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May 12, 2021, 12:41:35 am

Author Topic: VCE Chemistry Question Thread  (Read 1474888 times)  Share 

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Corey King

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Re: VCE Chemistry Question Thread
« Reply #9030 on: February 10, 2021, 06:38:59 pm »
0
you always use the lowest number of sig figs that were used in the calculation so if you used 55 in the calculation i would assume it would still be 2 sig figs not 3. Not sure

Yes, she stated this also. However, she said that if we use the data book provided for this years exam (we are using the data book for practise questions right now as well), then we dont cpunt the dig figs from the data book. I could get her to elaborate on why.


ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9031 on: February 10, 2021, 07:47:10 pm »
+1
If I remember correctly I don't think you use already calculated/standard values when looking at sig figs. I'm not sure how this affects the accuracy though.

For example, if you have a calculation that uses the molar mass of an element, this value does not come into play when you look at the sig figs. You use the given values in the stem of the question.

Hope this helps.

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #9032 on: February 10, 2021, 10:25:27 pm »
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would this not have any stereoisomers, doesn't it have to have a chiral carbon to have stereoisomers

Owlbird83

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Re: VCE Chemistry Question Thread
« Reply #9033 on: February 11, 2021, 08:04:55 am »
+4
would this not have any stereoisomers, doesn't it have to have a chiral carbon to have stereoisomers

Under the category of stereoisomers, there's optical isomers (which have the chiral carbon, enantiomers), and geometric isomers (cis-trans, double bond).

So this molecule would have a stereoisomer where the double bond has rotated (rotation of double bond requires a lot more energy than rotation around a signal bond, so it mostly stays how it is I think unless something is done to it). So right now it's in the cis form because the high priority (heavier) atoms (the oxygens ) are on the same side of the double bond (looking from an imaginary line drawn down double bond). The isomer would be a trans version, so the oxygens would be on opposite sides (so if the OH, and CH3 on the right side swapped places).
Hope that makes sense!
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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #9034 on: February 11, 2021, 09:33:20 am »
+5
Top level answer from Owlbird, just want to clarify: it's not that rotating around double bonds takes more energy, you just actually can't rotate around double bonds because of the way they're made (requires first or second year uni Chem to properly explain, but think of it like one bond would have to physically go through the second bond if you tried to twist the molecule to make the rotation happen)

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #9035 on: February 11, 2021, 04:15:25 pm »
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thanks,
for this why is one of the CHCH3CH3 groups ranked higher and the other lower if they are the same thing shouldn't they both be ranked higher priority or does it not matter which one is higher and which one is lower

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #9036 on: February 11, 2021, 05:09:44 pm »
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Hey guys, I was wondering why biodiesel produces less energy than petrodiesel? I've read it's because biodiesel is more hygroscopic and partially oxidised but how does this relate to the energy content?

Owlbird83

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Re: VCE Chemistry Question Thread
« Reply #9037 on: February 11, 2021, 06:08:48 pm »
+8
thanks,
for this why is one of the CHCH3CH3 groups ranked higher and the other lower if they are the same thing shouldn't they both be ranked higher priority or does it not matter which one is higher and which one is lower
You rank the high vs low priority by comparing the two groups bonded to each carbon (do each side at a time and ignore the other), so on the left it can be higher than the other group bonded, and right it can be lower than the other group on that side. (random analogy I just thought of: It's like how maybe the same person can be less skilled when compared to one person, and then in another situation they are more skilled than another person, despite being the same person).

Hey guys, I was wondering why biodiesel produces less energy than petrodiesel? I've read it's because biodiesel is more hygroscopic and partially oxidised but how does this relate to the energy content?
I am not confident, but my understanding is that because it's hygroscopic it absorbs some water/moisture, which would lower some of the energy that can be used because some energy of combustion is absorbed by the water. And I think some of the energy in the molecule is released in the partial oxidation so there's less left in the system when it's biodiesel.
Would prefer someone who knows more to answer this in case I'm wrong haha ^
« Last Edit: February 11, 2021, 06:10:39 pm by Owlbird83 »
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #9038 on: February 12, 2021, 10:23:15 pm »
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How would you draw a potential energy diagram for a reaction that is endothermic and has 3 steps with the 2nd step being the rate determining step . is it supposed to look like this

Jinju-san

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Re: VCE Chemistry Question Thread
« Reply #9039 on: February 13, 2021, 11:27:28 am »
+2
How would you draw a potential energy diagram for a reaction that is endothermic and has 3 steps with the 2nd step being the rate determining step . is it supposed to look like this

Hi Chocolatepistachio,
That looks about right, but I think the tail at the very end of the graph should be a bit higher up, so that you can clearly tell that the energy of the products is higher than the energy of the reactants (and that it is therefore an endothermic reaction). Also, since you said that the 2nd step was the rate determining step (hence the slowest step that requires the greatest amount of activation energy), the 2nd peak should be a little bit higher.

I actually havenít learnt a lot about energy profile diagrams, so someone please correct me if Iím wrong here..

Hope this helps!  :)

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #9040 on: February 14, 2021, 06:29:54 pm »
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Thanks

if someone could help with this question
a. From the graph determine the instantaneous rate when the concentration of o2 is at 8x 10-3M
b. Between 60 seconds and 300seconds, which instantaneous rate was higher? why?


For b would this be right
60 seconds is steeper that means it has a higher gradient than the 300 secs. Higher gradient means higher rate

For c from the graph determine the instantaneous rate at 60 seconds and at 300 seconds

Would this be right
300- 6/500= 0.012
60-13/40= 0.325

Would a be pretty much the same as the 60 secs


Mod edit: Merged Double posts
« Last Edit: February 17, 2021, 11:11:14 pm by Erutepa »

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #9041 on: February 17, 2021, 10:04:40 am »
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Is temperature included for sig figs?

ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9042 on: February 17, 2021, 10:06:31 am »
+3
Is temperature included for sig figs?

Yes - as long as you've used the value in your calculations.

Yemily

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Re: VCE Chemistry Question Thread
« Reply #9043 on: February 17, 2021, 03:02:28 pm »
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Need a bit of a refresher regarding SF :D If we were calculating the amount in mole of 3.45g of H2, would we only be able to go to 2SF due to the molar mass of H being 1.0 in the data booklet?

colline

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Re: VCE Chemistry Question Thread
« Reply #9044 on: February 17, 2021, 04:30:22 pm »
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Need a bit of a refresher regarding SF :D If we were calculating the amount in mole of 3.45g of H2, would we only be able to go to 2SF due to the molar mass of H being 1.0 in the data booklet?

If I remember correctly, there's a lot of ambiguity around this and VCAA accepts both, however for your internal SACs at school you should get this checked with your teacher.

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