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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • imagehttps://mothership.sg/

      A BedokFunland JC H2 Chemistry Challenge Question for you :

      Amos Yee drinks Teh-O-Ice-Limau according to 1st order kinetics...

      http://www.bedokfunlandjc.com/

    • A BedokFunland JC H2 Chemistry Question

      The amount of CN−(aq) ions in an analyte solution can be determined by titration against Ag+(aq), during which the soluble dicyanoargentate(I) coordination complex is first generated, until sufficient Ag+(aq) is added from the burette to generate the insoluble white precipitate of the silver(I) dicyanoargentate(I) coordination compound. If x mol of Ag+(aq) had been added at first sighting of the white precipitate, what was the amount of CN−(aq) ions present in the analyte solution?

      A) 0.5 x
      B) 1.0 x
      C) 2.0 x
      D) None of the above

    • 2017 JC2 students, which Singapore JC's 2017 Prelim papers is in your opinion the toughest paper amongst all the JCs?

      (Every year, all the Singapore JCs are fiercely competing against each other to set the most impossibly sadistic Prelim papers, tis a traditional badge of pride & honor, you understand).

    • If anyone has any 2017 Prelim Paper question that you wish to discuss or seek help on (specify what is it about the question or mark scheme that you do not understand or disagree with), just post your question here. If I need you to take a photo of the question, I’ll say so. For now, just post the JC, Paper no. and Qn no.

    • I’m so glad to see the quality and standard of all Singapore JCs Prelim Papers increased significantly this year (no doubt in anticipation of the rigor of the new syllabus). It’s heartening and affirming to observe that many higher-level questions (eg. H3 content and Olympiad level) which would in previous years only be seen in my own BedokFunland JC materials, are now appearing in (almost all) Singapore JCs’ Prelim Papers! Very well done, all Singapore JCs! Excellent work setting the 2017 new syllabus tougher Prelim Papers! Hopefully Cambridge doesn’t disappoint us this year (and every year from now on)! Very Happy

    • An Original BedokFunland JC H2 Chemistry Challenge

      image

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

      Q1. Explain fully the relative basicities of the 3 N atoms in Histidine (ie. explain which N atom on the R group imidazole ring is more basic, and explain whether the alpha N atom or the R group imidazole ring is more basic).

      Q2. Given pKa1 = 1.60, pKa2 = 5.97 and pKa3 = 9.28, calculate the isoelectric point of Histidine.

      Q3. Calculate the pH at all significant points (ie. initial, 1st maximum buffer capacity, 1st equivalence point, 2nd maximum buffer capacity, 2nd equivalence point, 3rd maximum buffer capacity, 3rd equivalence point) in
      (i) the titration of the fully deprotonated form of Histidine against a strong monoprotic acid added in excess, as well as in
      (ii) the titration of the fully protonated form of Histidine against a strong monoprotic base added in excess.
      Use 0.05M as the molarities for Histidine and both titrants, and 25cm3 as the analyte volume of the Histidine solution for both titrations. Include a sketch of both titration curves, with all significant pH values labeled.

      Q4. By considering the enthalpy and entropy changes during decarboxylation, explain how temperature affects the thermodynamic feasibility and Kc value for the decarboxylation reaction.

      Q5. Suggest and draw out the curved-arrow electron-flow reaction mechanism for the (non-enzyme catalyzed) decarboxylation of Histidine.

      Edited by UltimaOnline 22 Oct `17, 4:19AM
  • Zzzfrendzzz's Avatar
    93 posts since Oct '12
    • hello, with the h2 chem prac coming this thursday, i would like to ask how does one prepare for the inorganic planning section of the paper? All of the answers suggested have very very detailed amounts / conc of the reagents and steps required to be taken as well as the 'pre-calculation'. Considering that i would most likely be short of time in the actual exam, how am i suppose to reproduce it?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Zzzfrendzzz:

      hello, with the h2 chem prac coming this thursday, i would like to ask how does one prepare for the inorganic planning section of the paper? All of the answers suggested have very very detailed amounts / conc of the reagents and steps required to be taken as well as the 'pre-calculation'. Considering that i would most likely be short of time in the actual exam, how am i suppose to reproduce it?


