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March 28, 2024, 09:28:44 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313034 times)  Share 

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wingdings2791

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Re: VCE Chemistry Question Thread
« Reply #9195 on: September 24, 2021, 10:57:25 pm »
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Hi everyone,
I'm doing my U4 AOS3 (prac) SAC on fuels, as my school has not assessed most of U3 AOS1. The prac focuses on producing a biodiesel from vegetable oil, then determining and comparing the density and energy content on each fuel.

Because of lockdown, we weren't able to complete the experiment ourselves, so the teachers provided the following sample data (to analyse as practice):

Spoiler
For the energy content determination (combusting fuels with a spirit burner to heat water, recording \(\Delta T\) of water):
- \(v(H_2O)=50\ mL\)
- \(v(fuel)=10\ mL\)
- \(\Delta m(fuel)=3.66g\) (for both fuels)
- \(\Delta T=+28.5^\circ C, +31.0^\circ C\) (oil and biodiesel respectively)

Using this data, I determined the energy content of the oil and biodiesel each as follows:



These values seem very inaccurate, given that fats/oils typically have a heat of combustion \(\sim 37kJg^{-1}\). Am I forgetting something and doing a step wrong, or is this data just unrealistic? I've checked over the units, given data, and calculations a hundred times but still can't figure out what's wrong. Under the results column, the rubric does specify that 'any outliers should be identified (if appropriate)', but I'm hesitant to declare this a huge systematic error caused by a terribly calibrated thermometer or similar.

Any help would be greatly appreciated.
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Mackenzie Aps

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Re: VCE Chemistry Question Thread
« Reply #9196 on: September 28, 2021, 09:06:01 am »
0
Does anyone have the NEAPS Chemistry 3/4 Trial exam and solutions

Mackenzie Aps

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Re: VCE Chemistry Question Thread
« Reply #9197 on: September 28, 2021, 09:06:42 am »
0
Does anyone have the NEAPS Chemistry 3/4 Trial exam and solutions for 2021

lm21074

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Re: VCE Chemistry Question Thread
« Reply #9198 on: September 28, 2021, 12:51:11 pm »
+1
Does anyone have the NEAPS Chemistry 3/4 Trial exam and solutions for 2021
Hi Mackenzie,

It is against the forum rules to distribute copyrighted materials, including the NEAP trial exams and solutions.

Does your school have access to these resources?
2021: VCE
2022: Science / Arts @ Monash

Mackenzie Aps

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Re: VCE Chemistry Question Thread
« Reply #9199 on: September 28, 2021, 03:26:54 pm »
0
Apologies for that, they don't have this one. Not to worry, will use others.

Mackenzie Aps

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Re: VCE Chemistry Question Thread
« Reply #9200 on: September 28, 2021, 05:51:07 pm »
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Hi Mackenzie,

It is against the forum rules to distribute copyrighted materials, including the NEAP trial exams and solutions.

Does your school have access to these resources?

Just wondering how other exam material requests and questions are posted but mine was not appropriate or against forum rules?

valjaybj

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Re: VCE Chemistry Question Thread
« Reply #9201 on: October 04, 2021, 10:39:57 pm »
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hi can someone help me with U4 AOS3 (fuels prac) - my sac is in 2 days and I am extremely confused (as I haven't been formerly assessed on this part of the course and so I haven't bothered to study for it either) - what do I talk about in my discussion? The prac is about making biodiesel from a vegetable oil, determining the density and energy content of the biodiesel and comparing it to the original oil/ethanol. What limitations can I talk about? And how do I calculate the density and energy content? Oh and what would the DV and IV be? (I'm utterly lost so) any help would be GREATLY APPRECIATED!!!!!!!!!!!!!!

squid_2021

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Re: VCE Chemistry Question Thread
« Reply #9202 on: October 06, 2021, 12:17:24 am »
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Just wondering, is this thread only for VCE chem 3&4? Looking for the thread (if there is one) for 1&2....

ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9203 on: October 06, 2021, 11:09:48 am »
+4
Just wondering, is this thread only for VCE chem 3&4? Looking for the thread (if there is one) for 1&2....

