ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: psyxwar on June 13, 2013, 07:47:29 pm
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Since I'll probably be asking a fair few chem questions, might as well start a thread now.
For propene, how would you describe its shape (assuming that you have to describe the whole molecule and not the individual carbons?)
I'm asking because this was on my exam and I said it was trigonal planar, but other people said it was linear/ tetrahedral.
Thanks.
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Also, under what circumstances exactly do resonance structures occur? I'm curious.
Will they always occur if there's a negative one formal charge and a double bond nearby?
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Yelp, suddenly feeling very doubtful in my knowledge of chemistry... And I'm doing 3/4!
I would have said linear for propene because I have no idea what trigonal planar is
And if resonance structures are on the study design there are huge gaps in my knowledge.
Sorry I couldn't be more helpful. :-\
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Yelp, suddenly feeling very doubtful in my knowledge of chemistry... And I'm doing 3/4!
I would have said linear for propene because I have no idea what trigonal planar is
And if resonance structures are on the study design there are huge gaps in my knowledge.
Sorry I couldn't be more helpful. :-\
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I also would have said linear... fml!
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Yelp, suddenly feeling very doubtful in my knowledge of chemistry... And I'm doing 3/4!
I would have said linear for propene because I have no idea what trigonal planar is
And if resonance structures are on the study design there are huge gaps in my knowledge.
Sorry I couldn't be more helpful. :-\
Haha I'm learning for fun, rather than because it's in the study design or whatever (so it probably isn't haha, don't worry!)
Thanks for your help though!
I was told if you were to describe individual carbons, carbon 1 would be trigonal planar (all in the same plane, triangle shaped), carbon 2 also trigonal planar, and the third carbon would be tetrahedral. But it's kind of hard to describe the whole molecule.
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Why is the cathode considered positive in a voltaic cell and negative in a electrolytic cell?
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As far as I know, the cathode is considered the positive electrode in a voltaic cell because the reaction of 'reduction' occurs at the cathode. But this happens in electrolytic cells as well... and yet the cathode is the negative electrode there :-\ . However, the cathode terminal itself is negatively charged in a voltaic cell because it attracts positively charged ions. That I know for sure. 8)
The electrolytic cell converts electrical to chemical energy whereas the voltaic cell does the opposite by converting chemical energy to electrical energy. Perhaps, this has something to do with the roles of the cathode being switched over??
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mhm, because the first two carbons have a double bond, they become trigonal planar.
as the last carbon had four bonds with other molecules, its tetrahedral.
you don't really describe the shape of the whole molecule. it's like if someone gave you something like 2,3-dimethyl-4-ethyl-heptane how would you describe the shape? you wouldnt. but you could go and say that all the carbons are tetrahedral.
the cathode is always where reduction occurs, and the anode where oxidation occurs.
In a galvanic cell, the reaction is spontaneous. Electrons are being produced at the anode and travel to the cathode. Hence the anode is negative and the cathode is positive.
In an electrolytic cell, you're using a power supply to force electrons to go from positive to negative. the cathode is still where reduction occurs/where electrons are being gained. Since the electrons are being pumped to the cathode (from the anode), it becomes negative.
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mhm, because the first two carbons have a double bond, they become trigonal planar.
as the last carbon had four bonds with other molecules, its tetrahedral.
you don't really describe the shape of the whole molecule. it's like if someone gave you something like 2,3-dimethyl-4-ethyl-heptane how would you describe the shape? you wouldnt. but you could go and say that all the carbons are tetrahedral.
the cathode is always where reduction occurs, and the anode where oxidation occurs.
In a galvanic cell, the reaction is spontaneous. Electrons are being produced at the anode and travel to the cathode. Hence the anode is negative and the cathode is positive.
In an electrolytic cell, you're using a power supply to force electrons to go from positive to negative. the cathode is still where reduction occurs/where electrons are being gained. Since the electrons are being pumped to the cathode (from the anode), it becomes negative.
Close enough :P
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Is the "positive" designation just an arbitrary one based upon the fact that electrons travel there?
Why is carbon able to double bond, but silicon not? (eg. CO2's structure vs. SiO2)
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This might help :) http://www.chem.queensu.ca/chembook/articles/Silicon.htm
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Well...the positive terminal is also analogous to the sign designation for batteries as well.
