Lurkers, register with SgForums and ask ur H2 Chem qns here!
-
Hi Ultima, rlly appreciate what you are doing here! Hoping that you can help me clarify some doubts:
1) Regarding the coordination number of a complex, I understand that it is defined as the number of coordination bonds around the central metal atom/ion. For the case of a porphyrin ring complexed with Mg2+ in chlorophyll a, I understand that 2 of the N forms ionic bonds while the other 2 N coordinate bonds with the Mg2+ ion. In such a case, would the coordination number be 2 or 4? Also, since complex is when the no. of coordination bonds exceed the O.S of the central metal ion, would this compound even be a complex since it only has 2 coordinate bonds?
2) Regarding optical isomerism in metal complexes, while i am aware of the propeller chirality in compounds such as (cr(cro4)3)3-, I was wondering if there would also be such isomerism in tridentate or even hexadentate ligand complexes like fe edta complex.
3) Regarding cyanohydrins, will basic hydrolysis of it yield coo- or will the original aldehyde be produced instead (Oh- extracts H from Oh in cyanohydrin which results in formation of c=o bond again and the elimination of cn-)
4) For the amino acid tyrosine, the OH in the phenol is apparently neutral and tyrosine is not ionised at physiological ph of 7. However, we learnt that phenol usually exists as phenoxide at ph7. Is there a particular reason why tyrosine is neutral at ph7 and if we are asked to circle an ionisable r grp at ph7, can we circle the phenol in tyrosine?
5) When asked for possible interactions between R groups, do we assume they are protonated or not, because does assuming that they are not protonated / charged rule out interactions like ionic interactions? This is especially so if the question does not give the amino acid residues in their charged form yet it is quite obvious that ionic
interactions are possible
6) Pertaining to questions that ask for the reactions of period 3 elements with oxygen and chlorine, do we give the reaction that least oxidised the element or the reaction that most oxidises the element when it is not specified. Eg
should we give the formation of p4o6 or p4o10 when p4 is burnt in oxygen?
Thank you!
-
Hi MagicLeprechaun,
Maybe Ultima can check if Im correct, but I was thinking that in response to qn 3 of your list of qns, the OH- nucleohhile will attack the delta positive H of the -O-H group instead of the delta positive C in the -C-N group. This is because the electronegativity difference between O and H is greater than between C and N. Hence, since O is more electron-deficient, OH- will be attracted to the -O-H group first. This reaction causes the formation of the original aldehyde which then causes nucleophilic addition to occur again between the aldehyde and the CN- nucleophile.Ultima I would appreciate it if you could tell me if Im right! :) Thank you!
-
Hey Ultima!
Thank you so much for doing this :) Really enjoy trying to solve all the challenging chem questions you post on your website
My question pertains to the oxidation of a benzene ring with an alkene side chain. When we add acidified KMnO4, do we oxidise the alkene first before oxidising the side chain or do we simply oxidise off the entire side chain without bothering about the alkene inside?Thank you!
-
Ultima u are god, your positive karma points is definitely off the chart you'll definitely ascend to heaven ultima
-
Originally posted by MagicLeprechaun:
Hi Ultima, rlly appreciate what you are doing here! Hoping that you can help me clarify some doubts:
1) Regarding the coordination number of a complex, I understand that it is defined as the number of coordination bonds around the central metal atom/ion. For the case of a porphyrin ring complexed with Mg2+ in chlorophyll a, I understand that 2 of the N forms ionic bonds while the other 2 N coordinate bonds with the Mg2+ ion. In such a case, would the coordination number be 2 or 4? Also, since complex is when the no. of coordination bonds exceed the O.S of the central metal ion, would this compound even be a complex since it only has 2 coordinate bonds?
2) Regarding optical isomerism in metal complexes, while i am aware of the propeller chirality in compounds such as (cr(cro4)3)3-, I was wondering if there would also be such isomerism in tridentate or even hexadentate ligand complexes like fe edta complex.
3) Regarding cyanohydrins, will basic hydrolysis of it yield coo- or will the original aldehyde be produced instead (Oh- extracts H from Oh in cyanohydrin which results in formation of c=o bond again and the elimination of cn-)
4) For the amino acid tyrosine, the OH in the phenol is apparently neutral and tyrosine is not ionised at physiological ph of 7. However, we learnt that phenol usually exists as phenoxide at ph7. Is there a particular reason why tyrosine is neutral at ph7 and if we are asked to circle an ionisable r grp at ph7, can we circle the phenol in tyrosine?
