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• UltimaOnline

  27 Oct 15, 08:40

  Originally posted by Flying grenade:

  Thank you!!!!

   

  Why is the delT argument wrong /not valid?

  
  Wot is a "deIT"?
• Flying grenade

  27 Oct 15, 09:34

  Delta T 🔺T

   

  Based on 🔺T = q /mc ,

  As mass doubled , delta T halved

   

  Why is this argument not valid /wrong?

• Flying grenade

  27 Oct 15, 09:41

  CCl4 readily undergoes hydrolysis, while SiCl4 is hydrolysis resistant; the availability of 3d orbitals in Si (but not C) creates the potential for addition-elimination hydrolysis mechanisms with lower activation energy).

   

  So does sicl4 undergo hydrolysis?

  Thought it does,  to form sio2 , and a solution of pH approx 2

• UltimaOnline

  27 Oct 15, 12:55

  Originally posted by Flying grenade:

  CCl4 readily undergoes hydrolysis, while SiCl4 is hydrolysis resistant; the availability of 3d orbitals in Si (but not C) creates the potential for addition-elimination hydrolysis mechanisms with lower activation energy).

   

  So does sicl4 undergo hydrolysis?

  Thought it does,  to form sio2 , and a solution of pH approx 2

  
  Stupid typo, yeah I meant CCl4 is hydrolysis resistant while SiCl4 readily hydrolyzes. The reason I gave for this remains correct. Typos like this, added up, can cost you an A grade.
• UltimaOnline

  27 Oct 15, 12:58

  Originally posted by Flying grenade:

  Delta T 🔺T

   

  Based on 🔺T = q /mc ,

  As mass doubled , delta T halved

   

  Why is this argument not valid /wrong?

  
  In this case, 2 things doubles simultaneously : heat given off doubled, and mass of water to absorb the heat also doubled, hence delta T remains unchanged. Originally there are 5 guys who have to split $5 between them. In the new scenario, now there are 10 guys, who have to split $10 between them. In both cases, each guy still gets $1, no change.
• Flying grenade

  27 Oct 15, 14:40

  Thank u god

   

  2010 p3 qn 5d

  Ok we know yellow [cucl4]^2- ions formed 

   

  How do we know why no reaction occur with h2so4?

  Are we supposed to know that no complexes are formed with so42- ion???

  And why??

   

• Flying grenade

  27 Oct 15, 14:45

  2014 p2 qn 6ii)

   

  There are 2 possible sets of optical isomers,  1 chiral carbon at position 1, another chiral c at position 2.

  At there can be cis trans geometrical isomer between the 2 substituted C, carbon 1 and carbon 2

   

  So do we draw all?? 2 sets of optical isomer and 1 cistrans geometric isomers? ?

   

   

  Update : Eh??seems like there's many possible pairs of optical isomers

  https://www.dropbox.com/s/6a3zojrz50o8ckw/20151027_151538.jpg?dl=0

  Sir can help me check my drawing?

  Are the 3rd and 4th pair i drew the same?

   

  Thank you ultima

• Flying grenade

  27 Oct 15, 14:53

  We know that solubility of group 2 hydroxides increases down the group 

   

  How about hydroxides in general?

  Are all soluble /sparingly soluble? 

  From 2009 paper 2 qn 2g)

  How do we know that zn2+ + OH- -> zinc hydroxide? 

  I only know this happens to Al , and becos this is learnt in periodicity ,.

   

   

  Update : a very huge sorry for my stupid question. 

  All hydroxides are insoluble except group 1 and ammonium hydroxides

   

   

   

   

  How about solubility of oxides?

  https://en.m.wikipedia.org/wiki/Solubility_chart

  ^ indicates most oxides *reacts* with water. So does it means its soluble?

  Or is it for example, zinc oxide reacts with water to produce zinc hydroxide(insoluble). So no OH- in solution 

  One exception. ^above wiki link state that barium oxide is soluble. So it means BaO dissociates in water, forming Ba2+ and O2-??

   

  Help!

   

  Thanks a million by saving countless students

   

• UltimaOnline

  27 Oct 15, 19:31

  Yo Flying Grenade, you're welcome. Keep your qns coming in this format (where you post the question reference, the answer given, and your thoughts / queries / doubts about it). It's good.
  
  2010 p3 qn 5d - It's not that the sulfate ion can't act as ligand at all, it's just that it doesn't act as a ligand here in this question. How you know? Coz the question said, "no such colour change occurred" which means no ligand substitution occurred. Different ligands have different equilibrium affinities to different metal cations, for reasons too complex to be dealt with at A levels.
  
