VCE Chemistry Question Thread

[keltingmeith]

Oh okay thanks! I was just wondering since one of my textbook questions was: Barium and calcium are both Group II elements. Account for the observation that samples of barium and calcium compounds produce different colours when they are held in a flame.

And the answer was: As a consequence of the presence of different numbers of protons in the nuclei of barium and calcium atoms, the energies of electrons in the shells of the two atoms are not the same. Different amounts of energy are released when an electron moves from a higher energy level to a lower one in a barium atom compared with when the same process occurs in a calcium atom. These different energies are seen as light of different colours.

Does this correspond with MO theory, or..?

Sort of. It's not a complete explanation, but VCAA won't ask this anyway.

[thushan]

Oh okay thanks! I was just wondering since one of my textbook questions was: Barium and calcium are both Group II elements. Account for the observation that samples of barium and calcium compounds produce different colours when they are held in a flame.

And the answer was: As a consequence of the presence of different numbers of protons in the nuclei of barium and calcium atoms, the energies of electrons in the shells of the two atoms are not the same. Different amounts of energy are released when an electron moves from a higher energy level to a lower one in a barium atom compared with when the same process occurs in a calcium atom. These different energies are seen as light of different colours.

Does this correspond with MO theory, or..?

There is a simplified explanation which I would say you SHOULD be able to know for VCAA - barium and calcium have different electronic configurations and different nuclear charges. Hence, the distance between the shells will be different, therefore the energy differences between them, thus the frequencies of light emitted by the excited electrons.

MO theory will give a more correct explanation, but this is well out of the course.

[grannysmith]

MO theory will give a more correct explanation, but this is well out of the course.

This reminds me of revising for something rather important which I cannot quite identify at this very moment :p

By the way Thushan, does the thermodynamics covered in CHM1011 help with unit 4 chemistry?

[lzxnl]

Can someone please help me out!

 I don't get why alkanoic acids are generally more soluble than alkanols. I know it's something to do with the fact that the COOH group of the alkanoic acid contains the double covalent bond between C and O, but I'm a little unsure.

Additionally, in my chemistry textbook it is written: "Alkanoic acids are stronger acids than their corresponding alcohols because the —OH group is more polarised in the —COOH group by the presence of the highly electronegative O atom of C=O. This =O atom attracts the electrons towards it and away from the —OH group. Therefore, the H is more weakly bonded to O and is more easily donated."

Since the electrons are attracted towards the C=O and away from the -OH group, does this mean electrostatic forces of attraction in the -OH decrease, resulting in H being more weakly bonded to O and thus more easily able to be donated?

Cheers

What happens is that if you have more electron density dragged out of the carbon, it will then attract electrons from the OH group. The oxygen atom, now having less electron density, pulls electron density from the hydrogen, reducing the electron density in the actual bond. As a result, the OH bond weakens a lot. You can see this in that the alkanol OH stretch is at a higher IR absorption than that of the acid OH stretch.

This reminds me of revising for something rather important which I cannot quite identify at this very moment :p

By the way Thushan, does the thermodynamics covered in CHM1011 help with unit 4 chemistry?

Assuming Monash CHM1011 is anything at all like first year chem at UoM, maybe a bit. I mean you'll get better with adding/manipulating enthalpies and you'll understand the true meaning of electrode potentials, so that will help.

[keltingmeith]

This reminds me of revising for something rather important which I cannot quite identify at this very moment :p

By the way Thushan, does the thermodynamics covered in CHM1011 help with unit 4 chemistry?

(having actually done CHM1011... ) Not really. It's not like it'll be useless, but unit 4 is less thermodynamics and more enthalpy/making energy. You'll find the equilibria section quite useful for unit 4, though - the formula you learn aren't all used, but quite a few of the equations are very insightful to some contextual stuff that VCE ignores. Also next semester in CHM1022, the organic stuff basically works as revision to unit 3 organic chem, so look forward to that.

[Macka598]

Hi, im just wondering is Ethanoic acid and Methyl Methanoate considered structural isomers or not??

[keltingmeith]

Hi, im just wondering is Ethanoic acid and Methyl Methanoate considered structural isomers or not??

Yes. They have the same molecular formula, this makes them structural isomers - that's all you need to be a structural isomer.

[IndefatigableLover]

From STAV 2008:
Propan-1-01 (Molar mass = 60) undergoes complete fragmentation in a mass spectrometer. What is the m/e value of the base peak in its mass spectrum?

The answer gives me a m/e value of 31 however the answer which reads:
"The base peak (that representing the most abundant fragment formed and recorded by a mass spectrometer) for a small primary alcohol corresponds to M-18 (due to the loss of water) or has a m/e value of 31 corresponding to the CH3O+ species" which I don't entirely understand (I get how they get 31 but why choose that rather than say 29 for C2H5)?

[lzxnl]

From STAV 2008:
Propan-1-01 (Molar mass = 60) undergoes complete fragmentation in a mass spectrometer. What is the m/e value of the base peak in its mass spectrum?

The answer gives me a m/e value of 31 however the answer which reads:
"The base peak (that representing the most abundant fragment formed and recorded by a mass spectrometer) for a small primary alcohol corresponds to M-18 (due to the loss of water) or has a m/e value of 31 corresponding to the CH3O+ species" which I don't entirely understand (I get how they get 31 but why choose that rather than say 29 for C2H5)?

You do NOT need to know this for VCE. Don't worry. I hear you do mass spec analysis in a third year chemistry subject but I'm not taking that subject.
You also don't need to know which fragments are stable.

[RazzMeTazz]

In UV-Visible spectroscopy, since the chosen wavelength is one that is highly absorbed by the chemical you are analysing for and not so highly absorbed by the other irrelevant matter in your substance, how would you find out the absorption for the  different wavelengths by the other irrelevant matter in your substance?

Thanks

[keltingmeith]

In UV-Visible spectroscopy, since the chosen wavelength is one that is highly absorbed by the chemical you are analysing for and not so highly absorbed by the other irrelevant matter in your substance, how would you find out the absorption for the  different wavelengths by the other irrelevant matter in your substance?

Thanks

Why would you want to?

[wunderkind52]

In UV-Visible spectroscopy, since the chosen wavelength is one that is highly absorbed by the chemical you are analysing for and not so highly absorbed by the other irrelevant matter in your substance, how would you find out the absorption for the  different wavelengths by the other irrelevant matter in your substance?

Thanks

As Euler said, if you're determining the concentration of some sample, then you really don't care about what other impurities are in your sample - that's the whole point of UV-Vis. Of course, you could just change the wavelength and chart the absorption of different wavelengths, but that's quite pointless and redundant.

[I am a unicorn]

Hello
Why doesn't NMR spectroscopy work for nuclei with an even number of nucleons?

[RazzMeTazz]

Can someone please explain why homonuclear molecules such as O2 and N2 don't absorbed infrared radiation?

Thankyou!

[Sundal]

What is meant by "For a molecule to absorb infrared radiation the bending or stretching vibrations must change the overall dipole moment of the molecule." By dipole moment is that referring to polarity?

Any help appreciated.