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• Flying grenade

  19 Apr 16, 18:48

  Originally posted by UltimaOnline:

  
  But if the ozone layer is thin, it will have holes. You can't have your cake and eat it too. You can't lepak for A levels and still score distinction A grade (unless you're me, Mr BedokFunland JC).

  Thanks for witty humor and writings for your many posts ^^

• Flying grenade

  19 Apr 16, 19:35

  Why we haven't ever use the sections :  "Characteristic values for infra-red absorption (due to stretching vibrations in organic molecules)." And "typical proton chemical shift value relative to TMS." In the data booklet before?

• UltimaOnline

  19 Apr 16, 19:40

  Originally posted by Flying grenade:

  Why we haven't ever use the sections :  "Characteristic values for infra-red absorption (due to stretching vibrations in organic molecules)." And "Characteristic values for infra-red absorption (due to stretching vibrations in organic molecules)." In the data booklet before?

  
  That's only tested in the H3 syllabus under spectroscopy, specifically : UV, IR, NMR.
• Flying grenade

  20 Apr 16, 08:39

  Hi ultima, hope u could help out over here, 

  Is Na2CO3 a strong or weak base?

  Strength of base depends on stability of conjugate acid 

  Is the cj acid Na+?  Na+ is very stable,  hence Na2CO3( or any group one salts, are strong bases, is it?

  Actually i have difficulty identifying the cj acid, is the cj acid the species that gains H+?

  Based on B(aq) +H2O(l) 》《 BH+ (aq) + OH-(aq) , im not sure how to write the above eqn for Na2CO3, is it Na gains the H+? And idk where write the OH- and CO32- ..

• Flying grenade

  20 Apr 16, 08:46

  Is it Na+ extracts H+ from water, hence OH- formed. So i know products have NaH, OH-, but what about CO32-?

   

  Na+ , not Na.  albeit, still wrong. No lone pair on Na+, cannot grab H+

• UltimaOnline

  20 Apr 16, 16:33

  Originally posted by Flying grenade:

  Hi ultima, hope u could help out over here, 

  Is Na2CO3 a strong or weak base?

  Strength of base depends on stability of conjugate acid 

  Is the cj acid Na+?  Na+ is very stable,  hence Na2CO3( or any group one salts, are strong bases, is it?

  Actually i have difficulty identifying the cj acid, is the cj acid the species that gains H+?

  Based on B(aq) +H2O(l) 》《 BH+ (aq) + OH-(aq) , im not sure how to write the above eqn for Na2CO3, is it Na gains the H+? And idk where write the OH- and CO32- ..

  
  CO3 2- is a weak diprotic base, HCO3- is amphiprotic, and H2CO3 is a weak diprotic acid. Hydrolysis equation is CO3 2- + H2O (double-half arrow) HCO3- + OH-

  You need to *practice* doing *lots* of *hands-on* calculation questions to master this topic, you can't just read / type out on the concepts involved.

• Wingzhan

  21 Apr 16, 15:52

  Very good effort. Clap Clap

• Flying grenade

  21 Apr 16, 20:09

   

  Atoms(e.g. Cl) of Halogen molecules(e.g. Cl2) already have complete octet, why are they still reactive? 

   

  Update : (Noticed bonds between halogens are quite weak, as seen from bond strength from data booklet),  but still, principal question above^

• UltimaOnline

  21 Apr 16, 20:19

  Originally posted by Flying grenade:

   

  Atoms(e.g. Cl) of Halogen molecules(e.g. Cl2) already have complete octet, why are they still reactive? 

   

  Update : (Noticed bonds between halogens are quite weak, as seen from bond strength from data booklet),  but still, principal question above^

  
  Because the Cl atom in covalent molecules (ie. bond-Cl), and the Cl- ion in ionic compounds, also have a stable octet, plus an even more stable OS of -1 (instead of 0 in Cl2).
• Flying grenade

  21 Apr 16, 22:03

  Why O.S. of -1 more stable? 

  Doesn't mean the more -ve o.s. is, more stable the atom is correct? 

• UltimaOnline

  22 Apr 16, 00:58

  Originally posted by Flying grenade:

  Why O.S. of -1 more stable? 

