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  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
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      June 2017 Special Conditional Offer for JC2 Students who are interested in H2 Chemistry tuition.

      If you cannot afford tuition, feel free to (make a SgForums account) and post your H2 Chem questions here, I'll help you out for free here.

      image

      This thread is open to all 2017 Singapore JC 1 and JC 2 students studying H1 and H2 Chemistry, whether you're from RJC or MI, whether you already have your own private tutor or have no tuition at all. You just need to register an SgForums account (it's free!), and you get to enjoy the benefit of getting high quality, trustworthy and reliable help with your Chemistry questions free-of-charge.

      Terms & Conditions : Only genuine and reasonable questions from genuine and reasonable students asked in a genuinely and reasonably respectful manner will be replied to. And you should post your own attempted answers, and/or the given answers, and/or your queries on specifically what you don't understand about the question or don't agree with the given answers. I won't do your homework for you, instead I'll comment, guide and advise you on your question.

      2017 JC1 & JC2 students, do use SgForums to support your studies. And if you haven't yet made an SgForum account, you can go ahead and do so now. Post any H2 Chemistry questions you have here in this thread.

      From the number of views my 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 my H2 Chemistry posts.

      Well, to all you lurkers out there (especially if you're a genuine JC student), here's a suggestion : Why don't *you* go ahead and make a SgForums account so you can post your own H2 Chemistry queries here.

      Just register an account with SgForums, it's easy and free (of course, asking for free online help here... or for that matter anywhere... isn't as effective as actually joining my BedokFunland JC 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. "2016 RJC P3 Q2" or "TYS 2015 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.

      New members of SgForums won't be able to start new threads until you've reached a certain post count, so go ahead and post your H2 Chem qns in this thread till then.

      Edited by UltimaOnline 20 May `17, 7:04PM
    • Mothership.sg translates for the general public how a trio of data scientists from GovTech solved the mystery of the "Rogue One MRT Train" (actually the Mothership article missed out on several key steps, read the original report written by the heroes themselves here). Daniel Sim, Clarence Ng and Lee Shangqian are εΈ…ε“₯ Singapore heroes who are role models for a scientific career.

      http://mothership.sg/2016/12/unsung-heroes-who-found-the-rogue-circle-line-train-at-3am-tell-the-real-story-behind-how-the-problem-was-solved/

  • JJ_DHS's Avatar
    1 post since Dec '16
  • BryanWong_DHS's Avatar
    1 post since Dec '16
  • Jh2424's Avatar
    19 posts since Nov '16
    • Hello, I have a question regarding melting points of HI vs HCl

      I know that melting point of HI is higher, is this due to the greater size of HI/number of electrons, less impact nuclear charge has on charge distribution hence greater polarizibility of electrons , leading to greater magnitude of partial charges on HI compared to HCl? 

      But the electronegativity difference of HCl is greater than HI, so what effect does this have on the strength of the Van der Waals forces?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • This year's upcoming A level results will be 1 week earlier, ie. last week of Feb instead of 1st week of March, as was the case in the last few years.

      http://www.seab.gov.sg/pages/important_dates.asp

      Edited by UltimaOnline 04 Dec `16, 8:20PM
    • Originally posted by Jh2424:

      Hello, I have a question regarding melting points of HI vs HCl

      I know that melting point of HI is higher, is this due to the greater size of HI/number of electrons, less impact nuclear charge has on charge distribution hence greater polarizibility of electrons , leading to greater magnitude of partial charges on HI compared to HCl? 

      But the electronegativity difference of HCl is greater than HI, so what effect does this have on the strength of the Van der Waals forces?


      The magnitude of electronegativity difference, ie. polarity, has the opposite effect (ie. for the same molecular size and no. of electrons, the more polar the molecule, the stronger the intermolecular van der Waals interactions) but is strongly outweighed by the greater molecular size and no. of electrons, and hence polarizability of electron charge clouds and hence the magnitudes of partial charges and dipoles, hence strength of the van der Waals interactions, for HI over HCl. Consequently and concordantly, all factors considered, HI has a higher melting and boiling point compared to HCl.

      Edited by UltimaOnline 04 Dec `16, 5:48PM
  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      The magnitude of electronegativity difference, ie. polarity, has the opposite effect (ie. for the same molecular size and no. of electrons, the more polar the molecule, the stronger the intermolecular van der Waals interactions) but is strongly outweighed by the greater molecular size and no. of electrons, and hence polarizability of electron charge clouds and hence the magnitudes of partial charges and dipoles, hence strength of the van der Waals interactions, for HI over HCl. Consequently and concordantly, all factors considered, HI has a higher melting and boiling point compared to HCl.

      You mentioned 'the more polar the molecule, the stronger the intermolecular van der Waals interactions' Which van der Waals interactions are you talking about?( permanent dipole or instantaneous-induced dipole)

      And do you mean that you can only consider electronegativity difference if both molecules have same molecular size/electrons?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

      You mentioned 'the more polar the molecule, the stronger the intermolecular van der Waals interactions' Which van der Waals interactions are you talking about?( permanent dipole or instantaneous-induced dipole)

      And do you mean that you can only consider electronegativity difference if both molecules have same molecular size/electrons?


      Yes, of course.