5) When asked for possible interactions between R groups, do we assume they are protonated or not, because does assuming that they are not protonated / charged rule out interactions like ionic interactions? This is especially so if the question does not give the amino acid residues in their charged form yet it is quite obvious that ionic
interactions are possible
6) Pertaining to questions that ask for the reactions of period 3 elements with oxygen and chlorine, do we give the reaction that least oxidised the element or the reaction that most oxidises the element when it is not specified. Eg
should we give the formation of p4o6 or p4o10 when p4 is burnt in oxygen?
Thank you!
No problem, MagicLeprechaun.
Q1. In complex (no pun intended) and unusual cases such as these, the proper chemist and exam-smart student would specify exactly which type of bonds involved in calculating the coordination number. Because different chemists and different authors have a different take on whether ionic bonds should be included or not. For instance, in a P2 or P3 qn, you should write, "coordination number is 2 if we only count the dative covalent bonds; coordination number is 4 if we also include the 2 ionic bonds." and Cambridge would go, "now there's a smart student.". If it were an MCQ however, then it would be the question's fault for not specifying this, then the exam-smart student would deduce the best option by eliminating more implausible options. But all things considered, as far as the underlying definitional purpose of "coordination" is concerned (as opposed to a shallow, dogmatic, specific interpretation), it's actually better (more sensible) chemistry to include *all* bonds (after all, all of the bonds herein, whether ionic or dative covalent, are in truth part ionic, part covalent anyway) that the central metal cation is coordinated with. Concordantly, the coordination number of Mg2+ in chlorophylls (ignoring any other ligands not included in the simplified diagrams given in A level exams) is better regarded as 4 (ie. including ionic bonds) instead of 2. And yes, the Mg-chlorin ring in chlorophylls is considered a coordination complex.
Q2. Yes, it's possible, as long as the coordination complex is overall tetrahedral or octahedral, and the mirror image is non-superimposable (or more strictly, non-superposable) on the molecule itself. But assuming you're a H2 Chem student (are you? or do you take H3 Chem as well?) this is beyond H2 Chem syllabus and shouldn't be your concern.
Q3. MagicLeprechaun and Twe375, because the nucleophilic addition of CN- to a carbonyl compound is reversible (HCN is a slightly stronger acid than H2O, so CN- conjugate base is slightly more stable than OH- conjugate base because even though the negative formal charge is on C which is less electronegative than O, but the C atom being sp hybridized has high % s orbital character), hence under alkaline hydrolysis conditions, you do get a mixture of both (carbonyl and amide) products initially. But because the first hydrolysis product of the nitrile group is an amide, which is itself further hydrolyzed eventually to a carboxylic acid (acidic hydrolysis) or carboxylate ion (alkaline hydrolysis), this pulls the position of equilibrium eventually to the RHS, which is why for the H2 Chem syllabus, you simply have to state that the final product of cyanohydrin / hydroxynitrile hydrolysis is a hydroxycarboxylic acid (acidic hydrolysis) or hydroxycarboxylate ion (alkaline hydrolysis).
Q4. "We learnt that phenol usually exists as phenoxide at pH 7". This is not correct. The pKa of phenol is 9.95 (in aqueous solvent), which means at pH 7, which is more acidic than the pKa value of 9.95, the majority of phenol molecules exist as the protonated conjugate acid form, instead of the deprotonated conjugate base form. Notice that the pKa of the phenol side chain in Tyrosine is 10.07, which reflects its slightly weakened acidity (compared to phenol by itself), as a result of the benzylic sp3 C atom which donates electrons slightly by induction (but only slightly, because of the electronegative N atom of the alpha amine). If by "ionizable" you mean a small percentage exists in it's anionic deprotonated form at pH 7, that's still technically correct, by definition of acid-base equilibria. But for A levels, it wouldn't be acceptable, because if Cambridge asks the question, it implies "circle the most ionizable group(s) at pH 7" and the phenolic side chain of Tyrosine ranks a distant 3rd place behind the the alpha COOH/COO- and alpha NH2/NH3+ groups, and Cambridge would think, "this student's answer implies he/she doesn't know the alpha groups ionize to a far greater extent than the R group, so zero marks for him/her!".
