VCE Chemistry Question Thread

[wobblywobbly]

Again, my question is: is this method of using the half-equations the proper approach to answering this type of question?

You used the right method, but just make sure you're balancing your H+ and H2O, which you didn't do in your half equations.

It should really be (I've used brackets for legibility, but of course don't use them around the charges)
Cr2O7(2-) -> Cr(3+) + H2O which when correctly balanced (remember the order: balance non H/O then O then H then electrons)

Cr2O7(2-) + 14H(+) + 6e(-) -> 2Cr(3+) + 7H2O
6Fe2+  ---> 6Fe3+  +  6e- (doubled so that we have six electrons)

1:6 -> C would be your answer.

[Sine]

Guys, the textbook has a compound named as "2-Methylpropene".
I think it is redundant to put the 2 there since it doesn't add anything. Methylpropene is just enough as the numbering always starts from the double bond and the methyl group cannot be attached anywhere else (if it was one or three, it would be called butene).
I have heard it said that vcaa assessors consider putting a number that is not required in the IUPAC name as a mistake. What do you think guys??   

yes 2-methylpropene is redundant It should be methylpropene as you said.

I'm not sure for something like this molecule but you will probably lose marks for naming carboxylic acids with an additional 1. In reality they should be taking marks off for adding extra numbers. Just name it properly without redundant numbers so you are guaranteed marks. 

[klippo]

Hi everyone,
I'm just a bit confused about how this equation works:
K₃PO₄ + 3HNO₃ -> H₃PO₃ +3KNO₃
The question stem involving this equation says that phosphate has been completely protonated to phosphoric acid (H₃PO₄) Shouldn't there be interim reactions that lead to this final equation?
Any help would be greatly appreciated! 

[qazser]

Whats in Chem 1/2 that is important for 3/4, might start learning early

[HopefulLawStudent]

If we had a polar molecule and non-polar mobile phase, would there be more interaction than if we had a non-polar mobile phase and non-polar molecule?

Anyone?

[grannysmith]

Anyone?

If you've got a non polar mobile phase and a polar molecule, there'd be less interaction than if it was a non polar molecule. This is because the polar molecule would prefer to interact with the (presumably) polar stationary phase.

[HopefulLawStudent]

Gotcha. Thank you so much!

[keltingmeith]

Hi everyone,
I'm just a bit confused about how this equation works:
K₃PO₄ + 3HNO₃ -> H₃PO₃ +3KNO₃
The question stem involving this equation says that phosphate has been completely protonated to phosphoric acid (H₃PO₄) Shouldn't there be interim reactions that lead to this final equation?
Any help would be greatly appreciated! 

There can be, but there definitely doesn't need to be. For example, consider the basic addition reaction:

Ethene + Bromine -----> 1,2-dibromoethane
CH2CH2 + Br2 ------> BrCH2CH2Br

This reaction actually goes through the following intermediate:

BrCH2CH2+ + Br- -----> BrCH2CH2Br

So, the full reaction would actually be:

CH2CH2 + Br2 -----> BrCH2CH2+ + Br- ------> BrCH2CH2Br

But, do we write the middle reaction? No, because there's no need to. Same idea here - yes, there are intermediates that form between the initial and final step, but we don't see them, so we don't care about them.

Whats in Chem 1/2 that is important for 3/4, might start learning early

Things that you will be explicitly tested on: redox, stoichiometry (+the mole, as it's called in unit 1... Really, it's mass stoichiometry) and gas laws, naming organic molecules, chemical equation writing, and a little bit of acids and bases.

In reality, though, it's good to have a good grasp on all the concepts in units 1 and 2, as all of it helps with your conceptual understanding. Most importantly, IMO the best way to learn chemistry is to think about what's happening on the chemical level - this might seem a little obvious, but I mean going so far as drawing the chemistry that's happening. If you can draw the chemicals interacting, then you are on your way to doing really well in VCE chemistry (obviously not necessary). This requires having a good grasp of everything you learn in unit 1 and 2.

Having said all of that, every school does units 1 and 2 differently - particularly in this new study design, everyone's going to be guessing at what realllllly carries over, I wouldn't be surprised some schools skipping over stuff that doesn't pop up again (I'm looking at you, spectroscopy) and even teaching stuff from units 3/4.

[klippo]

There can be, but there definitely doesn't need to be. For example, consider the basic addition reaction:

Ethene + Bromine -----> 1,2-dibromoethane
CH2CH2 + Br2 ------> BrCH2CH2Br

This reaction actually goes through the following intermediate:

BrCH2CH2+ + Br- -----> BrCH2CH2Br

So, the full reaction would actually be:

CH2CH2 + Br2 -----> BrCH2CH2+ + Br- ------> BrCH2CH2Br

But, do we write the middle reaction? No, because there's no need to. Same idea here - yes, there are intermediates that form between the initial and final step, but we don't see them, so we don't care about them.

Okay awesome, thanks so much! So are the hydrogen and potassium ions switched one by one? (if that makes sense)

[lzxnl]

Okay awesome, thanks so much! So are the hydrogen and potassium ions switched one by one? (if that makes sense)

Yes. I'd say protons fall off one by one. Remember that the potassium ions are not actually bound to the anions in solution.

[NerdyPi]

Just a quick organic chem question, in 3/4 VCE chem, would we ever have to worry about an alkene which also has a functional group (such as alkenols, or any other molecule with both a double bond and a function group like OH, COOH, ect). Thanks

[Sine]

Whats the catalyst for Alkene-->ChloroAlkane currently i've got AlCl3 but many resources just leave it as "catalyst" also does anyone have a list of catalysts for organic reactions?

Also doing the classic aspirin sac (reaction pathways) what sort of questions would normally come up on this sac? Other than reaction pathway diagrams?

when drawing structural formula of species with a benzene ring can we just draw the hexagon with the circle or do we need to specify all the bonds

[HasibA]

Whats the catalyst for Alkene-->ChloroAlkane currently i've got AlCl3 but many resources just leave it as "catalyst" also does anyone have a list of catalysts for organic reactions?

Also doing the classic aspirin sac (reaction pathways) what sort of questions would normally come up on this sac? Other than reaction pathway diagrams?

when drawing structural formula of species with a benzene ring can we just draw the hexagon with the circle or do we need to specify all the bonds

1) not too sure, but i'm pretty sure this is correct- iffy area for me
2) questions about why your mass of aspiring  was higher or lower, sources of error etc. all your basic ones, but there may be some questions (haven't had this sac , but some friends have) relating to why the process of aspirin works and what bonds are broken/formed when your reactants mixed etc. (this was worth like 15/60 or so marks on friends' sacs) sorry if this wasn't too helpful
3) dont have to specificy all the bonds, unless it specifically says 'show all bonds' in which case you do; i use he hexagon with the circle in assessments and it's been fine for me

sorry if i've confused you or wasn't specific enough :/ :3

[@#035;3]

For the Zwitterion structure of the amino acid aspartic acid, would you only deprotonate one of the carboxy groups? If so which one?
 

[blacksanta62]

I have 2 questions:
1) Does anyone have suggestions for sketching what a molecule could potentially look like, I feel like I could use a bit more practice (to do this would we normally be given some spectrums which would assist us e.g. a 1HNMR spectrum, an infrared spectrum and a 13CNMR spectrum?)
2) What chapter is your class up to? Not a huge deal but some of my friends schools are light years ahead. I don't want to be cramming in important chapters at the end of the year, should I just work ahead?

Thank you