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Author Topic: VCE Chemistry Question Thread  (Read 2313098 times)  Share 

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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #855 on: May 13, 2014, 06:16:11 pm »
+1
The problem is, if your sodium acetylsalicylate (called sodium AS from here on in) was to not dissolve, it would remain as crystals. Now, the crystals have strong ionic bonds between them, that is true.
However, when the crystal breaks up, there is a lot of energy released by the hydration of sodium cations (your ion-dipole bonds mentioned). Ion dipole bonds are only really meaningful when the crystal has dissolved in water (in the solid form, sure, the sodium cations may attract the oxygens, but the AS anion will repel the oxygens; it gets trippy) as then, each ion can form bonds to multiple water molecules. Sodium, for instance, bonds to six water molecules when dissolved. That's a lot of energy. I'm not exactly sure how many the AS forms.
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ETTH96

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Re: VCE Chemistry Question Thread
« Reply #856 on: May 13, 2014, 08:41:25 pm »
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Hey guys can someone help me with this question?

The oxidation number of manganese changes as MnO2 (s) is converted to MnO42- how many electrons are gained or lost when one mole of MnO2 is completely converted into MnO2 2- (aq)?

And this one too:
when a polyester forms from reaction between the monomers HOCH2CH2OH and HOOCC6H4COOH the percentage, by mass, of carbon in the polyester is
a) greater than in the monomers
b)) less then in the monomers
c) the same as in the monomers
d) dependent on the length of the polymer chain

can you explain how to figure this out as well please!! thank you
« Last Edit: May 13, 2014, 08:45:40 pm by ETTH96 »

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #857 on: May 13, 2014, 09:32:31 pm »
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Hey guys can someone help me with this question?

The oxidation number of manganese changes as MnO2 (s) is converted to MnO42- how many electrons are gained or lost when one mole of MnO2 is completely converted into MnO2 2- (aq)?

And this one too:
when a polyester forms from reaction between the monomers HOCH2CH2OH and HOOCC6H4COOH the percentage, by mass, of carbon in the polyester is
a) greater than in the monomers
b)) less then in the monomers
c) the same as in the monomers
d) dependent on the length of the polymer chain

can you explain how to figure this out as well please!! thank you

First one: manganese in manganese dioxide is in oxidation state +4, but in your manganate ion (lol interesting manganese species there), it is oxidation state +6, so it loses two electrons.
You made a typo I think; MnO22- does not exist from my knowledge.

As for the second one, think about it. In your polymerisation, the number of moles of carbon stays the same. However, you lose water when polymerising at each step. Therefore, you may have joined two pairs of monomers, meaning you have twice the amount of carbon as one pair of monomers (I'm considering the collection of diol and diacid as a pair of monomers), but as you've lost water, the total mass of the molecule is less than the mass of two pairs of monomers. Twice the carbon for less than twice the total mass => larger percentage by mass of carbon than in monomers.
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ETTH96

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Re: VCE Chemistry Question Thread
« Reply #858 on: May 13, 2014, 09:42:25 pm »
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Sorry! i did make a typo! I was meant to type:
The oxidation number of manganese changes as MnO2 (s) is converted to MnO42- how many electrons are gained or lost when one mole of MnO2 is completely converted into MnO42- (aq)?

So, for MnO2, the oxidation number is +4, and for MnO4 2-, the oxidation number is +6.. so 2 electrons are lost?

This makes total sense but, its not one of the choices for the multiple choice :/

a) 3
b) 1.2 x 1024
c) 4
d) 1.8 x 1024

Could you help me again? thanks!

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #859 on: May 13, 2014, 10:14:56 pm »
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It's 2 electrons for every manganese atom. So one mole of manganese atoms...? You can do the rest.
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Re: VCE Chemistry Question Thread
« Reply #860 on: May 14, 2014, 11:44:41 pm »
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I have a few queries that I'd like to clarify:

• How can we calculate the m/e value of the base peak in the mass spectrum of a compound?
• Are we required to know different tests we can perform to determine one organic compound from another? If so, could someone give me a run down of the best tests to perform?
• What is the significance of TMS? Could I say that it's used in NMR spectroscopy as a point of reference? I know the answer, I just don't know how to properly phrase it.

Help would be greatly appreciated :)


« Last Edit: May 14, 2014, 11:46:12 pm by Yacoubb »

jgoudie

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Re: VCE Chemistry Question Thread
« Reply #861 on: May 15, 2014, 07:40:40 am »
+1
1) The base peak is the one with the greatest intensity, the biggest peak. All you need to do is look at the spectrum and read off the m/e value.

2) Yes, you should know enough about organic reaction to explain tests which will be able to identify compounds:
   - Alkenes - bromine water (is normally brown if an alkene is present it goes clear)
   - Carboxylic acids - React with Na2CO3 to form carbon dioxide (acid base reaction)
   - 1-alkanols - React with dichromate ions to form carboxylic acids (dichromate is orangish will change to greenish)

3) Yep TMS is added for a point of reference.

