VCE Chemistry Question Thread

[Owlbird83]

Plenty of people at my school use a clear plastic pocket to put their pens in if they forget to bring a ziploc bag. I'm not sure what the examiners would do if you start highlighting it though...

I agree, the examiners always made us leave plastic pockets on the floor we weren't allowed to have them on the desk, so highlighting wouldn't be allowed.

[Chocolatepistachio]

Which of the following solutions would have the highest pH?
A 0.150mol-1 solution of barium hydroxide
B 0.150 moL-1 solution of sodium hydroxide
C 0.150 mol-1 solution of sulfuric acid
D 0.150 moL-1 solution of ammonia

Would this be c or would it be a if you do 14-pOH but it’s not an acid

[Billuminati]

Which of the following solutions would have the highest pH?
A 0.150mol-1 solution of barium hydroxide
B 0.150 moL-1 solution of sodium hydroxide
C 0.150 mol-1 solution of sulfuric acid
D 0.150 moL-1 solution of ammonia

Would this be c or would it be a if you do 14-pOH but it’s not an acid

This question is no longer examinable for VCE because acid base equilibria ie Ka, Kb, pKa and pKb have been taken out of the course since 2017. You'll need a Ka or Kb value to figure this out. C is incorrect because the acid has the LOWEST pH.

[Chocolatepistachio]

How do you remember that if delta G is negative the reaction is spontaneous and it’s exergonic

[Sine]

How do you remember that if delta G is negative the reaction is spontaneous and it’s exergonic

For the most part I think it is something that you should see enough times that you remember it
I think some people think of the graph as a hill so with the delta G < 0 you go downhill. So it is much easier to go down a hill than up.

[Billuminati]

How do you remember that if delta G is negative the reaction is spontaneous and it’s exergonic

Sine explained it very well, however I don't think you need to know about ΔG as I didn't know what that was until 1st year uni chem. At VCE level I'm pretty sure you're supposed to explain it with ΔH (change in enthalpy) which follows the same "up the hill down the hill" rule Sine mentioned. ΔG calculations include change in enthalpy, but they also take into account something called entropy (ΔS). The formula for ΔG is ΔG= ΔH- TΔS where T is the temperature in Kelvins. As you can see, the more exothermic ie more negative ΔH value a reaction has, the more negative ΔG is and hence the more likely it is to be spontaneous given that we ignore entropy effects.

[eloisegrace]

Hello AN,

I was wondering if esters were only able to be made from primary alcohols, or if secondary and tertiary alcohols can undergo esterification?

Thanks!

Eloise

[Billuminati]

Hello AN,

I was wondering if esters were only able to be made from primary alcohols, or if secondary and tertiary alcohols can undergo esterification?

Thanks!

Eloise

Esterification can certainly happen with 2° and 3° alcohols, you'll end up with a branched ester. You can even have a lactone (a cyclical ester) forming if you have a molecule that has a COOH and an OH within itself (self esterification). I should highlight that 3° alcohol esterification takes forever because it's very crowded and has trouble accessing the acid.

[aspiringantelope]

Hi,
Is there a quick method of determining the number of structural isomers a compound has, for example: \(C_{4}H_{9}OH\) Butanol?
Or we do have to draw all the ones we can think of and then find out? Like is it similar to math where the exponential hints the number of solutions? If so, would this be different for different functional groups?
Thanks!

edit:
Thanks for the response Billuminati! (didn't want to make excessive posts)

[Billuminati]

Hi,
Is there a quick method of determining the number of structural isomers a compound has, for example: \(C_{4}H_{9}OH\) Butanol?
Or we do have to draw all the ones we can think of and then find out? Like is it similar to math where the exponential hints the number of solutions? If so, would this be different for different functional groups?
Thanks!

Unfortunately there are no formulas that allow you to determine the number of structural isomers for any given molecule, so it's back to good old trial and error. They won't be so mean on the final exam to ask you to draw all structural isomers of something, but I know a lot of SACs are dodgy and may very well ask you to.

If you're interested, there's actually a formula for determining the number of OPTICAL isomers. It's given by 2^(n-1) where n is the number of chiral (or stereogenic) centres. The only exception to this rule is in meso compounds which are achiral due to having 2 axes of symmetry, but still possess chiral centres.

[Chocolatepistachio]

would this be right for the stereoisomer

[Billuminati]

would this be right for the stereoisomer

Don't use Z and E notation, please use cis and trans respectively for these in high school.

The trans isomer should look like this because your highest 2 priorities (the F and COOH) are opposite each other and you must show the groups with the correct trigonal planar, sp2 hybridised geometry (ie bond angles of 120°) around that double bond: https://drive.google.com/file/d/1XGaUh8H2ToUzKq9vrBGOawaCjMQxlKke/view?usp=drivesdk

[Chocolatepistachio]

wait what happened to the double bonded oxygen

 but if you're using E and Z how would you draw E of that molecule

[Billuminati]

wait what happened to the double bonded oxygen

 but if you're using E and Z how would you draw E of that molecule

In skeletal formulas it's customary to condense a carboxyl into COOH (without showing the double bond), but in high school you're expected to draw all bonds. The red structure I drew is the E isomer, cuz E= trans and Z= cis

[sweetcheeks]

This compound is a good example of the issues with the cis/trans nomenclature. Depending on which chemist you talk to, you will get a different answer. For the Z configuration (the original molecule) some might say it is the cis isomer because the two groups of highest priority are on the same side. However, other chemists may say that it is the trans isomer because the two longest chains are on opposite sides.