brightsky's Chem Thread

[lzxnl]

By definition, a zwitterion is neutral.

[brightsky]

just to confirm: insight seems to think that phenolphthalein is not a suitable indicator for strong base vs. strong acid reaction...but surely, even though pH = 7 does not fall within the pH range of phenolphthalein, due to the steepness of the curve around equivalence, phenolphthalein is still suitable? we've been using phenolphthalein as indicator for strong base vs. strong acid reactions since preschool...

[lzxnl]

just to confirm: insight seems to think that phenolphthalein is not a suitable indicator for strong base vs. strong acid reaction...but surely, even though pH = 7 does not fall within the pH range of phenolphthalein, due to the steepness of the curve around equivalence, phenolphthalein is still suitable? we've been using phenolphthalein as indicator for strong base vs. strong acid reactions since preschool...

LMAO preschool
I don't know what preschool you went to, but when I was in preschool I couldn't spell phenolphthalein.

As I maintain, if you add half a millidrop to the equivalence point, the pH shoots up a lot. Within the accuracy of two decimal places (when measuring titres in mL), the endpoint approximates the equivalence point well.

[brightsky]

LMAO preschool
I don't know what preschool you went to, but when I was in preschool I couldn't spell phenolphthalein.

As I maintain, if you add half a millidrop to the equivalence point, the pH shoots up a lot. Within the accuracy of two decimal places (when measuring titres in mL), the endpoint approximates the equivalence point well.

damn it, insight!

thanks again nliu!

[brightsky]

a few more:

1. i think this question has been asked before, but how does vcaa define concordant titres? this is the definition on which i'm currently running: a set of titres are concordant if and only if the lowest titre and the highest titre differ by no more than 0.10 mL. insight, however, seems to run on a completely different definition...
2. for ionisation reaction in the context of mass spec, is "X(g) --> X+* (plus-dot) (g) + e-" acceptable? or do we have to write "X(g) + e- --> X+* (g) + e-"?
3. also, say you run a sample of ethanoic acid through a mass spectrometer. you see a peak at m/z 45. is the fragment lost CH3 or (CH3)* (with a dot representing electron), strictly speaking?

[lzxnl]

damn it, insight!

thanks again nliu!

LOL insight has done many funny things
There was a question in which we were meant to have a neutral solution of a salt that when electrolysed, would produce an acidic solution around the anode.
Unfortunately, the only possible choice was KF. And guess what?  F- is basic as it is the conjugate base of HF, a weak acid, and after calculations I showed that the pH would be out of the indicator's neutral pH range. God damn it Insight.

a few more:

1. i think this question has been asked before, but how does vcaa define concordant titres? this is the definition on which i'm currently running: a set of titres are concordant if and only if the lowest titre and the highest titre differ by no more than 0.10 mL. insight, however, seems to run on a completely different definition...
2. for ionisation reaction in the context of mass spec, is "X(g) --> X+* (plus-dot) (g) + e-" acceptable? or do we have to write "X(g) + e- --> X+* (g) + e-"?
3. also, say you run a sample of ethanoic acid through a mass spectrometer. you see a peak at m/z 45. is the fragment lost CH3 or (CH3)* (with a dot representing electron), strictly speaking?

Insight I think did that too; they forgot about concordant titres in one question. I'm fairly sure VCAA defines concordant titres they way you have.

I think for the ionisation, you're best of with the second one. That's what actually happens.

Saying that the fragment lost is CH3 is perfectly fine IMO. The dot just stresses that you have a free radical, although it's not needed; NO2 is often written as NO2 without a dot.

[brightsky]

okay a few questions:

1. are we meant to know the common isotopes of elements and their relative abundances off by heart? for instance, 35Cl and 37Cl, etc.?
2. CSE 2011 Q6 a. Image below. how are we supposed to use the data to work out what alkanol 1, 2, 3, 4 are? why can't alkanol 1 be methanol, for instance?
3. why is it important that the structure of ibuprofen is similar to the structure of aspirin in treating headache and pain? the answer talked about active sites, but of what? are they talking about the active sites of the problem area, or the active site of some enzyme? (I have a feeling this question has something to do with protein as markers of disease...)
4. CSE 2011 Q7 k. Image below. how are we supposed to draw the splitting pattern? given that the two peaks will be fairly close to each other at around 7.3 ppm? surely they won't expect us to draw this to scale?

thanks!