Q5. Again, be exam-smart. If the pH is given, use the predominant form (ie. either protonated or not at that pH). If the pH is not given, then you should take the initiative to state "at physiological pH, these groups exists as..." and label the R group interactions accordingly. Or if there isn't an obvious predominant form at the pH indicated, then you should (be exam-smart and) give both answers : eg. as conjugate base form, NH2 can donate and accept H bonds ; as conjugate acid form, NH3+ can donate H bonds and participate in ionic bonding. If it's an MCQ, then (be exam-smart and) eliminate the less plausible choices, and the remaining option will clue you in on what the Cambridge question setter wanted.
Q6. Again, be exam-smart. Give both eg. "in limited oxygen P4O6 is formed ; in excess oxygen P4O10 is formed", if you're uncertain on which one the question wanted. You won't be penalized because both equations would be correct. Cambridge would accept the one they wanted, and ignore the one they didn't want (in some cases, Cambridge will accept either, eg. P4O6 or P4O10, unless the question precluded one of them in its description). You'll only be penalized if one of your answers contradict or nullify the other point (in which case Cambridge would go "KNN this bugger trying to cheat!"). But in the context of your question, and especially if you're too short of time to write both answers, you should give the form of the compound that is more thermodynamically stable under standard conditions and thus more common, eg. SO2 instead of SO3, and/or the form that assumes excess O2 and Cl2 is present and available for complete reaction, ie. P4O10 instead of P4O6, and PCl5 instead of PCl3. -
Originally posted by chemistryiskool:
Hey Ultima!
Thank you so much for doing this :) Really enjoy trying to solve all the challenging chem questions you post on your website
My question pertains to the oxidation of a benzene ring with an alkene side chain. When we add acidified KMnO4, do we oxidise the alkene first before oxidising the side chain or do we simply oxidise off the entire side chain without bothering about the alkene inside?Thank you!
U're welcome, and glad to hear you enjoyed my BedokFunland JC's website questions :)
With hot acidified KMnO4, oxidize the alkene side chain (with oxidation cleavage) first, then oxidize the benzene ring together with its alkyl side chain (provided a benzylic H atom is present) to benzoic acid. So your answer (if this was an A level exam qn) should include at least 2 different organic products (ie. excluding inorganic CO2 and H2O), unless of course the alkene side chain upon oxidative cleavage is oxidized to methanoic acid or ethandioic acid (both of which would be further oxidized to carbonic(IV) acid, which exists in equilibrium with, and hence can decompose into, CO2 and H2O, with thermodynamically favourable positive entropy change, and with the position of equilibrium pulled over to the RHS as predicted by Le Chatelier's principle, as gaseous CO2 leaves the reaction mixture), or if the other side of the alkene side chain is also another benzene ring which is also oxidized to the same product benzoic acid (eg. if the reactant molecule is symmetrical or close to symmetrical about the alkene double bonds outside the benzene rings). -
Note : Fyi, I've just updated H2Chem_Special_Exceptions_for_Copper(I)Iodide_Fehings_Tollens on my BedokFunland JC website, reflecting Mr Worry's underlying concern about copper(I) iodide, ie. why the redox reaction proceeds despite the standard cell potential being negative.
-
Originally posted by UltimaOnline:
Note : Fyi, I've just updated H2Chem_Special_Exceptions_for_Copper(I)Iodide_Fehings_Tollens on my BedokFunland JC website, reflecting Mr Worry's underlying concern about copper(I) iodide, ie. why the redox reaction proceeds despite the standard cell potential being negative.
Hi Ultima, pertaining to the above concern, how do we reconcile the possibility of the precipitation of insoluble CuI also lowering [I-] and thus shifting the position of eqm of I2/I- reduction equation to the right hence making it more positive and in that sense still make the cell potential for the reaction negative?
Thanks so much!
-
Originally posted by Sugarfortress :
Hi Ultima, pertaining to the above concern, how do we reconcile the possibility of the precipitation of insoluble CuI also lowering [I-] and thus shifting the position of eqm of I2/I- reduction equation to the right hence making it more positive and in that sense still make the cell potential for the reaction negative?
