VCE Chemistry Question Thread

[MisterNeo]

Any idea what type of graph is best to represent titrations?

Hi!
Titrations are graphed using equivalence curves (aka titration curves or pH curves) to show the neutralisation being completed.
This website has a good explanation of equivalence curves.

[amigos]

Question: do only alkanes have chain isomers?

[Syndicate]

Question: do only alkanes have chain isomers?

No.
Chain isomers are molecules that have an alkyl group attached to the primary chain. It can be alcohols, esters, amines etc... (ie. 3-methyl butanoic acid)

[MisterNeo]

Question: do only alkanes have chain isomers?

Straight-chain alkanes do have isomers.

-An isomer of butane is methylpropane.
-An isomer of pentane is 2-methylbutane, or ethylpropane.
Alkenes and alkynes also have chain isomers, but that becomes more complex with the addition of groups such as chlorine, benzene rings, etc.

[Lavar Big BBB Balls]

Hi,

For this http://imgur.com/a/Mofm0
Can someone confirm that the answer is 55.7% instead of 55.9%?

Thanks

[BlinkieBill]

Hey,
I am still unsure of how much about pH we need to know about in this new study design. What types of questions could it come up in?

[Sirius]

Hi,
For this http://imgur.com/a/Mofm0
Can someone confirm that the answer is 55.7% instead of 55.9%?
Thanks

Hey 
The answer varies due to a very small rounding issue when solving the question!. It depends on whether we round the number of mole of propan-1-ol (underlined below) to 3 sig fig or more. If I worked with 1.33 mol of propan-1-ol then percentage yield will be 55.88%. But like me you probably did not round off this number ( i.e  you used 1.3333 or 4/3) therefore you got 55.7%. Rounding always gets me  so I never round off my answers during working.

Working out :
n(C₃H₇OH) = 80/60 = 1.333 mol
Mole ratio is 1:1 therefore n(C₂H₅COOH) = 1.3333 mol
Now m(C₂H₅COOH) = n * M = 1.3333 * 74 = 98.664 g    Ahhh... Here is the problem. Now if you used 1.3333 *74 you will get 98.642 grams (percentage yield would be 55.74%) But if you round the number of mol to 1.33 * 74 it will give you 98.43 grams meaning percentage yield would be 55.87%. Therefore your answer would be different by a very small amount.

Wouldn't mind being corrected here but I believe 55.7% would be a more accurate answer.   
Hope this helps!

[vasuk]

my school provided a powerpoint about organic isomers etc which said this: that cis isomers have a higher boiling point but also a lower melting point in comparison to trans isomers.

For lower melting point it says that due to the shape of cis isomers the molecules cannot pack as well intermolecular forces are not working as effectively as they should be so less energy is needed to separate the molecules, but for higher boiling point it says the stronger intermolecular forces are the reason that more energy is needed to break these bonds.

I'm having trouble differentiating these two and understanding why shape and packing matters more for melting but then the strength of the intermolecular forces matters more for boiling? could someone please explain this? I understand the concept of packing and intermolecular forces but don't understand why it's one way and then the other for melting vs boiling.

thank you

[cloudz99]

I'm getting quite low scores in chemistry (50s) and I require a study score of 30 for my course. I have numerous notes, resources and a tutor (both private and tutesmart) but nothing seems to be sticking in my head. Practice exams and corrections don't seem to be helping either. Any tips on how I can try to improve my grades to at least 70s?

[Bri MT]

I'm getting quite low scores in chemistry (50s) and I require a study score of 30 for my course. I have numerous notes, resources and a tutor (both private and tutesmart) but nothing seems to be sticking in my head. Practice exams and corrections don't seem to be helping either. Any tips on how I can try to improve my grades to at least 70s?

What specifically are you having trouble with?
Understanding? Remembering? Applying?

[Sine]

I'm getting quite low scores in chemistry (50s) and I require a study score of 30 for my course. I have numerous notes, resources and a tutor (both private and tutesmart) but nothing seems to be sticking in my head. Practice exams and corrections don't seem to be helping either. Any tips on how I can try to improve my grades to at least 70s?

hmm how have you been learning your content?
and how have you been utilising your resources?

 The best way would be to learn via first principles so that you actually understand everything and don't really have to memorise much since you can derive any concept in your head.
 
Even if practice exams aren't "working" just do a tonne of them since a lot of the content on your final exam will be stuff you have seen before (maybe in a different scenario).

Since you have a tutor, utilise them to their full extent by asking any type of questions even if they may sound simple.

[pmmenotes]

Why do stereoisomers have different chemical/physical properties

[cloudz99]

hmm how have you been learning your content?
and how have you been utilising your resources?

 The best way would be to learn via first principles so that you actually understand everything and don't really have to memorise much since you can derive any concept in your head.
 
Even if practice exams aren't "working" just do a tonne of them since a lot of the content on your final exam will be stuff you have seen before (maybe in a different scenario).

Since you have a tutor, utilise them to their full extent by asking any type of questions even if they may sound simple.

I write my notes and examples and have cue cards with concepts using class and textbook notes, I also have a poster/mind map linking all the ideas in the unit. I'll attempt practice exams open book to boost my confidence and still end up with a less than reasonable score. When I see my tutor she tells me how to answer a question and what is required to get the marks and I'll write it down but once I get home it all leaves my head. Maybe it's just a learning issue :/

[VanillaRice]

Why do stereoisomers have different chemical/physical properties

In general terms, since their structures are different (atoms are arranged differently), the interactions between and with other molecules will also be different.

[Bri MT]

Why do stereoisomers have different chemical/physical properties

I'm going to try and explain this with the example of 1,2dichloroethane. The molecule will be slightly negative where the chlorine atoms are. If this is the cis isomer, both chlorine atoms are on the same side so this will mean that overall the molecule will be more negative on that side. Whereas in the trans isomer the chlorine atoms will be on different sides and so the resulting polarity of the molecule will be quite different.

Hope this helps