VCE Chemistry Question Thread

[lzxnl]

*Bump previous question Could someone answer this one?

With NMR, does it work like I think it does?
[Proton NMR]
We know that the majority of H nuclei will just be protons, and a minority will be deuterium.
Therefore, the majority of H nuclei will have an overall spin, hence overall magnetic field, and are therefore able to absorb energy to promote spin of a nucleon, and then to release that amount when the nucleon defaults back to its original aligned spin. slight mistake; energy is only absorbed when you flip the spin from aligning with the external magnetic field to being against it

We are told that "samples may be dissolved in a solvent which will not give a signal, such as D2O".
D2O is heavy water, i.e. the hydrogens in it are all deuterium.
But why does D2O not give out a signal?
Deuterium has two nucleons, therefore there is a possibility if the states are (u, d) that there will not be any net spin. My textbook says Does this mean that all nuclei with an even nucleon count have paired spins? Or could a deuterium nucleus have both nucleons as up/down (hence getting a net spin)? I wouldn't say deuterium doesn't give out a signal; rather, the amount of energy absorbed isn't of the same order of magnitude as that absorbed by a proton. For deuterium to be NMR active, it'd have to flip one nucleon's spin only, and given that deuterium nuclei in their ground state have opposite spins, making them have parallel spins isn't energetically favourable (think two magnets with the same poles directed at each other)
OR could a low energy (2x low energy nucleons) deuterium nucleus absorb one quantum of energy, promote one nucleon to a higher energy state, and then stop responding to EMR (as it is now stable, with no net spin)?

[13-C NMR]
Ok, and this one. We know that 12-C is the most common isotope, and that 13-C is less common.
From what I can understand, the fact that 13-C NMR can work is because of the large number of molecules in the sample. With billions of sample molecules in the sample, there will be many occurrences of 13-C in different parts of the molecule. 12-C atoms will not have net spin, and hence will not absorb EMR. However, 13-C will always have a net spin, so it will be continually absorbing/releasing EMR.
In short, if we could guarantee that all instances of carbon atoms in a sample were 12-C, 13-C NMR wouldn't work. Well yeah...C-13 NMR is there to analyse for C-13, not C-12

Have I got all this correct?

You do NOT need to understand how NMR works only until like second year uni will you need to know at all.

[lArcdeTriomphe]

With subshells I know they go in order of: s,p,d,f but what comes after these?
Are there any others?
Thanks

The subshells are s,p,d,f(,g,h...) (theoretically it goes on and on, but, in practice, we limit ourselves to s,pd,f). The only problem is that, in ground states of all atoms, we never reach the g or h subshells (at least, we haven't gotten up to it yet on the periodic table - why? because the Aufbau Principle). Basically, let's just say that anything beyond the f subshell is irrelevant (technically, g,h,... can come into play with atoms in excited states, but let's not worry about that.)

Just as a sidenote, subshells, structure of an atom, etc. aren't examinable and aren't taught in chemistry 3/4 (although they are covered in 1/2)

[paper-back]

Oh I see. Thank you RazzmeTazz and sin0001!

[paper-back]

Is the wavelength emitted when a metal is burnt different to the one it absorbs? or are they the same?

[Kel9901]

Is the wavelength emitted when a metal is burnt different to the one it absorbs? or are they the same?

For the purposes of VCE Chemistry they are the same (technically a metal can emit more wavelengths than it absorbs)

[nerdmmb]

So what is the purpose of spectroscopy? Obviously it's used to detect different wavelengths of light, but why?

Thanks!

[keltingmeith]

So what is the purpose of spectroscopy? Obviously it's used to detect different wavelengths of light, but why?

Thanks!

It's so that we can qualitatively (and quantitatively in most cases, to memory) figure out what it is in an unknown sample. An example of this from a lab I did last year, we had a set of cymbals and had to use UV-vis spectroscopy to figure out the metal content of the alloys that made up the cymbals. In another example, we had samples from a lake, and had to find the salt-content in the lake to see if it was safe for wildlife.

The amount of applications of spectroscopy is endless, but I think you can gather the general situations one might use it for based on the above. It's important to note that spectroscopy that rely on an emission or absorption spectrum can only be used on metal samples.

[thushan]

So what is the purpose of spectroscopy? Obviously it's used to detect different wavelengths of light, but why?
Thanks!

Basically it's because different substances absorb different wavelengths of light (good for qualitative analysis). Also, because the amount of light absorbed (measured in absorbance) is approximately proportional to the concentration of the substance (quantitative analysis).

[Zues]

if the question has 2SF, e.g. given something to 2 sf, but our final answer is 1234.568789, whats 2sf here?

also whn can the "rules for how many sf" change? e.g. if they have moles to 2sf, but also a dilution will the change in mole due to ratios and whatever effect how many moles you need to express answer?

[nerdmmb]

if the question has 2SF, e.g. given something to 2 sf, but our final answer is 1234.568789, whats 2sf here?

also whn can the "rules for how many sf" change? e.g. if they have moles to 2sf, but also a dilution will the change in mole due to ratios and whatever effect how many moles you need to express answer?

You would round your answer to the nearest hundred. So 1234.568789 would become 1200. Since the zeros after a non-zero number are not considered to be significant figures, you would leave it at that --> 1200  which is the same as 1.2 x 10^3

[Zues]

i think the zeros after a non zero are significant, 1200 = 4sf, but 0.0012 is 2sf, since the 0's are placeholders.

1.2 x 10^3 would be correct, i see.

also

1) what is the mole fraction of sulfuric acid in a solution made by adding 3.4 grams of sulfuric acid to 3500ml of water.
2) what will the volume of a 0.50 M solution be if it contains 25g of calcium hydroxide.

also how can you manipulte molar. its the mols per liter, so how could we use it in  n = m/M, is there restrictions? can we treat molar as mol?

[nerdmmb]

i think the zeros after a non zero are significant, 1200 = 4sf, but 0.0012 is 2sf, since the 0's are placeholders.

1.2 x 10^3 would be correct, i see.

also

1) what is the mole fraction of sulfuric acid in a solution made by adding 3.4 grams of sulfuric acid to 3500ml of water.
2) what will the volume of a 0.50 M solution be if it contains 25g of calcium hydroxide.

also how can you manipulte molar. its the mols per liter, so how could we use it in  n = m/M, is there restrictions? can we treat molar as mol?

Woops sorry Zeus! Glad you pointed it out

[nerdmmb]

How is percentage composition different from percentage by mass? Or are they the same?

[paper-back]

Does OH always produce a long broad trough in IR spectra? Why does it do this?

[paper-back]

In an IR spectra, when there is a carboxylic acid present the OH group is often merged to the C-H trough
However when there is no carboxylic acid, then the OH group is not merged to the C-H trough
Why is this?

Does anyone know any sites with good IR spectra questions?