VCE Chemistry Question Thread

[dpwad3]

I'm confused about this question from the 2014 exam, its a 3 marker about why the graphite anode cannot be replaced with iron anode.
I understand that Fe is a stronger reductant than Cl- and will be preferentially oxidised, but why is it that the iron produced at the anode will eventually migrate to the CATHODE, and thus since Fe2+ ions are stronger oxidants than Mg2+ ionds they would be preferentially reduced (only 3% of the state got this point). So, should we always consider this possibility of migration when answering such type of questions?

In the diagram for this question the half cells in the electrolytic cell are not separated, therefore migration can occur. Make sure you always look at the diagram and any information in the question, if the half cells are separated migration cannot occur.

[turtlebanana]

Hey guys!

For HNMR, does an ester always have a singlet for the (n+1) rule?

Due to the carbon (in the ester bond) that is attached to the single oxygen atom?

[3086]

Hey!
In HPLC, why is it important to use standard solutions of many concentrations to construct a calibration graph?
Why is extrapolation outside the range of the calibration graph 'unreliable'?

As standard solutions are known concentrations and hence it is much easier to draw graphs and is, therefore, more reliable.

Thanks!

[KiNSKi01]

Do we never consider fuels in an aqueous form?

I'm asking cos I answered a question about viscosityof biodiesel in terms of hydrogen bonding between the ester group and hydrogen of water molecules and mark scheme only discussed in terms of dipole dipole interactions, this brought to my attention that I don't think I have come across a VCAA question where fuels are in the aqueous state

[johnnyboi]

How would the retention time of butane compare to that of methyl-propane? Given that they are structural isomers, would the linearity of each molecule be considered? if this is the case, would butane (being more linear) have a lower Rt?

[Matthew_Whelan]

Do we never consider fuels in an aqueous form?

I'm asking cos I answered a question about viscosityof biodiesel in terms of hydrogen bonding between the ester group and hydrogen of water molecules and mark scheme only discussed in terms of dipole dipole interactions, this brought to my attention that I don't think I have come across a VCAA question where fuels are in the aqueous state

I’m pretty sure fuels are only gases or liquids since this is the state that ones like methane, ethanol exist as. I could be wrong though.

[alanihale]

Hey guys!

For HNMR, does an ester always have a singlet for the (n+1) rule?

Due to the carbon (in the ester bond) that is attached to the single oxygen atom?

I don't think so. HNMR splitting (the n+1 rule) is based on the number of hydrogens on the neighbouring carbons. The carbon attached to the single oxygen (I assume you mean from the alcohol side) may have no neighbouring carbons with hydrogens bonded to them, hence producing a singlet; however, it may also be bonded further (eg. ethyl group) and therefore it would produce a doublet as oppose to a singlet. (Carboxylic acids will always have a singlet in the HNMR due to the lone hydrogen on the -COOH group).
Hope this helps!! (Someone please correct me if I'm wrong)

[turtlebanana]

Anyone know why ''grease'' affects the reaction of galvanic cells?

Eg. Greasy electrode (like copper)

[turtlebanana]

I don't think so. HNMR splitting (the n+1 rule) is based on the number of hydrogens on the neighbouring carbons. The carbon attached to the single oxygen (I assume you mean from the alcohol side) may have no neighbouring carbons with hydrogens bonded to them, hence producing a singlet; however, it may also be bonded further (eg. ethyl group) and therefore it would produce a doublet as oppose to a singlet. (Carboxylic acids will always have a singlet in the HNMR due to the lone hydrogen on the -COOH group).
Hope this helps!! (Someone please correct me if I'm wrong)

Makes much more sense now
But if the (n+1) rule is based on number of Hydrogens on neighbouring carbons, why does the lone H in the carboxylic acid produce a singlet? It is not a carbon atom and has no carbon atoms attached to it?

When you use the (n+1) rule, don't you look at a specific carbon atom, then the number of hydrogens on neighbouring/adjacent carbon atoms?

