Post by: kenhung123 on May 22, 2010, 08:34:24 pm
Feel free to ask your questions here too..
Umm I'm wondering what does it mean when asked to discuss the physical and chemical properties of a molecule? What should I be thinking when asked that?
Post by: kenhung123 on May 26, 2010, 06:36:21 pm
Post by: superflya on May 26, 2010, 06:39:14 pm
Post by: Martoman on May 26, 2010, 06:47:03 pm
Post by: kenhung123 on May 26, 2010, 09:13:32 pm
Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?
Post by: Martoman on May 26, 2010, 09:31:00 pm
Post by: kenhung123 on May 26, 2010, 09:37:02 pm
Post by: Martoman on May 26, 2010, 09:41:31 pm
Post by: Potter on May 26, 2010, 11:19:28 pm
As requested.. I don't know if that's helpful or not.
Btw, N(alpha) = adjacent environment 1. N(beta) = adjacent environment 2.
Disclaimer - I'm not 100% sure on what I've written.. I just kinda put two and two together, so don't take this religiously. It was taken with a crappy phone camera, forgive me. Cbf to bring out the scanner.
Post by: darkphoenix on May 26, 2010, 11:21:49 pm
Or do we have to sketch out each structural formula?
Post by: superflya on May 26, 2010, 11:23:54 pm
Is there meant to be a specific way to determine the number of isomers of a molecule?
Or do we have to sketch out each structural formula?
just have to draw em out, theres no formula.
Post by: Potter on May 26, 2010, 11:24:20 pm
Is there meant to be a specific way to determine the number of isomers of a molecule?
Or do we have to sketch out each structural formula?
I would say you would just have to sketch out each one..
-Edit
Beaten
Post by: stonecold on May 26, 2010, 11:24:34 pm
Is there meant to be a specific way to determine the number of isomers of a molecule?
Or do we have to sketch out each structural formula?
There was a thread on this a few weeks back. Apparently if you can devise a formula that will give you the number of isomers for a molecule, then you would win the Nobel prize or something.
That's what TT said... :P
Post by: Martoman on May 26, 2010, 11:27:30 pm
http://yfrog.com/16image0043kj
As requested.. I don't know if that's helpful or not.
Btw, N(alpha) = adjacent environment 1. N(beta) = adjacent environment 2.
Disclaimer - I'm not 100% sure on what I've written.. I just kinda put two and two together, so don't take this religiously. It was taken with a crappy phone camera, forgive me. Cbf to bring out the scanner.
If this is a general rule which works for spin spin shit, me loves you long time.
Is there meant to be a specific way to determine the number of isomers of a molecule?
Or do we have to sketch out each structural formula?
http://vcenotes.com/forum/index.php/topic,25608.0.html
Usually if its
ie for butane you can wrench the carbon off the end to branch off the middle one. So if Cl is a functional group, then you write out the backbone and put a "dot" where the Cl should be.
it can be on the end, and on the next carbon for the straight chain.
Then for the branched it can be on the first carbon on the branched or the other carbon in the straight 3 chain giving a total of 4.
Post by: darkphoenix on May 26, 2010, 11:34:37 pm
Post by: kenhung123 on May 27, 2010, 12:03:25 am
Post by: m@tty on May 27, 2010, 12:07:04 am
Post by: kenhung123 on May 27, 2010, 12:11:31 am
What chromatographic and spectroscopic techniques do not destory a sample?
What effects are there on the state of hydrocarbon when functional groups OH and Br are added?
Post by: Martoman on May 27, 2010, 07:44:40 am
Spec: UV, IR, NMR
The chemical properties change. Becoming more polar (and in the case of OH becoming soluble on a sufficiently short hydrocarbon) and more reactive. Again, on a relatively short chain, bioling, melting and viscoscity would increase and volatility would decrease.
Post by: kenhung123 on May 27, 2010, 07:11:23 pm
Just to confirm in organic pathways, Br, HCl reactants are (aq) and caroxbylic acids, alcohol are liquid and esters are generally (g)?
