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2017 H2 Chemistry JC1 & 2 students post your questions here

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  • Jh2424's Avatar
    19 posts since Nov '16
  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Jh2424:

      Hello, does water form 2 or 4 hydrogen bonds? Im not too sure about this, thanks


      Depends on state. As gaseous (assuming ideal gas behavior), 0. As liquid, fluctuates (many times per second) between 2 to 4. As solid, 4 (for the specific crystalline structure of ice relevant to A level syllabus).

      Edited by UltimaOnline 17 Dec `16, 6:26PM
  • Jh2424's Avatar
    19 posts since Nov '16
    • Thank you, I have another question, for ammonia, i dont understand why it forms just 2 hydrogen bonds, why can't the two hydrogens(left over after nitrogen and hydrogen) take part in hydrogen bonding?

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Jh2424:

      Thank you, I have another question, for ammonia, i dont understand why it forms just 2 hydrogen bonds, why can't the two hydrogens(left over after nitrogen and hydrogen) take part in hydrogen bonding?


      They can, but when you're talking about intermolecular H bonding between NH3 molecules, notice the ratio of H bond acceptors (ie. the N atoms) to H bond donors (ie. the H atoms) is 1:3, while the required ratio per H bond is 1:1.

      Analogously, in a class of 1 guy and 3 girls, how many (monogamic heterosexual) couples can you have at most? Concordantly, on average only 2 atoms (1 N and 1 H) can participate in H bonding between NH3 molecules.

  • Jh2424's Avatar
    19 posts since Nov '16
    • Sorry, i dont quite understand your explanation , going by your analogy, if 1 'guy' gets 1 'girl', whats going to stop the other 'girls' from getting other 'guys'? (unless you are talking about just 3 ammonia molecules)

  • Moderator
    H2 Chemistry @ BedokFunland JC (near VJC & TJC)
    UltimaOnline's Avatar
    13,909 posts since May '05
    • Originally posted by Jh2424:

      Sorry, i dont quite understand your explanation , going by your analogy, if 1 'guy' gets 1 'girl', whats going to stop the other 'girls' from getting other 'guys'? (unless you are talking about just 3 ammonia molecules)


      Then you'll run out of guys much sooner than girls, in a school of 10 classes, each class with 1 guy and 3 girls.

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


      Then you'll run out of guys much sooner than girls, in a school of 10 classes, each class with 1 guy and 3 girls.

      Hmm, I think I understand what you mean, thanks!

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

      Edited by UltimaOnline 13 Oct `17, 2:28AM
    • "READ THIS BEFORE YOU SHOP FOR CNY MANDARIN ORANGES, FULL OF SULPHUR & CHEMICALS"

      https://www.allsingaporestuff.com/article/read-you-shop-cny-mandarin-oranges-full-sulphur-chemicals

      Note : it's now spelled sulfur, not sulphur. The Brits (including Cambridge) have agreed to the American spelling. So JC students reading this, don't get penalized, yo.

    • _

      Edited by UltimaOnline 13 Oct `17, 2:30AM
    • In the 1st experiment, what's cool is that the slug fragmented into 2 equally lethal parts *in mid air* before reaching the target, *both* fragments subsequently penetrating the solid metal armor. In the 2nd experiment, what's cool is that upon hitting the solid metal armor, the slug fragmented into many dozens of molten metal shrapnel fragments, radiating spectacularly sideways (imagine if you were standing just beside where the slug hit), as illustrated by the explosions of the 2 filled soft drink bottles to the sides of the metal armor. Chemistry note : a "carbide" is a compound composed of carbon and a more electropositive element (eg. a metal), and isn't a metal per se, as erroneously stated by the YouTube video host. But let's not fault him, and appreciate his video making and sharing efforts instead. He can leave the Chemistry checks to us... makes you appreciate taking H2 Chemistry more and more everyday, don't it? ;Þ

      https://www.youtube.com/watch?v=voM13PNFYZI

      Edited by UltimaOnline 02 Jan `17, 3:45AM
  • ArJoe's Avatar
    877 posts since Nov '13
    • Hi ultimaonline, sorry for disturbing you again.

      in the recent 2016 h2 chem paper 2, there was one qn on explain how heterogeneous catalyst works. Would it be sufficient if one just writes partially filled 3d orbitals, reactant molecules adsorbed onto xatalyst surface, weakening the bonds, lower ea, increase frequency of effective collisions, without saying desorption? Qn was 3 marks for explanations

      cu is less reactive than k. Because cu have higher nuclear charge amd shielding effect than k, but 3d electrons in cu do not provide as effeftive shielding effect as compared to s or p electrons, since 3d orbitals are more diffused. So valence electrons in cu are more strongly attracted to the nucleus, hence cu less reactive. Is this correct?

