VCE Chemistry Question Thread

[daniel.h]

You just haven't gone high enough - I did some maths, and uh, the coefficients for this one is stupidly high. If you're comfortable with maths (the content is essentially methods level, the actual solving of the maths approaches uni levels though), I can show you how to do this if you want, but realistically they won't give you such a stupid equation to balance at high school level (unless you manage to find an exam question that proves me wrong, I am a bit out of touch at this point)

Chemistry is a physical science - never be afraid to draw on real life experience, and I highly encourage you do this! Think about this - whenever you've seen a fire, have you seen a pool for fire starting to form?

The question relating to oxidation is a bit more complicated - it depends on how hot that oxidation reaction is, but typically the way to see is if one of your products is an oxide - eg, sodium oxide, carbon dioxide, potassium oxide, etc. If you see any of these, likely the oxidation reaction is hot enough to form steam instead of liquid water, unless the oxide being formed is a transition metal oxide. But even that isn't a particularly good rule to follow. Again, though, don't be scared to consider the context and draw on life experience - for example, if the oxidation reaction is in a battery in aqueous medium, it's not likely that you'll be producing steam, or the battery would explode. If, however, you're burning glucose for a calorimetry experiment, makes perfect sense that it wouldn't form liquid water.

What photo? I think I missed something, sorry.

Ah sorry, for Q3 here's the photo! Thanks a bunch man, really kind of you! I'll read your explanations and just send you the photo for now

[keltingmeith]

Ah sorry, for Q3 here's the photo! Thanks a bunch man, really kind of you! I'll read your explanations and just send you the photo for now

Yup, I see the problem now. You're right, glucose isn't the only product of this reaction - but this is a synthetic scheme. Typically in synthetic schemes like these, the chemist isn't particularly interested in the balancing of equations or by-products, they're only concerned with the focus of the scheme, and the rest are details to be figured out later. That's why the O2 isn't listed.

[daniel.h]

Yup, I see the problem now. You're right, glucose isn't the only product of this reaction - but this is a synthetic scheme. Typically in synthetic schemes like these, the chemist isn't particularly interested in the balancing of equations or by-products, they're only concerned with the focus of the scheme, and the rest are details to be figured out later. That's why the O2 isn't listed.

Ah yup, I guess glucose is more important to the chemists than O2! Cheers man, started questioning everything I learnt about glucose lol

For Q1, I was taught that the ratio of coefficients in formation of triglyceride was 3 fatty acids + 1 glycerol ---> 1 trigylceride + 3 H2O. Guess that's incorrect?

For Q2, yupp I never thought of it like that- I've never seen water as a liquid when I imagine a fire, so water would be a gas?

For Q2, When oxidation is specifically adding O2 (rather than removing H2 etc), those equations would also classify as combustion. So is that why you said that if products are an oxide (except for transition metal oxide), water would be a gas?

New question: In the attached photo of the biuret test, I understand that the peptide/ amide link is replaced with the biuret compound. But where does the CHR on the left of the peptide/ amide link go?

[keltingmeith]

Ah yup, I guess glucose is more important to the chemists than O2! Cheers man, started questioning everything I learnt about glucose lol

For Q1, I was taught that the ratio of coefficients in formation of triglyceride was 3 fatty acids + 1 glycerol ---> 1 trigylceride + 3 H2O. Guess that's incorrect?

Loool that's what I get for not reading the context - yeah, that absolutely should be what's happening. Check that you get the triglyceride formula correct, that'll be where your downfall is. At a quick glance, it looks like you've only given the formula for the addition of just one of the fatty acids, as opposed to all three attaching.

For Q2, yupp I never thought of it like that- I've never seen water as a liquid when I imagine a fire, so water would be a gas?

Correctamundo!

For Q2, When oxidation is specifically adding O2 (rather than removing H2 etc), those equations would also classify as combustion. So is that why you said that if products are an oxide (except for transition metal oxide), water would be a gas?

