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November 29, 2020, 12:33:48 pm

Author Topic: VCE Chemistry Question Thread  (Read 1320218 times)  Share 

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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8880 on: October 28, 2020, 09:19:53 pm »
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i need help with this question  i am so confused

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8881 on: October 28, 2020, 09:32:42 pm »
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i need help with this question  i am so confused

That is a toughie! What have you managed to come up with so far?
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8882 on: October 28, 2020, 10:07:03 pm »
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There is a triple bond so it is an alkyne
There is an alcohol group on the first carbon

The longest carbon chain is 5?and I have no idea what that wavy line is

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8883 on: October 28, 2020, 10:13:51 pm »
+2
There is a triple bond so it is an alkyne
There is an alcohol group on the first carbon

The longest carbon chain is 5?and I have no idea what that wavy line is

Alright, so it LOOKS like the alcohol is on the first carbon, but the picture is actually being really tricky! The alcohol is on the second carbon. The way you're number this is actually as attached - not quite sure how you got 5? Also, don't forget that fluorine in there, as well as the methyl group!

I don't see any wavy lines, though, so not sure what you're talking about?
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8884 on: October 28, 2020, 10:39:47 pm »
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Is it
   5, floro methyl Hept -2 ynol

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8885 on: October 29, 2020, 12:00:51 am »
+5
Is it
   5, floro methyl Hept -2 ynol

Close! First, it's spelled fluoro. Next, you haven't told us the position of the methyl group. Finally, where you've put the 2 would mean that the alkyne starts at the second carbon - but it doesn't, it starts at the third carbon. You also need to indicate what carbon the alcohol is attached to. After you do that, you should be all good!
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Moonblossom

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Re: VCE Chemistry Question Thread
« Reply #8886 on: October 29, 2020, 07:47:04 am »
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So I actually answered a very similar question just a few pages ago. Have a quick squiz through that and let me know if you're still confused. As for whether R-groups should be joined - no, the peptide bond is ONLY ever made through the alpha amino and carboxyl groups. You are correct that those side-chains can react - they are chemically reactive. Hell, that's how disulphide bridges are made - by the R-groups of cysteine residues reacting with each other. But remember that proteins are made by enzyme catalysed reactions, and that enzymes are stupidly specific - because they are SO specific, they won't react with the terminal side chains. This IS a problem for other polymerization reactions, but that's university level chemistry there (and I only learned about and touched on it when I hit 3rd year).

That answer was pretty helpful but after reading this answer as well, I had another question. So if the variable groups don't react during the condensation reaction, how do they make react to make bonds between them during the secondary structure? I don't know how clear that was, sorry!

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8887 on: October 29, 2020, 05:07:15 pm »
+5
That answer was pretty helpful but after reading this answer as well, I had another question. So if the variable groups don't react during the condensation reaction, how do they make react to make bonds between them during the secondary structure? I don't know how clear that was, sorry!

Yeah - so remember the secondary structure does not include covalent bonds between amines or carboxyls. They're formed purely by hydrogen bonding. Yes, the amines and carboxylic acids are involved in that hydrogen bonding, but the energy required to form a hydrogen bond is MUCH less than the energy required for a reaction to occur. And again, it's not that those R groups are inert and can't react. They absolutely can (in fact, I have some colleagues who react the amino acid side-chains so that they can attach things to proteins). The fact is that there's not enough energy for them to react, and so they require either the body to heat up a lot (like, get to 100 degrees. Not going to happen, the person would die), or for a catalyst to become involved. There are enzymes around, but the enzymes are designed such as to not react with the R groups when making proteins. Hence, secondary structure is all hydrogen bonds - things that the R groups can form with a limited supply of energy - and not covalent bonds - which the R groups can't form with their limited supply of energy.

Hopefully that makes sense :)
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ErnieTheBirdi

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Re: VCE Chemistry Question Thread
« Reply #8888 on: October 30, 2020, 10:06:45 am »
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Someone please help me! I honestly just cannot do well in Chem. And when I say do well I mean pass.
I do all the revision and everything, and I do all the work. I get the questions correct when doing revision but somehow I do it all the same way and yet I still fail. Someone please help me. I need to do this subject as I want to do medicine. I donít know what to do, I will talk to my teacher about this later today. I just need some advice on what to do, and what I can do to not fail / do well / ace 3/4 chemistry next year. Thanks in advance everyone !

sazabo_c

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Re: VCE Chemistry Question Thread
« Reply #8889 on: October 30, 2020, 11:07:49 am »
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I definitely agree that rote learning is not the answer although for me personally, it found it really beneficial to categorise the reactions and just write a list (previously all the reactions felt muddled in my brain and it was difficult to reach to certain information when I needed it. After having written it out I found it much easier to organise the content in my brain and thus gain a better understanding of it. This won't work for everyone but maybe it'll work for you). Although I've written out the products of the reactions in this list, as keltingmeith indicated, there's no need to rote learn this as it should just come naturally with the understanding of each reaction. Anyway see below...

