VCE Chemistry Question Thread

[keltingmeith]

Wait wait wait what? The middle CH2 hydrogens all have different proton environments...for starters the CH3-CH2 protons (the CH2 ones) will be split into 12 peaks whereas the other CH2 ones will be split into 9.

... I think you'll need to explain this one to me, because I can't see any environment that'll split even close to 9, let alone 12.

[thushan]

Guys, don't sweat it.

The splitting rules can be quite tough to navigate, but you're only expected to know the n+1 rule at the year 12 level (NOT the multiplicity rule, which I mention briefly below).

You learn that n = number of H atoms on neighbouring carbons (within 3 bonds of the H atom responsible for the peak), but the caveat is that those H atoms must be in the same environment as one another.

For example, if we take CH3-CH2-COOH, the H atoms that are bolded have 2 H atoms within three bonds (on the neighbouring carbon). Both those H atoms are in the same environment as one another. The CH2 protons have 3 H atoms on the neighbouring carbon (within 3 bonds), all which are on the same environment. You may ask "why is the H in COOH not counted as a neighbour?" The reason is because the H atom there is not within 3 bonds of the H atom in the CH2 group. In any case, the O atom bonded to the H prevents any "communication" between the H in COOH and any other H atom.

Now let's take CH3-CH2-CH2-COOH and focus on the bolded protons. Here, the 5 H atoms are in different environments. This requires the use of the multiplicity rule, which is not taught in year 12. FYI the multiplicity rule states that if n1 is the number of H atoms on one neighbouring carbon, and n2 is the number of H atoms on the other neighbouring carbon, and the two groups of H atoms are in different environments to one another, then the concerned peak will be split into (n1 + 1)(n2 + 1) peaks. In this example, n1 = 3 and n2 = 2. Therefore, the peak corresponding to the bolded protons will be split into 4 x 3 = 12.

HOWEVER you will not be given NMR spectra of substances where you need to apply the multiplicity rule at VCE level.

[Rishi97]

Hey everyone

If anyone wants some trial chemistry exams (non-VCAA) , pls inbox me

Rishi97

[lzxnl]

In any case, the O atom bonded to the H prevents any "communication" between the H in COOH and any other H atom.

You sure you'd even have COOH and not COOD if you're using a deuterated solvent to make the NMR?

... I think you'll need to explain this one to me, because I can't see any environment that'll split even close to 9, let alone 12.

Essentially, if you have the CH3-CH2-CH2 protons, for instance, the middle CH2 is split into 3 by the CH2 on its right. But this triplet is itself split into four by the CH3 on its right. Hence the 12 peaks. In practice they're going to look REALLY messy.

HOWEVER you will not be given NMR spectra of substances where you need to apply the multiplicity rule at VCE level.

Yeah...wait until maybe second or third year uni for that. I've had to analyse a few doublets of doublets but that's it; analysing a triplet of quartets requires really high resolution NMR to be able to resolve the individual peaks. On most machines you just get a huge blob of peaks.

[cosine]

A certain brand of washing soda contains partially hydrated sodium carbonate solid. A 0.300g sample completely reacts with 20.00mL of 0.250M hydrochloric acid.

How do I write an equation for this?

[Nora1234]

Hi guys can someone please help with this question

Explain why GLC is suitable for the determination of blood alcohol levels but not the separation of sugar samples?

Thanks

[lzxnl]

A certain brand of washing soda contains partially hydrated sodium carbonate solid. A 0.300g sample completely reacts with 20.00mL of 0.250M hydrochloric acid.

How do I write an equation for this?

It's acid-base with sodium carbonate as the base and hydrochloric acid as the acid.
The HCl is going to act as a proton donor and the carbonate is going to accept the protons. When carbonate accepts two protons, it decomposes into water and CO2. Is that enough info?

Hi guys can someone please help with this question

Explain why GLC is suitable for the determination of blood alcohol levels but not the separation of sugar samples?

Thanks

GC requires vaporising things. Sugars can be quite large and if you try to vaporise them, the sugar polymers may thermally decompose, which isn't good. GC isn't possible for large molecules that may decompose.
On the other hand, ethanol is quite a small molecule that can be vaporised without decomposition.

[cosine]

It's acid-base with sodium carbonate as the base and hydrochloric acid as the acid.
The HCl is going to act as a proton donor and the carbonate is going to accept the protons. When carbonate accepts two protons, it decomposes into water and CO2. Is that enough info?

GC requires vaporising things. Sugars can be quite large and if you try to vaporise them, the sugar polymers may thermally decompose, which isn't good. GC isn't possible for large molecules that may decompose.
On the other hand, ethanol is quite a small molecule that can be vaporised without decomposition.

Okay I give up can you show me the equation, I still don't get it

[Rishi97]

Okay I give up can you show me the equation, I still don't get it

Na2CO3(s) + 2HCl(aq) --> Na2Cl2(aq) + CO2(g) +H2O(l)

I'm pretty sure it's this...when any acid reacts with a carbonate, carbon dioxide and water are always produced along with a salt

[cosine]

Na2CO3(s) + 2HCl(aq) --> Na2Cl2(aq) + CO2(g) +H2O(l)

I'm pretty sure it's this...when any acid reacts with a carbonate, carbon dioxide and water are always produced along with a salt

Wait but it said hydrous Sodium Carbonate? This was the part i was stuck at haha

[keltingmeith]

Wait but it said hydrous Sodium Carbonate? This was the part i was stuck at haha

So the state on that should be aq (also, fix-up from Rishi, because it should be 2NaCl not Na2Cl2):

Na2CO3(aq) + 2HCl(aq) --> 2NaCl(aq) + CO2(g) +H2O(l)

[Rishi97]

So the state on that should be aq (also, fix-up from Rishi, because it should be 2NaCl not Na2Cl2):

Na2CO3(aq) + 2HCl(aq) --> 2NaCl(aq) + CO2(g) +H2O(l)

oh yeh lol haha time to go to bed :p

[tiff_tiff]

can someone please help me here

how does addition of NaOH affect pH of water? How will i explain this in terms of H+, b/c it is the H+ that affects the pH?

[lzxnl]

Water dissociates naturally into H+ and OH- ions. However, this process is reversible.
If you add NaOH, this dissociates into Na+ and OH- ions. The OH- ions then react with H+ to reform water.

[IndefatigableLover]

can someone please help me here

how does addition of NaOH affect pH of water? How will i explain this in terms of H+, b/c it is the H+ that affects the pH?

NaOH is a strong base and when added to water, it will turn more basic than acidic (so its pH will be greater than 7). This is due to there being less H+ ions and more OH- ions present in the solution of NaOH and H2O (where the Na+ and OH- ions are floating around in water which naturally dissociates into H+ and OH- ions).

EDIT: Beaten by lzxnl