2017 H2 Chemistry JC1 & 2 students post your questions here

• Jh2424

  17 Dec 16, 18:02

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

• UltimaOnline

  17 Dec 16, 18:23

  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).
• Jh2424

  17 Dec 16, 20:02

  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?

• UltimaOnline

  17 Dec 16, 20:19

  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

  17 Dec 16, 21:48

  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)

• UltimaOnline

  17 Dec 16, 22:25

  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

  17 Dec 16, 22:42

  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!

• UltimaOnline

  18 Dec 16, 19:22

  ^_^
• UltimaOnline

  31 Dec 16, 14:06

  "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.

• UltimaOnline

  2 Jan 17, 01:32

  ^_^
• UltimaOnline

  2 Jan 17, 03:43

  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

• ArJoe

  4 Jan 17, 11:53

  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!
• UltimaOnline

  4 Jan 17, 12:05

  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

  6 Jan 17, 14:11

  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.

• UltimaOnline

  6 Jan 17, 15:13

  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.

• UltimaOnline

  9 Jan 17, 06:00

  ^_^
• UltimaOnline

  15 Jan 17, 00:25

  ^_^
• UltimaOnline

  20 Jan 17, 01:58

  ^_^
• UltimaOnline

  22 Jan 17, 01:52

  ^_^
• UltimaOnline

  24 Jan 17, 06:58

  ^_^
• UltimaOnline

  30 Jan 17, 12:22

  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.

• UltimaOnline

  30 Jan 17, 13:17

  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

  12 Feb 17, 20:36

  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
• UltimaOnline

  12 Feb 17, 22:40

  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

  13 Feb 17, 11:18

  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!