      Anytime a polar molecule is involved, all 3 types of van der Waals interactions are relevant : Keesom, Debye, and London Dispersion forces.

      Edited by UltimaOnline 04 Dec `16, 6:04PM
  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      Yes, of course.

      Anytime a polar molecule is involved, all 3 types of van der Waals interactions are relevant : Keesom, Debye, and London Dispersion forces.

       Thank you so much :)

      Edited by Jh2424 04 Dec `16, 6:23PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

       Thank you so much :)


      No prob.

      You mentioned that you're a 2017 J1 student, implying you're now self-studying the A level syllabus before school begins.

      Are you from a top 5 JC, a middle-tier JC, or a neighbourhood JC?

      You intend to self-study all the way, not intending to join any tuition for H2 Chem, correct?

  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      No prob.

      You mentioned that you're a 2017 J1 student, implying you're now self-studying the A level syllabus before school begins.

      Are you from a top 5 JC, a middle-tier JC, or a neighbourhood JC?

      You intend to self-study all the way, not intending to join any tuition for H2 Chem, correct?

      I've actually just finished my O levels, currently self-studying as ive nothing much to do anyway, and judging from my papers, im most likely going to a mid tier- neighbourhood jc

      Regarding tuition, i might go for it actually, as based on my self-studying, i feel that im pretty slow :(

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

      i feel that im pretty slow :(


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      image

      Edited by UltimaOnline 04 Dec `16, 6:58PM
  • Jh2424's Avatar
    19 posts since Nov '16
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

       Wut ._.


      It'll be good and productive to have self-motivated tuition students like yourself. Lemme know anytime in the future if you're interested.

    • image

  • Jh2424's Avatar
    19 posts since Nov '16
    • Sorry, i have another query regarding the abovementioned issue

      Then in the case where electronegativity does play a role ( both molecules are of same size) then what actually happens?(as in does partial charges get stronger or something else comes into play)

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

      Sorry, i have another query regarding the abovementioned issue

      Then in the case where electronegativity does play a role ( both molecules are of same size) then what actually happens?(as in does partial charges get stronger or something else comes into play)


      Yes, of course. That's why (assuming similar molecular size) the more polar the molecule, the higher the melting & boiling points.

  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      Yes, of course. That's why (assuming similar molecular size) the more polar the molecule, the higher the melting & boiling points.

      Many thanks:)

    • Hello, I have a question, how exactly does a non-polar solvent dissolve a non-polar solute?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

      Hello, I have a question, how exactly does a non-polar solvent dissolve a non-polar solute?


      Whether the solute and solvent are polar or non-polar, the same consideration below applies. It's just that "like dissolves like" is a shortcut guideline that usually works.

      In terms of enthalpy change (delta H), heat released is comparable with heat absorbed (overall enthalpy change preferably exothermic, but can be slightly endothermic), in terms of physical interactions (van der Waals, and if applicable, hydrogen bonds or ionic bonds) formed and broken.

      In terms of entropy change (delta S), there will be a favourable positive entropy change upon mixing.

      Overall quantitatively, Gibbs free energy change must be negative for the dissolving to be thermodynamically favourable, according to the formula delta G = delta H - T x delta S.

      When you (ie. all students using this forum) ask me qns, and if you don't fully understand my replies, I won't spoonfeed you on the forum, so you'll have to research it further online (Google by thy sword, Wikipedia be thy shield), and/or ask your school teacher or private tutor, and/or join my BedokFunland JC tuition for more detailed, personalized guidance.

      Edited by UltimaOnline 10 Dec `16, 5:44PM
  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      Whether the solute and solvent are polar or non-polar, the same consideration below applies. It's just that "like dissolves like" is a shortcut guideline that usually works.

      In terms of enthalpy change (delta H), heat released is comparable with heat absorbed (overall enthalpy change preferably exothermic, but can be slightly endothermic), in terms of physical interactions (van der Waals, and if applicable, hydrogen bonds or ionic bonds) formed and broken.

      In terms of entropy change (delta S), there will be a favourable positive entropy change upon mixing.

      Overall quantitatively, Gibbs free energy must be negative for the dissolving to be thermodynamically favourable, according to the formula delta G = delta H - T x delta S.

      When you (ie. all students using this forum) ask me qns, and if you don't fully understand my replies, I won't spoonfeed you on the forum, so you'll have to research it further online (Google by thy sword, Wikipedia be thy shield), and/or ask your school teacher or private tutor, and/or join my BedokFunland JC tuition for more detailed, personalized guidance.

      So the 'energy' explaination for why something dissolves is more relevant than describing the physical process?( like how the polar water molecules pulls away the NaCl crystals)

      Edited by Jh2424 10 Dec `16, 4:44PM
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
    • Originally posted by Jh2424:

      So the 'energy' explaination for why something dissolves is more relevant than describing the physical process?( like how the polar water molecules pulls away the NaCl crystals)


      Of course, because the physical process is able to occur only because the overall thermodynamics is favourable enough to allow it.

  • Jh2424's Avatar
    19 posts since Nov '16
    • Originally posted by UltimaOnline:


      Of course, because the physical process is able to occur only because the overall thermodynamics is favourable enough to allow it.

      Oh I see.. Thanks :)

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    12,334 posts since May '05
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