Lurkers, register with SgForums and ask ur H2 Chem qns here!
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Originally posted by theophilus:
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. -
Wah 70+, okay wow. But thanks so much for the tip friend! good to hear opinions from a veteran. CHEERS!
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We are a group of students from MAD School trying to get survey answered for our marketing project.
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2012/p3/qn 2aiii serious conceptual qn when we construct the electrochemical cell fully already, there will be a emf value shown on the voltmeter, doesn't this mean redox reaction already take place? why in part iii, when qn say 'write the full balanced redox reaction that would take place if the electrodes of the cell were connected by an external circuit' , as if the reaction will only happen after connected to an external circuit ,and hence the full redox balanced eqn can only be written after connecting to an external circuit?? also, what does connecting the electrodes to an external circuit means? change from an electrochemical cell to an electrolytic cell?? the electrochemical cell is already complete, not sure why need 'connect to an external circuit' ? https://www.dropbox.com/s/h69nei3bcfvmcom/20160922_090807-1.jpg?dl=0
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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
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2015 p2 h2 chem cl2 increase rate of rxn qn https:// en.m.wikipedia.org/wiki/Chain_reaction http://www.chemguide.co.uk/mechanisms/freerad/ch4andcl2.html . yes, FRS is a chain reaction is what my cher say true?he said that if 2 cl radicals forms in the beginning ,there will be only 2 cl atoms substituted in the end. means it's not like nuclear fission, whereby one U-238 splits to give 3 neutrons and as a result hit three more U-238 to give a total of 9 neutrons in the 2nd stage does chain reaction means no. of reactants increase? my jc school teacher say *Cambridge examiner reports(CER) says* : *Since the rxn is exo, the heat involved increases the temp of environment , hence, results in an increase in the rate of rxn* (CER/answer didnt include the increasing number of cl• radicals, nor discussed it. based on my memory, take it with a pinch of salt. Frustratingly they didnt print for us as they say they couldn't, and they only show us for awhile, i couldn't take photo nor copy down in time. But i did finish reading it but did not retain 100%, no photographic memory, but did retain more than 80% ) initiation: uv light provides energy to overcome the ea for the homolytic fission of cl2 to form cl• radicals [endo] propagation overall endo or exo? termination [exo] whole FRS overall is Exothermic? Thanks God Ultima for settling and addressing difficult questions for us
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my cher also says since rate of rxn = k [reactants]^× , since [reactants] are constant, rate proportional to k, which is affected by Temp.
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2012 p3 qn 2eii either base or acid hydrolysis of phenobarbital the product have CO2 and NH3 but no H2O right?
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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.
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Originally posted by Flying grenade:
2012/p3/qn 2aiii serious conceptual qn
when we construct the electrochemical cell fully already, there will be a emf value shown on the voltmeter, doesn't this mean redox reaction already take place? as if the reaction will only happen after connected to an external circuit ,and hence the full redox balanced eqn can only be written after connecting to an external circuit??also, what does connecting the electrodes to an external circuit means?
change from an electrochemical cell to an electrolytic cell??
the electrochemical cell is already complete, not sure why need 'connect to an external circuit' ?
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*. -
Originally posted by Flying grenade:
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
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. -
Originally posted by Flying grenade:
2015 p2 h2 chem cl2 increase rate of rxn qn
https:// en.m.wikipedia.org/wiki/Chain_reactionhttp://www.chemguide.co.uk/mechanisms/freerad/c... . yes, FRS is a chain reactionis what my cher say true?he said that if 2 cl radicals forms in the beginning ,there will be only 2 cl atoms substituted in the end. means it's not like nuclear fission, whereby one U-238 splits to give 3 neutrons and as a result hit three more U-238 to give a total of 9 neutrons in the 2nd stagedoes chain reaction means no. of reactants increase?my jc school teacher say Cambridge examiner reports(CER) says : Since the rxn is exo, the heat involved increases the temp of environment , hence, results in an increase in the rate of rxn(CER/answer didnt include the increasing number of cl• radicals, nor discussed it. based on my memory, take it with a pinch of salt. Frustratingly they didnt print for us as they say they couldn't, and they only show us for awhile, i couldn't take photo nor copy down in time. But i did finish reading it but did not retain 100%, no photographic memory, but did retain more than 80% )initiation: uv light provides energy to overcome the ea for the homolytic fission of cl2 to form cl• radicals [endo] propagation overall endo or exo? termination [exo] whole FRS overall is Exothermic?Thanks God Ultima for settling and addressing difficult questions for us
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).
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Originally posted by Flying grenade:
my cher also says since rate of rxn = k [reactants]^× , since [reactants] are constant, rate proportional to k, which is affected by Temp.
Obviously. -
Originally posted by Flying grenade:
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. -
Originally posted by UltimaOnline:
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*.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
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Originally posted by UltimaOnline:
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).
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)
OR
the increasing number of radicals reason is wrong, i.e. number of radicals is constant
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Originally posted by Flying grenade:
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
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)
OR
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. -
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•
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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?
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Originally posted by Flying grenade:
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. -
Why can ethanedioic acid be oxidised?
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how is C-H(in HCFCs) more reactive than C-Cl or C-F(in CFCs) ???
BE(kjmol-1) C-F 486 , C-H 410, C-Cl 340
https://www.dropbox.com/s/jod3jnsesmr9c7m/20160923_170437-1.jpg?dl=0
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Originally posted by 8truthseeker8:
Why can ethanedioic acid be oxidised?
Because of the high OS of both C atoms in it, and the high ratio of O to C in it. -
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