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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...

UltimaOnline SG

Oh no as in not fixed chem questions, but like they have only fixed library of experiments for the a level syllabus? I actually have the planning book and there are only 10 experiment types. Generally these 10 will do?

haha how not to! bell curve man, but yeah la I guess will have to theoretically study practical

Each of the 10 types in the book, is further subdivided into many different individual experiments, for a total of approx 70+ experiments. So yeah, Chan Kim Seng's Planning Book covers all the possible experiments that can be asked at A levels.

theophilus SG

Wah 70+, okay wow. But thanks so much for the tip friend! good to hear opinions from a veteran. CHEERS!

MADstud SG

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Flying grenade SG

2012/p3/qn 2aiii serious conceptual qn

https://www.dropbox.com/s/h69nei3bcfvmcom/20160…

Flying grenade SG

why H2 is more flammable than CH4? what determines flammability of a gas or liquid?
is it H2 more volatile?
lower Ea to combust?
B.E. H-H less than 4xB.E. CH4?
enthalpy change combustion deltaHc(H2) = -286kjmol-1
deltaHc(CH4) = -818kjmol-1

Flying grenade SG

2015 p2 h2 chem cl2 increase rate of rxn qn

Flying grenade SG

my cher also says since rate of rxn = k [reactants]^× , since [reactants] are constant, rate proportional to k, which is affected by Temp.

Flying grenade SG

2012 p3 qn 2eii
either base or acid hydrolysis of phenobarbital
the product have CO2 and NH3 but no H2O right?

UltimaOnline SG

I count over 20 question marks in 5 posts. That’s considered spamming, you coconaden. I’ll address your posts only after midnight. If you want a faster reply in future, make just 1 post at a time, and/or less multiple sub-questions per post.

UltimaOnline SG

2012/p3/qn 2aiii serious conceptual qn

https://www.dropbox.com/s/h69nei3bcfvmcom/20160…

Before you connect the 2 half-cells, no redox reaction occurs. The moment you connect the wire (including voltmeter), then the redox reaction begins *at that moment), which is why you immediately see a voltmeter reading *at that moment*.

UltimaOnline SG

Yes, lower Ea, due to less covalent bonds required to be broken. Being more volatile helps, as the heat from combustion can more readily vaporize the species being combusted, which (being gaseous) in turn allows for more access to atmospheric oxygen during combustion process.

UltimaOnline SG

2015 p2 h2 chem cl2 increase rate of rxn qn

You're confusing answering the question "Why only a short flash of UV light required" versus the question "Why rate of reaction increases".

The former has to do with free radical substitution being a chain reaction ensuring the continued existence (ie. propagation) of free radicals as a result of the propagation steps. The latter has to do with the fact that stronger bonds are formed while weaker bonds are broken, and hence the reaction is exothermic (which is why iodination does not occur, because the reaction is endothermic).

UltimaOnline SG

my cher also says since rate of rxn = k [reactants]^× , since [reactants] are constant, rate proportional to k, which is affected by Temp.

Obviously.

UltimaOnline SG

2012 p3 qn 2eii
either base or acid hydrolysis of phenobarbital
the product have CO2 and NH3 but no H2O right?

Acidic hydrolysis will generate H2O (if you dunno why then *facepalm*). Alkaline hydrolysis will not (if you dunno why then *facepalm*). But Cambridge doesn't care if you include H2O or not (if you dunno why then *facepalm*) when answering this question.

