VCE Chemistry Question Thread

[Bri MT]

You need to consider both yield and collision theory. The reverse reaction will be favoured by Le Chatlier's principal and both reactions will increase in their rate (it'll just be that the reverse one increases more)

[EllingtonFeint]

Hey, I've got lots of questions! Sorry!

*Q: Each compound in a particular selection of a primary alcohol, a secondary alcohol and a tertiary alcohol has the same no of carbon atoms per molecule.
The tertiary alcohol
A Has a diff molecular formula to the primary alcohol
B Has a diff empirical formula to the secondary alcohol
C Can be oxidised partially to give an aldehyde or ketone
D Has 3 C atoms and an-OH group bonded to one C atom

Correct answer: A

How can you determine this?? Surely the formulas are different?


*Are we expected to know the P1 x V1 /T1 = P2 x V2/T2 formula?

*Also, do we need to memorise the M = (m x Vm) / V formula??

*If an equilibrium equation is not shown as exo/endothermic, would the yield increase with a higher temperature? Surely, only rate is increased?

*Q: the reaction bw CH2CH2OH and Cu at 300 degrees C prods:
A ethane
B ethanoic acid
C ether
D methyl ester

Um. I don't understand this. Does the Cu react w the OH or something? (answer is B)


I think that's all for now! Would be so grateful for any help!

[Erutepa]

Hey, I've got lots of questions! Sorry!

*Q: Each compound in a particular selection of a primary alcohol, a secondary alcohol and a tertiary alcohol has the same no of carbon atoms per molecule.
The tertiary alcohol
A Has a diff molecular formula to the primary alcohol
B Has a diff empirical formula to the secondary alcohol
C Can be oxidised partially to give an aldehyde or ketone
D Has 3 C atoms and an-OH group bonded to one C atom

Correct answer: A
How can you determine this?? Surely the formulas are different?

Do you mean the correct answer is D?

Primary alcohols have and OH that is bonded to a carbon which is bonded to only 1 other carbon
secondary alcohols have and OH that is bonded to a carbon which is bonded to 2 other carbons
tertirary alcohols have and OH that is bonded to a carbon which is bonded to 3 other carbons
It might help to visulise these differences in the image here:

Spoiler

Hence D would be correct.

The reason A is incorrect is that all of these alcohols do have the same molecular formula. Knowing that each alcohol has the same carbons, all the the tertiary, secondary and primary forms of the alcohol are really just different isomers and will have the same molecular formula. You can also come to this conclusion by drawing a primary, seconday and tertiary alcohol with the same number of carbons, and deriving the molecular formula for each one. You will find that the molecular formula in each case is the same.

*Are we expected to know the P1 x V1 /T1 = P2 x V2/T2 formula?

You are not expected to know that as a formula, however you are expected to know that PV=nRT which you can transpose to give nR=PV/T. If the moles of gas are constant, and a change in occuring in the system, then you can say that PV/T (before change) = PV/T (after change) since nR is constant.
So that formula is really just an application of PV=nRT. If it helps you to remember it, then I would reccomend memorising it, but its not essential since you can just derive it if neccasary.

*Also, do we need to memorise the M = (m x Vm) / V formula??

I have never used this formula - its really just combining the two formulas M=m/n and V=n*Vm
You don't need to remember the formula specifically - you can just use the two seperately. I recomend doing this becuase If you start using derived formulas and make mistakes, I am not sure you will get the marks for working out --> its best to not overcomplicate things.

*If an equilibrium equation is not shown as exo/endothermic, would the yield increase with a higher temperature? Surely, only rate is increased?

When they ask this question they should indicate that the reaction is exothermic/endothermic. However the question might excpect you to determine that the reaction is endothermic/exothermic from the nature of the reaction. However, I don't this for sure since I am lacking the context of the question.

*Q: the reaction bw CH2CH2OH and Cu at 300 degrees C prods:
A ethane
B ethanoic acid
C ether
D methyl ester

Um. I don't understand this. Does the Cu react w the OH or something? (answer is B)

I think this last question is just wrong. From what I can find on the internet, these conditions will dehydrogenate the ethanol to produce an aldehyde. firstly this is a reaction pathway which you don't need to know, and secondaly the given isn't correct.
I don't think you should worry youreself about this question.

[EllingtonFeint]

Thank you so much!! ^ 
 I have a few more questions!
 
So,
-Are enantiomers the same as optical isomers or are they a subset (?) ?

-What happens to amino acids in low/high pH solutions?

-^Are these zwittterions or is that something different?

-Do we need to know about anhydrous primary standards?

-What is the formula(s) for fatty acids. Like CnHn=??? And it would be different for fatty acids made from like double bonded alcohols, right?

Thank you

[turtlebanana]

Question related to flashpoints:

What is the difference between the flashpoint and autoignition point?

Is it just that the flashpoint requires an ignition source whereas the autoignition point does not?

So for both, it is the lowest temperature at which a liquid will form sufficient vapour to ignite (one requires a source such as a flame whereas the other does not?) Or for the autoigntion, is it strictly for ''fuels'' only, and the lowest temperature at which they will spontaneously ignite without an ignition source?

[Erutepa]

Thank you so much!! ^ 
 I have a few more questions!
 
So,
-Are enantiomers the same as optical isomers or are they a subset (?) ?

-What happens to amino acids in low/high pH solutions?

-^Are these zwittterions or is that something different?

-Do we need to know about anhydrous primary standards?

