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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by glitter58:

      when do i need to write state symbols for chemical equations? for example in the topic of periodicity, do i have to write state symbols even when the qn doesnt require me to? 


      1. See if the question specifically asks for it.

      2. See if it's directly relevant to the topic (eg. energetics directly relevant, periodicity not directly relevant, except some parts eg. hydration then hydrolysis of AlCl3).

      3. When in doubt, go ahead and write it.

      4. For ionic equations, always write state symbols.

  • glitter58's Avatar
    18 posts since Mar '17
    • do both the statements mean the same thing?

      -compound A has intermolecular permanent dipole–permanent dipole attractions 

      and 

      -pd-pd attractions exist between A molecules

       

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

       

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

       

      Edited by glitter58 16 May `17, 2:06PM
    • also do i call pd-pd and id-id as interactions or attractions or forces of attraction?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by glitter58:

      do both the statements mean the same thing?

      -compound A has intermolecular permanent dipole–permanent dipole attractions 

      -pd-pd attractions exist between A molecules

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

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

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

       


      Just to check, are you a school candidate or private candidate, and are you taking A levels this year? You've been asking a mixture of J1 and J2 topics.

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

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

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

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

  • glitter58's Avatar
    18 posts since Mar '17
    • i am a school candidate taking alevels this year. my foundations and j1 topics are quite weak so i have started revising both j1 and j2 topics

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by glitter58:

      i am a school candidate taking alevels this year. my foundations and j1 topics are quite weak so i have started revising both j1 and j2 topics


      Alright, 6 months left to A level exams. Go ahead and continue to post your questions here. If it's a TYS or Prelim qn, just specify the paper, eg. 2016 VJC P3 Q4.

      May 2017 Special Conditional Offer for JC2 Students looking for H2 Chemistry tuition

      To everyone reading this : if you're a JC student who cannot afford tuition, you're welcome to make an SgForums account and post your H2 Chem qns here.

  • Acebrainery.sg's Avatar
    1 post since May '17
    • Hey, Ultima and all the chemistry lover students, this is great way to help any one. Keep going buddy....!!!!

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by Acebrainery.sg:

      Hey, Ultima and all the chemistry lover students, this is great way to help any one. Keep going buddy....!!!!


      Congrats on your new premises @ The Riverwalk, good central location.

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

      A BedokFunland JC Original H2 Chemistry Question

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

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

      Edited by UltimaOnline 20 May `17, 7:18PM
  • H4x0ru5's Avatar
    8 posts since Mar '12
    • 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? 

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • 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 :

      image

      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).

      Edited by UltimaOnline 23 May `17, 6:55AM
  • H4x0ru5's Avatar
    8 posts since Mar '12
    • Originally posted by UltimaOnline:

      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?

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

      image

      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).

      Ok if that's the case, what about changing the electrolyte to aq NaCl? So now I have two different metals dipped into the same beaker containing aq NaCl. Do I need a porous pot/any concerns of charge built up? In fact, by taking away the salt bridge/porous pot, and changing the electrolyte from aqueous salt of the metal to aq NaCl, is it still a Daniel Cell?

      Edited by H4x0ru5 23 May `17, 6:40AM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by H4x0ru5:

      Ok if that's the case, what about changing the electrolyte to aq NaCl? So now I have two different metals dipped into the same beaker containing aq NaCl. Do I need a porous pot/any concerns of charge built up? In fact, by taking away the salt bridge/porous pot, and changing the electrolyte from aqueous salt of the metal to aq NaCl, is it still a Daniel Cell?


      If NaCl(aq) is used, no redox reaction occurs, not even between the electrolyte and Zn electrode, as would have occurred if an acid was used. Either way, no electrons would flow in the external wire, so the setup would still be a failure.

      You would then not need a porous pot, nor would you have any concerns of a charge build up, because no redox reaction would occur, as the setup would fail.

      It would no longer be a Daniell cell, or any type of Galvanic-Voltaic cell. It would be a failure.

  • H4x0ru5's Avatar
    8 posts since Mar '12
    • Originally posted by UltimaOnline:


      If NaCl(aq) is used, no redox reaction occurs, not even between the electrolyte and Zn electrode, as would have occurred if an acid was used. Either way, no electrons would flow in the external wire, so the setup would still be a failure.

      You would then not need a porous pot, nor would you have any concerns of a charge build up, because no redox reaction would occur, as the setup would fail.

      It would no longer be a Daniell cell, or any type of Galvanic-Voltaic cell. It would be a failure.

      Why would the set up fail then? This set up is always seen in typical O/A level guidebooks. 

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by H4x0ru5:

      Why would the set up fail then? This set up is always seen in typical O/A level guidebooks. 


      Understandable, you tutor both O and A levels.

      At O levels, the diagram of the setup is often simplified, because O level students are not expected to understand why or how a salt bridge is necessary.

      At A levels, the student must draw a proper setup separating the cathode and anode (to force electrons to flow in the external wire), and a salt bridge is necessary to allow the electrons to flow.

      That's why your previously suggested setup (which doesn't separate cathode and anode) fails.

  • glitter58's Avatar
    18 posts since Mar '17
    • for reaction between conc. sulfuric acid and hcl, will the salt formed be na2so4 or nahso4? which reaction is more widely used?

    • also, if the qn part iii states "using the answers in part i and ii" do i use the 3sf answer or the 5sf intermediate working for calculation in qn part iii?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • Originally posted by glitter58:

      for reaction between conc. sulfuric acid and hcl, will the salt formed be na2so4 or nahso4? which reaction is more widely used?


      Depends on which reactant is in excess. For Group 17 purposes, use NaHSO4. In other contexts, see which equation is more relevant to the question and/or final products.

    • Originally posted by glitter58:

      also, if the qn part iii states "using the answers in part i and ii" do i use the 3sf answer or the 5sf intermediate working for calculation in qn part iii?


      For A level Chemistry purposes, Cambridge will accept either. No worries.

    • Originally posted by H4x0ru5:

      Why would the set up fail then? This set up is always seen in typical O/A level guidebooks. 


      Also, it's likely that some setups you're referring to, do not need salt bridges or separate compartments, because they're not Galvanic-Voltaic cells, but are electrolytic cells.

      O levels test mainly on the latter, while A levels test equally on the former and the latter both.

  • glitter58's Avatar
    18 posts since Mar '17
    • why doesnt fluorine follow the trend of decreasing bond energy from cl2 to br2 to i2?

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

      Edited by glitter58 24 May `17, 7:23PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,547 posts since May '05
    • 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".

    • image

      http://mothership.sg/2017/04/we-found-that-colourful-talad-drink-in-toa-payoh-so-you-dont-have-to-squeeze-at-artbox/

      image

      Tuk Tuk Cha now selling colour-changing gradient drinks too

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

      Edited by UltimaOnline 26 May `17, 2:51AM
    • 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.

      Edited by UltimaOnline 26 May `17, 2:54AM
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