Reaction pathways
SUBSTITUTION reactions (no C=C bonds)
- alkane + halogen: UV light/heat --> haloalkane
- haloalkane + NaOH --> alcohol
- haloalkane + ammonia --> amine

ADDITION reactions (C=C involved)
- alkene + hydrogen gas: metal catalyst--> alkane
- alkene + halogen --> haloalkane
- alkene + water: H3PO4/Heat --> alcohol
- alkene/alkyne molecules can form a polymer in presence of a catalyst

OXIDATION reactions --> alcohols oxidised in presence of H+/Cr2O72- or H+/MnO4-
- Primary alcohols --> aldehyde --> carboxylic acid
- Secondary alcohols --> ketone
- Tertiary alcohols --> don't oxidise

CONDENSATION reaction
- carboxylic acid + alcohol: H2SO4 (l) --> ester (esterification)

HYDROLYSIS reaction (pretty much opposite of above)
- ester + water: catalyst --> carboxylic acid + alcohol

As keltingmeith, no use rote memorising this but writing it out like this helps me (and it also shows that there actually aren't that many reactions you need to know). Another tip is when given a question figure out your reactant and what is the desired product. Generally once you've sorted these out in your brain, the required reaction comes naturally.

Yeah I completely agree with you and keltingmeith in regard to understanding to pathways. I think this is honestly the key to succeeding in chem on a general level. That resource is super helpful also so thank you. I shall sit down and ensure I understand it all. Thanks a million both of you :)

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8890 on: October 30, 2020, 03:05:22 pm »
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For this question why is it b
Which of the following solvents do you predict ethyl acetate(ethyl ethanoate) would have the highest solubility
A water
B acetone( 2-propanone)
C hexane

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8891 on: October 30, 2020, 04:36:24 pm »
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For this question why is it b
Which of the following solvents do you predict ethyl acetate(ethyl ethanoate) would have the highest solubility
A water
B acetone( 2-propanone)
C hexane


Interesting! Very outside 3/4 chem, so don't stress if you're struggling with this. I'm curious, have you given this question much thought? What do you think the answer might be, and why?
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8892 on: October 30, 2020, 04:45:47 pm »
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The intermolecular forces in ethyl acetone are dispersion and hydrogen bonding and acetone has the same

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8893 on: October 30, 2020, 05:24:54 pm »
+6
The intermolecular forces in ethyl acetone are dispersion and hydrogen bonding and acetone has the same

Yeah, so the thing with solubility is that the highest determining factor for it is polarity. Hydrogen bonding is also important to consider, but dispersion forces are not. Dispersion forces are really weak, remember, so if something is polar, the energy for that is going to dominate in something dissolving. The other thing with hydrogen bonding, is you need to figure out if something is a hydrogen bonding donor or acceptor, or both. Water, for example, is both a hydrogen bond donor, and a hydrogen bond acceptor. Methanol is also both a hydrogen bond donor and acceptor. Ammonium, however, is only a hydrogen bond donor, and not a hydrogen bond acceptor. The reasons for these are to be a hydrogen bond donor, you need a hydrogen attached to your strongly electronegative atom (usually O and N). To be a hydrogen bond acceptor, you need a lone pair attached instead (which ammonium does not have). As another example, if you look at this molecule (which has a lone pair on the nitrogen, not shown), it is not a hydrogen bond donor, because it doesn't have a hydrogen attached to the nitrogen. It is, however, a nitrogen bond acceptor.

Knowing this now, would you still say ethyl acetate is going to be most soluble in acetone, or would you change your mind, and why?
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55232Jeremy

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Re: VCE Chemistry Question Thread
« Reply #8894 on: November 05, 2020, 07:43:37 pm »
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Hi guys,
Just a quick question on reagents and catalysts for reaction pathways! Is it necessary to indicate states for them? given that the question is not asking to write a balanced equation. Some of the examiner reports indicate states in their answers such as NaOH(aq) or H+(aq)/Cr2O72-(aq) while other examiner reports just leave it simply as NaOH, H20, etc.