this so-called grade inflation over the years is not because the papers are getting easier, it's rather that more Singaporeans are studying more desperately to do better for the sake of their futures, but that's harsh cruel reality of physical incarnation for you : like what elite PAP ministers say : Singapore can't have all doctors and lawyers and graduates, the economy needs lower non-graduate qualifications for lower pay jobs as well.

      C'est la vie.

  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by UltimaOnline:


      Hate to be a bearer of bad news here, but you *might* get penalized for all 3 of the questions you asked on, but not for the reasons you think. See below.

      Q1. Similar to the discussion on how I had doubts over how most students chose to answer the acid-base nature of oxides across Period 3 (although on 2nd thoughts, I've recanted that one, Cambridge was probably just being generous with those 6 marks), it seems most students merely described the trend of thermal stabilities down Group 17, rather than the visual observation of their individual decompositions, as Cambridge probably wanted.

      As such, most students see 3 marks allocated and try to fill in more points relating to the trend (such as effectiveness of orbital overlap), but still missing out the individual thermal decomposition descriptions. Quoting Data Booklet bond dissociation enthalpy values for this question gets no marks, unfortunately for you (and many students).

      Based on my interpretation of what Cambridge probably wanted (as opposed to most students' interpretation), Cambridge could or should have fairly allocated 4 marks for this question (in contrast, Cambridge was inconsistently generous to allocate 6 marks for the acid-base nature of oxides across Period 3) ; that they stingily allocated only 3 marks here (and as such misleading many students) is likely thus : 1 mark for describing the trend down the Group (including a generic decomposition equation and stating that HCl is thermally stable and does not decompose), 1 mark for explaining the trend (just relationship between bond length and ease of bond dissociation will suffice, no need effectiveness of orbital overlap, no need quote Data Booklet values), and 1 mark for describing the different visual observations for HI and HBr upon heating.

      Organic Questions :

      "Limited" is actually indeed required to ensure monochlorination, and should have been included for a good answer. Cambridge has in some past papers let it slide, but in some other past papers penalized it. So you may or may not get away with this one.

      For nitration of a benzene (whether activated or deactivated), what Cambridge required was "heat under reflux" ideally, although they *may* accept "maintained at 55 degrees Celsius", so you *may* get away with this one.

      The irony is that while many (but certainly not all) Singapore students managed to correctly recognize that the aryl ketone substituent was deactivating, they erroneously thought that Cambridge was being sneaky-cunning and had required "more than 55°C" for the answer, when ironically, retired A level examiner-marker Jim Clark warns that :

      "Benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric (sic) acid at a temperature not exceeding 50°C. At higher temperatures there is a greater chance of getting more than one nitro group substituted onto the ring. You will get a certain amount of 1,3-dinitrobenzene formed even at 50°C."

      That's for benzene, so for deactivated benzene, 55°C is actually (and ironically for you) reasonable. But then again, that's Jim Clark's opinion (different A level syllabuses in the UK used slightly different temperatures) ; in the pre-2017 H2 syllabus, Cambridge actually wanted "50°C < T < 60°C" as the ideal student answer for nitration of unsubstituted benzene.

      But for the 2017 P3 Qn, Cambridge actually only required (and wanted) "heat under reflux" (more correct than just "heat", since this is a synthesis question, but fortunately for many students, Cambridge will probably accept just "heat", but seriously you students should have known better), since this new syllabus no longer requires (or even wants, ie. Cambridge will ignore) specific temperature values submitted by the student.

      Dang so for the p3 q3 if i wrote 'temperature more than 55 degrees' is 0?

       

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,890 posts since May '05
    • Originally posted by MapPwner:

      Dang so for the p3 q3 if i wrote 'temperature more than 55 degrees' is 0?

       


      Cambridge probably will accept lah, no worries.

      Edit : At least for this year (1st year of new syllabus), they'll probably accept. If too many students all write the same answer which is not the Mark Scheme answer, especially if Cambridge recognizes it's a new syllabus teething problem caused by an entire country of teachers still teaching from the old syllabus, the Cambridge Markers will submit this issue to the Cambridge Chief Marker for him/her to make a decision on the matter, and the Cambridge Chief Marker will usually make a fair decision, so as not to unfairly penalize an entire country of students.

      Edited by UltimaOnline 12 Nov `17, 7:11PM
    • C'mon people, faster finish up ALL the 2017 Prelim Paper 1s, and any MCQ you need help with, faster post here to ask.

      As long as you score near full marks for MCQ, there's still hope for that A grade.

      This is your future at stake, this is your fight. Do right by yourself.

  • do12453's Avatar
    13 posts since Nov '17
    • haha, this week got other subs leh. and got 2weeks+ before chem mcq. Plan to clear everything else in the way first

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,890 posts since May '05
  • kelvinlee200's Avatar
    7 posts since Nov '17
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