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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

White Dust SG

Can I ask questions from my university Chemistry modules here too?

I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.

Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?

UltimaOnline SG

Can I ask questions from my university Chemistry modules here too?

I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.

Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?

Sorry, all Chemistry qns beyond the Singapore A level H2 Chemistry syllabus should be asked on ChemicalForums.com instead :
http://www.chemicalforums.com/

Just as some people will find Physics naturally easier than Biology, some others will find Biology naturally easier than Physics. Every soul is unique, with unique interests, strengths and (if you're free-thinking enough to appreciate this) unique past life backgrounds and soul karma. Infinite Diversity in Infinite Creation, as the Vulcans (from Star Trek) would say.

Of course, it's entirely possible that you're exactly equally excellent (or sucky) at both Physical and Organic Chemistry, but that's pretty rare (fact is, almost everyone has a personal preference between dogs and cats; very few people love or hate both dogs and cats exactly equally).

To make a prudent and judicious decision, you should explore the syllabus details of both alternative modules before making a decision. But I leave you with a (rather obvious) tip : if you're more Physics and Mathematics inclined, you'll enjoy Physical Chemistry better ; If you're more Biology and Medicine inclined, you'll enjoy Organic Chemistry better.

Personally, at the Uni level, I much preferred (and still do prefer) Organic Chemistry over Physical Chemistry.

qdtimes2 SG

Can I ask questions from my university Chemistry modules here too?

I don't major in Chemistry but I do have to take a couple of those modules to fulfill the elective requirements.

Also, assuming I had to choose either Organic Chemistry (CM1121) or Physical Chemistry (CM1131), would it be correct to say that Organic Chemistry would be a lot more dificult?

I personally agree that CM1121 is more difficult. It is based on some understanding of thermodynamics and kinetics (CM1131) and even greater understanding of Atomic Orbitals, HOMO-LUMO Theory, HSAB Theory etc (CM1111 Inorganic I). However there are other factors to consider as well, like workload, bell-curve, lesson plan, grading and exam formats etc.... If you are really interested I can tell you more about these on CM1131 and CM1121 here.

I'm a Chem Major and I think majority of my peers are more of the memory-workers, and this observation probably extends to batches after and before mine. So in this case, CM1131 is more scorable. Most people would feel that CM1131's contents is boring though.

p.s.I like Inorg and Org Chem much more but I suck much more at both.

Light5 SG

http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_11.pdf

^ How to solve Q28 from this paper, the one about ladenberg structure ....i cant find a way to name the isomers drawn ..... and how to know whether the isomers are distinct from each other or not.?

http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_12.pdf

^ Q30...there is an ester linkage that will break to form CO2Na but what about the CON linkage.....i havent done this linkage in AS chemistry so how can i know whether it will hydrolyse or not?

^Q18 of the same paper: Dosent CO oxidise to CO2 in the atmosphere naturally, i cant find it written in any book that CO dosent oxidise spontaneously in the atmosphere?

^Q37 of the same paper: To me, it seems none of the C atoms in all 3 structures lie in the same plane...so how to solve this?

Following are the mark schemes for both papers :

http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_11.pdf

http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_12.pdf

Thanks a lot !

UltimaOnline SG

Q28. bottomleft + topleft ; bottomleft + bottomright ; bottomleft + topright.

Q30. Google "hydrolysis of amide".

Q18. CO is *eventually* oxidized to CO2 in the atmosphere, not *spontaneously*. Spontaneity implies both kinetic feasibility as well as thermodynamic feasibility. Here, while it's thermodynamically feasible to oxidize CO to CO2, the activation energy is fairly high. Nonetheless, I agree that this is one of those lousy Cambridge MCQs where more than 1 option might be arguably acceptable.

Q37. Options 1 and 2 include at least two sp2 C atoms (with 2 sp3 C atoms). Option 3 has only one sp2 C atom (with three sp3 C atoms) ; sp2 C atoms are planar, while sp3 C atoms are tetrahedral, ie. non-planar.

