Chemistry 3/4 2013 Thread

[Alwin]

What is the likelihood that we will have to draw actual semi-structural formulae for DNA (either a strand of nucleotides or the specific hydrogen bonding between complementary bases)? I don't really remember them that specifically, but it's come up on a couple of papers.

Firstly, I assume you mean just drawing, since the semi-structural formula of benzene rings are not in VCE chem... neither is naming or aromatic compounds but meh.

It takes quite a bit of space and time, so I don't think it will be that high on VCAA's priorities. Like they might, but all the pieces of DNA are in the databook, just have to work out how they join + where the hydrogen forces occur.

Personally, if it's a drawing question I think it'll be about esters giving the IR and NMR stuff and we have to work out what happens where. They also seem to really like asking us to draw dipeptides or sections of polymers. Sketching sections of DNA are kinda pointless, a better question would be to give a diagram and ask us to label sections, functional groups, etc. Or, a partial diagram and we have to fill in the bonds or groups of atoms.

As for DNA questions, I'm guessing something about how AT and CG proportions are equal, and how higher proportions of CG lead to higher boiling point (3 hydrogen bonds as opposed to 2). But since electrophoresis is off the course this year (restriction enzymes, etc) your guess is as good as mine for what they'll test for DNA.

Just be prepared for everything

[ahat]

It takes quite a bit of space and time, so I don't think it will be that high on VCAA's priorities. Like they might, but all the pieces of DNA are in the databook, just have to work out how they join + where the hydrogen forces occur.

Where do they occur, if I may ask?

[Stick]

Firstly, I assume you mean just drawing, since the semi-structural formula of benzene rings are not in VCE chem... neither is naming or aromatic compounds but meh.

It takes quite a bit of space and time, so I don't think it will be that high on VCAA's priorities. Like they might, but all the pieces of DNA are in the databook, just have to work out how they join + where the hydrogen forces occur.

Personally, if it's a drawing question I think it'll be about esters giving the IR and NMR stuff and we have to work out what happens where. They also seem to really like asking us to draw dipeptides or sections of polymers. Sketching sections of DNA are kinda pointless, a better question would be to give a diagram and ask us to label sections, functional groups, etc. Or, a partial diagram and we have to fill in the bonds or groups of atoms.

As for DNA questions, I'm guessing something about how AT and CG proportions are equal, and how higher proportions of CG lead to higher boiling point (3 hydrogen bonds as opposed to 2). But since electrophoresis is off the course this year (restriction enzymes, etc) your guess is as good as mine for what they'll test for DNA.

Just be prepared for everything

Sorry, I meant just structural formulae. If we had to draw a segment of DNA or show how two complementary bases bonded with each other, would it be simple diagrammatic representations, or the actual structural formula? I wouldn't know the latter at this stage. :|

[Alwin]

Where do they occur, if I may ask?

Of course you can ask! Isn't that the point of Atarnotes   (I even colour coded it on paint for you!)


EDIT: all fixed up now, thanks ECheong!

Sorry, I meant just structural formulae. If we had to draw a segment of DNA or show how two complementary bases bonded with each other, would it be simple diagrammatic representations, or the actual structural formula? I wouldn't know the latter at this stage. :|

I think that it would want to you draw out the structures and indicated (with dotted lines) where hydrogen bonds occur. Look at my pic above

You just have to rotate the nitrogen bases or reflect them correctly if need be (look at the textbook for more examples)

[ECheong]

Of course you can ask! Isn't that the point of Atarnotes   (I even colour coded it on paint for you!)

(Image removed from quote.)

Just a quick note that the bonds to the 3' and the 5' carbons from the phosphate are off the O- parts of phosphate (i.e. not off the double bond). Other than that I liked the colour coded-ness of the image haha!

[ahat]

Of course you can ask! Isn't that the point of Atarnotes   (I even colour coded it on paint for you!)

(Image removed from quote.)

