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• UltimaOnline

  28 Oct 15, 18:10

  Originally posted by Flying grenade:

  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 ?? ):

  
  If you didn't change conditions, and Qc < Kc, then you say position of equilibrium LIES to the RHS. If you didn't change conditions, and Qc > Kc, then you say position of equilibrium LIES to the LHS.
  
  If at first Qc = Kc, ie. system is at equilibrium, but then you changed experimental conditions, then you say position of equilibrium SHIFTED to either RHS or LHS.
  
  If you increased or deceased the moles or molarities or partial pressures of reactants or products, then it's Qc which changes, not Kc.
  
  If you or increased or decreased temperature, then it's Kc which changes, not Qc.
  
  If Qc now < Kc, then you say position of equilibrium has SHIFTED to the RHS.
  If Qc now > Kc, then you say position of equilibrium has SHIFTED to the LHS.
  
  All these your own JC never teach you meh? That proves that you (yo JC students reading this) should come for my BedokFunland JC tuition. *hmmph*
• Flying grenade

  28 Oct 15, 18:30

   

  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)

   

   

  Attempted to apply this for

  Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

  I got stuck , pls help :

  1) NH3 (aq) +H2O -> NH4+ + OH-

  2) Ag+ +Cl- -> Agcl (s)

  3)Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

  Is this correct?

• UltimaOnline

  28 Oct 15, 18:35

  Originally posted by Flying grenade:

   

  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)

   

   

  Attempted to apply this for

  Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

  I got stuck , pls help :

  1) NH3 (aq) +H2O -> NH4+ + OH-

  2) Ag+ +Cl- -> Agcl (s)

  3)Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

  Is this correct?

  
  Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)2]+(aq)
• Flying grenade

  28 Oct 15, 18:36

  Yes i will spread the word of the majestic chem guru in the east to my friends, promise

  Still unsure about the concept of Qc..

  Qsp i know but what is Qc?

• UltimaOnline

  28 Oct 15, 18:41

  Originally posted by Flying grenade:

  Yes i will spread the word of the majestic chem guru in the east to my friends, promise

  Still unsure about the concept of Qc..

  Qsp i know but what is Qc?

  
  Eh, "chem guru" is the title used by Maverick Puah lah. He gives tuition at Bishan, while I give tuition at Bedok / Marine Parade. Just tell ur friends (in the eastern part of Singapore) to google "BedokFunland JC".
  
  Qc has the same formula as Kc, but Qc is at initial and can take on any mathematical value, while Kc is at equilibrium, and has a fixed constant mathematical value for any reaction at a given temperature.
• Flying grenade

  28 Oct 15, 18:44

  Yea this the ionic eqn

  Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)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.

   

   

  I attempted to rephrase this to suit agcl, but couldn't, cos there's no oh- and deleted the chunk i typed

  Seems like theres a gap cos no oh- in eqn 2

  How?

• Flying grenade

  28 Oct 15, 18:49

  Okay will do so to instruct them to search up your website

  Seems like youre advocating mainly people in the east to sign up for yr tui

  Perhaps thats unnecessary, those who see the treasure of knowledge : 40+ pages chem information on this forum and many more on ur website that you have shared for public use, think those that seek enlightenment wouldn't mind the travelling task 

• UltimaOnline

  28 Oct 15, 18:49

  -_-"
  
  Eqn 1 - Ag+(aq) + Cl-(aq) --> AgCl(s)
  Eqn 2 - Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)2]+(aq)
  
  When you add excess NH3(aq), position of equilibrium for Eqn2 shifts to RHS, hence position of equilibrium for Eqn1 shifts to LHS, hence AgCl(s) ppt dissolves.
  
  With Br-, you need concentrated NH3(aq) because the Ksp for AgBr is much smaller, ie. less soluble. With I-, the Ksp for AgI is so damn small, that no amount of concentrated NH3(aq) or even NH3(l) can dissolve the AgI(s) ppt.
• Flying grenade

  28 Oct 15, 19:30

  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 ))):

   

  I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

  Val and tyr aren't polypeptide chains, their discrete amino acids

  Aren't the groups on val and tyr considered R groups? 

