Lurkers, register with SgForums and ask ur H2 Chem qns here!
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Originally posted by Flying grenade:
ok dafuq, means we do tick the option as Correct in the exam, that anhydrous socl2 will result in a decrease in pH of the soln , even though we know that it won't, in a inert soln(e.g. ccl4)? force yourself get confused like this, not healthy
and is it like Since we know socl2 or pcl5 must react in anhydrous condition, if Cambridge present the question similar to above, do u think they expect us to think that the solvent must be inert?
wow, i hope they want us to select the correct Chemistry option, if this unfortunate event happens. i will take my chances and gamble or give or take that 1 mark away and shade the correct Chemistry answer
Eh don't OCD and worry so much lah. It's a bell-curve afterall. If it's not fair to everyone, it's still fair. Be like me, any ambiguous qn like these, just make a relaxed, stress-free educated guess on what Cambridge wants (based on what Cambridge thinks A level students know) and make ur choice, or (if u're really good enough like me), skip the qn also still can get A grade. *evil laugh* -
okay thanks Ultima. you are my inspiration
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Ex-RJC students post their condolences on Facebook RJC Confessions, and hope to guide current RJC students stressed out with life.
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means the cobalt form covalent bond with both the O atoms? in some organometallic complex like these, the central transition metal share e- to form covalent bond , instead of donating it as in ionic bonding
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https://www.dropbox.com/s/z3toihywdv16i6j/20160914_122128-1.jpg?dl=0
for test using barium nitrate, why need acidify?
ok got it. cos white ppt might be zinc hydroxide . zinc hydroxide dissolves in excess hno3 when OH- not enough such that I.P. > ksp.
why need to test if the sulfate solid salt that is white in color, will decompose under heat or not?
oh cos if it's PbCl2, PbCl2 can decompose, or rather, more likely/easier to decompose compared to PbSO4?
so if test for SO42- using pb(no3)2 , its like both pbcl2 and pbso4 will be precipitated. both are white in color. then u heat it. then after pbcl2 all thermally decomposed alr, then if have a white solid remaining that does not decompose under heat, is pbso4?
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Originally posted by Flying grenade:means the cobalt form covalent
bond with both the O atoms?
in some organometallic complex like these, the central transition metal share e- to form covalent bond , instead of donating it as in ionic bonding
Covalent with ionic character. -
Originally posted by Flying grenade:
https://www.dropbox.com/s/z3toihywdv16i6j/20160914_122128-1.jpg?dl=0
for test using barium nitrate, why need acidify?
ok got it. cos white ppt might be zinc hydroxide . zinc hydroxide dissolves in excess hno3 when OH- not enough such that I.P. > ksp.
why need to test if the sulfate solid salt that is white in color, will decompose under heat or not?
oh cos if it's PbCl2, PbCl2 can decompose, or rather, more likely/easier to decompose compared to PbSO4?
so if test for SO42- using pb(no3)2 , its like both pbcl2 and pbso4 will be precipitated. both are white in color. then u heat it. then after pbcl2 all thermally decomposed alr, then if have a white solid remaining that does not decompose under heat, is pbso4?
Unlike silver(I) halides, lead(II) halides are soluble in hot water. -
if Cambridge ask us to write eqn with state symbols including p4o6,
if we write (l) do we get penalised?
from wiki, m.p. p4o6 = 23.8°c , so p4o6 exists as liquid in r.t. liao
cs toh book writes it as p4o6 in solid state
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why does most ppt dissolve in higher temp? as temp increase, posn of eqm shift right, favours dissociation?
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Originally posted by Flying grenade:
if Cambridge ask us to write eqn with state symbols including p4o6,
if we write (l) do we get penalised?
from wiki, m.p. p4o6 = 23.8°c , so p4o6 exists as liquid in r.t. liao
cs toh book writes it as p4o6 in solid state
By convention, use (s). If u wish, u can write (l) with an annotation that "since m.p. p4o6 = 23.8°c". But bear in mind that "room temp" can be either 20°C or 25°C in Chemistry (hence should always specify exactly which). -
Originally posted by Flying grenade:
why does most ppt dissolve in higher temp? as temp increase, posn of eqm shift right, favours dissociation?