No problem, good that you provide links to both Qns and Mark Schemes for ease of discussion.

Light5 SG

General question : What would be the equation for the reaction of ICl (iodo chloride) with KBr ....now Cl is more reactive than Br so it should replace it but Iodine in ICl isnt so would I be converted to I2 or will it remain in ICl.....?

UltimaOnline SG

I-Cl may be conceptualized as I+ and Cl-, due to the greater electronegativity of Cl over I. Br- from KBr is a reducing agent (since it's electron rich), and can be oxidized to Br2, while reducing I+ to I2.

I+ from I-Cl is an oxidizing agent (since it's electron deficient) and can be reduced to I2, while oxidizing Br- to Br2.

Br- isn't able to further reduce I2 to I-, because I- is itself a stronger reducing agent (due to having a larger ionic radius and hence also having a more positive oxidation potential compared to Br-).

I+ isn't able to further oxidize Br2 to positive OSes of +1, +3, +5 or +7, which requires a stronger oxidizing agent and/or self-disproportionation under alkaline conditions and/or heating.

Hence the overall reaction for this question should be 2 I-Cl + 2 KBr --> I2 + 2 KCl + Br2.

Light5 SG

Q1.)

a.) How the interchain hydrogen bonds in Nomex differ from those in Kevlar?

b.) How might the flexibility of Nomex compare to that of Kevlar?

Q2.) I know that cycloalkenes from C3 to C7 dont exhibit cis-trans isomerism due to ring strain but CycloOctene does show cis trans isomerism. However, if , lets say CylcoOctene was attached with other rings in a larger structure, will it still exhibit cis trans isomerism?

Q3.) From which book or website can i read about how different groups attached to benzene affects its reactivity with certain compounds and how to predict whether these groups will be 2,4 directing or 3-directing?

Thank You.

UltimaOnline SG

Q1.)

a.) How the interchain hydrogen bonds in Nomex differ from those in Kevlar?

b.) How might the flexibility of Nomex compare to that of Kevlar?

Thank You.

Q1. The hydrogen bonds in Nomex are longer, more strained, and hence weaker compared to those in Kevlar ; hence Nomex possesses greater flexibility compared to Kevlar.

Q2. Depends on the larger structures and the other rings to which cyclooctene was attached to. If the cis isomer generates an unacceptable level of ring strain in the other connected rings, then it's a no-go.

Q3. https://www.khanacademy.org/science/organic-chemistry/aromatic-compounds/deffect-2013-07-03T14:44:54.851Z/v/ortho-para-directors-i

BCME SG

With reference to JJC 2014 P3 Q2(c)(i),

Can we say that due to more electron donating akyl groups on TEA, the lone pair of electron on N is more available for hydrogen bonding as compared to DEA?

UltimaOnline SG

With reference to JJC 2014 P3 Q2(c)(i),

Can we say that due to more electron donating akyl groups on TEA, the lone pair of electron on N is more available for hydrogen bonding as compared to DEA?

No, as that's an insignificant point relative to the more relevant points required by the mark scheme.

However, the JJC mark scheme can nonetheless be further improved in 2 aspects :

1) Intramolecular hydrogen bonding is possible for both species, hence you should not use it as a reason for why TEA has a higher boiling point than DEA.

2) To explain the more extensive hydrogen bonding between TEA molecules : while both DEA and TEA have 3 protic groups that can function as both hydrogen bond donors and acceptors, but only TEA has an additional 4th group that can function as a hydrogen bond acceptor.

Mrworry SG

Regarding Organic chem made easy by George Chong

Page 228, Example 7.17

For isomer III why CH3CH2CH(OH)CH3 is not allowed?

Xie xie ~

UltimaOnline SG

Regarding Organic chem made easy by George Chong

Page 228, Example 7.17

For isomer III why CH3CH2CH(OH)CH3 is not allowed?