      The vast majority of JC students cannot reproduce the full details, nor score anywhere close to full marks for the Planning. This is expected, and should not be a cause for worry. As long as you (ie. the student), understand what the experiment is about, just do your best to come up with an appropriate Plan for the experiment, and just fill in as many details (with reasonable values for volumes, molarities, etc, based on the apparatus involved) as you can along the way, and work according to the values you've provided. In other words, see the big picture, don't sweat the small stuff. If you can do this, you can pass your Planning (don't try to score full marks, it's not realistic). And together with near full marks for your Theory Papers (that's way more realistic), an overall A grade is easily within reach (as I personally demonstrate every year).

    • I haven't gone through all of the JCs yet, but so far, RJC has (by far) the highest quality Prelim Paper questions amongst all the 2017 Prelim Papers.

      Anyone want to recommend any particular JC's 2017 Prelim Paper? Eg. that you found particularly challenging or educational or unique? (in a good way hor, not uniquely atrocious).

    • I'm skipping this year's A levels due to a scheduling conflict, and if you miss just 1 paper, you miss all papers for that subject, because you'll be awarded an "ABSENT" grade for that subject.

      On an unrelated note, after or alongside RJC, DHS also has one of the highest quality 2017 Prelim Papers amongst all the Singapore JCs' 2017 Prelim Papers. If I come across any more top quality 2017 Prelim Papers (which all Singapore JC students are recommended to learn from), I'll mention them (though I probably won't go through all of the Singapore JCs' papers, so if you've a particular JC to recommend, please make a post here to share your opinion with us).

  • MapPwner's Avatar
    138 posts since Aug '15
    • Can i ask for the prac enthalpy change of decomposition,must we take into account stoichiometric coefficient,whereby -(heat change)/amount of h2o2 x2

      Then fe(h2o)6 3+ catalyst role is intermediate(i forgot the compound) formed in step 1,it decomposes in step 2 and then fe(h2o)6 3+ regenerated  in step 3.

      Fe(h2o)6 3+ +H2o->fe(h2o)5(oh) +2 +h3o+

      Fe3+ has high charge density than polarise.(I careless,said H2O act as base)

      H2o2 is first order

      Planning didnt bother doing

      Then inorganic qa observation

      Naoh soln turns purple to colourless aft addin kmno4,and turns blue aft some time.Deionised remains purple.

       

      Then inorganic qa,observation is redox,ligand exchage reaction,reducing agent for h2o2.

    • Compared my answers with many sources(classmates,friends from other schs like AJ/HCJC),excluding planning my score is about 30/41 for the 15% of practical,which is supposedly rather decent among those i asked so far(my friends from AJ/HCJC supposedly did worse than this score out of 41,and some skipped planning altogether).

      I counted my score rather stringently so yeah,somewhat disappointed but oh well,at least i didn't do any worse as the practical really looked simple yet tricky,which was maybe why a lot of people lost a lot of marks due to careless mistakes,which I did too.

      Lost approximately 4 marks in energetics component(-1 for failure to write down o2 evolved relights glowing splint,-1 for not taking into account x2 stoichiometric coefficient for enthalpy change of decomposition in both cases-assuming there is ecf,-1 for possibly inefficiently explaining how fe(h2o)6 3+ acts as catalyst and -1 for giving wrong reason for fe(h2o)6 3+ forming fe(h2o)5(oh) 2+ and h3o+ due to high charge density)

      Kinetics lost 3 marks due to calculation misunderstanding,otherwise was relatively manageable,such as quoting of 2 consecutive t1/2 to prove whether is it first order or not(for me it is yeah).

      Inorganic QA probably lost 3 marks,-1 due to failure of writing down blue ppt formed for test 2 but just wrote blue soln instead,-1 for not accurately explaining why the second test tube with naoh evolves o2,merely stated redox and mno4- reduced while OH- oxidised to O2 which im unsure whether thats correct so assumed wrong.Also -1 for incomplete observation of compound formed and reasoning for part ii,only said Co(NH3)6 2+ formed due to ligand exchange reaction,never accounted for Co(OH)2.