Hope this helps, pretty simplistic i know, if anyone else wants to add more detail go for it.

I have a few queries that I'd like to clarify:

• How can we calculate the m/e value of the base peak in the mass spectrum of a compound?
• Are we required to know different tests we can perform to determine one organic compound from another? If so, could someone give me a run down of the best tests to perform?
• What is the significance of TMS? Could I say that it's used in NMR spectroscopy as a point of reference? I know the answer, I just don't know how to properly phrase it.

Help would be greatly appreciated :)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #862 on: May 15, 2014, 08:03:15 pm »
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1) The base peak is the one with the greatest intensity, the biggest peak. All you need to do is look at the spectrum and read off the m/e value.

2) Yes, you should know enough about organic reaction to explain tests which will be able to identify compounds:
   - Alkenes - bromine water (is normally brown if an alkene is present it goes clear)
   - Carboxylic acids - React with Na2CO3 to form carbon dioxide (acid base reaction)
   - 1-alkanols - React with dichromate ions to form carboxylic acids (dichromate is orangish will change to greenish)

3) Yep TMS is added for a point of reference.

Hope this helps, pretty simplistic i know, if anyone else wants to add more detail go for it.

You could also just use indicator paper for the water-soluble acids.

As for how TMS works, its purpose is to produce some number that doesn't change depending on your machine. The energy absorbed by a nucleus in NMR to change spin state is directly proportional to the external magnetic field. Now, the TMS and the proton in question experience slightly different magnetic fields due to this phenomenon called "shielding". Let's call the constant of proportionality k, the magnetic field experienced by TMS to be B(TMS), the magnetic field experienced by the proton in question be B(H) and E(TMS), E(H) to be the energy absorbed by the respective nuclei.
Then, the chemical shift is defined as
Clearly the k's cancel. This suggests that the chemical shift is really just the ratio of magnetic field strengths. If you can accept that doubling the external magnetic field will double the shielding effect, this last fraction really is constant. That is the effect of defining chemical shifts so weirdly.

Bit of a hand-wavy explanation but it hopefully shows what chemical shifts were designed for.
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Re: VCE Chemistry Question Thread
« Reply #863 on: May 15, 2014, 11:05:41 pm »
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Currently in year 11 near the end of organic chem, i want to make a start on unit 3/4 stuff, and was wondering best way to start? since half of the course will require stoich etc which we learn near term 3.

Really, what can i do in 3/4 book that can extend my knowledge of 1/2, for example do 3/4 organic chem to extend me 1/2 knowledge if you understand where im going,
Thanks

jgoudie

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Re: VCE Chemistry Question Thread
« Reply #864 on: May 16, 2014, 10:59:24 am »
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I would say your best bet look at the organic reactions area of chemistry.  Chapter 9/10 in the Heinemann textbook is really just an extension of year 11, so start there, it that is okay have a look at biofuels and biomolecules.  It is all based around the same idea, how function groups react together.

Currently in year 11 near the end of organic chem, i want to make a start on unit 3/4 stuff, and was wondering best way to start? since half of the course will require stoich etc which we learn near term 3.

Really, what can i do in 3/4 book that can extend my knowledge of 1/2, for example do 3/4 organic chem to extend me 1/2 knowledge if you understand where im going,
Thanks
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speedy

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Re: VCE Chemistry Question Thread
« Reply #865 on: May 16, 2014, 03:47:56 pm »
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What differences would you expect to see in the HNMR and IR spectrum for aspirin vs. salicylic acid?
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Rishi97

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Re: VCE Chemistry Question Thread
« Reply #866 on: May 16, 2014, 04:50:56 pm »
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I have my aspirin sac on Monday and I have answered some revision questions
Could someone please check that I am correct?
- What's the purpose of adding Sulfuric acid?
The H2SO4 acts as a catalyst and speeds up the reaction
- What is the limiting reagent in the reaction?
The limiting reagent is salicylic acid as ethanoic anhydride is added in excess
- Why is water added at the end of the reaction?
Water is added at the end of the reaction to allow for crystallisation to occur and it cools down the reaction
-Why is cold water used?
Cold water is used to lower the solubility of aspirin. Water is also used to ensure that all other solutes and reactants are washed away.

Feel free to add anything
Much appreciated :)
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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #867 on: May 17, 2014, 01:02:43 pm »
+2
- Why is water added at the end of the reaction?
Water is added at the end of the reaction to allow for crystallisation to occur and it cools down the reaction
The water also converts any excess acetic anhydride to acetic acid.
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Re: VCE Chemistry Question Thread
« Reply #868 on: May 18, 2014, 11:02:20 am »
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How should i prepare for my EEI, i do have the results and my SAC is next monday . My cohort has performed our experiments on an energy drink

Thanks in advance

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Re: VCE Chemistry Question Thread
« Reply #869 on: May 18, 2014, 12:25:34 pm »
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can someone explain to me how the answer is C, much thanks :)