   

   

• UltimaOnline

  28 Oct 15, 19:40

  Originally posted by Flying grenade:

   

  I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

  Val and tyr aren't polypeptide chains, their discrete amino acids

  Aren't the groups on val and tyr considered R groups? 

   

   

  
  Aspartic acid's R group is a COO- group. Valine's R group is an alkyl group. Tyrosine's R group is a phenol group. The question isn't asking for interactions between discrete amino acids. It's asking for interactions between R groups of these amino acid residues within a polypeptide chain. Don't lose your marks carelessly like this, it'll cost you at least 1 grade (with steep bell-curves).
• Flying grenade

  28 Oct 15, 19:44

  Originally posted by Flying grenade:

   

  I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

  Val and tyr aren't polypeptide chains, their discrete amino acids

  Aren't the groups on val and tyr considered R groups? 

   

   

  Update 

  Is my understanding correct, and can help improve my explanation? 

  Sian, think question interpretation might also be a problem

  Is it because :in this qn, the amino acids are part of a polypeptide chain, e.g. val-tyr-asp- etc etc

  Then, both val and tyr's nh2 and cooh is used up in amide bond linkage

  Only aspartic acid has the extra cooh group, hence can interact with the qn?

  But then wouldnt it be Hbond interactions?

   

   

   

  I thought of another : 

  Is it because ph 7, so means its in water? ( thats why the acid and basic group will deprotonate and protonate??)

  Thats why will have ionic interactions instead of h bonding?

  But if its in water, then hydrolysis would occur,  then wouldnt the nh2 and cooh group on val and tyr able to interact? ?

  Help!

• Flying grenade

  28 Oct 15, 19:50

  If ph7 in water,  will it cause the polypeptide chain to undergo hydrolysis and cause it to break into small portions of amino acid residues?

• UltimaOnline

  28 Oct 15, 19:55

  Originally posted by Flying grenade:

  Update 

  Is my understanding correct, and can help improve my explanation? 

  Sian, think question interpretation might also be a problem

  Is it because :in this qn, the amino acids are part of a polypeptide chain, e.g. val-tyr-asp- etc etc

  Then, both val and tyr's nh2 and cooh is used up in amide bond linkage

  Only aspartic acid has the extra cooh group, hence can interact with the qn?

  But then wouldnt it be Hbond interactions?

   

   

   

  I thought of another : 

  Is it because ph 7, so means its in water? ( thats why the acid and basic group will deprotonate and protonate??)

  Thats why will have ionic interactions instead of h bonding?

  But if its in water, then hydrolysis would occur,  then wouldnt the nh2 and cooh group on val and tyr able to interact? ?

  Help!

  
  1st of all, hydrolysis only occurs partially, it's an acid-base *equilibrium* remember?
  
  2ndly, technically (Singapore JCs don't teach this well so it's not surprising JC students don't really understand this), while COOH and NH2 groups are only capable of (both donating and accepting) H bonds, COO- and NH3+ groups are actually capable of participating in both H bonds (COO- can only accept H bonds, while NH3+ can only donate H bonds) as well as ionic bonds, depending on who the other party is.
  
  And yes, Cambridge expects you to automatically first deprotonate COOH groups at pH7, and protonate NH2 groups at pH7.
• Flying grenade

  28 Oct 15, 19:58

  Can tyrosine's phenol group interact with nh3+?

  If cannot, is it because phenol doesn't ionise in water?

• UltimaOnline

  28 Oct 15, 19:59

  Originally posted by Flying grenade:

  If ph7 in water,  will it cause the polypeptide chain to undergo hydrolysis and cause it to break into small portions of amino acid residues?

  
  Oi!!! Wah biangz eh, if at pH7 and room temperature, proteins spontaneously "break into small portions of amino acid residues", then you won't be able to remain in 1 piece typing this post with your phone liao (you won't even be able to hold your phone), you'll disintegrate instantly into a sludge. Sludge man!
• UltimaOnline

  28 Oct 15, 20:02

  Originally posted by Flying grenade:

  Can tyrosine's phenol group interact with nh3+?

  If cannot, is it because phenol doesn't ionise in water?

  
  Hydrogen bonding is possible, between the partial positive H atom of NH3+ (the H bond donor), and the partial negative O atom of phenol (the H bond acceptor).
  