I think that it would want to you draw out the structures and indicated (with dotted lines) where hydrogen bonds occur. Look at my pic above

You just have to rotate the nitrogen bases or reflect them correctly if need be (look at the textbook for more examples)

Thankyou! I could never remember where the bonds were, but that is excellent - easy to remember! Thanks!

[Alwin]

Just a quick note that the bonds to the 3' and the 5' carbons from the phosphate are off the O- parts of phosphate (i.e. not off the double bond). Other than that I liked the colour coded-ness of the image haha!
[/quote

whoopise! didn't notice that it was rotated in the databook awks thanks ECheong!

[ahat]

In a galvanic cell, if we're asked to order the 'reactivity of metals', does this refer to strongest oxidant or reductant, and what is the reason?
I believe it is the strongest reductant as most reactive as it is most likely to give up electrons, ie. undergo oxidation?
Thanks.

[brightsky]

well I don't believe there are very many metals out there which can function as oxidants. Consider Mg(s). if Mg is an oxidant, it must lose electrons. so the oxidation number must decrease from 0, which is nonsense for a metal. so yepp, most reactive metals are those which are the strongest reductants. in general however reactivity can encompass oxidation, I believe. if you take something like F2, it WILL want to grab electrons off anything in the immediate vicinity. so F2 is very reactive, although it's a strong oxidant not a strong reductant.

[ahat]

well I don't believe there are very many metals out there which can function as oxidants. Consider Mg(s). if Mg is an oxidant, it must lose electrons. so the oxidation number must decrease from 0, which is nonsense for a metal. so yepp, most reactive metals are those which are the strongest reductants. in general however reactivity can encompass oxidation, I believe. if you take something like F2, it WILL want to grab electrons off anything in the immediate vicinity. so F2 is very reactive, although it's a strong oxidant not a strong reductant.

Thank you

[xenial]

Hey this is just a question about sig figs.

In the 2008 chemistry exam 1, question 8, we make the calculation

0.0105 - 0.00497
= 0.00553

Shouldn't this be given to two sig figs? I thought if you subtract two figures you give the answer to the number of decimal places of the figure with the smallest number of dps. Is this wrong? VCAA have given it to three, and have continued this on in subsequent calculations.

Also!

From memory, can anyone tell me how VCAA 2008 exam 1 compared to more recent chem exams? Was it harder or easier than your average VCAA exam? Just wondering what to expect on the day.

Thanks

[thushan]

You're absolutely right. The answer should be 1.5 x 10^2 g mol^-1. VCAA were a little slacky on that question there. But in subsequent years VCAA seems to have adhered to it, in fact it made a mention of the rule in the 2009 assessment report.

VCAA 2008 Exam 1 was on the relatively easy side. If you want something difficult, VCAA 2009 Exam 1 and VCAA 2011 Exam 1 would do it, as well as VCAA 2008 Exam 2 and VCAA 2012 Exam 2.

[Mafioso]

So if ALL my calculation questions are expressed to ridiculous sig figs, I only get 1 mark taken off for the whole paper right (assuming that all my answers were correct)?

[thushan]

So if ALL my calculation questions are expressed to ridiculous sig figs, I only get 1 mark taken off for the whole paper right (assuming that all my answers were correct)?

I know that they look at a particular question for sig figs, and they need to be fully accurate. However, I'm not sure how lenient they would be for the other questions.

[Edward21]

You're absolutely right. The answer should be 1.5 x 10^2 g mol^-1. VCAA were a little slacky on that question there. But in subsequent years VCAA seems to have adhered to it, in fact it made a mention of the rule in the 2009 assessment report.

VCAA 2008 Exam 1 was on the relatively easy side. If you want something difficult, VCAA 2009 Exam 1 and VCAA 2011 Exam 1 would do it, as well as VCAA 2008 Exam 2 and VCAA 2012 Exam 2.

I did the VCAA 2009s last night, I'm pretty sure they made a list of common chemistry mistakes and made sure each and every single one was scrutinised across both papers!! Although I feel like checkpoints ruined them for me, because I had already seen some of the questions so the 2011/12s will be the most accurate I guess