Psyxwar's Chemistry Thread

[psyxwar]

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.

[psyxwar]

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?

[pterozachtyl]

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. 

[Yacoubb]

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. 
[/quote
 
I also would have said linear... fml!

[psyxwar]

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.

[psyxwar]

Why is the cathode considered positive in a voltaic cell and negative in a electrolytic cell?

[Guest]

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.
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??

[scribble]

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.

[Guest]

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 

[psyxwar]

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)

[pi]

This might help http://www.chem.queensu.ca/chembook/articles/Silicon.htm

[lzxnl]

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 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.

[psyxwar]

Edit: wrong thread.

Thanks nliu though. You sure know heaps about chem!

[lzxnl]

Well...you'd expect the summer school to have been somewhat useful in teaching me something

[psyxwar]

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.