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March 29, 2024, 04:45:47 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313600 times)  Share 

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Jason12

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Re: VCE Chemistry Question Thread
« Reply #1005 on: June 15, 2014, 04:16:13 pm »
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what does CO2 evolved mean?

when analysing an NMR spectra how do you know which chemical shift is right? e.g. 4.1 ppm can match different things in the data book. This concept is still confusing for me so can anyone explain
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Re: VCE Chemistry Question Thread
« Reply #1006 on: June 15, 2014, 04:28:00 pm »
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what does CO2 evolved mean?

when analysing an NMR spectra how do you know which chemical shift is right? e.g. 4.1 ppm can match different things in the data book. This concept is still confusing for me so can anyone explain

When a gas evolves, this means that a gas is produced from a reaction. So for instance, carbon dioxide evolves in the fermentation of glucose.

You're often given background information which should guide you into choosing the right chemical shift. On exams, you will find that questions on this topic give you background information on the compound in the form of it's chemical formula, an IR spectrum, a mass spectrum of the compound, and a proton/Carbon-13 NMR spectrum. Through this, you can properly identify the molecule.

Hannibal

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Re: VCE Chemistry Question Thread
« Reply #1007 on: June 15, 2014, 05:22:12 pm »
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Not sure if this is the right place to ask this, I hope so :)

What's the difference between a dipole and a polar molecule?
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Reus

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Re: VCE Chemistry Question Thread
« Reply #1008 on: June 15, 2014, 05:33:10 pm »
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Not sure if this is the right place to ask this, I hope so :)

What's the difference between a dipole and a polar molecule?
Perfect place to ask!

A polar molecule is where there is separation of charge in the chemical bonds. One region of the molecule will have a slight positive charge and another with a slight negative charge. Think of it like the North and South Pole. A common example of a polar molecule is H2O. The O is negative yet the H2 is positive.
In comparison to dipole which also have a positive and a negative portion to the molecule. However these occur in cases of symmetry where dipole vectors cancel each other out. An example of this is all four bonds in carbon tetrachloride (CCl4) are polar because chlorine is very electronegative. This then results in four dipoles where all four vectors point from the central carbon to one of the four chlorines. Due to its tetrahedral shape, the vectors face in opposite directions hence cancel each other out.
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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #1009 on: June 15, 2014, 05:42:46 pm »
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Not sure if this is the right place to ask this, I hope so :)

Look, if you can't get help with chemistry in here, I think a lot of us are in for some trouble, hahah.

Hannibal

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Re: VCE Chemistry Question Thread
« Reply #1010 on: June 15, 2014, 05:49:34 pm »
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Look, if you can't get help with chemistry in here, I think a lot of us are in for some trouble, hahah.
Thanks Reus! Haha yeah, I was just wondering because it looked like it was for Year 12's. I also wanted to ask if this was an accurate description of metallic bonding:

This bond requires large amounts of atoms, and occurs in most metals. This is because the atoms are structured in a way that the electrons are free to move, forming the electron sea model. This is the reason metals have high conductivity, ductility, malleability and low volatility.
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Bestie

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Re: VCE Chemistry Question Thread
« Reply #1011 on: June 15, 2014, 05:51:43 pm »
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hello,
i was wondering what the answer to this question should be cause in the worked solution (funny enough didnt give any explaination of how they got their ans) got B but the back of the book said C and i said D???

thank you

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Re: VCE Chemistry Question Thread
« Reply #1012 on: June 15, 2014, 05:55:46 pm »
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Hannibal - seems fairly accurate, minus the most metals part. The idea of "metallic bonding" is that it occurs in all metals, hahah.

Bestie - I'm with you there, I'd say D. Probably more correct to say B in an experimental sense, but given the data they've given you, certainly D is correct.

Bestie

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Re: VCE Chemistry Question Thread
« Reply #1013 on: June 15, 2014, 06:25:25 pm »
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yup thank you :)

i also found another one:
More energy is required to vibrate:
A C–C compared to C=C
B C–Cl compared to C–Br
C C–O compared to C–H
D C=C compared to C≡C

shouldn't the answer be B, the back of book says C?

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Re: VCE Chemistry Question Thread
« Reply #1014 on: June 15, 2014, 07:40:18 pm »
+1
Thanks Reus! Haha yeah, I was just wondering because it looked like it was for Year 12's. I also wanted to ask if this was an accurate description of metallic bonding:

This bond requires large amounts of atoms, and occurs in most metals. This is because the atoms are structured in a way that the electrons are free to move, forming the electron sea model. This is the reason metals have high conductivity, ductility, malleability and low volatility.

One point of dispute: metallic bonding involves metal IONS, not atoms. Atoms are neutral (:

yup thank you :)

i also found another one:
More energy is required to vibrate:
A C–C compared to C=C
B C–Cl compared to C–Br
C C–O compared to C–H
D C=C compared to C≡C

shouldn't the answer be B, the back of book says C?

The problem with B is that although C-Cl has a stronger bond, Br is much larger.
Mass also affects the amount of energy required
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Bestie

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Re: VCE Chemistry Question Thread
« Reply #1015 on: June 15, 2014, 07:55:13 pm »
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wasn't that with lower mass, more energy required?

psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1016 on: June 15, 2014, 08:50:49 pm »
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wasn't that with lower mass, more energy required?
yeah I thought it was this too. The C-H band has a higher wavenumber (and thus energy) than the the C-O band, doesn't that mean more energy is needed to vibrate it?
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Bestie

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Re: VCE Chemistry Question Thread
« Reply #1017 on: June 15, 2014, 08:54:57 pm »
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same here... but.. i don't know :(

can i ask another question then?
The amount of alcohol, CH3CH2OH, in your breath can be determined by
blowing into a tube containing acidified potassium dichromate, K2Cr2O7. Which
substance has been oxidised and which has been reduced?

thank you :)
« Last Edit: June 15, 2014, 08:57:36 pm by Bestie »

psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1018 on: June 15, 2014, 09:01:17 pm »
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same here... but.. i don't know :(

can i ask another question then?
The amount of alcohol, CH3CH2OH, in your breath can be determined by
blowing into a tube containing acidified potassium dichromate, K2Cr2O7. Which
substance has been oxidised and which has been reduced?

thank you :)
Alcohol gets oxidised to a carboxyllic acid (CH3COOH), meaning the K2Cr2O7 gets reduced (from unit 3 area of study 2).
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Bestie

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Re: VCE Chemistry Question Thread
« Reply #1019 on: June 15, 2014, 09:45:32 pm »
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so alcohols are never reduced?