2016 A Level H2 Chemistry Paper 2, 3, 1 discussions

last SG · 2016-11-08T20:10:45+00:00

I would think they should accept only NO2+S02=SO3+NO then regeneration bla bla blaso its like a homogeneous catalyst and Oxidising agent i would think elect...

Thefire521 SG

Hi. Saying alkanes are inert because c-h bond is strong is not a valid reason. Alkenes, for example have c=c bonds which are even stronger (in terms of bond energy), but they are more reactive.

I would more or less think there are many many ways to justify, whether is it by polarity or the fact that they have no lone pair of electrons available for donation (so rarely becomes nucleophile nor electron deficient to become electrophile...)

If you say carbon cannot expand octet, i would think they might give it to you but to think of it the same reasoning-that alkenes are also made up of carbons which cannot expand their octet, so this reason isnt very important.. (i believe students probably give this answer alot when asked why ccl4 cannot hydrolyse in water, but should not for this question)

If you did not label full set of standard conditions, then that should be 1 mark gone.

Also as for cycle, if they are lenient they will only penalise 1 mark and ecf you for knowing how to apply hess law from cycle. If they are stricter, they might decide to take away 2.

ArJoe SG

Hi, is there ecf available for deduction in qn 5??

ArJoe SG

And if i accidentally spell electrophillic instead of electrophilic from naming the reaction in the first part and naming the mechanism in the second part, they would not penalise me twice right? I hoped they didnt spot that error omg

Thefire521 SG

Hi, is there ecf available for deduction in qn 5??

QN5 is a 7 mark question so marks are awarded for each correct answer. Should be no ECF for deduction

since they asked you to name the mechanism alr, they prob wont give any more marks to naming the mechanism again in the next part...

Acerbicia SG

Damn it about the alkanes question hahaha. Couldn't really think of any other reasons, my 2nd was just to explain lack of lone pair of electrons/electron deficient atom/lack of partial positive or negative charge due to non-polarity and close electronegativity of C-H bonds. Initially split up the partial positive/negative charge as my first reason but changed it because I felt it was similar (lack of reactive sites). Hmm if those are separate points I wonder if they'll be lenient? LOL.

I finished Q3 fast simply because I have fun doing structure elucidation (reason I chose Q5 over Q1, although I regretted it in the end). I'm weird, I know.

@Fire, I feel that strength would be a valid reason as the high BDE makes any reactions energetically unfavoured thus alkanes are unreactive. Your argument about C=C bonds doesn't seem to hold well since e.g. for halohydrogenation, 1 C-Cl, C-H and 1 C-C bonds are added when removing 1 C=C bond and enthalpy change is exothermic. For alkanes, 1 C-H bond is broken to form 1 C-Cl bond and the tradeoff is endothermic. Hence alkenes are more likely to react in terms of energetics. Perhaps it would be a clearer answer to state that "C-H and C-C bonds are strong, making reactions energetically unfavourable thus alkanes are unreactive" rather than just mentioning strength.

James Pek SG

Hi, regarding the VSEPR theory, I only mentioned that the electron and bond pairs are structured in a way such that minimal repulsion occurs, do I get any mark for this? And would 65/80 be a safe A?

Acerbicia SG

Hi, regarding the VSEPR theory, I only mentioned that the electron and bond pairs are structured in a way such that minimal repulsion occurs, do I get any mark for this? And would 65/80 be a safe A?

Minimal repulsion should be 1m. My teachers say best bet for a "safe" A is at least 80% across all papers, and you've hit the mark for P3 (64/80). That's a 32/40 for P1 and 58/72 for P2. Have a feeling P1 is going to mega-screw everyone over though LOL.

UltimaOnline SG

Hi, regarding the VSEPR theory, I only mentioned that the electron and bond pairs are structured in a way such that minimal repulsion occurs, do I get any mark for this? And would 65/80 be a safe A?

1 mark out of 2 marks.

Overall (combining all papers), you need approx 75% (depending on difficulty of papers, we haven't seen P1 yet, and performance of Singapore cohort, which we won't know) to get A for H2 Chem.

65/80 is 81.25%, but that's just P3. As long as your overall is > 75% your A grade is safe. Between 70 to 75%, it's either a A or B, depending on the bell-curve for this year.

UltimaOnline SG

Have a feeling P1 is going to mega-screw everyone over though LOL.

If only. A super-killer P1 (eg. RJC Prelim P1) that might drive the A grade boundary to 70% would be lovely.

Thomas00 SG

I think indeed Paper 1 might be a killer. (Hopefully still decent) But still got time to study for that :)

Anyone found answers to chem P3, care to share? ^^

James Pek SG

Thanks for the replies! Also for question 5, regarding the different reactivities to water and Aqueous NaOH, are they asking about the ease of hydrolysis of the three compunds stated?

zenthyl SG

^ Yeah that question was weird. Didn't know which they were referring to. I just explained ease of hydrolysis of the compounds then explained that reacting with NaOH will be more reactive since OH- is a better nucleophile than water.

And omg. how to study for paper 1. LOL sucks to have a geog paper in the morning.

Thefire521 SG

And omg. how to study for paper 1. LOL sucks to have a geog paper in the morning.

Since it was 4 marks, i think there should be sufficiently enough to talk about the 3 things. And it wouldn't hurt either elaborating on NaOH

though i highly doubt paper 1 will be hard.. they know that it has the biggest time constraint in 40 minutes, which is why next year's syllabus is changed to 1 hour with the same number of questions.

They recognise the time constraint and therefore will try their best to make it as straightforward as possible

Northernblart SG

Hi for Q4 c part ii, is it each half cell has ch4 and o2 bubbled in respectively? or does one half cell need to have hydrogen gas? Also for Q1 e part i can i say its heterogenous catalysis a HI is gas and CH3OH is liquid? Or are they refering to the rhodium catalyst? Thanks so much!

Thefire521 SG

I would say Carbon Dioxide should be gassed in as well. The hydrogen ions should be in H+ 1 moldm form.

No it is homogeneous catalysis.

James Pek SG

Sorry but for Q4, why would there be hydrogen gas? Also would I be marked wrong if I wrote methane as a liquid? Completely slipped my mind that methane is a gas zz

Thefire521 SG

Sorry but for Q4, why would there be hydrogen gas? Also would I be marked wrong if I wrote methane as a liquid? Completely slipped my mind that methane is a gas zz

Yes haha if you look at the beginning of the question, they did tell you methane's boiling point. So at standard conditions of electrochem cell 298k, it is going to be a gas.

This is the tricky part about the paper expecting you to synthesize info.

Jlptj SG

For why alkanes are unreactive how many marks will i get if i say that alkanes are not reactive becos they are saturated compounds and they are non polar.

Northernblart SG

I would say Carbon Dioxide should be gassed in as well. The hydrogen ions should be in H+ 1 moldm form.

No it is homogeneous catalysis.

Ah thanks...what abt 4 part d i and ii? any clue of the answers?

Thefire521 SG

For why alkanes are unreactive how many marks will i get if i say that alkanes are not reactive becos they are saturated compounds and they are non polar.

they should give you one mark for non polar. but you should have gone on to explain why non-polar=not reactive

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