-What is the formula(s) for fatty acids. Like CnHn=??? And it would be different for fatty acids made from like double bonded alcohols, right?

Thank you

1)enatiomers are a pair of optical isomers

2)
In low pH solution (acidic) --> lots of H+ ions --> amine group will accept H+ and becomes positively charged
In high pH solution (basic) --> lots of OH- ions --> carboxylic group will donate H+ to OH- ions and become negatively charged

Spoiler

3) you need to know what anhydrous means (a compound that doesn't contain water) and you need to know that the primary standard is the solution of known concentration which is used (in titrations) to determine the concentration of an unknown solution
I don't think theres anything you need to know about anhydrous primary standars in particular though

4) The general formula for a saturated fatty acid is CnH2n+1COOH. For every 2 less hydrogens in the molecule, there will be 1 double bond.
So if a hypothetical fatty acid had the formula:
C20H41COOH --> you could tell that it was saturated since it obeys the above general formula fo a saturated fatty acid
If it had the formula C20H39COOH --> you could tell that it has 1 double bond since it has 2 hydrogens less than the general formula for a saturated fatty acid
If it had the formula C20H37COOH --> it would have 2 double bonds since it has 4 hydrogens less than the general formula for a saturated fatty acid

Question related to flashpoints:

What is the difference between the flashpoint and autoignition point?

Is it just that the flashpoint requires an ignition source whereas the autoignition point does not?

So for both, it is the lowest temperature at which a liquid will form sufficient vapour to ignite (one requires a source such as a flame whereas the other does not?) Or for the autoigntion, is it strictly for ''fuels'' only, and the lowest temperature at which they will spontaneously ignite without an ignition source?

Flashpoint is the point is the minimum temperature at which vapours of the fuel will ignitie in the presence of an ignition source while the autoingition point is the temperature at which a fuel will spontaneously ignite without the presence of an ignition source.

[Bri MT]

Irt standards the only thing I can think of for that is that it's important that you know how hydrated or not the standard is otherwise you will be unable to accurately determine the number of moles from the mass.

[pugs]

hey! i just have a few general questions that i was hoping people would be able to help me clarify

- for questions that have several parts (part a, b, c etc.) do you use rounded values from a previous part (eg use the rounded value from part a) or do we continue to use the decimal value in our calculator? i know you have to continue to use the decimal value for methods & the rounded one for further, but what about chem?

- when do you use PV=nRT and n=V/Vm? are they interchangeable? (because for many of the questions i've done, it seems like either could work, but the answers for each formula have been different – these questions have been either at STP or SLC btw which is why the second equation could also work)

- do significant figures include temperature values? in some questions that i've done, they haven't included the temperature values (usually 2 figures) as significant figures even though you use them in the calculations (if so, what else do sig figs not include?)

- for electrolysis, if a solution of 1.0M HCl is in question and we are asked to write the oxidation reaction, do we need to write the equations for both Cl and H2O?

(and just wanted to check one last one: is the current that flows through an electrolytic circuit the same value for all half-cells if multiple half-cells are included in the circuit? (eg you don't need to add up the Q value for each half cell individually))

thank you so so much! apologies for the influx of questions!!

[turtlebanana]

Question about titration!

Do we have to know what is the ''most suitable indicator'' for different reactions?

For example:
The most suitable indicator for a titration of NaOH against benzoic acid, C₆H₅COOH, is:
A. bromophenol blue
B. methyl orange
C. thymol blue
D. phenol red

I don't recall learning how to do this in class. If this is still in the study design, how do we know which indicator to use? How do we know if a certain compound is a ''weak'' or ''strong'' acid/base?

[jollyboat]

I'm a bit confused about batteries! Was wondering if anyone could help.

I thought that galvanic cells/batteries needed to have the oxidant and reductant seperate so that they don't react directly and are instead forced to react through the wire. I thought that this was the reason galvanic cells need two beakers/half cells.

But apparently batteries often don't have a separator to keep the two half equations apart? There are lots of VCAA questions where batteries appear to only have a single electrolyte solution that isn't separated into two seperate solutions.

Thanks for any replies!!!

[turtlebanana]

Do primary alcohols always oxidise to aldehydes first, and then carboxylic acids after? (For reaction pathways)

Or can a primary alcohol go straight to a carboxylic acid?

[sweetcheeks]

I'm a bit confused about batteries! Was wondering if anyone could help.

I thought that galvanic cells/batteries needed to have the oxidant and reductant seperate so that they don't react directly and are instead forced to react through the wire. I thought that this was the reason galvanic cells need two beakers/half cells.

But apparently batteries often don't have a separator to keep the two half equations apart? There are lots of VCAA questions where batteries appear to only have a single electrolyte solution that isn't separated into two seperate solutions.

Thanks for any replies!!!

Do you have examples where they use the same solution? Generally you would need some way of separating the electrolytes so you don't get spontaneous reactions.

Do primary alcohols always oxidise to aldehydes first, and then carboxylic acids after? (For reaction pathways)

Or can a primary alcohol go straight to a carboxylic acid?

In the reaction pathway you need to know for VCE, they always go to the aldehyde first.

[jollyboat]

Do you have examples where they use the same solution? Generally you would need some way of separating the electrolytes so you don't get spontaneous reactions.

The question attached was the one that confused me. There's a similar one in Q9 of the VCAA 2019 northern hemisphere exam I think

[Stargirl113]

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?

[anoushka_iyer]

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'?



Thanks!