Post by: Mao on May 27, 2010, 07:16:36 pm
Thanks guys
Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?
This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:
Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.
The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.
Post by: Mao on May 27, 2010, 07:18:14 pm
HPLC, Paper/TLC.
Spec: UV, IR, NMR
The chemical properties change. Becoming more polar (and in the case of OH becoming soluble on a sufficiently short hydrocarbon) and more reactive. Again, on a relatively short chain, bioling, melting and viscoscity would increase and volatility would decrease.
Try to avoid statements like this. Methanol and ethanol are much less reactive than methane and ethane (in terms of combustion), but much more reactive in terms of substitution pathways.
Post by: kenhung123 on May 27, 2010, 07:48:42 pm
Thanks that clears things up!Thanks guys
Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:
Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.
The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.
Post by: kenhung123 on May 27, 2010, 07:53:30 pm
What best describes this reaction:
acid base
precipitation
redox
gravimetric
Post by: Martoman on May 27, 2010, 08:01:31 pm
Thanks that clears things up!Thanks guys
Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:
hahaa I try :-[
Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.
The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.
hahaa I try :-[
Post by: Martoman on May 27, 2010, 08:03:10 pm
Zn(s) +2HCl(aq)=>ZnCl2(aq)+H2(g)
What best describes this reaction:
acid base
precipitation
redox
gravimetric
Redox Zn changes from 0 to +2
H changes from +1 to 0
Post by: kenhung123 on May 27, 2010, 08:04:44 pm
Post by: Mao on May 27, 2010, 08:07:34 pm
Thanks that clears things up!Thanks guys
Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:
hahaa I try :-[
Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.
The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.
Don't stress miss, you are doing excellent. You won't pick up these quirky stuff until you're well settled in uni [if you even do chemistry at uni, that is]. I didn't know any of these back in year 12.
Post by: Mao on May 27, 2010, 08:09:22 pm
Can you also say its acid base ? metal cloride+H gas?
For the sake of VCE (Bronsted-Lowry definition), no, you cannot say it's acid-base reaction. It is redox.
Eventually though, you'll learn acid-base is almost the same thing as redox reaction (Lewis and Arrhenius definition of acid/base).
Post by: kenhung123 on May 27, 2010, 08:15:02 pm
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
2. Is the OH in carboxy group considered a hydroxy group i.e. can be ionised?
3. Do I need to use words to describe an organic pathway e.g. oxidise ethanol to form ethanoic acid with the use of dichromate oxidising agent
Post by: kenhung123 on May 27, 2010, 10:07:19 pm
4. Can fractional distillation extract pure compounds?
5. Do we need to know about the laboratory fractional distillation or industrial?
Post by: Martoman on May 27, 2010, 10:09:00 pm
5. Both. Although I have never seen a question on the laboratory one. (*waits to be corrected*)
Post by: kenhung123 on May 27, 2010, 10:41:32 pm
Post by: Martoman on May 27, 2010, 10:48:55 pm
Post by: kenhung123 on May 27, 2010, 10:56:27 pm
OK another question: Can water (H2O can form H+ and OH- right) be considered a reagent for reaction of chloroalkane to product alkanol? E.g. CH3CH2Cl+H2O=>Ch3CH2OH commonly the exams say NaOH or KOH but does it mean H2O is wrong?
Post by: Martoman on May 27, 2010, 11:04:49 pm
Post by: olly_s15 on May 27, 2010, 11:07:06 pm
4. In the industrial one its usually a mixture of hydrocarbons with similar bioling points.
5. Both. Although I have never seen a question on the laboratory one. (*waits to be corrected*)
laboratory one was on VCAA 2009 Q19 MC
you stand corrected :)
Post by: Martoman on May 27, 2010, 11:15:27 pm
Post by: kenhung123 on May 28, 2010, 12:12:42 am
You need at least 300-400 degrees of heat and a solidI think that is the condition+catalyst for addition of water to ethene not substitution?catalyst to do so. It really involves a lot of effort.