  2014 p2 qn 6ii - No need to draw all. Just draw a pair of geometric isomers correctly, can get the marks liao. Cambridge testing to see who so coconaden as to draw all the possible geometric isomers and thus sabo himself to have no time to finish the paper.
  
  Your 3rd and 4th pairs are wrong. You 'violated' your C atom with 5 bond pairs. Period 2 element cannot accommodate expanded octet dah dey!
  
  2009 paper 2 qn 2g - "So it means BaO dissociates in water, forming Ba2+ and O2-??" No lah, it means BaO(s) hydrolyses in water to generate Ba(OH)2(aq), so from (s) to (aq), we can still use the term "dissolve" or "soluble". Hydroxides are the 'dissolved' or 'solubilized' form of oxides in the 1st place. So when (and if) an oxide dissolves, it first forms a hydroxide, which may or may not be further soluble (ie. ppt vs dissociating into separate hydrated ions).
  
  On solubility of Group II hydroxides vs carbonates / sulfates, which *can* come out for Singapore H2 Chem A levels (even though many JCs don't teach it) as an extension type of question, go read Jim Clark's webpage :
  
  http://www.chemguide.co.uk/inorganic/group2/problems.html
• Flying grenade

  27 Oct 15, 21:17

  Thank you sir. Would have a chance to learn from you personally. Might have a chance to seek knowledge and wisdom from you personally, regarding h2 chem,

  and/or, further/higher chem for uni( if possible  (:   

   

   

  Funny then why wikipedia described Baoh2 as soluble, while the rest as 'reacts' , If both means to say that the oxide turn to hydroxide in water

   

  https://www.dropbox.com/s/3omf49ye1bm4r2v/20151027_210223-1.jpg?dl=0

  Can help ascertain part ii) ?

  Source:

  https://www.dropbox.com/s/u6cqbnrr4gdp7zg/Screenshot_2015-10-27-21-04-11-1.jpg?dl=0

• Flying grenade

  27 Oct 15, 21:40

  That 2009 qn,

  So the ammonia ligand replaced oh ligand?

  So zinc hydroxide is soluble?

  Probably sparingly?

• UltimaOnline

  28 Oct 15, 00:41

  No chance liao. You're already taking your A levels next week. And I have chosen to specialize only in A level H2 Chem. But good to hear you may be interested in studying Chem at Uni levels after your NS.
  
  "Funny then why wikipedia described Baoh2 as soluble, while the rest as 'reacts' , If both means to say that the oxide turn to hydroxide in water."
  
  In this instance, why bother with Wikipedia when CS Toh has already succinctly summarized all you need to know about Group II metal reactions, metal oxide reactions, oxide solubilities and hydroxide solubilities? The only additional material related to this topic I recommend Singapore H2 Chem students to explore, which is not in CS Toh, is the following Jim Clark webpage, as mentioned in my previous post :
  
  http://www.chemguide.co.uk/inorganic/group2/problems.html
  
  So you don't understand Sean Chua's answer, and when you drew your erroneous 3rd & 4th pairs, you thought you were drawing the same as what Sean Chua drew? No, the error is yours, not Sean Chua's. His diagram is fine, his C atom has 4 bonds (including 1 wedge bond, that is part of the ring) and thus isn't 'violated', while yours had 5 bonds and is thus 'violated'.
  
  From 2009 paper 2 qn 2g - Eh jialat lah, you really don't understand coordination complex equilibria, izzit? You made 2 errors right here : 1st of all, in this question OH- isn't a ligand at all. It's a counter ion which precipitates out Zn2+ as Zn(OH)2(s). *If* you had added excess NaOH(aq), giving a sufficiently high molarity of OH-, then the excess OH- can start to function as ligands, and that's how you get the tetrahydroxozincate(II) coordination complex.
  
  But in this qn, you did *not* add excess NaOH(aq), rather you added excess NH3(aq), giving a sufficiently high molarity of NH3, then the excess NH3 can start to function as ligands, and that's how you get the tetraamminezinc(II) coordination complex.
  
  All 'insoluble' salts are actually 'sparingly soluble'. But most importantly, you need to be able to explain to Cambridge *why and how* the 'sparingly soluble' metal hydroxide solid precipitate, eg. Cu(OH)2(s) dissolves when adding in excess a solution containing a suitable ligand, eg. NH3(aq), in terms of shifting of positions of equilibria across 2 (or more) reaction equations.
  
