2016 H2 Chemistry JC1 & 2 students post your questions here

last SG · 2016-02-03T02:17:14+00:00

Hello, would like to ask a questionAs to why c=c is stronger than c-c bond, is it because of the additional pi bond, or because the sigma bond in c=c is shor...

UltimaOnline SG

Hi ultimaonline.

Very good, Rapidestlime. That is correct.

Ephemeral SG

RI 2015 Prelim

Paper 1

Q12. How do I infer option B?

Q36. Why is option 2 wrong? Cl is reduced frm 0 in cl2 to -1 in cl- and oxidised to +5 in cl03-, so technically it is reduced right?

Paper 2

Q4bi. Why is the rate determining step not the step involving the formation of nitronium ion? Generation of nitronium ion involves 2 neutral molecules while the next step involves a positively charged ion so isn't the reaction faster in second step?

Ephemeral SG

2015 HCI Prelim paper 1

Q25. Why is option D wrong? I mean to me both option C and D are correct...

UltimaOnline SG

RI 2015 Prelim

Paper 1

Q12. How do I infer option B?

Q36. Why is option 2 wrong? Cl is reduced frm 0 in cl2 to -1 in cl- and oxidised to +5 in cl03-, so technically it is reduced right?

Paper 2

2015 RJC P1 Q12. Vol of vessel = 2dm3, hence 0.1mol = 0.05M. Concordantly, at 4 mins, molarity increases from 0.05 to 0.10.

2015 RJC P1 Q36. Yes, but "reduced" implies "reduced only" while here Cl is "disproportionated", ie. "both reduced and oxidized". Blame RJC teacher if you disagree, but even Cambridge often 'chut' ambiguous stunts like these, and Singapore JC teachers often disagree on the best answer.

2015 RJC P2 Q4bi. Because even temporarily losing aromaticity is a hugely destabilizing, reluctant step.

2015 HCI P1 Q25. The MCQ asks for "best describes". NO2 isn't itself a greenhouse gas (that would be N2O, not NO2), it only contributes to the formation of tropospheric ozone which is a greenhouse gas (in contrast, the beneficial ozone layer refers to stratospheric ozone). Instead, NO2 pollutant is far more damaging to humans and the environment as a major contributor to acid rain.

ACA-Milkshake SG

2012 TYS Paper 2

Qns 5b(i): The answer that my school gave was that "All molecules present in the same chirality, that us, only one enantiomer is present." But can I just say that the lactic acid contains chiral carbons that's why it causes the rotation of th eplane polarised light?

UltimaOnline SG

2012 TYS Paper 2

Your school teacher's answer and your own answer are both incomplete. You also need to define what a chiral carbon is, and/or what an optically active molecule entails, as part of your answer.

Fxwhy SG

MJC 2015 Prelims Paper 2

1e) For the safety hazard, can I write chemical reagents like ethanal are corrosive when it is in contact with the skin and eyes?

3a) Where did the ans get 2/4 from? I am also quite weak at approaching this type of qns where do you start?

3d) Why doesnt the ans include bond length? How do you know when the ans requires you the write bond length or bond energy?

Will I be penalised if I write PCl5 at rtp instead of anhydrous PCl5 at rtp?

UltimaOnline SG

MJC 2015 Prelims Paper 2

1e) For the safety hazard, can I write chemical reagents like ethanal are corrosive when it is in contact with the skin and eyes?

3a) Where did the ans get 2/4 from? I am also quite weak at approaching this type of qns where do you start?

3d) Why doesnt the ans include bond length? How do you know when the ans requires you the write bond length or bond energy?

Will I be penalised if I write PCl5 at rtp instead of anhydrous PCl5 at rtp?

Q1e) Ethanal is toxic and carcinogenic, not corrosive.
https://en.wikipedia.org/wiki/Acetaldehyde#Toxicity

Q3a) Instead of blindly memorizing and applying a mathematical formula, use common sense. For a uninegatively charged species : when mass is 37g, deflection is merely -1 degrees. Hence when mass is 1g (ie. much lighter), deflection should concordantly be -37 degrees (ie. larger magnitude deflected). Since mass of He is 4g, hence deflection should be -9.25 degrees (if uninegatively charged). Since He nuclei is dipositively charged, hence deflection should be +18.5 degrees.

Q3d) Bond length here is only indirectly relevant (since bond length is inversely proportional to bond strength, ceteris paribus). It is bond strength which is directly relevant. Hence, there is no need to (ie. no marks awarded for) mentioning bond length.

Q4ai) It's ok if you don't specify "anhydrous", as long as you do *NOT* specify aqueous (aq) (which would be marked wrong).

Same for all reagents which need to be anhydrous, eg. LiAlH4 reducing agent, acyl halides for nucleophilic acyl substitution, Grignard nucleophilic & reducing reagents, AlCl3 or FeBr3 Lewis acid catalysts for halogenation via electrophilic aromatic substitution and for Friedel-Crafts alkylation & acylation, etc.

LavXuan SG

(George Chong Inorganic Chemistry pg 4) when to apply I3- (aq) and I2 (aq) ?

UltimaOnline SG

(George Chong Inorganic Chemistry pg 4) when to apply I3- (aq) and I2 (aq) ?

Pg 4 is the contents page lah.

When you carry out *oxidation* of the iodine, you can oxidize either I- or I3- into I2 (balance accordingly). When you carry out *reduction* of the iodine, you can reduce I2 into I- or I3- (balance accordingly). Depends on which formula the question uses. But both are correct stoichiometrically, so Cambridge will accept either (do you understand why?).

3.1415926525 SG

yes

yulemugging SG

I love chem

MrKelvinOng SG

I love chemistry!

Ng.keebin SG

In grp ll, there is greater covalent character in mgcl2 than those below it but in group Vll, there is greater covalent character in silver astatide than in silver iodide though astatine is below iodine. Is it something about those cations in group ll having smaller radius and hence higher polarising power while for the anions under group Vll, larger anions are more polarisable...?

UltimaOnline SG

Correct.

Because Group II or 2 ions are the cations with the polarizing power, hence the higher the cationic charge density and concordantly the greater the polarizing power up the Group, the higher the covalent character for the ionic compound.

Conversely Group VII or 17 ions are the anions with the polarizable electron charge clouds, hence the lower the anionic charge density and concordantly the greater the polarizability down the Group, the higher the covalent character for the ionic compound.

Flying grenade SG

Correct.

AlI3 has weaker Al-hal bond(since more covalent) compared to Alcl3?

UltimaOnline SG

AlI3 has weaker Al-hal bond(since more covalent) compared to Alcl3?

That, plus less effective overlap of hybridized orbitals to form the sigma bond due to more diffused valence orbitals of the halogen atom down the Group.

Flying grenade SG

so AlCl3 has lower BP, but AlI3 is more susceptible to thermal decomposition?

UltimaOnline SG

so AlCl3 have lower BP than AlI3 (since less electrons, less extensive id-id forced of attn) ,but AlI3 more susceptible to sublime/undergo thermal decomposition due to weaker Al-hal bond?

Partly right, partly wrong. The chemistry (including physical properties) of the different aluminium halides are not so straightforward (hence not in H2 syllabus), and will be confusing for A level students.

If you're still interested, go through with your private tutor or school teacher.

Flying grenade SG

nice! ! Thanks , Ultima.

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