      On top of those mistakes would also want to ask the following questions:

      In part iii of role of H2O2,its a reducing agent if one deduces that from equation 2 of page 8 and quoting oxidation states of elements affected?Then explaining fe(h2o)6 3+ as a catalyst,is this the structure as follows:

      In step 1,Fe(H2O)6 3+ reacts to form an intermediate Fe(H2O)5 something (1m)

      The intermediate complex decomposes to form 2 products in step 2,which subsequently reacts with h2o2 to regenerate Fe(h2o)6 3+ in step 3(1m)

      technically,such an explanation is 2/3m only right?I assume the last mark is with respect to colour change of solution during the steps.

       

       

  • Ellipsisss's Avatar
    4 posts since Oct '17
    • Paper 4 practical was a disaster. I couldn't finish the entire planning question, only managed to write the first 5 steps, 20 marks gone. 

      First experiment was on catalysis of H2O2 decomp using Fe3+ as catalyst. As i was calculating the enthalpy change of decomposition: mcT/n, I wasn't sure whether to multiply by 2, because according to the equation, they wanted us to calculate the enthalpy change of this reaction 2H2O2 -> 2H2O + O2, where the stoic is 2. I decided not to multiply in the end, hence another 2 marks lost. 

      Next was a kinetics experiment for the decomp of H2O2, continuous method, extracting aliquots during timed intervals from the reaction. Drew a graph, deduced first order wrt H2O2.

      Planning was a similar kinetics experiment but to investigate how temperature affects catalysed decomp of H2O2. (COULDN'T reach past 5 steps)

      Next it was QA for redox of transition metals.

      In the first test, there were 2 tubes.

      1st tube: Deionised water + KMnO4

      2nd tube: NaOH, heat, + KMnO4

      The 2nd tube, small amts of O2 produced, purple turned blue then green. I wrote that the MnO4- was reduced to green MnO42-.

       

      In the second test, Co2+ + NH3(aq) + H2O2

      Effervescence of O2 gas, insoluble green/blue ppt, that turned to brown ppt on addition of H2O2. Co2+ in Co(OH)2 must have been oxidised to brown Co(OH)3. But i wrote that it was reduced to Co(OH) instead sobs. 

       

      Overall tons and tons of mistakes, i'm just a soppy sobbing mess. Am from HCJC and i must admit, our prelim Paper 4 was 10 times more manageable IMO than the A levels Paper 4. I just don't know if my Paper 1, 2, 3 will be able to make up for this. 

      Edited by Ellipsisss 26 Oct `17, 4:40PM
  • MapPwner's Avatar
    138 posts since Aug '15
    • Originally posted by Ellipsisss:

      Paper 4 practical was a disaster. I couldn't finish the entire planning question, only managed to write the first 5 steps, 20 marks gone. 

      First experiment was on catalysis of H2O2 decomp using Fe3+ as catalyst. As i was calculating the enthalpy change of decomposition: mcT/n, I wasn't sure whether to multiply by 2, because according to the equation, they wanted us to calculate the enthalpy change of this reaction 2H2O2 -> 2H2O + O2, where the stoic is 2. I decided not to multiply in the end, hence another 2 marks lost. 

      Next was a kinetics experiment for the decomp of H2O2, continuous method, extracting aliquots during timed intervals from the reaction. Drew a graph, deduced first order wrt H2O2.

      Planning was a similar kinetics experiment but to investigate how temperature affects catalysed decomp of H2O2. (COULDN'T reach past 5 steps)

      Next it was QA for redox of transition metals.

      In the first test, there were 2 tubes.

      1st tube: Deionised water + KMnO4

      2nd tube: NaOH, heat, + KMnO4

      The 2nd tube, small amts of O2 produced, purple turned blue then green. I wrote that the MnO4- was reduced to green MnO42-.