  Cambridge already stated in the question, regarding extent of deprotonation of phenol at pH 7. Read the question carefully lah!
• Flying grenade

  28 Oct 15, 20:08

  Wtf sia, then why we learn polypeptide linkages and proteins undergo hydrolysis and break into smaller portions?

  And thought ph7 the acid and basic groups ionise?

   

• UltimaOnline

  28 Oct 15, 20:19

  You need to either heat under reflux or use biological enzymes, plus at strongly acidic or strongly basic pH lah. That's how your strongly acidic stomach digests the meat you eat, with proteases enzymes.
  
  Acidic COOH groups and basic NH2 groups do ionize via hydrolysis to generate COO- groups and NH3+ groups.
  
  Different from hydrolysis of the protein's peptide bond within the amide group in the polypeptide chain of a protein (the word "peptide" refers to *either* the peptide bond within the amide group in a polypeptide chain of a protein, *or* the amino acid residues in a polypeptide chain of a protein). Because the amide group within a polypeptide chain bonds two amino acid residues or 'peptides' together, the amide group in proteins is called the peptide group, and the C-N bond within the amide group is called the peptide bond, linking two 'peptides' or amino acid residues together.
  
  Hydrolysis means "chemical reaction with water" (where chemical reactions involve breaking or forming of sigma bonds, while resonance involves breaking or forming of pi bonds only), while hydration means "physical interaction with water" (physical, inorganic & organic chemistry) or "addition of water" (in some organic chemistry contexts, eg. hydration of alkenes to alcohols).
• Flying grenade

  28 Oct 15, 21:08

  2013 paper 1 qn 27

  Are we supposed to know if pcl5 reacts with oh or cooh group?

   

  I only know pcl5 is a acidic chloride and it may react with oh group 

• UltimaOnline

  29 Oct 15, 01:02

  Originally posted by Flying grenade:

  2013 paper 1 qn 27

  Are we supposed to know if pcl5 reacts with oh or cooh group?

   

  I only know pcl5 is a acidic chloride and it may react with oh group 

  
  -_-" Have you even memorized your CS Toh A level Study guide? You *must* memorize the entire book, if you expect to even *pass* your A levels (coming up in the next few days). Instead of spoonfeeding you (basic stuff like these), you go ahead and look up (in CS Toh book, or google or Wikipedia) PCl5 and tell me what it reacts with.
• Flying grenade

  29 Oct 15, 02:42

  Pcl5 reacts with acid to form acyl chloride

  Reacts with alcohol to from halogenoalkane

  ^both via nucleophilic substitution 

• UltimaOnline

  29 Oct 15, 04:16

  Originally posted by Flying grenade:

  Pcl5 reacts with acid to form acyl chloride

  Reacts with alcohol to from halogenoalkane

  ^both via nucleophilic substitution 

  
  Correct.
  
  PCl5 (and for that matter, SOCl2 as well) doesn't react with phenol to generate aryl halides though, for 2 reasons.
  
  1stly, the unlike aliphatic alcohols, the lone pair on the phenolic sp2 O atom is delocalized by resonance to form a pi bond with the sp2 C atom of the benzene ring in some resonance contributors), exactly as is the case with aryl and vinyl halides, and thus the resonance hybrid's phenolic O atom is electron-deficient (has a positive formal charge on some contributors, and hence a partial positive charge in the resonance hybrid), and thus is non-nucleophilic and cannot attack the electrophilic P atom of PCl5 (or the electrophilic S atom of SOCl2).
  
  2ndly (even for the small % of phenols which happen to possess sufficient activation energy to attack PCl5 or SOCl2), the electron-rich Cl- nucleophile (eliminated from PCl5 or SOCl2 after reacting with ROH) is electrostatically repelled by the pi electrons of the electron-rich benzene ring nucleophile, and thus PCl5 and SOCl2 cannot complete the mechanism to convert phenol to aryl halides (which therefore does not happen).
  
  If descriptions of the elementary steps are provided in the question, Cambridge could ask you (eg. in this year's A level exam) to draw the (partial or full) mechanism for PX5 or PX3 or SOX2 reacting with alcohols and carboxylic acids to generate alkyl halides and acyl halides respectively. So for those of you interested (and ready ; if any of these confuses you, then you're not ready and you should skip this entirely), here are the mechanisms
  
  (Bonus teaser challenge to all JC students : the use of SOCl2 is often favoured over PCl5, because the SOCl2 reaction has an important advantage over the PCl5 reaction, can you guess what it is? Cambridge could ask this question sooner or later, and I won't reveal the answer, up to those aiming for a distinction to figure it out for yourselves.)
  