1. I thought if products where Acid+metal=Salt+H2 or Salt+H2O or salt+H2O+CO2 is an acid-base?
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
2. Is the OH in carboxy group considered a hydroxy group i.e. can OH (hydroxy) be ionised in basic solution to form O-?
3. Do I need to use words to describe an organic pathway e.g. oxidise ethanol to form ethanoic acid with the use of dichromate oxidising agent
4. In organic reactions, quite a lot of the time we may deal with small molecules in (g) state. When forming bromo- and alcohols, do we retain the (g) state or would it be silly to say a gaseous alcohol is produced?
5. Is fermentation oxidation or a reduction reaction?
Post by: Mao on May 28, 2010, 01:38:24 pm
1. I thought if products where Acid+metal=Salt+H2 or Salt+H2O or salt+H2O+CO2 is an acid-base?False. Acid + metal --> salt + H2 is always a redox reaction at VCE level.
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
base/carbonate/hydrogen carbonate + acid --> salt + h2O [ + CO2 ] are acid/base reactions
Quote
2. Is the OH in carboxy group considered a hydroxy group i.e. can OH (hydroxy) be ionised in basic solution to form O-?No, it does not ionise (at VCE level). In reality, it can ionise, but you require a VERY strong base (pH > 14 if I remember correctly)
Quote
4. In organic reactions, quite a lot of the time we may deal with small molecules in (g) state. When forming bromo- and alcohols, do we retain the (g) state or would it be silly to say a gaseous alcohol is produced?Depends on what temperature. Generally, light alcohols become gaseous at temperatures above 100 degrees (they boil before water does).
Quote
5. Is fermentation oxidation or a reduction reaction?C6H6O6 --> 2CO2 + 2C2H6O
It is both an oxidation and a reduction reaction, where glucose is both the oxidant and the reductant.
If that is a bit of a leap:
Redox reactions usually concern only two atoms, one of them accepting electrons, one of them donating electrons. these two atoms may be bonded to a range of other things, thus making two molecules reacting together, when it is essentially two centers exchanging electrons.
In large organic molecules, redox reaction may occur between different parts of the molecule. E.g. the 1' C acts as an oxidant and the 3' C acts as a reductant (just an example, this may not be true for fermentation of glucose). the actual reaction pathway for fermentation of glucose is quite complicated with NADH and ATP floating around everywhere, but the key is, different parts of the sugar have different chemical properties, and may react with each other.
Post by: kenhung123 on May 28, 2010, 03:05:10 pm
Post by: kenhung123 on May 28, 2010, 03:23:38 pm
Does that mean metal+acid=>salt +H2O [+CO2] is not acid-base?1. I thought if products where Acid+metal=Salt+H2 or Salt+H2O or salt+H2O+CO2 is an acid-base?False. Acid + metal --> salt + H2 is always a redox reaction at VCE level.
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
base/carbonate/hydrogen carbonate + acid --> salt + h2O [ + CO2 ] are acid/base reactions
6. Given that each carbon has at least 1 hydrogen, would the number of carbon and hydrogen environments be same for the molecule?
7. Is bromine used in bromine tests in state (g) (l) or (aq)
Post by: Mao on May 29, 2010, 02:28:16 am
Correct, it is not acid/base.Does that mean metal+acid=>salt +H2O [+CO2] is not acid-base?1. I thought if products where Acid+metal=Salt+H2 or Salt+H2O or salt+H2O+CO2 is an acid-base?False. Acid + metal --> salt + H2 is always a redox reaction at VCE level.
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
base/carbonate/hydrogen carbonate + acid --> salt + h2O [ + CO2 ] are acid/base reactions
6. Given that each carbon has at least 1 hydrogen, would the number of carbon and hydrogen environments be same for the molecule?
Not necessarily, OH and NH groups would increase the number of H environment. E.g. for ethanol, 2 C environment, 3 H environments.