Thanks so much!
Excellent point, Sugarfortress (are you a H2 or H3 student? or not even a student?).
This *appears* to be irreconcilable at A levels, given the stoichiometry and magnitudes of the relevant oxidation and reduction potentials. A easy workaround would be to suggest that excess I- be required (again, non-standard molarities). However, as it turns out, there's actually a more important reason that enables the redox reaction to proceed to generate CuI(s) even without excess I-. Which is, the reduction potential of Cu2+(aq) to Cu+(s) precipitate is actually far more positive than given in the Data Booklet, which is for aqueous species, ie. Cu2+(aq) to Cu+(aq). So much so that as CuI(s) precipitates out, the magnitude of the decrease in oxidation potential of I- to I2, is still far outweighed by the magnitude of increase in reduction potential of Cu2+(aq) to Cu+(aq) to Cu+(s). As such, admittedly my earlier explanation is simplified and incomplete, but would still suffice for A levels, should Cambridge ever ask this question. -
Hi Ultima,
Some questions here which I hope I can seek your advice :)
1. With reference to 2014 TYS Paper 2, why is FeS2 considered a "complex" with a yellow / gold colour even though it is supposed to be ionic (Fe2+ and (S2)2- ions)?
2. Also with reference to the trend of boiling points of the Transition Metals, why does chromium have a higher boiling point of 2672 than that of Mn of 2061 despite it having less 4s electrons that Mn? Shouldn't having more electrons in Mn result in stronger metallic bonding?
3. Generally, what is the safest / most efficient bet when Cambridge asks for the "structural formula" of a compound? What about "displayed formula"? In such circumstances, should we draw 1) condensed 2) all bonds 3) skeletal? Or is a hybrid even accepted by Cambridge?
4. Regarding the usage of "in paper" and "out of paper" wedges in illustrating Walden's Inversion in Sn2 Mechanism, do the orientations of the wedges (how they are placed before and after reaction i.e. "do they flip or stay the same") matter and does Cambridge see it as a marking point?
5. In Sn2 Mechanism, in the transition state, is it a must to label the nucleophile and leaving group as delta minus and the attacked carbon atom as delta plus? Are there any circumstances where we do not label as such?
6. I understand that one can use "arrow pushing" to illustrate "inorganic mechanisms" such as how SO2 becomes H2SO3 and how CO2 becomes H2CO3 when attacked by H2O. In seeking a better understanding of other similar reactions, how can we also use "arrow pushing" to illustrate how giant covalent SiO2 becomes ionic Sio3 2- when reacted with concentrated NaOH?
7. I understand that we often use terms like pi / π electron cloud / ring system / aromatic ring system interchangeably. I would like to ask if all of these terms are accepted by Cambridge and if not, which would be the safest bet to use in elaboration in Paper 2 and 3 (for instance when illustrating partial double bond character in chlorobenzene)
8. With regards to the graph of rate of reaction against concentration of substrate, we often say that at low concentrations of substrate, the order of rxn is 1 wrt substrate. When at high concentrations of substrate, the order of rxn is 0 wrt substrate due to catalyst saturation. However, what if even without the substrate, the order of reaction wrt to the reactant (our substrate) is non-zero. How would that change how the graph looks like at low and high reactant (substrate) concentrations in the presence of catalyst?
9. In illustrating complex ions, say CuCl4 2-, which would be the preferable way of writing in equations: 1) [CuCl4(H2O)2] 2- or 2) [CuCl4-]2-. If it is the former, should we be writing precipitates that we commonly know of (like CuOH2) as [Cu(OH)2(H2O)4] from then on? Also, will we be penalised when we state the molecular geometry of the complex based on our answer in 1) and 2)? i.e. when we write 1), we would then proceed to write "octahedral" but when we write 2), we would tend to write "square planar" later on if the question requires.
10. In solving "calculate pH" questions given the intial acid/base concentration and the ka / kb values respectively, what would be the most correct statement to use/write to assume that [acid/base intial] =approx= [acid/base eqm]?
11. In questions where Cambridge asks why compounds like Al2O3 react with acid and base, will it suffice to just say high charge density = acidic, and ionic oxide = basic? Must we go in depth to talk about how Al3+ deprotonates coordinating H2O molecules to make Al2O3 acidic and how O2- is a strong base and thus this makes Al2O3 basic?
