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Author Topic: VCE Chemistry Question Thread  (Read 2313345 times)  Share 

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hobbitle

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Re: VCE Chemistry Question Thread
« Reply #735 on: April 23, 2014, 08:34:11 pm »
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Anyone got any particularly fantastic ways of remembering functional groups / nomenclature?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #736 on: April 23, 2014, 09:01:13 pm »
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alcohol => ol is at the end of the word, so if the alcohol group is the most important one, the name ends in ol
If it's a substituent, use 'hydroxy' as there is a 'hydrogen' and 'oxygen' making up 'hydroxy' and 'hydroxy' is like 'hydroxide' except shortened at the end, so it makes sense to put it at the front
oic acid you just have to remember
When naming esters, you have alkyl carboxylate. Example: methyl ethanoate can be thought of as a methyl group bonded to an ethanoate group, which is ethanoic acid minus a hydrogen, for instance
Ketones => 'one' at the back, like propanone; note how 'one' is also at the back of 'ketone'
Aldehydes have 'al', which is a bit weird given that the name begins with 'al'
The two above begin with 'oxo' if not the primary functional group, making sense as these functional groups only consist of oxygen atoms.
Halogen groups are easy xP
Methyl, ethyl etc you just have to know. It's only really up to butyl that you must rote learn as pentyl, hexyl are easier to work out.
Amides are fairly simple as they're named "ethanamide", for instance, with the only real difference being you name substituents attached to the nitrogen as N-methyl, for instance, instead of 2-methyl.

Nomenclature rules? Hmm. Longest hydrocarbon chain is the backbone. Your most important functional group is, well, the most distinguishing feature of your molecule. Substituents are like extra bits and the numbers tell you where they are. Then, order the substituents by alphabetical order. Anyone else you're unclear of?
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DJA

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Re: VCE Chemistry Question Thread
« Reply #737 on: April 23, 2014, 10:35:03 pm »
+1
How many moles of CH3COOH (acetic acid) are required to neutralise 0.14 moles of NaOH (sodium hydroxide)?


This question confuses me - what do you do when you have a weak acid like acetic acid?
« Last Edit: April 23, 2014, 10:42:58 pm by DJALogical »
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #738 on: April 23, 2014, 11:18:21 pm »
+2
How many moles of CH3COOH (acetic acid) are required to neutralise 0.14 moles of NaOH (sodium hydroxide)?


This question confuses me - what do you do when you have a weak acid like acetic acid?


By definition (which is a pretty stupid one IMO) neutralisation is when n(acid) = n(base)
Why do I think it's stupid? Well, if you add 0.14 moles of NaOH to 0.14 moles of CH3COOH, you're left with 0.14 moles of CH3COONa (and some water), which is a weak base. If you're carrying out the experiment in water, you don't have a neutral pH at the end because all you have left is base. Therefore I think neutralisation is a misnomer :P
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DJA

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Re: VCE Chemistry Question Thread
« Reply #739 on: April 23, 2014, 11:42:24 pm »
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By definition (which is a pretty stupid one IMO) neutralisation is when n(acid) = n(base)
Why do I think it's stupid? Well, if you add 0.14 moles of NaOH to 0.14 moles of CH3COOH, you're left with 0.14 moles of CH3COONa (and some water), which is a weak base. If you're carrying out the experiment in water, you don't have a neutral pH at the end because all you have left is base. Therefore I think neutralisation is a misnomer :P

Thats LITERALLY exactly why I was so confused.

Thanks for clarifying!
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Re: VCE Chemistry Question Thread
« Reply #740 on: April 24, 2014, 08:26:41 pm »
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Does anyone have any links for practice spectrums (IR/NMR) I can do: identifying bonds/molecules etc? Thanks :)
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Re: VCE Chemistry Question Thread
« Reply #741 on: April 25, 2014, 11:02:16 am »
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Why are alkanols such as methanol and ethanol soluble in water, but octanol is not?
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thushan

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Re: VCE Chemistry Question Thread
« Reply #742 on: April 25, 2014, 11:09:51 am »
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Put simply, the hydrocarbon chain in octanol is very long, meaning lots of hydrogen bonds between water molecules have to be broken if dissolution were to occur. In return, only a small number of hydrogen bonds can be formed by the single OH group in the octanol with water. Hence, it is unfavourable for octanol to dissolve into water.
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Re: VCE Chemistry Question Thread
« Reply #743 on: April 25, 2014, 12:37:43 pm »
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Put simply, the hydrocarbon chain in octanol is very long, meaning lots of hydrogen bonds between water molecules have to be broken if dissolution were to occur. In return, only a small number of hydrogen bonds can be formed by the single OH group in the octanol with water. Hence, it is unfavourable for octanol to dissolve into water.

Thushan, could we say that because octanol is a longer molecule, there are more non-polar alkyl regions within the compound relative to say methanol or ethanol, and that as a result, shorter alcohols are more soluble than longer alcohols?

thushan

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Re: VCE Chemistry Question Thread
« Reply #744 on: April 25, 2014, 12:52:15 pm »
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That's correct.
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Re: VCE Chemistry Question Thread
« Reply #745 on: April 26, 2014, 12:10:56 am »
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Concerning resonance, do nucleons that are already aligned against the external magnetic field flip, when exposed to radio waves, too? So, resonance doesn't just concern the alignment of nucleons with the external magnetic field?  :-\
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Re: VCE Chemistry Question Thread
« Reply #746 on: April 26, 2014, 12:23:08 am »
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Concerning resonance, do nucleons that are already aligned against the external magnetic field flip, when exposed to radio waves, too? So, resonance doesn't just concern the alignment of nucleons with the external magnetic field?  :-\

Not in the slightest bit relevant to the VCE course.
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soNasty

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Re: VCE Chemistry Question Thread
« Reply #747 on: April 26, 2014, 12:58:07 pm »
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what makes sulfuric acid such a good dehydrating agent to then be used as a catalyst in many reactions? eg esterification
Also, will the absence of sulfuric acid in a reaction of just say methanol and salicylic acid yield a higher percentage of water rather than methyl salicylate? if so, why?

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Re: VCE Chemistry Question Thread
« Reply #748 on: April 26, 2014, 01:04:22 pm »
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Not essential to know for the exam - concentrated sulfuric acid is a very strong acid, and it also has a very high affinity for water. In other words, if exposed to hydrogen and oxygen atoms, it will often try to pull them out of the organic molecule in its attempt to find water to mix in (this is a highly simplified explanation). It uses its property as an acid to pull out O and H atoms from molecules.

If we mixed just methanol and salicylic acid without adding a catalyst, we'll get the same percentage yield of methyl salicylate, except it will take over 9000 years for the reaction to occur to get that yield of methyl salicylate. You'll learn why to an extent in Unit 4.
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Re: VCE Chemistry Question Thread
« Reply #749 on: April 26, 2014, 01:10:38 pm »
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thanks thushan!