Feel free to post your 1/2 questions here as well! This thread is for both :)

Corey King

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Re: VCE Chemistry Question Thread
« Reply #9204 on: October 12, 2021, 12:25:19 pm »
+1
Hey guys,
I'm struggle to see where I went wrong for a past question. I sent it to my teacher, but I think she gave up explaining it to me :P. Any help appreciated :)

https://gyazo.com/2cff1b72ae64054c76159fbb51cb3723
https://gyazo.com/b3c2ca680c5225f756c075d05e8ae6fe  (teacher response)
https://gyazo.com/383ac80b0f1724cf9a79db1e88c3f0ad
https://gyazo.com/5e3b50a0a422e5188dd2e6c46abcad1d
https://gyazo.com/c9d87dc26b5c7c434adbdeac50305c16

Thanks,
Corey

wingdings2791

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Re: VCE Chemistry Question Thread
« Reply #9205 on: October 12, 2021, 04:26:23 pm »
+3
Hey guys,
I'm struggle to see where I went wrong for a past question. I sent it to my teacher, but I think she gave up explaining it to me :P. Any help appreciated :)

https://gyazo.com/2cff1b72ae64054c76159fbb51cb3723
https://gyazo.com/b3c2ca680c5225f756c075d05e8ae6fe  (teacher response)
https://gyazo.com/383ac80b0f1724cf9a79db1e88c3f0ad
https://gyazo.com/5e3b50a0a422e5188dd2e6c46abcad1d
https://gyazo.com/c9d87dc26b5c7c434adbdeac50305c16

Thanks,
Corey

Hey Corey,
I think the reason behind this incongruence of data might be this sentence in the data book:
'Typical 13C shift values relative to TMS = 0
These can differ slightly in different solvents'

Hence, I suspect that the disconnection between the C NMR and the lack of \(R-CH_2-R\) is because of different range standards (and we should discount this by thinking of the potential for variation in the range). Here's the C NMR spectrum for methylpropan-1-ol, which indicates a similar chemical shift (~30 ppm) as the question indicates for a carbon bonded to two methyl groups and one hydrogen atom (which would be exactly the same environment)

Methylpropan-1-ol


       ppm   Int.  Assign.

       69.47   691      1
       30.84   634      2
      18.99   1000     3
Since this NMR spectra and the ones on VCAA exams seem to be from the same source, perhaps the standard range for \(R_3-CH\) simply differs between VCAA and the labs that make these. I think VCAA possibly just wants students to account for slight variation in chemical shift ranges, focusing more on determining structure from the molecular formula and number of environments. Sorry for not being able to answer this better, but I think that may be what's going on :)
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miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #9206 on: October 16, 2021, 04:11:45 pm »
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Hey everyone!
I'm just wondering why rinsing the conical flask and volumetric flask with water in a titration has NO EFFECT on the calculated concentrations of titrant and aliquot solutions? Sources say that this is because the amount of moles are not affected. However, wouldn't having slightly more water make the solutions diluted?
Thank you
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james.358

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Re: VCE Chemistry Question Thread
« Reply #9207 on: October 16, 2021, 05:02:27 pm »
+5
I'm just wondering why rinsing the conical flask and volumetric flask with water in a titration has NO EFFECT on the calculated concentrations of titrant and aliquot solutions? Sources say that this is because the amount of moles are not affected. However, wouldn't having slightly more water make the solutions diluted?

Hey there,

Rinsing the conical flask with water definitely affects the concentration of the aliquot solution. However, it doesn't affect the accuracy of the titration as the moles of the reactants are unchanged, its just that the concentration is lower as you correctly stated

However, if you rinse the volumetric flask with water, it will not affect the concentration of the standard solution. Since you know how much of the primary standard you have, and you're just filling the flask up with water anyways.

I think what you were thinking of is rinsing the burette with water. That will lower the concentration of the titre, so you definitely cannot do it, or else it would become a systematic error.

Hope this helps!
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reverie

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Re: VCE Chemistry Question Thread
« Reply #9208 on: October 16, 2021, 06:19:47 pm »
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Hey guys,

Not a very important question but why don't coefficients affect oxidation numbers?

Thanks  :)

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #9209 on: October 25, 2021, 10:07:48 am »
+1
Hey guys,

Not a very important question but why don't coefficients affect oxidation numbers?

Thanks  :)

I think of the coefficients as merely a way of balancing the equation, to give the correct stoichiometric ratio for the reaction. This doesn't affect the oxidation numbers of the individual atoms because the coefficients don't affect the extent to which a compound is oxidised or reduced. This is my understanding of it anyway.
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