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Originally posted by UltimaOnline:
nice
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Originally posted by UltimaOnline:
By convention, use (s). If u wish, u can write (l) with an annotation that "since m.p. p4o6 = 23.8°c". But bear in mind that "room temp" can be either 20°C or 25°C in Chemistry (hence should always specify exactly which).oh wow ok i see, thanks god !
yes, should always indicate. my school trains all of us always write standard conditions, of 298K and 1 atm.
cs toh writes standard cond as 25°c, 1atm.
https://en.m.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure
writes
- standard temperature and pressure(STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 100 kPa (1 bar).
- standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of exactly 100 kPa (1 bar).
chemguide writes1 mole of any gas occupies 22.4 dm3at stp (standard temperature and pressure, taken as 0°C and 1 atmosphere pressure). You may also have used a value of 24.0 dm3 at room temperature and pressure (taken as about 20°C and 1 atmosphere).http://www.chemguide.co.uk/physical/kt/idealgases.html -
LiAlH4 can reduce everything except alkenes
NaBH4 can only reduce carbonyl compounds
H2, Ni , heat or H2,Pt can reduce everything(except carboxylic acid? idk?) ?
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2009 p3 qn 1a bi)
given pka of glutamic acid is 4.3
of course many of us know the COOH R group of glutamic acid will be deprotonated above pH of 4.3
i know pH above 4.3 is alkaline relative to the pka of 4.3 of glutamic acid
but i cant answer when my friend ask me why
why a pH above the pka of an acidic(cooh) group will cause it to be deprotonated?
no good definition of pka online
how to answer the above question using chemistry concepts using chemistry terms?
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Originally posted by Flying grenade:
2009 p3 qn 1a bi)
given pka of glutamic acid is 4.3
of course many of us know the COOH R group of glutamic acid will be deprotonated above pH of 4.3
i know pH above 4.3 is alkaline relative to the pka of 4.3 of glutamic acid
but i cant answer when my friend ask me why
why a pH above the pka of an acidic(cooh) group will cause it to be deprotonated?
no good definition of pka online
how to answer the above question using chemistry concepts using chemistry terms?
For every acidic group (this includes the conjugate acids of basic groups), there will be a specific pH of the solution in which you've equal molarities of conjugate acid and conjugate base (for this particular acidic group), correct? (this is common sense).Chemists call this pH (which is unique for all acidic groups) the pKa value (for this particular acidic group).
Hence, when pH starts getting more alkaline, you get higher molarity of conjugate base, lower molarity of conjugate acid. Hence, when pH starts getting more acidic, you get higher molarity of conjugate acid, lower molarity of conjugate base.
If you and your friend still don't understand after reading the above, then you and your friend have no chemistry sense, no hope liao.
And bear in mind this is about equilibria, which means no matter what the pH is, you'll always have some molarity of all possible conjugate acids and conjugate bases, eg. H3A2+, H2A+, HA, A-, etc, it's only a matter of correctly identifying the major species present at any given pH for A levels (at Uni levels, you need to identify the 1st most major species, the 2nd most major species, the 3rd, and so on, at any given pH).
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Originally posted by UltimaOnline:
For every acidic group (this includes the conjugate acids of basic groups), there will be a specific pH of the solution in which you've equal molarities of conjugate acid and conjugate base (for this particular acidic group), correct? (this is common sense).Chemists call this pH (which is unique for all acidic groups) the pKa value (for this particular acidic group).
Hence, when pH starts getting more alkaline, you get higher molarity of conjugate base, lower molarity of conjugate acid. Hence, when pH starts getting more acidic, you get higher molarity of conjugate acid, lower molarity of conjugate base.
