VCE Chemistry Question Thread

[keltingmeith]

The intermolecular forces in ethyl acetone are dispersion and hydrogen bonding and acetone has the same

Yeah, so the thing with solubility is that the highest determining factor for it is polarity. Hydrogen bonding is also important to consider, but dispersion forces are not. Dispersion forces are really weak, remember, so if something is polar, the energy for that is going to dominate in something dissolving. The other thing with hydrogen bonding, is you need to figure out if something is a hydrogen bonding donor or acceptor, or both. Water, for example, is both a hydrogen bond donor, and a hydrogen bond acceptor. Methanol is also both a hydrogen bond donor and acceptor. Ammonium, however, is only a hydrogen bond donor, and not a hydrogen bond acceptor. The reasons for these are to be a hydrogen bond donor, you need a hydrogen attached to your strongly electronegative atom (usually O and N). To be a hydrogen bond acceptor, you need a lone pair attached instead (which ammonium does not have). As another example, if you look at this molecule (which has a lone pair on the nitrogen, not shown), it is not a hydrogen bond donor, because it doesn't have a hydrogen attached to the nitrogen. It is, however, a nitrogen bond acceptor.

Knowing this now, would you still say ethyl acetate is going to be most soluble in acetone, or would you change your mind, and why?

[55232Jeremy]

Hi guys,
Just a quick question on reagents and catalysts for reaction pathways! Is it necessary to indicate states for them? given that the question is not asking to write a balanced equation. Some of the examiner reports indicate states in their answers such as NaOH(aq) or H+(aq)/Cr2O72-(aq) while other examiner reports just leave it simply as NaOH, H20, etc.

[colline]

Hi guys,
Just a quick question on reagents and catalysts for reaction pathways! Is it necessary to indicate states for them? given that the question is not asking to write a balanced equation. Some of the examiner reports indicate states in their answers such as NaOH(aq) or H+(aq)/Cr2O72-(aq) while other examiner reports just leave it simply as NaOH, H20, etc.

States are definitely required for reaction pathways. If I remember correctly, the only questions where states aren't required are electrolysis questions.

[55232Jeremy]

Hi all,

I'm a bit confused with Significant figure marks. Is it true you only lose one mark for a sig fig mistake on the exam? Also, if a question, for example, gave data with sig figs of 2 but then a subsequent question like 1b or 1c uses sig figs of 3 in that question, should I be using Sig figs of 2 or 3 in my answer?

[dylan.kumar21]

Can someone help me with question 5d of 2019 NHT?

[55232Jeremy]

Can someone help me with question 5d of 2019 NHT?

Yo, if you are talking about the Question,'Identify Electrode P as either the anode or the cathode.', the Answer is Anode as in the diagram the electrons are flowing from Electrode P and as we know, electrons always flow from the anode to the cathode or from the oxidation half cell to the reduction half cell. In terms of Eocell of the half reactions, the electrons will flow from the more negative half reaction to the more positive half reaction

[dylan.kumar21]

Yo, if you are talking about the Question,'Identify Electrode P as either the anode or the cathode.', the Answer is Anode as in the diagram the electrons are flowing from Electrode P and as we know, electrons always flow from the anode to the cathode or from the oxidation half cell to the reduction half cell. In terms of Eocell of the half reactions, the electrons will flow from the more negative half reaction to the more positive half reaction

thanks man but i was supposed to say Q5e soz :p

btw how are we supposed to identify the electrolyte for a fuel cell?

[ArtyDreams]

thanks man but i was supposed to say Q5e soz :p

btw how are we supposed to identify the electrolyte for a fuel cell?

In a fuel cell, you should know that Oxygen always gets REDUCED at the CATHODE. If you see on this question, Oxygen is getting reduced to O2- ions (as you can see in the diagram.)

Therefore, you should have an equation like this:
O₂ + 4e- ---> 2O^2-

Hope this helps!

[Coolgalbornin03Lo]

In a fuel cell, you should know that Oxygen always gets REDUCED at the CATHODE. If you see on this question, Oxygen is getting reduced to O2- ions (as you can see in the diagram.)

Therefore, you should have an equation like this:
O₂ + 4e- ---> 2O^2-

Hope this helps!

To add onto what Arty said just think oxygen= oxidant, so it’s reduced at cathode and oxidising another species(?)

[dylan.kumar21]

To add onto what Arty said just think oxygen= oxidant, so it’s reduced at cathode and oxidising another species(?)

Thank you both for your responses but shouldn't the half reaction be O2 + 4H+ 4e- -> 2H20 as that reaction is highest on the top left of the series?

[55232Jeremy]

Thank you both for your responses but shouldn't the half reaction be O2 + 4H+ 4e- -> 2H20 as that reaction is highest on the top left of the series?

A little hint about this question is that it is called an ‘oxide’ fuel cell indicating that the oxygen ion is important in its production. If oxide is used as a reactant at the anode, it is most likely a product at the cathode to keep it balanced.

After looking at the diagram and their answers, I cannot see why there is a hydrogen ion moving from left to right because this is neither a product at the anode nor is it a reactant at the cathode (according to their half equations). I think this is what causes the confusion and why you have given the answer you did (and I would have given the same). But for the future, make sure to look at the electrolyte and see whether it contains any oxide. e.g acidic fuel cell or alkaline fuel cell or oxide fuel cell.

[dylan.kumar21]

A little hint about this question is that it is called an ‘oxide’ fuel cell indicating that the oxygen ion is important in its production. If oxide is used as a reactant at the anode, it is most likely a product at the cathode to keep it balanced.

After looking at the diagram and their answers, I cannot see why there is a hydrogen ion moving from left to right because this is neither a product at the anode nor is it a reactant at the cathode (according to their half equations). I think this is what causes the confusion and why you have given the answer you did (and I would have given the same). But for the future, make sure to look at the electrolyte and see whether it contains any oxide. e.g acidic fuel cell or alkaline fuel cell or oxide fuel cell.

thank you, that clears it up

[Chocolatepistachio]

Hi all,

I'm a bit confused with Significant figure marks. Is it true you only lose one mark for a sig fig mistake on the exam? Also, if a question, for example, gave data with sig figs of 2 but then a subsequent question like 1b or 1c uses sig figs of 3 in that question, should I be using Sig figs of 2 or 3 in my answer?

Yes there is generally 1 mark for the answer and 1 mark for the correct sig figs . Your final answer would be using 2 sig figs

[55232Jeremy]

Yes there is generally 1 mark for the answer and 1 mark for the correct sig figs . Your final answer would be using 2 sig figs

Hi, thanks for replying. Do we only lose one mark for significant figures for the ENTIRE paper though? Or would I keep losing marks each question if I forget to use the right significant figures?

[Evolio]

Hi, thanks for replying. Do we only lose one mark for significant figures for the ENTIRE paper though? Or would I keep losing marks each question if I forget to use the right significant figures?

Hi!

I'm pretty sure we lose one mark for sig figs for the whole paper.