  Mechanism for SOCl2 and PBr3 reacting with alcohols to generate alkyl chloride and alkyl bromide :
  https://www.khanacademy.org/science/organic-chemistry/alcohols-ethers-epoxides-sulfides/reactions-alcohols-tutorial/v/preparation-of-alkyl-halides-from-alcohols
  
  Mechanism for SOCl2 reacting with carboxylic acids to generate acyl chloride :
  https://www.khanacademy.org/science/organic-chemistry/carboxylic-acids-derivatives/reactions-carboxylic-jay/v/preparation-of-acyl-acid-chlorides
  
  Mechanism for PCl5 reacting with carboxylic acids to generate acyl chloride :
  https://upload.wikimedia.org/wikipedia/commons/7/71/Phosphorus_pentachloride_mechanism.png
  
  Note that the phosphoryl chloride generated can be further hydrolyzed, this is covered in the H2 syllabus under the inorganic chemistry topic : periodicity. In fact, the mechanism for both water H2O (inorganic chemistry) and alcohol ROH (organic chemistry) reacting with PCl5, is exactly the same (up to a point), because did you notice that water (H-O-H) is a 'type' of alcohol (R-O-H) in which the R group is a H atom, and thus it should make natural and logical sense to you (did your school teach this to you?) that the reaction mechanism (up to a point) is exactly the same : HOH with PCl5 gives POCl3 and 2HCl, while ROH with PCl5 gives POCl3 and RCl + HCl.
  
  You can try drawing the mechanism for further hydrolysis of POCl3 to give H3PO4 + 3HCl (notice that the OS of all elements remain unchanged during hydrolysis reactions, so based on the starting OS, you can deduce the final OS which will help you deduce the structure or formula of the final hydrolysis product). The mechanism involves 3 water molecule nucleophiles (one by one) attacking the electrophilic P atom of POCl3, thereby eliminating the 3 Cl- ion leaving groups one by one, followed by proton transfers to generate the final products of H3PO4 and 3 HCl. Cambridge could ask you to draw out this fairly simple mechanism (of which the balanced equation you have to memorize under the Periodicity topic anyway, so might as well you draw out the mechanism to make sense of it for yourself instead of blindly memorizing the equation like most JC students do... which is just sad. Learning should always be fun and meaningful.)
  
  ---------------------------------------------------------------------------------------------
  
  Btw, the following is something extra, a bonus for those of you (eg. Flying Grenade) who may be interested in studying Chemistry at the Uni level. Did you know that other than SN1 (racemization) and SN2 (inversion of configuration), there's a 3rd mechanism called SNi (retention of configuration) whose secret existence, bewildered chemists figured out, when trying to figure out why some reactions like SOCl2 with alcohols mysteriously resulted in alkyl chlorides with retention of configuration instead of the expected racemization or inversion of configuration (ie. meaning it was neither SN1 nor SN2). If hearing this excites you, then yup, you should certainly consider studying Chemistry at Uni level. Enjoy! :)
  
  SOCl2 and the SNi Mechanism
  http://www.masterorganicchemistry.com/2014/02/10/socl2-and-the-sni-mechanism/
• Flying grenade

  29 Oct 15, 10:16

  ^ thank you ultima, for everything.

  Socl2 thionyl chloride might be preferably used than pcl5 or pcl3 for the Nu substn for the conversion of alcohol to haloalkane is because the other products formed are gases: so2 and hcl that would escape from the soln, hence haloalkane is easily obtained,

  Rather than an additional step of fractional distillation needed if used phosphorus chloride

  But hcl and so2 are also poisonous, might nt be advantage in all occasions?

• Flying grenade

  29 Oct 15, 12:37

  Btw what/how do you think about the 'thomas bond, chris hughes A level books, by the themis yellowreef publisher  haha

• Flying grenade

  29 Oct 15, 15:32

  Is there any significance of pyridine in H2 chem that we need to know?