Thx alot! :)
-
Originally posted by Pandankaya123:
Hi Ultima,
Some questions here which I hope I can seek your advice :)
1. With reference to 2014 TYS Paper 2, why is FeS2 considered a "complex" with a yellow / gold colour even though it is supposed to be ionic (Fe2+ and (S2)2- ions)?
2. Also with reference to the trend of boiling points of the Transition Metals, why does chromium have a higher boiling point of 2672 than that of Mn of 2061 despite it having less 4s electrons that Mn? Shouldn't having more electrons in Mn result in stronger metallic bonding?
3. Generally, what is the safest / most efficient bet when Cambridge asks for the "structural formula" of a compound? What about "displayed formula"? In such circumstances, should we draw 1) condensed 2) all bonds 3) skeletal? Or is a hybrid even accepted by Cambridge?
4. Regarding the usage of "in paper" and "out of paper" wedges in illustrating Walden's Inversion in Sn2 Mechanism, do the orientations of the wedges (how they are placed before and after reaction i.e. "do they flip or stay the same") matter and does Cambridge see it as a marking point?
5. In Sn2 Mechanism, in the transition state, is it a must to label the nucleophile and leaving group as delta minus and the attacked carbon atom as delta plus? Are there any circumstances where we do not label as such?
6. I understand that one can use "arrow pushing" to illustrate "inorganic mechanisms" such as how SO2 becomes H2SO3 and how CO2 becomes H2CO3 when attacked by H2O. In seeking a better understanding of other similar reactions, how can we also use "arrow pushing" to illustrate how giant covalent SiO2 becomes ionic Sio3 2- when reacted with concentrated NaOH?
7. I understand that we often use terms like pi / π electron cloud / ring system / aromatic ring system interchangeably. I would like to ask if all of these terms are accepted by Cambridge and if not, which would be the safest bet to use in elaboration in Paper 2 and 3 (for instance when illustrating partial double bond character in chlorobenzene)
8. With regards to the graph of rate of reaction against concentration of substrate, we often say that at low concentrations of substrate, the order of rxn is 1 wrt substrate. When at high concentrations of substrate, the order of rxn is 0 wrt substrate due to catalyst saturation. However, what if even without the substrate, the order of reaction wrt to the reactant (our substrate) is non-zero. How would that change how the graph looks like at low and high reactant (substrate) concentrations in the presence of catalyst?
9. In illustrating complex ions, say CuCl4 2-, which would be the preferable way of writing in equations: 1) [CuCl4(H2O)2] 2- or 2) [CuCl4-]2-. If it is the former, should we be writing precipitates that we commonly know of (like CuOH2) as [Cu(OH)2(H2O)4] from then on? Also, will we be penalised when we state the molecular geometry of the complex based on our answer in 1) and 2)? i.e. when we write 1), we would then proceed to write "octahedral" but when we write 2), we would tend to write "square planar" later on if the question requires.
10. In solving "calculate pH" questions given the intial acid/base concentration and the ka / kb values respectively, what would be the most correct statement to use/write to assume that [acid/base intial] =approx= [acid/base eqm]?
11. In questions where Cambridge asks why compounds like Al2O3 react with acid and base, will it suffice to just say high charge density = acidic, and ionic oxide = basic? Must we go in depth to talk about how Al3+ deprotonates coordinating H2O molecules to make Al2O3 acidic and how O2- is a strong base and thus this makes Al2O3 basic?
Thx alot! :)
Q1. Unlike O levels, now at A levels you begin to appreciate that Chemistry, a microcosm of real-life, is not so clear-cut black-&-white, but many fascinating shades over the entire spectrum. While it's still not wrong to consider FeS2 as ionic, it's also not wrong for Cambridge to call it a complex, because there's an area of overlap between the 2, for which FeS2 falls in. Firstly, unlike O, S atoms have a significantly more polarizable electron charge clouds (due to larger ionic radius and shielding effect), and hence the bonds for FeS2 are actually closer to dative covalent than ionic. Secondly, in the crystal lattice structure of FeS2, each S atom (of S2 2- disulfide anions) donates 3 bond pairs unto each Fe2+ cation, concordantly each S2 2- dilsulfide anion donates 6 bond pairs unto each Fe2+ cation, while each Fe2+ cation accepts dative bonds from, and is thus (distorted octahedrally) coordinated with, 6 S atoms (of S2 2- disulfide anions), giving rise to the stoichiometry of 1 Fe2+ : 1 S2 2-, ie. FeS2. Cambridge chose to call it a coordination complex, simply out of convenience in the context of the (coordination complex topical) questions they chose to ask the A level candidate.