Personally, if I had to describe the shape of propene, I'd say that carbon 1 has trigonal planar geometry, carbon 2 has trigonal planar geometry and carbon 3 is tetrahedral (propene is CH2=CHCH3), and the carbon skeleton is trigonal planar.
Resonance occurs whenever you can write a second equally valid Lewis structure where the only difference between the two structures is a shift in electron density (a distinction must be made between resonance, where the resonance forms are only an average of reality, and tautomerism, where the difference forms both exist separately and the compound actually shifts between the forms). It is common with double bonds and formal charges, yes. However, they can occur with triple bonds too :P just try N2O. You can have N=N=O or N=-N-O.
You don't need to know resonance for VCE.
For two elements to form a double bond, their orbitals must be of similar energy and size. Silicon is a fair bit larger than oxygen, so generally Si=O does not form.
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Edit: wrong thread.
Thanks nliu though. You sure know heaps about chem!
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Well...you'd expect the summer school to have been somewhat useful in teaching me something :P
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Okay so when solving equilibrium problems (where I need to work out equilibrium concentrations after being given the K value and initial concentrations), at what point can I forgo the quadratic equation and all that and just assume the "x" value is negligible and can be discounted?
eg. say when K is 1.6x10^-5, and the equilibrium expression is ((2x)^2(x))/(0.50-2x)^2. In this case we can just go "oh since the K is so small x must be really small so let's just reduce that to (0.50)^2".
Is there some sort of K value I should use as a rough guideline, like once the K value is smaller than 10^-4 I can start discounting the x or something?
I hope that makes sense to whoever reads this.
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in my course, we've been told that you can make that assumption as long as you the percentage ionisation is <5%
so you do your calculation and assume that 0.50-2x = 0.5 (with squiggly equals if you want to be pedantic)
and then if x/[acid] * 100 is less than 5, then all is fine and well.
as far as VCE is concerned though, you'll ALWAYS be allowed to make the assumption. because chem students aren't expected to be able to do math and giving quadratics with decimals on a scientific calc exam isn't very nice.
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Ah the good old days of assumptions in analytical chemistry.
This is what I do. Let's say you're trying to work out the % dissocation of 1M ethanoic acid, pKa 4.76.
Then if x M ethanoic acid dissociates, you form x M Ch3COO-
You have 1-x M ethanoic acid left
and you have formed x M H+ (forget the auto dissociation of water in these; water's a shoddy acid)
So 10^-4.76 = x^2/(1-x)
x = sqrt(10^-4.76*(1-x))
Now this is where it gets trippy. In every single question I've ever met at the summer school, this next step has been valid. It's fast on scientific calculators and it's very accurate.
First, type in 0 and press enter to clear your answer.
Then, type in the right hand side of the equation while letting x = Ans
Finally, keep jamming the enter button until your answer converges. It WILL converge.
The first time, you sub in x = 0 on the right hand side. You get some small value of x, and sub this into the right hand side again. If your approximation the first time is valid, the second time won't change much. Try it.
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Also, under what circumstances exactly do resonance structures occur? I'm curious.
Will they always occur if there's a negative one formal charge and a double bond nearby?
Resonance structures are a lie. These are the results of using simple Lewis structures to explain "intermediate" bonds.
The fact is, bonds are never strictly pairs of electrons. The electron density is instead smeared over the molecule, though most of its density is localised between two atoms, like a bond. In the case of "resonance structures", these are ways to represent groups of electron that don't "belong" to a particular bond, but rather is smeared over a larger group of molecules (e.g. in benzene, 6 electrons in the ring are smeared over the ring, rather than belonging to any C-C bond). A much better representation would be to show these delocalised electrons as they are.
These delocalised electrons are often represented in Lewis structures as alternating double/single bonds, usually called "conjugation". Sometimes (especially in hydrocarbons), delocalised orbitals do not necessarily mean there are multiple resonant Lewis structures (e.g. hexan-1,3,5-triene has a certain degree of delocalisation, but no resonant structures).
tl;dr, resonant structures are a lie.
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i feel like the last six months of my life has been a lie. </3
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LOL unfortunately a bit of first-year uni chem is still slightly simplified.
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LOL unfortunately a bit of first-year uni chem is still slightly simplified.
haha such a nick thing to say. :p
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i'm pree sure more than just "a bit" of it is simplified. but hey! we'll get there one day.
until then, i'll survive with my life being a lie. :'D
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who's this "nick" guy you speak of?