       

      In the second test, Co2+ + NH3(aq) + H2O2

      Effervescence of O2 gas, insoluble green/blue ppt, that turned to brown ppt on addition of H2O2. Co2+ in Co(OH)2 must have been oxidised to brown Co(OH)3. But i wrote that it was reduced to Co(OH) instead sobs. 

       

      Overall tons and tons of mistakes, i'm just a soppy sobbing mess. Am from HCJC and i must admit, our prelim Paper 4 was 10 times more manageable IMO than the A levels Paper 4, and I only attained 35/55. I just don't know if my Paper 1, 2, 3 will be able to make up for this. 

      If i'm not wrong,the brown one is Co(NH3)6 3+,using data booklet equations,and 35/55 is already very good,excluding planning i only got 30/41.

      But i think u are probably correct.

       

      Edited by MapPwner 26 Oct `17, 3:58PM
    • ... enthalpy of decomposition: -(heat change)/amount of h2o2 x2 Rest...plotted graph,range of temperature change:10.5-11.5 degrees(most people i...) Why is there a change in colour?as there is...that's correct) Rate of change of KMno4- = concentration of KMnO4...

  • Ellipsisss's Avatar
    4 posts since Oct '17
    • On a side note, here is the link to my answers to the questions above. Can i trouble you to point out the mistakes? Thank you!!

      https://imgur.com/a/qS4uU

       

      Edited by Ellipsisss 26 Oct `17, 4:57PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • MapPwner, I didn't do the Practical today. I'm skipping this year's A levels due to a scheduling conflict, and if you miss just 1 paper, you miss all papers for that subject, because you'll be awarded an "ABSENT" grade for that subject. So I'm skipping all the Papers this year.

      So I'll leave it to your good hands, MapPwner, to lead the forum discussions on P2, P3 and P1. And since I won't have seen any of the Papers, I can't comment on the difficulty of the questions. If you'll like me to address any particular question, you'll have to type it out from memory (for P2) or upload an image of the question (for P3 and P1).

  • MapPwner's Avatar
    138 posts since Aug '15
    • For inorganic QA,we were tasked to add 1cm depth of CoCl2,followed by addition of NH3(aq) dropwise,forming a blue ppt and blue solution.Then thereafter,we are supposed to add H2O2(aq),resulting in formation of dark brown solution and dark brown precipitate.

      I identified dark brown solution as co(nh3)6 2+ but is probably wrong as it is co(oh)3 i think.Failed to identify co(oh)2 as i only saw a blue solution in my test tube.

      We were also tasked to compare role of h2o2 between the reaction mentioned above and role of h2o2 in reaction between MnO4- and H2O2.

      For energetics the practical was on determining enthalpy change of decomposition of H2O2,given by the equation 2 H2O2->2H2O+O2  Hdecomp=to be determined

      Many people could calc heat change and amt of h2o2 used,but failed to take into account the stoichiometric coefficient in energetics in calculating Hdecomp of H2O2,which is 2 moles,so -1/2 marks at least.

       

       

      Edited by MapPwner 26 Oct `17, 5:08PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Ellipsisss:

      Paper 4 practical was a disaster. Am from HCJC and i must admit, our prelim Paper 4 was 10 times more manageable IMO than the A levels Paper 4. I just don't know if my Paper 1, 2, 3 will be able to make up for this. 


      Damn, this makes me all the more regretful that I'll have to be skipping the A levels this year. If this year's A levels P2, P3 and P1 are also concordantly so much tougher than the Singapore Prelim Papers (as appears to be the case for P4), I would really have liked to take them. If this really turns out to be the case (which would be rather shocking to Singapore JC school teachers), don't feel too badly or worry too much : after Cambridge finishes marking and sends the raw scores to SEAB, SEAB will apply the bell-curve to assign grades after moderation.