Xie xie ~

Because your described structure fulfills the conditions for isomer I, and the question describes isomers I, II and III as *different* structures, hence the use of the term "isomer".

Light5 SG

Q1.)

(a) State the colour you would expect for the interhalogen compound BrCl

(b.) Predict the equation of the reaction between BrCl (interhalogen) and AgNO3.

(c.) Describe what you would observe when the products of (a) are added to Dilute NH3 in a test tube.

Q2.) http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_11.pdf

Please explain how to solve Q13 and Q24 from the above paper.

Mark Scheme : http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_11.pdf

Thank You !

UltimaOnline SG

Q1.)

(a) State the colour you would expect for the interhalogen compound BrCl

(b.) Predict the equation of the reaction between BrCl (interhalogen) and AgNO3.

(c.) Describe what you would observe when the products of (a) are added to Dilute NH3 in a test tube.

Q2.) http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_11.pdf

Please explain how to solve Q13 and Q24 from the above paper.

Mark Scheme : http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_ms_11.pdf

Thank You !

Where did you get the interhalogen qn from? Is it a CIE 'A' level Chem qn, or a school's exam qn (if so, which country is the school based in)?

Q1a) Somewhere between the colours of Cl2 (yellow-green) and Br2 (reddish-brown).

Q1b) Although AgBr is less soluble than AgCl, but due to the greater electronegativity of Cl over Br, we have Br+ (not Br-), hence the products would be AgCl(s) and (covalent) BrNO3. BrCl is a strong oxidizing agent, but neither Ag+ nor NO3- (with N already in it's most positive OS) can be readily oxidized (it is difficult to oxidize Ag+ and O atoms having OS of -2).

Q1c) The AgCl ppt would dissolve in NH3(aq), as the soluble ionic diamminesilver(I) coordination complex is generated. The Br atom of BrNO3 would also be nucleophilically attacked by NH3 and H2O, but the reaction products are not readily observable (hence irrelevant to the question).

Q13. This qn tests you on the thermal stabilities of Group 2 carbonates (determined by cationic charge densities). As a fire retardant, you would want two things : greater moles of CO2(g) generated per mole of retardant, and lower thermal decomposition temperature to release the CO2(g) more readily.

Q24. The mechanism involves 2 x nucleophilic substitution (first SN2 on the less sterically hindered terminal C atom, followed by deprotonation, then intramolecular SN2 or SN1 attack on the 5th C atom, followed by deprotonation) of the Br atoms on the 1st and 5th C atoms, by the NH3 nucleophile, to generate the required product Coniine.

Light5 SG

Where did you get the interhalogen qn from? Is it a CIE 'A' level Chem qn, or a school's exam qn (if so, which country is the school based in)?

Q1b) Although AgBr is less soluble than AgCl, but due to the greater electronegativity of Cl over Br, we have Br+ (not Br-), hence the products would be AgCl(s) and (covalent) BrNO3. BrCl is a strong oxidizing agent, but neither Ag+ nor NO3- (with N already in it's most positive OS) can be readily oxidized (it is difficult to oxidize Ag+ and O atoms having OS of -2).

Q1c) The AgCl ppt would dissolve in NH3(aq), as the soluble ionic diamminesilver(I) coordination complex is generated. The Br atom of BrNO3 would also be nucleophilically attacked by NH3 and H2O, but the reaction products are not readily observable (hence irrelevant to the question).

Q13. This qn tests you on the thermal stabilities of Group 2 carbonates (determined by cationic charge densities). As a fire retardant, you would want two things : greater moles of CO2(g) generated per mole of retardant, and lower thermal decomposition temperature to release the CO2(g) more readily.

Sorry if the interhalogen question was vague....i actually constructed the question by myself because cambridge has a knack of asking weird questions from time to time. They have already tested displacement reactions of interhalogen compounds so i wondered if they could test a question like this as well.