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who's this "nick" guy you speak of?
Clearly NOT you since it would be against forum rules to use ppl's real names ;)
(unless of course they are dumb and unimaginative enough to use their real name as their forum name.....)
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Well...we don't have a choice but to call you Alwin.
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https://www.asi.edu.au/userfiles/file/2010_Chemistry_NQE_Papers.pdf
q12, can someone explain to me how to work this out?
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So...substantially different pKa values? OK. That would generally imply that we would have many different equivalence points. Note how after the first equivalence point, there is still some acid left over so the first equivalence point can be below pH = 7. However, the last equivalence point is the point of where there's no acid left, only base. Something to think about.
Now if the acids have similar pKa values, then the neutralization of the acids will occur at a similar pH. Imagine the titration curves of the individual acids are superimposed together. The vertical bit just keep going. However, the vertical bit should start at pH<7 (as there still is excess acid left at that time) and end at pH>7, as we're left with only conjugate bases of the acids. Putting these together, B is the only possible answer.
I flicked through some of the questions. Wow. Even as a summer school student, some of those questions would be fun to do.
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https://www.asi.edu.au/userfiles/file/2012_Chemistry_ASOE_solutions.pdf
q18, f. Could someone explain why the symmetric stretch yields an IR active stretch?
EDIT: so I guess my question is more so why it yields an IR active structure here, but with say CO2, symmetrical stretching wouldn't?
EDIT2: Is it just because the vectors don't cancel out in this case (whereas for a linear/ planar molecule they would), and therefore increasing their magnitude affects the net dipole moment?
https://www.asi.edu.au/userfiles/file/2011Chemistry%20Exam%20Answers.pdf
q18, b. Answers here for O2 and C2H4 just don't make any sense to me...
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Does anyone have the textbook names for the HL IB texts for chemistry? (which one do you reckon has the best questions too?)
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Does anyone have the textbook names for the HL IB texts for chemistry? (which one do you reckon has the best questions too?)
Ask mr kemp nicely, he probably has it, and don't be late to class
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don't be late to class
Don't tell me what to do.
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Can someone list all the types of concentration (eg. M, %w/v, etc) that we're expected to know?
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oh my.
are you learning all this non-study design stuff for the funsies or.
are you doing 3/4 early. you are aren't you.
Can someone list all the types of concentration (eg. M, %w/v, etc) that we're expected to know?
my teacher said you didn't need to know about %w/v or that parts per million thing.
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oh my.
are you learning all this non-study design stuff for the funsies or.
are you doing 3/4 early. you are aren't you.
my teacher said you didn't need to know about %w/v or that parts per million thing.
who does work for funsies, pls.
Mmm i think it's still on my exam though, was in a few past papers for 1/2 anyway.
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who does work for funsies, pls.
Mmm i think it's still on my exam though, was in a few past papers for 1/2 anyway.
oh i think i know. were your previous questions for the chemistry olympiad?
because right now i am not sure what i need to know anymore.
bro all i'm seeing is a blur of magnitude/ resonance whats goin on.
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oh i think i know. were your previous questions for the chemistry olympiad?
because right now i am not sure what i need to know anymore.
bro all i'm seeing is a blur of magnitude/ resonance whats goin on.
yeah it was, don't worry, I don't think much of the stuff here is relevant
Also, for anyone who was also curious about my question (probs no one):
- w/w (percentage by mass)
- v/v (percentage by volume)
- w/v (percentage mass per volume)
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yeah it was, don't worry, I don't think much of the stuff here is relevant
Also, for anyone who was also curious about my question (probs no one):
- w/w (percentage by mass)
- v/v (percentage by volume)
- w/v (percentage mass per volume)
I'll send you a photo via email or something
Picture is too large on here and ceebs typing
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yeah it was, don't worry, I don't think much of the stuff here is relevant
Also, for anyone who was also curious about my question (probs no one):
- w/w (percentage by mass)
- v/v (percentage by volume)
- w/v (percentage mass per volume)
what does all that mean ;A;
eep
edit; nvm
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what does all that mean ;A;
eep
Let's put some possible units onto them...
% w/w = g per 100 g = kg per 100 kg and so on.
% v/v = mL per 100 mL = L per 100 L and so on.
% w/v = g per 100 mL = kg per 100 L and so on.