  Doesn't mean the more -ve o.s. is, more stable the atom is correct? 

  
  Edited to add : Ok fair enough, you were asking a question. But nonetheless, when answering questions in the A level exams, my point remains, don't lose marks by being ambiguous in your phrasing. Always be as specific as possible.

  Ya know, it's annoying when you (ie. Singapore JC students) say things like that, and you'll lose marks in the A level exams for such ambiguous phrasing. They way you phrase it, makes you appear simple minded (ie. oversimplifying matters due to lack of depth of understanding) or dogmatic (ie. blindly memorizing school notes without deeper understanding), because it makes you seem to think "the more -ve o.s. is, more stable the atom is" holds true for all elements.

  Obviously (at least it's obvious to me and should be obvious to anyone who truly understands Chemistry, but apparently this doesn't necessarily include the majority of Singapore JC students or even some teachers and tutors, does it?), metals being electropositive will find positive OSes more stable, and non-metals being electronegative will find negative OSes more stable.

  As for the exact unique OSes preferred by each individual element, you can go figure out for yourself, and/or research online, and/or ask your school teacher and/or private tutor. At A levels, if you really understand your A level Chemistry, then you'll begin to get a glimpse of understanding as to the myriad of chemistry factors (some opposing each other) contributing to why different elements (eg. different transition metals) have different unique preferred OSes.

  PS. Don't ask any further questions here on the forum about this subtopic, go research yourself or ask your school teacher or private tutor.

• Flying grenade

  22 Apr 16, 09:26

  My school's tcher said in lecture , for elimination of haloalkane to form alkene,

  The condition 'heat' is to supply Ea to break C-X bond, and 'reflux' is to increase yield of alkene formed

  Dyou agree with him?

• Flying grenade

  22 Apr 16, 10:37

  When do we write equation/reaction

  thermodynamically/entropically favourable

   thermodynamic/entropically feasible

   spontaneous ?

• UltimaOnline

  22 Apr 16, 11:24

  Originally posted by Flying grenade:

  My school's tcher said in lecture , for elimination of haloalkane to form alkene,

  The condition 'heat' is to supply Ea to break C-X bond, and 'reflux' is to increase yield of alkene formed

  Dyou agree with him?

  
  Yes, that's fine.
• UltimaOnline

  22 Apr 16, 11:25

  Originally posted by Flying grenade:

  When do we write equation/reaction thermodynamically/entropically favourable and thermodynamic/entropically feasible and spontaneous ?

  
  When delta H is -ve,=, or delta S is +ve, then the -ve delta H, or +ve delta S, is called thermodynamically favorable (but reaction may or may not be thermodynamically feasible). When delta G is -ve, then the reaction is thermodynamically feasible.
• Flying grenade

  22 Apr 16, 13:00

  Originally posted by UltimaOnline:

  
  Yes, that's fine.

  Why reflux increase yield?

• Flying grenade

  22 Apr 16, 13:19

  Hi Ultima, may i ask u, RVHS/2015/P1/Q35

  Qn here : https://www.dropbox.com/s/as9s9cyx2ng8p78/20160422_131408.jpg?dl=0

  Option 2. Based on numerous qns we did , we know by familiarity that ka of ch3cooh is 1.8×10^-5 , but Are we supposed to memorise it?? Btw, ka only changes if temp changes right?

  Option 3. Strong acids are defined as dissociates completely in water right?

   

• UltimaOnline

  22 Apr 16, 15:11

  Originally posted by Flying grenade:

  Why reflux increase yield?

  
  I wrote an entire essay about why reflux increases yield, go find it yourself. Or simply google out "why reflux?" or "purpose or reflux". Don't annoy me with questions like these, where you can easily settle yourself.
• UltimaOnline

  22 Apr 16, 15:30

  Originally posted by Flying grenade:

  Hi Ultima, may i ask u, RVHS/2015/P1/Q35

  Qn here : https://www.dropbox.com/s/as9s9cyx2ng8p78/20160422_131408.jpg?dl=0

  Option 2. Based on numerous qns we did , we know by familiarity that ka of ch3cooh is 1.8×10^-5 , but Are we supposed to memorise it?? Btw, ka only changes if temp changes right?