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


      Damn, this makes me all the more regretful that I'll have to be skipping the A levels this year. If this year's A levels P2, P3 and P1 are also concordantly so much tougher than the Singapore Prelim Papers (as appears to be the case for P4), I would really have liked to take them. If this really turns out to be the case (which would be rather shocking to Singapore JC school teachers), don't feel too badly or worry too much : after Cambridge finishes marking and sends the raw scores to SEAB, SEAB will apply the bell-curve to assign grades after moderation.

      I also felt that the chem prac was tough,but oh well i will probably end up with 31/55 at best,including planning,like my prelim p4 score of 31/55.

      From my discussion with my AJ/HC friends,some got the same score as me or way worse.

      Just quoting a Whatsapp message from my teacher in my chemistry class group:

      ''Hi all

      Mr Lee who did the practical today says it is "creative"... and creative for us in another words means the difficulty is high for students.

      No fear. Others will also find it the same

      Cheers.''

      Edited by MapPwner 26 Oct `17, 5:24PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Ellipsisss:

      On a side note, here is the link to my answers to the questions above. Can i trouble you to point out the mistakes? Thank you!!

      https://imgur.com/a/qS4uU

       


      Ahhh Ellipsisss of HCJC, I see you attempted my BedokFunland JC Challenge Qns... ok I'll have a look and give my comments, but you must understand, I won't reveal the full solutions here. I'll only give full solutions to my own BedokFunland JC students. I'll reply you on your attempted answers in a couple of hours. Check back later tonight.

    • ... larger than the change in molarity (ie....larger than the change, so you usually...effect of temperature change on the Gibbs...temperature. When temperature changes, you must consider...by the enthalpy change, while the effect...Gibbs free energy change is controlled by...

    • Alongside RJC and DHS, this year's VJC prelim papers also rank amongst the top of all Singapore JCs' prelim papers, in terms of quality of questions and standard of difficulty.

      Based on yesterday's P4, you can expect that this year's 2017 A level's P2, P3 and P1 to be comparable with the RJC, DHS and VJC papers. So be sure you (ie. all JC students intent of scoring a distinction for your H2 Chem) go through these 3 JCs' 2017 prelim papers very thoroughly.

      Disclaimer : I haven't, and probably won't, go through all of the other remaining Singapore JCs' 2017 prelim papers (I've gone through about 2/3 of all JCs, and find the quality of the RJC, DHS and VJC 2017 prelim papers in particular to be worthy of honorable mention). So it's entirely possible that some other JCs' 2017 prelim papers are every bit as good as RJC / DHS / VJC. And everyone is entitled to his/her own opinion. If you've any specific JC's 2017 prelim papers that you particularly enjoyed doing (helps if you're masochistic), do share your vote here.

      Overall, I'm honestly glad to see a substantial increase in both the quality and difficulty of all JCs prelim papers this year. Very good work done here setting the 2017 prelim papers, all Singapore JC teachers. And based on yesterday's P4, Cambridge certainly seems to have the same idea (ie. raising the quality, creativity and difficulty of the A level papers for the new syllabus). So be mentally (and emotionally!) well-prepared.

  • Ellipsisss's Avatar
    4 posts since Oct '17
    • ... larger than the change in molarity (ie. as... to neglect the change in molarity, and to...the effect of temperature change on the Gibbs ... temperature. When temperature changes, you must consider ... Gibbs free energy change is controlled by ...

      Edited by Ellipsisss 27 Oct `17, 9:37AM
    • Originally posted by MapPwner:

      I also felt that the chem prac was tough,but oh well i will probably end up with 31/55 at best,including planning,like my prelim p4 score of 31/55.

      From my discussion with my AJ/HC friends,some got the same score as me or way worse.

      Just quoting a Whatsapp message from my teacher in my chemistry class group:

      ''Hi all

      Mr Lee who did the practical today says it is "creative"... and creative for us in another words means the difficulty is high for students.

      No fear. Others will also find it the same

      Cheers.''

       

      Haha slightly comforting, just hope I don't get trampled on even in bell curve :/

       

      Oh MapPwner congrats on your reddit comment becoming famous xD

      Edited by Ellipsisss 27 Oct `17, 9:48AM
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