VCE Chemistry Question Thread

[Bri MT]

Hi! I was just wondering whether there was a dedicated post/resource to learn about all the chemical tests used for identifying substances (e.g. how to identify an ester or carboxylic acid, etc.?

Also, do we need to know the specific subunits that make up certain disaccharides (e.g. maltose)?

1. Not to my knowledge, but knowledge of reaction pathways should help you with this
2. Yes, but there's not much to remember so it's easy to learn

[Phenomenol]

Hi! I was just wondering whether there was a dedicated post/resource to learn about all the chemical tests used for identifying substances (e.g. how to identify an ester or carboxylic acid, etc.?

Did you do the STAV 2017 exam by any chance? A question in there asks for chemical tests for different functional groups. I don't particularly agree with their interpretation for some of the suggested "chemical tests" (e.g. smell, other physical observations) so if your query refers to that particular question on the STAV exam I wouldn't worry about it too much. It would have been clearer if the question had asked for an "observation to show evidence of product".

[Rieko Ioane]

Hi,

I've got a couple questions: https://imgur.com/a/BSaWN
Qb) Is my answer correct? The answer has 163L but I think they made a mistake when calculating the moles of decane and CO2

Q1a) If I wrote the equation like this without cancelling the spectator ions, would this be okay?

Q2b) Isn't this false? There aren't many amino acids that contain sulfur so I thought there wouldn't be many proteins that had this covalent bond?

Qd) Is the significant figures her 1? Since 1 tonne is one 1sf?

Q4a) How do we determine the state of H2O here? I wrote liquid but it was gaseous.

Q5e) I thought the concentration would just be 0.04M based off how the concentration of the other reactants change after t=30. We can see that C2H6 and H2 decrease/increase by 0.02M. Since the mole ratios are equal, I thought [C2H4] would decrease by 0.02M. Why does this not work here?

Thanks, (soz there's quite a few but most should be a quick answer)

[VanillaRice]

Hi,

I've got a couple questions: https://imgur.com/a/BSaWN
Qb) Is my answer correct? The answer has 163L but I think they made a mistake when calculating the moles of decane and CO2

Q1a) If I wrote the equation like this without cancelling the spectator ions, would this be okay?

Q2b) Isn't this false? There aren't many amino acids that contain sulfur so I thought there wouldn't be many proteins that had this covalent bond?

Qd) Is the significant figures her 1? Since 1 tonne is one 1sf?

Q4a) How do we determine the state of H2O here? I wrote liquid but it was gaseous.

Q5e) I thought the concentration would just be 0.04M based off how the concentration of the other reactants change after t=30. We can see that C2H6 and H2 decrease/increase by 0.02M. Since the mole ratios are equal, I thought [C2H4] would decrease by 0.02M. Why does this not work here?

Thanks, (soz there's quite a few but most should be a quick answer)

b) That looks fine to me (hopefully I'm not missing anything). Do the solutions just give the final answer, or is there working out as well?

1a) The question asks for a 'net ionic equation', that is, it should only involve the species that are actually involved in the reaction.

2b) Whether or not they are common, all interactions in the overall structure of a protein are important to its shape. In fact, changing even a single amino acid could have detrimental consequences. Also, consider that amino acids are not present in proteins in equal ratios - there are many proteins with a high proportion of cysteine residues.

d) I would also use 1 significant figure.

4a) Consider the temperature of combustion (which is quite high). In general, combustion reactions are written with gaseous water (steam).

5e) Not too sure on this one - it might have something to do with the concentration of ethene not actually being exactly 0.06M. Hopefully someone more knowledgeable can answer this question though!

Hope this helps

[Phenomenol]

Having been beaten to the punch to answer all your questions I will add extra comments to what has already been answered.

Qb) Is my answer correct? The answer has 163L but I think they made a mistake when calculating the moles of decane and CO2

Your answer is correct. They forgot to apply the stoichiometric ratio between decane and CO₂.

Q4a) How do we determine the state of H2O here? I wrote liquid but it was gaseous.

It depends on the temperature and pressure at which the combustion reaction is performed at, and in some situations it can be hard to judge these conditions. I think in most situations combustion is exothermic enough to produce the products at a high temperature - i.e. above boiling point and so water would be written as a gas. Having said that, a standard enthalpy of combustion is the enthalpy change for a combustion with all compounds in their standard state - i.e. water as a liquid.

Q5e) I thought the concentration would just be 0.04M based off how the concentration of the other reactants change after t=30. We can see that C2H6 and H2 decrease/increase by 0.02M. Since the mole ratios are equal, I thought [C2H4] would decrease by 0.02M. Why does this not work here?

Assuming you used the correct K_c value I think the problem lies with how you interpreted the graph. It was probably not right to assume any values based on the dotted lines that didn't line up with any axis label. It's hard to say exactly what's going on without the entire question. Perhaps you could provide the other sections?

However, I would think you should receive a mark for the correctly drawn curve.

[Rieko Ioane]

Thanks to both of you!

[Rieko Ioane]

Sorry, me again for Q5e) here's the full question which I'm still stuck on https://imgur.com/a/9chMh

The correct answer is 0.056M by calculating it with the new values in the Kc expression. But by using mole ratios to determine how the concentrations will change to get back to equilibrium, you get 0.04M.

also, from part b, you can see that the dotted lines between 2 labelled values is just the average of those.

[Phenomenol]

also, from part b, you can see that the dotted lines between 2 labelled values is just the average of those.

This is strange - they want you to interpret the given information differently for different parts of the question. The way the question is written is very sloppy, since if you take the values on the graph (correct stoichiometry) for t = 50 s you get a different equilibrium constant. Alternatively if you try to use the equilibrium constant from t = 25 s, you CANNOT use the values on the graph since you would end up with incorrect stoichiometry.

I wouldn't worry about it too much. There's no way to answer part e. without making a contradiction with something else in the situation.

[Butterflygirl]

Hey guys, do we need to know the steps for fractional distillation??

[simrat99]

hey guys,
can biodiesel be formed by a normal fatty acid and not a triglyceride?
thanks

[Phenomenol]

Hey guys, do we need to know the steps for fractional distillation??

It doesn't look like it's in the current study design. Perhaps it is sufficient to know that separation of components in crude oil is typically done with fractional distillation.

hey guys,
can biodiesel be formed by a normal fatty acid and not a triglyceride?
thanks

Yes. You can simply perform esterification from the fatty acid. In the case of the triglyceride, hydrolysis to the fatty acid first occurs, then esterification to afford the corresponding biodiesel.

[Butterflygirl]

Can someone please help me with this question (see attached).

Thanks!!

[Bri MT]

Can someone please help me with this question (see attached).

Thanks!!

1. Find the number of moles of cromium produced
2. Find the number of Faradays (mol of e-) needed per mole

[princessofpersia]

Hey guys, you know how the density of water is 0.997g/ml (from the data book) when do you use that in calculations?

[Rieko Ioane]

This is strange - they want you to interpret the given information differently for different parts of the question. The way the question is written is very sloppy, since if you take the values on the graph (correct stoichiometry) for t = 50 s you get a different equilibrium constant. Alternatively if you try to use the equilibrium constant from t = 25 s, you CANNOT use the values on the graph since you would end up with incorrect stoichiometry.

I wouldn't worry about it too much. There's no way to answer part e. without making a contradiction with something else in the situation.

Thanks.

Could you explain the contradiction that is occurring? I can't quite see it, you meant that the Kc value in part c is different to the one used in part e, I wrote the rounded value in part c. But then in part e, the unrounded value is used.