      Thanks!

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

      Hi ultimaonline, sorry for disturbing you again.

      in the recent 2016 h2 chem paper 2, there was one qn on explain how heterogeneous catalyst works. Would it be sufficient if one just writes partially filled 3d orbitals, reactant molecules adsorbed onto xatalyst surface, weakening the bonds, lower ea, increase frequency of effective collisions, without saying desorption? Qn was 3 marks for explanations

      cu is less reactive than k. Because cu have higher nuclear charge amd shielding effect than k, but 3d electrons in cu do not provide as effeftive shielding effect as compared to s or p electrons, since 3d orbitals are more diffused. So valence electrons in cu are more strongly attracted to the nucleus, hence cu less reactive. Is this correct?

      Thanks!


      Without desorption, you'll be penalized 1 mark, regardless of how many other points you write, because desorption is an essential point.

      Yes, correct for K vs Cu.

  • cienhan's Avatar
    2 posts since Dec '16
    • hello ultimaonline,

       

      when heated with carbon, azurite produces copper, carbon dioxide and steam as the only products. write a balanced eqn for the reaction of azurite with hot carbon. azurite is Cu3(OH)2(CO3)2

       

      thank you in advance.

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

      hello ultimaonline,

       

      when heated with carbon, azurite produces copper, carbon dioxide and steam as the only products. write a balanced eqn for the reaction of azurite with hot carbon. azurite is Cu3(OH)2(CO3)2

       

      thank you in advance.


      Decomposition :

      OH-, OH- [proton transfer] O 2-, H2O.

      CO3 2-, CO3 2- [elimination] O 2-, CO2, O 2-, CO2.

      Cu2+, Cu2+, Cu2+, O 2-, O 2-, O 2- as ionic solid + CO2, CO2, H2O as molecular gases.


      Redox :

      Cu2+, Cu2+, Cu2+, O 2-, O 2-, O 2- + 1.5 C [electron transfer + covalent bond formation] Cu, Cu, Cu as metallic solid + 1.5 CO2 as molecular gas.


      Hence overall balanced equation is :

      1 Cu3(OH)2(CO3)2 (s) + 1.5 C (s) ----> 3 Cu (s) + 3.5 CO2 (g) + 1 H2O (g)


      This is the BedokFunland JC way, seeing clearly the mechanisms involved (proton transfer, elimination, electron transfer, covalent bond formation) in the type of reactions occurring (decomposition, redox) and recognizing the underlying thermodynamic motivation (positive entropy change) for it.

      Edited by UltimaOnline 06 Jan `17, 5:04PM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:29AM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:29AM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:29AM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:28AM
    • _

      Edited by UltimaOnline 13 Oct `17, 2:28AM
    • To all JC students, here's a BedokFunland JC teaser for your entertainment :

      The N atom in both amines and amides have the same no. of lone pairs and bond pairs. Yet the hybridization of the N atom in both types of molecules are different. See if you can figure out why before googling out the answer.

    • If you're burning anything or have your stove on, always make sure the windows are open. I could explain to you the underlying chemistry involved (ie. as carbon is a relatively non-electronegative element, a uninegative formal charge on the sp hybridized C atom in the major resonance contributor of carbon monoxide results in an unusually strong Lewis base, which thus behaves as a competitive inhibitor ligand to nucleophilically attack the electrophilic Lewis acidic Fe atom in the haemoglobin molecule in red blood cells generating the kinetically and thermodynamically stable carboxyhaemoglobin with a much larger Kstab value than oxyhaemoglobin... etc) but just be sure you remind your family and friends to be mindful.

      https://sg.news.yahoo.com/six-teens-found-dead-garden-party-germany-192344411.html

  • ArJoe's Avatar
    877 posts since Nov '13
    • Why does compounds of transition elements form colours?

      If i wrote transition metal have partially filled 3d orbitals, presence of ligand causes the splitting of the energy level of the 5 3d orbitals…. blahhhh.. is it ok? They will accept regardless whether i write 3d or d orbital? It was a 3 mark qn in alevel

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

      Why does compounds of transition elements form colours?

      If i wrote transition metal have partially filled 3d orbitals, presence of ligand causes the splitting of the energy level of the 5 3d orbitals…. blahhhh.. is it ok? They will accept regardless whether i write 3d or d orbital? It was a 3 mark qn in alevel


      If it's 3rd, must specify 3d, not just d. Other than that, your answer (which you didn't write out in full here) should be ok.

  • ArJoe's Avatar
    877 posts since Nov '13
    • Cause the qn just said transition metal, and didnt specify which transition element, so i was wondering is it alright if i wrote 3d? Haha thanks!

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