  Eqn 1 : NH3(aq) + H2O(l) ---> NH4+(aq) + OH-(aq)
  
  Eqn 2 : Cu2+(aq) + 2OH-(aq) ---> Cu(OH)2(s)
  
  Eqn 3 : Cu2+(aq) + 4NH3(aq) ---> [Cu(NH3)4]2+(aq)
  
  When ammonia solution is added to copper(II) sulfate solution, the hydrolysis of ammonia (Eqn 1) generates OH-(aq), and when Qsp (aka Ionic Product) > Ksp (aka Solubility Product), position of equilibrium in Eqn 2 shifts to the RHS, generating Cu(OH)2(s) precipitate. When excess ammonia solution is added, the position of equilibrium in Eqn 3 shifts to the RHS, resulting in a decrease in molarity of Cu2+(aq), which causes the position of equilibrium in Eqn 2 to shift to the LHS, which causes the copper(II) hydroxide precipitate to dissolve.
• Flying grenade

  28 Oct 15, 01:14

  Omg i have learnt so much from you, my gratitude for you is overwhelming !!!!

  Thank you for all the explanations ^ !!!

  Wow, difficult to picture the ring, with one side the plane of the paper and the other side of the ring with plane out of the paper(wedge)

• UltimaOnline

  28 Oct 15, 01:44

  Originally posted by Flying grenade:

  Omg i have learnt so much from you, my gratitude for you is overwhelming !!!!

  Thank you for all the explanations ^ !!!

  Wow, difficult to picture the ring, with one side the plane of the paper and the other side of the ring with plane out of the paper(wedge)

  
  Well if any of your juniors are interested in H2 Chem tuition and they stay near VJC or TJC, you could recommend them to google "BedokFunland JC", thanks.
• Flying grenade

  28 Oct 15, 14:37

  2014 paper 3 qn 4c

  We are asked to draw the predominant species of amino acids at ph7 ,

  And given by the qn phenol side chain is negligible at pH7 

  Valine : one acid and amine group 

  Tyrosine : 1 co2h , 1 nh2 group

  Aspartic acid : 2 co2h group, 1 nh2 group

  From cs toh book,

  The pH at which an amino acid has a net charge of zero is the isoelectric point.

  {Amino acids with one co2h group and one nh2 group have isoelectric points in the ph range 5.5-6.5}

  Do we have to memorise the one above^ {in brackets}

   

   

  We know pH lower than pka, compound protonates 

  pH greater than pka, compound deprotonates 

  We aren't given the pka of the functional groups in this qn

  Are we supposed to memorise the pka of co2h and nh2 groups?

   

  Or we see ph 7 just draw every acidic group deprotonate and every basic group protonate?

   

  -Is it like oh, ph7, its like water, every acidic group and basic group ionises in water (does it?)

   

  Can ask chemistry questions here after 2015 chem a level papers and would u continue to reply?

• Flying grenade

  28 Oct 15, 14:42

  Originally posted by Flying grenade:

  2014 paper 3 qn 4c

  We are asked to draw the predominant species of amino acids at ph7 ,

  And given by the qn phenol side chain is negligible at pH7 

  Valine : one acid and amine group 

  Tyrosine : 1 co2h , 1 nh2 group

  Aspartic acid : 2 co2h group, 1 nh2 group

  From cs toh book,

  The pH at which an amino acid has a net charge of zero is the isoelectric point.

  {Amino acids with one co2h group and one nh2 group have isoelectric points in the ph range 5.5-6.5}

  Do we have to memorise the one above^ {in brackets}

   

   

  We know pH lower than pka, compound protonates 

  pH greater than pka, compound deprotonates 

  We aren't given the pka of the functional groups in this qn

  Are we supposed to memorise the pka of co2h and nh2 groups?

   

  Or we see ph 7 just draw every acidic group deprotonate and every basic group protonate?

  -Is it like oh, ph7, its like water, every acidic group and basic group ionises in water (does it?)

   

  Can ask chemistry questions here after 2015 chem a level qns and would u continue to reply?

   

• Flying grenade

  28 Oct 15, 14:42

  Originally posted by Flying grenade:

  2014 paper 3 qn 4c

  We are asked to draw the predominant species of amino acids at ph7 ,

  And given by the qn phenol side chain is negligible at pH7 

  Valine : one acid and amine group 

  Tyrosine : 1 co2h , 1 nh2 group

  Aspartic acid : 2 co2h group, 1 nh2 group

  From cs toh book,

  The pH at which an amino acid has a net charge of zero is the isoelectric point.

  {Amino acids with one co2h group and one nh2 group have isoelectric points in the ph range 5.5-6.5}

  Do we have to memorise the one above^ {in brackets}

   

   

  We know pH lower than pka, compound protonates 

  pH greater than pka, compound deprotonates 

  We aren't given the pka of the functional groups in this qn

  Are we supposed to memorise the pka of co2h and nh2 groups?