7. Is bromine used in bromine tests in state (g) (l) or (aq)
usually (aq) [or in an organic solvent such as CH2Cl2 (aka DCM), commonly used in organic chemistry]
Post by: kenhung123 on May 31, 2010, 11:53:54 pm
Another question I encountered:
If CH3CH(OH)CH3 reacted with CH3CH2CH2COOH (Is esterification possible when there is no primary alkanol?)
Isn't this meant to be the product:
Post by: m@tty on May 31, 2010, 11:59:26 pm
Post by: crayolé on June 03, 2010, 11:06:58 am
Sorry, maybe I'm reading the question wrong but doesnt the COOH of a COO- in any basic environment? i.e zwitterions in amino acids and amino acids in basic solutionQuote2. Is the OH in carboxy group considered a hydroxy group i.e. can OH (hydroxy) be ionised in basic solution to form O-?No, it does not ionise (at VCE level). In reality, it can ionise, but you require a VERY strong base (pH > 14 if I remember correctly)
Also whilst on this topic, at what pH do zwitterions occur?
The overall molecule is positive in acidic environments and negative in basic environments but when is it both?
Post by: kenhung123 on June 03, 2010, 11:38:41 am
Also I would like to ask, since amino acids turn positive in acidic environments, does it mean they are basic after this reaction? (They are conjugate acid base compounds)
Post by: crayolé on June 03, 2010, 12:59:13 pm
I think in general or at VCE level as Mao refers, it is 7 however in Neap 2010 they specified the isoelectric point of a few amino acids to be pH 6 (which is like their neutral point or something) but I can't believe they would expect us to know all the isoelectric points for each amino acid..Yep, they act as a base in acidic environments and vice versa
Also I would like to ask, since amino acids turn positive in acidic environments, does it mean they are basic after this reaction? (They are conjugate acid base compounds)
Post by: kenhung123 on June 03, 2010, 01:19:37 pm
Post by: m@tty on June 03, 2010, 04:12:44 pm
Post by: kenhung123 on June 03, 2010, 04:45:59 pm
# Identify the parent hydrocarbon chain (The longest continuous chain of carbon atoms)
# Identify the functional group, if any (If more than one, use the one with highest precedence as shown here)
http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry
Post by: m@tty on June 03, 2010, 05:41:51 pm
Post by: andy456 on June 03, 2010, 05:51:00 pm
I'm pretty sure this is how you would do it.
Post by: Mao on June 03, 2010, 07:43:31 pm
Sorry, maybe I'm reading the question wrong but doesnt the COOH of a COO- in any basic environment? i.e zwitterions in amino acids and amino acids in basic solutionQuote2. Is the OH in carboxy group considered a hydroxy group i.e. can OH (hydroxy) be ionised in basic solution to form O-?No, it does not ionise (at VCE level). In reality, it can ionise, but you require a VERY strong base (pH > 14 if I remember correctly)
Also whilst on this topic, at what pH do zwitterions occur?
The overall molecule is positive in acidic environments and negative in basic environments but when is it both?
OH, that was a misinterpretation on my part. You are correct in what you say.
An alkanol OH (hydroxy) cannot go to O-.
A carboxyl OH goes to O-. This is rarely referred to as a hydroxyl group.
Post by: Mao on June 03, 2010, 07:48:16 pm
But the IUPAC nomenclature classifies Haloalkanes as primary functional groups and is takes precendence over double and triple bonds..
# Identify the parent hydrocarbon chain (The longest continuous chain of carbon atoms)
# Identify the functional group, if any (If more than one, use the one with highest precedence as shown here)
http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry
Are you sure? I am fairly sure the correct precedence is multiple bond > halides:
http://vcenotes.com/forum/index.php/topic,25368.msg258296.html#msg258296
For that molecule, however, both names will be accepted at VCE level.
Post by: kenhung123 on June 03, 2010, 09:45:27 pm