VCE Chemistry Question Thread

[hums_student]

can someone real quick explain the bromine test and how it can be used to distinguish alkenes and alkanes

ty

Bromine water has a brown-ish colour; when you mix it with alkanes it won't react (so the colour will stay brown-ish) but it reacts with alkenes so the solution loses its colour and turns clear.

[turtlebanana]

can someone real quick explain the bromine test and how it can be used to distinguish alkenes and alkanes

ty

Bromine solution (aqueous) has an orange colour.
Bromine reacts relatively quickly with alkenes.
When you react something with Bromine, if it loses its colour (goes frown brown to clear), then it means an addition reaction has occurred - thus proving the presence of a carbon-carbon double bond (i.e. presence of alkene).
However, when an alkane for example is added to aqueous bromine, no addition reaction occurs, and thus there is no change in colour (i.e. bromine solution remains orange)

[Flyfishy]

do i need to know about quaternary structure of proteins and if so what about them?

[turtlebanana]

do i need to know about quaternary structure of proteins and if so what about them?

Yes, you need to know all 4 levels of protein structure (primary, secondary, tertiary, quaternary).
The quaternary structure of a protein is basically the structure formed when 2 or more polypeptide chains come together to form a large complex.
The interactions/bonds in a quaternary structure are the same as those in the tertiary structure (eg. dispersion, dipole-dipole, hydrogen, ionic, covalent) which occur between polypeptide chains (like how they occur between R groups in the tertiary structure).

[iroze007]

Small question about chromatography: if you have a polar stationary phase and a non-polar mobile phase, then will Ethanol or Butanol have a higher retention time?

Thank you

[hums_student]

do i need to know about quaternary structure of proteins and if so what about them?

Adding onto turtlebanana's really great response, the main thing VCAA tests you on is about how they're effected by temperature change (denaturation).

[turtlebanana]

In the data booklet (page 15):

For the C NMR data,
Does anyone know why the last one is a ketone? R₂C=O with a chemical shift of 205-220.

Why does the C=O seem like it's at the end of the chain when it's meant to be in between two carbons?
Pretty sure the C=O is in between two R groups, but i find it visually confusing.

[turtlebanana]

Small question about chromatography: if you have a polar stationary phase and a non-polar mobile phase, then will Ethanol or Butanol have a higher retention time?

Thank you

Since the mobile phase is non-polar, then the most non-polar molecule will travel slowest through the column.
In this case, ethanol and butanol are both polar molecules. However, butanol is more non-polar because of the additional C-H atoms.
Therefore, butanol will have the higher retention time.

[Swizzle10]

hi
does anyone have any electrolysis/galvanic tips for when in the exam room? they confuse me xD thank you

[Bri MT]

So zwitterions don't necessarily always have an overall charge of zero? Cos I've been taught if we are to draw out the zwitterion of an amino acid (one that has either a basic or acidic group) we ensure that it still has an overall charge of zero

Sorry I should have been more clear. The overall charge should be zero. I was trying to explain why they made the mistake but in all the definitions I've seen a zwitterion has no net charge.

[turtlebanana]

hi
does anyone have any electrolysis/galvanic for when in the exam room? they confuse me xD thank you

When referring to the electrochemical series:
- Electrolysis (increasing gradient)
- Galvanic (decreasing gradient)

Also:
Just remember that for galvanic cells, the sign/polarity of the anode is always negative. Then when it comes to electrolysis, you just switch the sign/polarity (i.e. anode becomes positive).

BUT no matter what always remember that:
- oxidation always occurs at the anode (AN OX) and reduction always occurs at the cathode (RED CAT)
- electrons flow from anode to cathode

[Bri MT]

In the data booklet (page 15):

For the C NMR data,
Does anyone know why the last one is a ketone? R₂C=O with a chemical shift of 205-220.

Why does the C=O seem like it's at the end of the chain when it's meant to be in between two carbons?
Pretty sure the C=O is in between two R groups, but i find it visually confusing.

They probably want to show that the oxygen isn't bonded to an R group. If the carbon wasn't bonded to two groups it would just be R rather than having the 2 there

Since the mobile phase is non-polar, then the most non-polar molecule will travel slowest through the column.
In this case, ethanol and butanol are both polar molecules. However, butanol is more non-polar because of the additional C-H atoms.
Therefore, butanol will have the higher retention time.

Increased affinity with the mobile phase decreases retention time whereas increased affinity with the stationary phase decreases retention time. Aside from that, your logic is sound

hi
does anyone have any electrolysis/galvanic tips for when in the exam room? they confuse me xD thank you

I found it useful to use diagrams (backwards Z for galvanic) to help me remember how to interpret the electrochemical series

[colline]

I found it useful to use diagrams (backwards Z for galvanic) to help me remember how to interpret the electrochemical series

Please tell me I'm not the only person who stupidly forgets which way a Z goes whenever I look at that page in the data book

Regarding the "extrapolation" of the graph for poorly insulated calorimeters, how accurate is the process, and do we calculate ΔT using the extrapolated line instead of the actual data? Thanks.

[Bri MT]

Please tell me I'm not the only person who stupidly forgets which way a Z goes whenever I look at that page in the data book

Regarding the "extrapolation" of the graph for poorly insulated calorimeters, how accurate is the process, and do we calculate ΔT using the extrapolated line instead of the actual data? Thanks.

You should've seen the confusion I had initially when dealing with the QCE data book (they order their electrochemical series the opposite way to VCE)

I can't confirm anything but my suspicion would be that they would accept either using the regression line or not. The accuracy of the process will depend on the level of insulation, time over which the reaction takes place etc. and the extrapolation will generally overstate the heat loss.

Imo the safest way to approach this type of situation if it comes up is to state the assumptions your using and do your calculation based on that

[pugs]

You should've seen the confusion I had initially when dealing with the QCE data book (they order their electrochemical series the opposite way to VCE)

I can't confirm anything but my suspicion would be that they would accept either using the regression line or not. The accuracy of the process will depend on the level of insulation, time over which the reaction takes place etc. and the extrapolation will generally overstate the heat loss.

Imo the safest way to approach this type of situation if it comes up is to state the assumptions your using and do your calculation based on that

yeah i think that they would accept either

i was just doing a question with a calibration curve that could have been extrapolated, but VCAA used the value from the curve :// so i think it's safe to just go with whatever value they give you from the curve

also! i don't know if anyone will be able to answer this before the exam lol, but for Cl(g) + 2e- <–> 2Cl-(aq) and O2(g) + 4H+(aq) + 4e- <–> 2H2O(l) in the electrochemical series, if a solution of HCl is in question and its concentration is 1.0M, is it necessary to write reduction equations for both Cl and H2O? Our teacher taught us to write both equations if the concentration of HCl is between 0.5M and 2M, but i'm not sure if it's required in the exam?

thank you!