   

  Or we see ph 7 just draw every acidic group deprotonate and every basic group protonate?

   

   

  Can ask chemistry questions here after 2015 chem a level papers and would u continue to reply?

   

• Flying grenade

  28 Oct 15, 14:46

  I sincerely apologise,  for the numerous duplicated post ^

  I'm using phone all along, so might be lag and result in these duplicated posts, sorry

  I Tried to delete the duplicated ones but not sure how

• UltimaOnline

  28 Oct 15, 15:02

  Originally posted by Flying grenade:

  2014 paper 3 qn 4c

  We are asked to draw the predominant species of amino acids at ph7 ,

  And given by the qn phenol side chain is negligible at pH7 

  Valine : one acid and amine group 

  Tyrosine : 1 co2h , 1 nh2 group

  Aspartic acid : 2 co2h group, 1 nh2 group

  From cs toh book,

  The pH at which an amino acid has a net charge of zero is the isoelectric point.

  {Amino acids with one co2h group and one nh2 group have isoelectric points in the ph range 5.5-6.5}

  Do we have to memorise the one above^ {in brackets}

   

   

  We know pH lower than pka, compound protonates 

  pH greater than pka, compound deprotonates 

  We aren't given the pka of the functional groups in this qn

  Are we supposed to memorise the pka of co2h and nh2 groups?

   

  Or we see ph 7 just draw every acidic group deprotonate and every basic group protonate?

   

  -Is it like oh, ph7, its like water, every acidic group and basic group ionises in water (does it?)

   

  Can ask chemistry questions here after 2015 chem a level papers and would u continue to reply?

  
  If Cambridge gives you the pKa of the amino acid's R group, use it to determine if at a particular pH, the R group is protonated or deprotonated. And as you say, "ph 7 just draw every acidic group deprotonated and every basic group protonated". That's right.
  
  Because after 2015 A levels, you might retake your A levels in 2016? Or some other reason?
• Flying grenade

  28 Oct 15, 15:06

  2009 paper 3 qn 4d

   

  Seems like all the amino acids can have ionic interactions+vdw interaction+hydrogen bonds with the given amino acid residues?? 

  Help!!

   

   

  What are the factors affecting hydrophobicity?

• Flying grenade

  28 Oct 15, 15:08

  Yes, might retake, and also for the pursuit of knowledge, and considering taking chemistry major in uni

• UltimaOnline

  28 Oct 15, 16:37

  Originally posted by Flying grenade:

  2009 paper 3 qn 4d

   

  Seems like all the amino acids can have ionic interactions+vdw interaction+hydrogen bonds with the given amino acid residues?? 

  Help!!

   

   

  What are the factors affecting hydrophobicity?

  
  List out specifically all the possibilities of which amino acid R group has which interactions with which amino acid R group, and I'll tell you if you're correct or wrong.
  
  Non-polar = hydrophobic. Go google yourself for generic conceptual qns like these.
• Flying grenade

  28 Oct 15, 17:00

  For di) amino acid residue,

  Seems like there is a nh3+ group at ph7, and a N-H group

  So since all val,tyr, aspartic acid has coo- at ph7, hence can interact with it via ionic interactions ?

  Err, no h bond involved becos at ph 7 all cooh group deprotonate to form coo-?

   

  Dii) amino acid residue

  Have a benzene side chain group and a N-H side chain group of interest

  Hence only hydrophobic methyl group on val can interact with it? Err, no compound can interact with the N-H here? Idk ))):

• Flying grenade

  28 Oct 15, 17:29

  What is the distinction between equilibrium shifts (to a side ) and lies (to a side) ??

   

  Is it that for *shift* ,

  Eqm alr shifted?

  While *lies* to a side is the eqm is more inclined towards that side? 

   

  When to use which term ?? ):

• UltimaOnline

  28 Oct 15, 18:01

  Originally posted by Flying grenade:

  For di) amino acid residue,

  Seems like there is a nh3+ group at ph7, and a N-H group

  So since all val,tyr, aspartic acid has coo- at ph7, hence can interact with it via ionic interactions ?

  Err, no h bond involved becos at ph 7 all cooh group deprotonate to form coo-?

   

  Dii) amino acid residue

  Have a benzene side chain group and a N-H side chain group of interest

  Hence only hydrophobic methyl group on val can interact with it? Err, no compound can interact with the N-H here? Idk ))):

  
  Oi, do you know the difference between amino acid residue and amino acid??? This question is about R group interactions lah!