[lzxnl]

okay a few questions:

1. are we meant to know the common isotopes of elements and their relative abundances off by heart? for instance, 35Cl and 37Cl, etc.?
2. CSE 2011 Q6 a. Image below. how are we supposed to use the data to work out what alkanol 1, 2, 3, 4 are? why can't alkanol 1 be methanol, for instance?
3. why is it important that the structure of ibuprofen is similar to the structure of aspirin in treating headache and pain? the answer talked about active sites, but of what? are they talking about the active sites of the problem area, or the active site of some enzyme? (I have a feeling this question has something to do with protein as markers of disease...)
4. CSE 2011 Q7 k. Image below. how are we supposed to draw the splitting pattern? given that the two peaks will be fairly close to each other at around 7.3 ppm? surely they won't expect us to draw this to scale?

thanks!

1. Relative abundances? Heck no. It's nice if you do remember them, but VCAA cannot possibly ask for that. It's VCE chemistry.
2. Increasing retention time=>more dispersion forces presumably=>larger molecule. I would have thought alkanol 1 was the smallest possible alkanol...they didn't give a range though.
3. Well...active sites of enzymes or proteins that can actually use the ibuprofen, otherwise it can't participate in chemical reactions
4. There are two substituents on the benzene ring, so there are only four hydrogens environments. Each hydrogen only splits the other into doublets. So you'd have four hydrogens, each peak split into two at around whatever.

[brightsky]

hmm...

re: q2 - the numbers of the table seems for the most part irrelevant, as well as the random volume and stuff they give at the top...I guessed methanol...but apparently alkanol 1 has to be ethanol...for reasons I've yet to discover...

re: q3 - so apparently proteins are markers of disease...am I right to think that ibuprofen enters the active site of the protein that is stuffed up and fixes it? or does it do something else?

re: q4 - yep so four peaks, all at around 7.3 ppm. but my question was more how do you even draw that to scale on the tiny scale they've given us? would you just draw two doublets of equal height crammed up in the region where chemical shift is 7.3 ppm?

[lzxnl]

4. I'm not sure. Just try and draw it. If you have eight squashed up peaks, I'm sure they'll let you off.

3. That's possible; I don't know what ibuprofen does so I can't help with that.

2. Because you can't have methanol in wine? There's a bit that says there's alkanol 1 in the wine.

[brightsky]

ahh yes right right...I see. thanks nliu! (also I think you overlooked the symmetry of benzene ring but I get what you mean :p)

[lzxnl]

I didn't bother with looking at where the substituents were on the wiki page. That wasn't my concern at the time

[ECheong]

re: q3 - so apparently proteins are markers of disease...am I right to think that ibuprofen enters the active site of the protein that is stuffed up and fixes it? or does it do something else?

We don't need to know this but just for curiosity's sake IIRC Aspirin and Ibuprofen are both NSAIDs (non-steroidal anti inflammatory drugs). They operate in a similar way by binding onto the active site of an enzyme (COX) to stop it catalysing a reaction that creates chemicals which trigger pain signals to be sent to the brain. Since these enzymes' active sites are blocked up by Aspirin/Ibuprofen they can't catalyse the reaction to make the chemicals and so you feel less pain.

[brightsky]

We don't need to know this but just for curiosity's sake IIRC Aspirin and Ibuprofen are both NSAIDs (non-steroidal anti inflammatory drugs). They operate in a similar way by binding onto the active site of an enzyme (COX) to stop it catalysing a reaction that creates chemicals which trigger pain signals to be sent to the brain. Since these enzymes' active sites are blocked up by Aspirin/Ibuprofen they can't catalyse the reaction to make the chemicals and so you feel less pain.

ahh ming bai le! thanks Echeong!

[brightsky]

1)

okay I've come across this question in a random practice exam:

NH4Cl(s) <--> NH3(g) + HCl(g), dH = +176 kJ mol-1

when solid NH4Cl is added to the equilibrium, what will happen to the forward and reverse rates?

now I know LCP might say that if you add stuff to the LHS, the equilibrium position will be shifted to the right, hence the rate of forward reaction will increase instantaneously and then decrease a little and the reverse reaction increase, before equilibrium is regained. but does adding solid NH4Cl really affect the equilibrium at all? if we consider the Kc of the reaction, NH4Cl is a solid, so the concentration of NH4Cl really isn't relevant. so Kc = [NH3][HCl]. now if we add NH4Cl, Qc still remains the same as Kc. in other words, it appears that adding NH4Cl doesn't really affect the equilibrium in any way, since the concentration of the solid more or less remains constant, if it is even right to speak of concentration of solid.

what are your thoughts? I may be missing something critical...in which case I apologise in advance for the stupid question :p

2) also just a quickie: if the question does not explicitly tell us to use LCP, can we argue using formulas such as the van't hoff equation? I don't understand how one can really use LCP to explain why, for an endothermic reaction, an increase in temp results in an increase in Kc. sure, the equilibrium position will shift to the right, but that doesn't mean Kc increases.

thanks!