VCE Chemistry Question Thread

[Yacoubb]

How do i approach questions like these:
For which of the following is the sign of the enthalpy change different from the other three?

A) CaCO3(s) ---> CaO(s) + CO2(g)
B) Na(g) -----> Na+(g) + e-
C) CO2(s) ------> CO2(g) <- I think this one is endothermic?
D) 2Cl(g) ------> Cl2(g)

Hmm...
D? Would D be endothermic given we're forming bonds between the chlorine atoms to form chlorine gas, and this involves a net requirement of energy ~ hence endothermic.

I'd just look at net energy requirement/expenditure. Other opinions?

[Sanguinne]

Hmm...
D? Would D be endothermic given we're forming bonds between the chlorine atoms to form chlorine gas, and this involves a net requirement of energy ~ hence endothermic.

I'd just look at net energy requirement/expenditure. Other opinions?

The answer says that A,B,C require heat and that D is exothermic. This is STAV 2012 btw.

[Yacoubb]

The answer says that A,B,C require heat and that D is exothermic. This is STAV 2012 btw.

Haha woops - so D is the right answer, but I've given you the wrong description as to why it is D.

[lzxnl]

Hmm...
D? Would D be endothermic given we're forming bonds between the chlorine atoms to form chlorine gas, and this involves a net requirement of energy ~ hence endothermic.

I'd just look at net energy requirement/expenditure. Other opinions?

Does this 'forming bonds requiring energy' mentality come from biology? If so...you'd do best to stay away from it, because forming bonds nearly ALWAYS releases energy. Otherwise why on earth would you form those bonds? In biology, you don't get energy by breaking a P-O-P bond in ATP; rather, you get energy by breaking the high energy P-O-P bond and replacing it with two O-P bonds that are lower in energy. Breaking bonds doesn't give you energy; forming better ones does.

Chlorine atoms are so incredibly reactive it's ridiculous. You can have methane in an oxygen atmosphere and it won't react under standard conditions. Chlorine atoms, however, will spontaneously and rapidly react with methane. They consist of an atom with 7 valence electrons, and this atom is the third most electronegative in existence after oxygen and fluorine. It's going to react fast

[Yacoubb]

Does this 'forming bonds requiring energy' mentality come from biology? If so...you'd do best to stay away from it, because forming bonds nearly ALWAYS releases energy. Otherwise why on earth would you form those bonds? In biology, you don't get energy by breaking a P-O-P bond in ATP; rather, you get energy by breaking the high energy P-O-P bond and replacing it with two O-P bonds that are lower in energy. Breaking bonds doesn't give you energy; forming better ones does.

Chlorine atoms are so incredibly reactive it's ridiculous. You can have methane in an oxygen atmosphere and it won't react under standard conditions. Chlorine atoms, however, will spontaneously and rapidly react with methane. They consist of an atom with 7 valence electrons, and this atom is the third most electronegative in existence after oxygen and fluorine. It's going to react fast

Yeah this mentality is derived from Biology, haha! Thanks for the explanation lzxnl!

[Edward Elric]

Hey chem folks

Are things like electronic configurations, shells and subshells and fractional distillation still examinable? as I found some of these Q of the VCAA 2006 exam. Should I just ignore them, or actually pay attention to them? Thanks any help would be much appreciated.

[thushan]

Ignore - 2006 is part of the old study design.

[Edward Elric]

Ignore - 2006 is part of the old study design.

THANK YOU THUSAN, from what year onwards do you recommend, I do it from? Thanks

[Robert123]

THANK YOU THUSAN, from what year onwards do you recommend, I do it from? Thanks

Last year was the first year under the new study design. You should still do all the other vcaa exams just don't stress if you are given questions that are not on the current study design. There is a docent in the VCAA website that should out line all the changes that had occur from the previous study design

[Crystall97]

THANK YOU THUSAN, from what year onwards do you recommend, I do it from? Thanks

I believe anything from 2009 onwards is pretty decent, in terms of the VCAA (:

[Robert123]

Do we have to be aware of secondary and tertiary amine/amino function groups or just the primary ones?

[Yacoubb]

Do we have to be aware of secondary and tertiary amine/amino function groups or just the primary ones?

For amino acids, I'm pretty sure we only need to know about alpha amino acids. You may be asked to name an organic compound with an amine group in it, so would that be considered an awareness of secondary/tertiary amine functional groups?

[Brunette15]

If a substance absorbs less strongly to the stationary phase in TLC does it move further or less from the solvent front?

[BLACKCATT]

Assuming both the substance and stationary phase are polar, then the substance will travel up more from the origin, so higher Rf value.

[psyxwar]

For amino acids, I'm pretty sure we only need to know about alpha amino acids. You may be asked to name an organic compound with an amine group in it, so would that be considered an awareness of secondary/tertiary amine functional groups?

Alpha amino acids are the biologically important ones (ie. the 20 used in proteins) so yup, just those.

Secondary/tertiary amine functional groups should be fair game, at least in naming.