In q1(c) , we would obviously have a colourless test tube as all nitrates are soluble and AgCl is also soluble but what would be the products of the nucleophilic attack of NH3 and H2O on BrNO3?

In q13(from the CIE paper), how can i predict the temperature required for decomposition of the 3 fire retardants since there are 2 Group 2 elements in one fire retardant.

Thanks Again.

UltimaOnline SG

In q1(c) , we would obviously have a colourless test tube as all nitrates are soluble and AgCl is also soluble but what would be the products of the nucleophilic attack of NH3 and H2O on BrNO3?

In q13(from the CIE paper), how can i predict the temperature required for decomposition of the 3 fire retardants since there are 2 Group 2 elements in one fire retardant.

Thanks Again.

No prob.

Q1c) NH2Br + HNO3, and BrOH + HNO3, all of which are colourless.

Q13. Each metal carbonate will decompose according to their own temperature, and you can treat each mineral as a mixture of two separate metal carbonates. Eg. For Mg3Ca(CO3)4, magnesium carbonate decomposes first, followed by calcium carbonate. Hence BaCa(CO3)2 is the worst fire retardant because Ba2+ has the lowest cationic charge density and thus requires the highest temperature to decompose, so by the time sufficient CO2(g) is released, your house would have burnt down completely.

Mrworry SG

Hi again :D

For Organic chem made easy page 258,

Example 8.4, part b)

Why cannot it be:

1. Add conc H2SO4(aq) 170C

2.Cl2 (aq), heat

3. Add Ethanolic KCN, heat

4. HCl (aq), reflux

xie2=3

UltimaOnline SG

You really love George Chong's book, doncha? He's busy climbing Mont Blanc right now even as we type, but up there in the lonely peaks of Mont Blanc, he'll surely appreciate the support of a loyal fan! ;Þ

Your proposed synthesis pathway will score some marks, but not full marks.

You'll be penalized *mostly* because your proposed synthesis pathway isn't the shortest possible (ie. George Chong's answer). Always give the shortest possible pathway, because using a longer-than-necessary synthesis pathway will result in a lower atom economy and/or equilibrium yield, and hence will always be penalized by Cambridge. (But hey, if you can't think of any shorter pathway, getting some marks is certainly preferable to getting no marks, so go ahead and just write the shortest pathway you can think of, even if it's 1 or 2 steps longer than specified by the qn).

In addition (this is a minor point), although your step 2 will indeed obtain the required intermediate (used in your step 3) as the major product, but nonetheless there will still be a mixture of products, which should still be avoided in synthesis qns whenever possible (it's perfectly fine to use it when there's no other choice, but in this qn, there *is* another, even shorter pathway, ie. George Chong's answer).

But I emphasize that the main problem with your answer is that it isn't the shortest possible synthesis pathway. If it had the same no. of steps as the shortest possible pathway, then Cambridge (in most cases) wouldn't penalize you for using electrophilic addition of aqueous halogen. No worries.

On this note, using halogenation via free radical substitution will always get you penalized for synthesis qns (as it generates a variety of products, hence low overall atom economy which means wasteful on resources), unless there is absolutely no other choice for that qn.

Hi again :D

For Organic chem made easy page 258,

Example 8.4, part b)

Why cannot it be:

1. Add conc H2SO4(aq) 170C

2.Cl2 (aq), heat

3. Add Ethanolic KCN, heat

4. HCl (aq), reflux

xie2=3

Light5 SG

Q.) http://gceguide.com/papers/A%20Levels/Chemistry%20(9701)/9701_s15_qp_11.pdf

In q 6 of the above paper, if volume is increasing from 1 to 2 , then pressure should decrease therefore equilibrium must shift to the RHS to increase No of Moles of PCl3 and Cl2 and decrease No of Moles of PCl5. This is reflected in both answer choices A and B yet the examiner has given B as the answer in the marking scheme? Can you please clarify this?

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