Q2. For a variety of reasons too complex to be dealt with at A levels, different metals across the d block have different metallic crystal lattice packing structures, with significantly closer packing for Cr compared to Mn. This is evidenced by the fact that in spite of the significantly higher molar mass of Mn over Cr, they have similar g/cm3 densities. Closer packing allows for stronger metallic bonds, which explains why Cr has a higher melting/boiling point and hardness compared to Mn.
Q3. When "structural formula" is asked, Cambridge accepts either displayed / full, skeletal, condensed, or a hybrid of all 3. Students should use whichever they're most comfortable with. More competent students will find skeletal structure to be advantageous, both in terms of being less time-consuming, as well as being clearer (for both the student and Cambridge marker).
Q4. Cambridge recommends A level students to use the following :
Q5. Yes it is required : all formal charges must always be shown, and partial charges must be shown when relevant. Although the positive (partial or occasionally formal) charge on the electrophilic C atom should be illustrated in the reactant rather than transition state. Understand the meaning of transition state, and apply chemistry sense to adjust accordingly for different cases or exceptions, eg. if the incoming nucleophilic atom has a negative formal charge, it'll have a partial negative charge in the transition state to reflect its process of donating its lone pair and losing its negative formal charge, eg. hydroxide ion ; if the incoming nucleophilic atom has a no formal charge and only a partial negative charge, it'll have a partial positive charge in the transition state to reflect its process of donating its lone pair and gaining a positive formal charge, eg. ammonia molecule. And likewise for the leaving group.
Q6. As far as helping you to understand the process and to elucidate the product is concerned, you can proceed with the mechanism by visualizing SiO2 as a simple covalent molecule rather than a giant covalent lattice. Just be aware that because the reactant exists as a giant lattice, so will (at least some of) the products (even as you draw it as simple molecular for the sake of simplification). After which, you can proceed to write out the overall balanced equation (which will be all that Cambridge would require from you in the A levels).
Q7. Yes, they are mostly all acceptable, and different JCs will use different terms and phrasing. A safe bet is to follow the phrasing of CS Toh, Chan Kim Seng & Jeanne Tan, and George Chong in their A level books. If you're still worried about this point, go look up the Cambridge Mark Schemes to learn exactly how they mark (as I've said many times before, Cambridge is more reasonable and less anal than JC teachers in marking students' answers).
Q8. You would have to mathematically adjust the graph of course, but which is beyond the A level Chem syllabus and will not be asked.
Q9. [CuCl4-]2- is [CuCl4-]2-, totally different from [CuCl4(H2O)2] 2-. The former is tetrahedral and the correct formula, while the latter is octahedral and the wrong formula. Other than the common coordination complexes from the A level H2 syllabus, for uncommon coordination complexes the Cambridge question will give you sufficient data for you to elucidate the correct formula and accordingly, the correct geometry. You should write the formulae of precipitates as Cu(OH2)(s) and not as [Cu(OH)2(H2O)4](s). There is usually error carried forward for such questions, but not always.
Q10. For most pH calculation questions, there is no need for a specific declaration of estimation (in spite of what your anal JC teacher might insist), unless of course, specifically required by the question, in which case any clear explanatory phrasing will suffice, eg. "Because the Ka value is very small *and* the initial molarity is fairly large, hence the change in molarity x, may be considered negligible relative to the initial molarity, and concordantly it will be mathematically valid to 3 significant figures to approximate the equilibrium molarities back to the initial molarities".
Q11. No details of the underlying mechanism to explain the acid-base nature of metal vs non-metal oxides are required in the basic A level H2 syllabus (but of course, it's entirely possible for distinction-type questions, for Cambridge might go with "Suggest a mechanism to explain..." etc). For the basic A level H2 syllabus, students need only state that "metal oxides, being ionic oxides, are basic oxides; non-metal oxides, being covalent oxides, are acidic oxides ; oxides with a high degree of both covalent and ionic character are amphoteric", and of course, the balanced equation must always be written for such questions, eg. Al2O3 reacting with acids, and Al2O3 reacting with alkalis.