  Option 3. Strong acids are defined as dissociates completely in water right?

   

  
  Option 2 : you're not supposed to memorize Ka values. You're supposed to calculate it out lah, from the pH and molarity given in the table. You dunno how to calculate? Then you jialat liao.

  Option 3 : the operative word here is "stronger", not "strong", ie. stronger dissociates more compared to weaker, but stronger is still not strong, geddit?.

• Flying grenade

  23 Apr 16, 18:09

  So for maximum buffer capacity (half neutralisation point) always can skip icf and ice table?

• UltimaOnline

  23 Apr 16, 18:13

  Originally posted by Flying grenade:

  So for maximum buffer capacity (half neutralisation point) always can skip icf and ice table?

  
  First, if you're not sure it's maximum buffer capacity (eg. because the question guai-lan and give different volumes and different molarities of both acid and base), do the ICF table to confirm it's maximum buffer capacity. With Cambridge questions getting tougher each year, it may be near, but not exactly maximum buffer capacity. So when in doubt, just do ICF table (which is arguably more important than ICE table, especially since Cambridge has stated they will accept the approximation of equilibrium molarity back to initial molarity).

  If it's really exactly maximum buffer capacity, obviously neither ICF nor ICE table is required, and not even the Henderson-Hasselbalch equation is required (since log 1 = 0). For maximum buffer capacity, pH = pKa1 or pKa2 or pKa3 (depending on which maximum buffer capacity it is during the titration of a polyprotic or multiprotic acid or base).

• Flying grenade

  25 Apr 16, 18:22

  Hi Ultima , can i ask,

  https://www.dropbox.com/s/tv6ihet7m88modj/20160425_181526.jpg?dl=0

  How option 2 and 3 affects this?

  Is this testing whether the ion would form a salt with a base added?

   

  (advanced) Cs toh book (pg 121) wrote "standard cell potential.. 1.00moldm^-3 concentration "

  Is it, 1.00moldm^-3 of anything( e.g. salt, acid) in a half cell ?

   

• UltimaOnline

  25 Apr 16, 18:33

  Originally posted by Flying grenade:

  Hi Ultima , can i ask,

  https://www.dropbox.com/s/tv6ihet7m88modj/20160425_181526.jpg?dl=0

  How option 2 and 3 affects this?

  Is this testing whether the ion would form a salt with a base added?

   

  (advanced) Cs toh book (pg 121) wrote "standard cell potential.. 1.00moldm^-3 concentration "

  Is it, 1.00moldm^-3 of anything( e.g. salt, acid) in a half cell ?

   

  
  All options 1, 2 and 3 are correct. It's testing you on applying Le Chatelier's principle.

  Yes, standard conditions refer to standard molarities of 1 M for all aqueous species, 1 atm (alternatively 1 bar) for all gases, and at 298 K.

• Flying grenade

  25 Apr 16, 19:09

  T-T my answer wrong means my working epic fail means my understanding epic fail T_T

  Pls help

  https://www.dropbox.com/s/3codbenfirr3luk/20160425_190417.jpg?dl=0

• UltimaOnline

  25 Apr 16, 19:50

  Originally posted by Flying grenade:

  T-T my answer wrong means my working epic fail means my understanding epic fail T_T

  Pls help

  https://www.dropbox.com/s/3codbenfirr3luk/20160425_190417.jpg?dl=0

  
  Q8) A student dissolved 10.5 g of sodium fluoride in 250 g of water. The following data is obtained:
  
  Lattice energy of NaF −918 kJ mol-1
  Enthalpy change of hydration of F- −457 kJ mol-1
  Enthalpy change of hydration of Na+ −390 kJ mol-1
  Given that the specific heat capacity of water is 4.2 J g-1 K-1, what would be the final temperature of the solution, if the initial temperature of water is 30 oC?
  
  BedokFunland JC guidance :
  
  Calculate mol of NaF.
  Hence calculate total enthalpy change for the following process :
  NaF(s) --> Na+(g) + F-(g) --> Na+(aq) + F-(aq)
  Plug Q, m, c into formula Q = m x c x delta T
  Solve for delta T.
  Hence obtain the final temperature, and answer for this MCQ.