If you and your friend still don't understand after reading the above, then you and your friend have no chemistry sense, no hope liao.
And bear in mind this is about equilibria, which means no matter what the pH is, you'll always have some molarity of all possible conjugate acids and conjugate bases, eg. H3A2+, H2A+, HA, A-, etc, it's only a matter of correctly identifying the major species present at any given pH for A levels (at Uni levels, you need to identify the 1st most major species, the 2nd most major species, the 3rd, and so on, at any given pH).
i got it liao! Thanks God !!!
Yes i see now. in a soln, some molecules present in *higher molarity* than some others. at pI, the zwitterion is present in the highest molarity
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Originally posted by UltimaOnline:
For every acidic group (this includes the conjugate acids of basic groups), there will be a specific pH of the solution in which you've equal molarities of conjugate acid and conjugate base (for this particular acidic group), correct? (this is common sense).Chemists call this pH (which is unique for all acidic groups) the pKa value (for this particular acidic group).
Hence, when pH starts getting more alkaline, you get higher molarity of conjugate base, lower molarity of conjugate acid. Hence, when pH starts getting more acidic, you get higher molarity of conjugate acid, lower molarity of conjugate base.
If you and your friend still don't understand after reading the above, then you and your friend have no chemistry sense, no hope liao.
And bear in mind this is about equilibria, which means no matter what the pH is, you'll always have some molarity of all possible conjugate acids and conjugate bases, eg. H3A2+, H2A+, HA, A-, etc, it's only a matter of correctly identifying the major species present at any given pH for A levels (at Uni levels, you need to identify the 1st most major species, the 2nd most major species, the 3rd, and so on, at any given pH).
all these knowledge you write here are godly knowledge. thanks for being so generous. i understand chemistry so much now. i will worship you
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a extract from my school's examiner report
'...candidates overlooked/missed out the nu sub on the chloro group. given alkaline conditions with warming, it is possible for the c-cl bond to cleave to produce alcohol.'
the qn asks us to draw the major product of a cpd that undergoes rxn with I2, in warm, alkaline conditions.
for haloalkane to alcohol, is naoh(aq) + heat
for haloalkane to alkene, is ethanolic naoh ,heat
a soln with aqNaOH , is already alkaline
i got it alr. adding ethanol, is Alcoholic conditions, not Alkaline conditions.
understand the function of ethanol from kschan book alr.
thanks
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if a cpd can undergo iodoform reaction and also have a c-cl bond, and cambridge ask us to draw the major product, do we draw that the Cl have been replaced by OH?
or do we draw both structures, one replaced, one isn't, then how explain? if temp sufficient, have substitution. reaction may or may not occur? (or is it will occur)
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Originally posted by Flying grenade:
if a cpd can undergo iodoform reaction and also have a c-cl bond, and cambridge ask us to draw the major product, do we draw that the Cl have been replaced by OH?
or do we draw both structures, one replaced, one isn't, then how explain? if temp sufficient, have substitution. reaction may or may not occur? (or is it will occur)
Check the question's exact phrasing. If "warm", then Cl not replaced by OH. If "heat", then Cl replaced by OH. -
Hi UltimaOnline, I am having trouble with qns 5 part a. Thanks
https://www.instagram.com/p/BKYb4KIBIEJ/?taken-by=lolwerwdr
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Originally posted by MightyBiscuits:
Hi UltimaOnline, I am having trouble with qns 5 part a. Thanks
https://www.instagram.com/p/BKYb4KIBIEJ/?taken-by=lolwerwdr
Step 1 : Cl2 in CCl4, in the dark at rtp, halogenation via electrophilic additionStep 2 : NaOH in ethanol, heat under reflux, dehydrohalogenation elimination via E2 mechanism
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for acid base reaction, not necessarily H2O is needed to be formed?
https://www.dropbox.com/s/end05thtc88s4jv/20160916_185541-1.jpg?dl=0
YJC 2016 prelim