No prob :) -
Why is it that formation of Cu+(s) is more thermodynamically feasible than forming Cu+(aq) ?
Is it cos solid has lower energy state
-
Originally posted by Flying grenade:
Why is it that formation of Cu+(s) is more thermodynamically feasible than forming Cu+(aq) ?
Is it cos solid has lower energy state
Yes, because lattice of ionic bonds formed = exothermic = thermodynamically favorable.
Although the negative entropy change involved is thermodynamically unfavorable, the fact that you can see a precipitate generated means that the magnitude of thermodynamically favorable exothermic enthalpy change outweighs the magnitude of thermodynamically unfavorable negative entropy change, in Gibbs free energy formula. -
Thank god
-
Page 329 inorganic chem kim seng chan's green book
Can we use the 'shorthand' notation or 'conventional notation' , instead of drawing the cell diagrams for exams?
-
Originally posted by Flying grenade:
Page 329 inorganic chem kim seng chan's green book
Can we use the 'shorthand' notation or 'conventional notation' , instead of drawing the cell diagrams for exams?
No, unless specifically required by the question. -
Ultima, can u explain the 'U' and the '+' in the benzene ring?
https://www.dropbox.com/s/ul2ufrclclwiphy/Screenshot_2015-11-19-14-33-23.jpg?dl=0
https://www.dropbox.com/s/tfmaugbgthzlr2x/Screenshot_2015-11-19-14-35-24.jpg?dl=0
-
Originally posted by Flying grenade:
Ultima, can u explain the 'U' and the '+' in the benzene ring?
https://www.dropbox.com/s/ul2ufrclclwiphy/Screenshot_2015-11-19-14-33-23.jpg?dl=0
https://www.dropbox.com/s/tfmaugbgthzlr2x/Screenshot_2015-11-19-14-35-24.jpg?dl=0
It represents the resonance hybrid of the tetrahedral sp3 hybridized intermediate of the benzene ring during electrophilic aromatic substitution, in which the positive formal charge is delocalized by resonance over the ortho, para and ortho C atoms.
If you want further clarification, join my BedokFunland JC tuition (assuming you're retaking your A levels) and discuss it during tuition. Alternatively, as is the case with 99% of Singapore JC students, just "memorize it and don't ask so much" as JC teachers would instruct. -
Would the positive formal charge be delocalised over the ortho, Meta, para C atoms as well?
-
For chemical equilibria, chemical energetics, electrochem topic
Must always write state symbol? Can we not write (aq), just write (s) , or (g) when needed?
-
Originally posted by Flying grenade:
Would the positive formal charge be delocalised over the ortho, Meta, para C atoms as well?
Not meta. -
Originally posted by Flying grenade:
For chemical equilibria, chemical energetics, electrochem topic
Must always write state symbol? Can we not write (aq), just write (s) , or (g) when needed?
Yes, must write (aq). -
molar mass of C, ie. 12g.
Edited by UltimaOnline 11 Nov `15, 10:12PMWhats the distinction between molar mass and atomic mass?Cs toh book doesnt have 'molar mass'https://www.dropbox.com/s/oqelub8jotpu7o1/Screenshot_2015-11-19-16-16-23-1.png?dl=0Is it 1 mol of C atoms weigh 12g,And One carbon atom weighs 12u, i.e. 12× 1.67×10^-27kg? -
Originally posted by UltimaOnline:
Not meta.Huh?? But the picture of the intermediate, shows the U across the 3rd C too, relative to the C atom that is fully filled
-
Originally posted by Flying grenade:
molar mass of C, ie. 12g.
Edited by UltimaOnline 11 Nov `15, 10:12PMWhats the distinction between molar mass and atomic mass?Cs toh book doesnt have 'molar mass'https://www.dropbox.com/s/oqelub8jotpu7o1/Screenshot_2015-11-19-16-16-23-1.png?dl=0Is it 1 mol of C atoms weigh 12g,And One carbon atom weighs 12u, i.e. 12× 1.67×10^-27kg?Oya, i tried taking 12u x 6.02e23, it gives 0.012kg, which equals to 12g