Why then does the lone H produce a singlet, if you are supposed to look at a carbon atom instead when using the (n+1) rule? Thanks

[alanihale]

Makes much more sense now
But if the (n+1) rule is based on number of Hydrogens on neighbouring carbons, why does the lone H in the carboxylic acid produce a singlet? It is not a carbon atom and has no carbon atoms attached to it?

It produces a singlet because it doesn't have any neighbouring carbon atoms w/ hydrogens bonded to it. Remember, it's the n+1 rule, so no neighbours (n=0) becomes 0+1 a.k.a a singlet. If you've got no neighbours, you're a singlet, if you've got two neighbouring hydrogens, you'll have triplet etc.

The lone hydrogen will be picked up on the HNMR because it's own hydrogen environment. The signal produced doesn't depend on the carbons, just the different hydrogen environments. It's the splitting of the peaks which are determined by the neighbouring hydrogens on the carbons.

I really hope this makes sense! I'm not the best at clear explanations!

When you use the (n+1) rule, don't you look at a specific carbon atom, then the number of hydrogens on neighbouring/adjacent carbon atoms?

Why then does the lone H produce a singlet, if you are supposed to look at a carbon atom instead when using the (n+1) rule? Thanks

You use the carbon atom, yes, to identify possible neighbours for peak splitting; however, that's it. The fact that the hydrogen is bonded to an oxygen rather than a carbon does not mean it won't produce a peak. It's still a different hydrogen environment. It's the fact that it has no neighbouring carbons which leads to it just being a singlet.

[3086]

Do we never consider fuels in an aqueous form?

I'm asking cos I answered a question about viscosityof biodiesel in terms of hydrogen bonding between the ester group and hydrogen of water molecules and mark scheme only discussed in terms of dipole dipole interactions, this brought to my attention that I don't think I have come across a VCAA question where fuels are in the aqueous state

well in the 2017 vcaa exam it specifically specified that fuels cannot be labelled as aqueous. This is because in aqueous reactions water is involved, therefore if we have a fuel mixed with water then we could write aqueous. Although fuels are never mixed with water as this would make them less efficient when they burn. So we always write liquid or gas. Plus vcaa will never give u a fuel u don't know the state of, as all common fuels and their States are in the data book. Those otherwise will be specified.

[3086]

Anyone know why ''grease'' affects the reaction of galvanic cells?

Eg. Greasy electrode (like copper)

grease adds impurities to the electrode and hence makes reactions less efficient due to interference with the grease.

[turtlebanana]

It produces a singlet because it doesn't have any neighbouring carbon atoms w/ hydrogens bonded to it. Remember, it's the n+1 rule, so no neighbours (n=0) becomes 0+1 a.k.a a singlet. If you've got no neighbours, you're a singlet, if you've got two neighbouring hydrogens, you'll have triplet etc.

The lone hydrogen will be picked up on the HNMR because it's own hydrogen environment. The signal produced doesn't depend on the carbons, just the different hydrogen environments. It's the splitting of the peaks which are determined by the neighbouring hydrogens on the carbons.

I really hope this makes sense! I'm not the best at clear explanations! You use the carbon atom, yes, to identify possible neighbours for peak splitting; however, that's it. The fact that the hydrogen is bonded to an oxygen rather than a carbon does not mean it won't produce a peak. It's still a different hydrogen environment. It's the fact that it has no neighbouring carbons which leads to it just being a singlet.

Ahh thank you! So as long as there is a hydrogen, regardless of it is bonded to a carbon, then it is considered a hydrogen environment right?
I thought the hydrogen needed to be bonded to a carbon. Thanks

[alanihale]

Ahh thank you! So as long as there is a hydrogen, regardless of it is bonded to a carbon, then it is considered a hydrogen environment right?

Yep! (Just keep a note of whether or not hydrogens are in the same environment or not. Whilst they may be on separate carbons, they may be in the same environment as in symmetrical molecules)

[anoushka_iyer]

Hello!
Do energy density, energy content and heat content need a sign? What is the difference between these terms and enthalpy, heat of combustion, molar heat of combustion? Also, in our calculations, do we write 'energy density= x MJ/Kg' OR '∆H=  x MJ/Kg' if the question specifically asks for energy density?

Thank you so much, y'all are life savers <3 <3