Flying grenade SG

ok i know why liao.

part 1 merely says what set up would you use, and draw.

part 2 just say what kind of measurements u will make, and explain the significance.

so part 3 just say what reaction will occur when the electrodes were connected to an external circuit(Which just simply means you finish setting up the electrochem cell setup now) and write the balanced chemical eqn.

cos part 1 involves drawing of the full set up and part 2 involves writing of the half eqns when answering the qn

so i find it strange and i was confused when asked to write the full balanced chemical eqn only in part 3

ok got it liao thanks ultima

Flying grenade SG

You're confusing answering the question "Why only a short flash of UV light required" versus the question "Why rate of reaction increases".

ok also cos the 2015 chem paper thread writes an increasing no. of radicals, as well as tys publishers

2015 sgforum thread :

seems like cambridge wants the rxn to be exothermic rxn increases rate of rxn.

seems like not many ppl thought about this

i guess the increasing no. of radicals reason is logical and reasonable. so cambridge might accept it. i didnt rmb fully what the Cambridge examiner report wrote. i only rmb they wrote the emphasis on exo, and also writes 'few candidates mentioned exo' . didnt write about increasing no. of radicals. so it could be right or wrong. cos for other qns, the report did write about some wrong answers written by candidates

So now, it could be the increasing no. of radicals reason is correct, and hence cambridge accept (so my sch cher wrongly say [cl•] or [rxts] is constant). And that's why im unsure whether does number of cl• stay constant or not(want to know the real and correct chemistry and happening ), after that one-time exposure to bright light (as opposed to in outdoors in sunlight, constantly exposed and hence radicals can continually form)

the increasing number of radicals reason is wrong, i.e. number of radicals is constant

UltimaOnline SG

ok also cos the 2015 chem paper thread writes an increasing no. of radicals, as well as tys publishers

2015 sgforum thread :

it's not the H radical, it's the Cl radicals than propagate the existence of radicals. Due to the large no. of different possible propagation reactions, free-radical concentrations will continue to increase, hence increasing rate of reaction, until the rate of termination reactions catch up.

This explanation seems really legit and chemically correct

seems like cambridge wants the rxn to be exothermic rxn increases rate of rxn.

seems like not many ppl thought about this

the increasing number of radicals reason is wrong, i.e. number of radicals is constant

No, this is my bad. My 2015 post is in error. I've just deleted it away. Based on stoichiometry, for every 1 free radical generated, 1 radical is also used up in each of the propagation steps. The initial flash of UV light generates a fixed no. of free radicals, and the propagation steps ensures this no. of free radicals remain constant, until the termination steps slowly uses them up.

Flying grenade SG

ok thanks GODDDDD!!!

SAP ans is wrong for this qn, as it writes increasing no. of free radicals generated.

yes, for every one free radical generated, e.g. H• , in this rxn between H2 and Cl2 , one Cl• is used up

initiation : Cl2 (under UV) -> 2Cl•

propagation : H2 + Cl• -> HCl + H•

Flying grenade SG

so no H• radical generated only when Cl2 reacts with Hydrocarbons e.g. CH4 under UV light (because weaker C-Cl bond preferentially clevaed compared to C-H bond)

in this case, its cl2 and h2, so no choice, H• is formed

for the propagation step CH4 + Cl• -> CH3• + HCl, isn't C-H bond cleaved?

when if asked why no H radical formed, my above reason correct right?

but the propagation step above has it being cleaved

ok does it mean like this, after any chlorinated molecule formed, CH3Cl or CH2Cl2, C-Cl bond preferably cleaved than C-H bond

for tri-substitution to form CCl3H and

tetra substitution to form CCl4 , no H• generated also?

UltimaOnline SG

so no H• radical generated only when Cl2 reacts with Hydrocarbons e.g. CH4 under UV light (because weaker C-Cl bond preferentially clevaed compared to C-H bond)

in this case, its cl2 and h2, so no choice, H• is formed

for the propagation step CH4 + Cl• -> CH3• + HCl, isn't C-H bond cleaved?

when if asked why no H radical formed, my above reason correct right?

but the propagation step above has it being cleaved

ok does it mean like this, after any chlorinated molecule formed, CH3Cl or CH2Cl2, C-Cl bond preferably cleaved than C-H bond

for tri-substitution to form CCl3H and

tetra substitution to form CCl4 , no H• generated also?

For A level purposes, H• is not generated during halogenation via free radical substitution. If you've further questions for this topic, go ask your school teacher or private tutor.

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