Not quite - the answer is honestly just thermodynamics, and starts to sneak into university level. The answer is actually just that the energy levels of most oxides is stupidly low compared to their precursors, and so oxidation reactions of them often result in flames occurring. The only reason I say not with transition metals isn't even that transition metal oxides won't cause flames, but just because they're more open to redox reactions, it's more likely that a random reaction referencing them won't produce steam than it is that the reaction will cause steam. Think of the oxidation of metallic iron to rust, as an example of this.

New question: In the attached photo of the biuret test, I understand that the peptide/ amide link is replaced with the biuret compound. But where does the CHR on the left of the peptide/ amide link go?

Leeel I'm not a protein chemist, gonna handball this to someone who knows the current curriculum 

[Azim.m]

Anyone have worked solutions for the 2017 sample exam (section B)?

[A TART]

Hi,

I'm unsure of the solution to this question:

It assumes that the k value is constant (even though it's reached a new equilibrium?) I tried to answer using an ICE table with a final concentration of 0.04M for ethylene.

Thanks

[Bri MT]

Hi,

I'm unsure of the solution to this question:

It assumes that the k value is constant (even though it's reached a new equilibrium?) I tried to answer using an ICE table with a final concentration of 0.04M for ethylene.

Thanks

K doesn't change unless the temperature is changed - even if the volume,  pressure, or amount of a reactant is changed

[tinashe91]

Hi everyone, i have been trying to figure out how to do the following questions, but i seem to have no luck. Any help would be greatly appreciated.

I need help with 11.101, 11.103 and 11.105

[daniel.h]

Hi everyone, i have been trying to figure out how to do the following questions, but i seem to have no luck. Any help would be greatly appreciated.

I need help with 11.101, 11.103 and 11.105

Hey is this year 11? Not sure but I thought ph was taken out of yr 12

[Bell9565]

Hey is this year 11? Not sure but I thought ph was taken out of yr 12

You still need to be able to do acid/base titrations in unit 4 I think and anyway, all 4 units are fair game for exam according to my teacher!

[Azim.m]

Isn’t both C and D correct?

[Yertle the Turtle]

Isn’t both C and D correct?

There is no such thing as a battery that can be used indefinitely. The course does cover a gradual loss of reusability, but I think that that would be seen as general knowledge, and therefore the answer would be D

[rani_b]

Hi, could someone help me with this question? I think I am doing all the right steps but my answer isn't matching up...

Water discharged from a mining plant contains silver ions present as silver nitrate (AgNO3). Excess potassium chromate (K2CrO4) solution is added to a 50.0g sample of water to precipitate the silver as silver chromate. The precipitate is heated to remove any water to produce 1.32g of silver chromate (Ag2CrO4). Calculate the percentage mass of silver in the water supply. (The molar mass of Ag2CrO4 is 331.7g/mol)

Thank you!

[Yertle the Turtle]

Hi, could someone help me with this question? I think I am doing all the right steps but my answer isn't matching up...

Water discharged from a mining plant contains silver ions present as silver nitrate (AgNO3). Excess potassium chromate (K2CrO4) solution is added to a 50.0g sample of water to precipitate the silver as silver chromate. The precipitate is heated to remove any water to produce 1.32g of silver chromate (Ag2CrO4). Calculate the percentage mass of silver in the water supply. (The molar mass of Ag2CrO4 is 331.7g/mol)

Thank you!

For this question you would use the formula n=m/M to find the number of moles of silver chromate in the sample. Then you find the mass of silver in that sample, given the mole ratio for silver of 1:2 in the following formula:

2(AgNO3) + K2CrO4 --> Ag2CrO4 + 2(KNO3)

Then you just divide that mass of silver (in grams) by 50, and multiply by 100 to find the mass percentage of silver.

Hope this helps, I haven't got a calc on me, but I think it will make more sense to you if you work it out numerically yourself.

[dimenc]

Hi everyone,
I know the exam is worth 60%, but i was wondering what a potential SS could be?
I don't have percents but I have grades
AOS 1
sac1 energy: B
sac2 rates and equillibirum: B+(I know that this was a high one)

AOS 2
sac 1 : food chem: A
sac 2 organic analysis A

Prac Poster A+