VCE Chemistry Question Thread

[peanut]

Is this type of question in this year's study design? E.g. things involving strong/weak acids, [H3O+][OH-] = 10^-14, pH calculations, Ka, etc.?

[peterpiper]

Is this type of question in this year's study design? E.g. things involving strong/weak acids, [H3O+][OH-] = 10^-14, pH calculations, Ka, etc.?

No it's not on the study design. Consult the study design over my word, but I am almost certain that it's not.

[ardria]

Is it worth knowing Gravimetric Analysis at all?

It's off the study design this year, yet my (extremely experienced and trustworthy) tutor is teaching it.

[tinagranger]

Hi Why would butan-1-ol have a singlet on its HNMR spectrum?
I thought there would be 3 triplets, a multiplet (5 splits) and a multiplet (6 splits)?

Also, I don't get why it has 5 H environments and not 4. I was told that Hs on the same C are always part of the same environment, but this contradicts that rule. Is that because the O-H on the first carbon is part of a different H environment than the other 2 Hs on that same C? (If so, why?)


Also if asked to draw more than 1 repeating polymer unit (e.g.: 4 repeating units all linked in a row with double edit SINGLE bonds), would you still have to have the brackets and 'n' subscript at the ends?

Thanks

[VanillaRice]

Hi Why would butan-1-ol have a singlet on its HNMR spectrum?
I thought there would be 3 triplets, a multiplet (5 splits) and a multiplet (6 splits)?

Also, I don't get why it has 5 H environments and not 4. I was told that Hs on the same C are always part of the same environment, but this contradicts that rule. Is that because the O-H on the first carbon is part of a different H environment than the other 2 Hs on that same C? (If so, why?)


Also if asked to draw more than 1 repeating polymer unit (e.g.: 4 repeating units all linked in a row with double bonds), would you still have to have the brackets and 'n' subscript at the ends?

Thanks

You'll hear the general rule that when counting the number of neighbouring hydrogens to predict the splitting of a signal is that you can't "go through" an oxygen atom. The basic concept (not in VCE) behind splitting is that neighbouring hydrogens exert their own magnetic field onto the hydrogens in that environment. However, in this case, the highly electronegative oxygen atom has enough of a deshielding effect on the hydrogen which is bonded to it, that the effect of the neighbouring hydrogens is neglible. This is why the signal does not split.

When intepreting the number of unique hydrogen environments, the hydrogen in the alcohol is counted as its own environment (since it produces it own peak on the NMR spectrum). We don't consider it to be "connected" to the carbon, or in the same environment. So, yes, the hydrogen in the hydroxyl group is in its own environment.

Unless the question specifically said the polymer was only 4 units long, you would still have to draw the brackets and the 'n' subscript to show that those 4 units are repeating. Without them, your structure tells the reader that the polymer only has 4 units.

Hope this helps

[tinagranger]

You'll hear the general rule that when counting the number of neighbouring hydrogens to predict the splitting of a signal is that you can't "go through" an oxygen atom. The basic concept (not in VCE) behind splitting is that neighbouring hydrogens exert their own magnetic field onto the hydrogens in that environment. However, in this case, the highly electronegative oxygen atom has enough of a deshielding effect on the hydrogen which is bonded to it, that the effect of the neighbouring hydrogens is neglible. This is why the signal does not split.

When intepreting the number of unique hydrogen environments, the hydrogen in the alcohol is counted as its own environment (since it produces it own peak on the NMR spectrum). We don't consider it to be "connected" to the carbon, or in the same environment. So, yes, the hydrogen in the hydroxyl group is in its own environment.

Unless the question specifically said the polymer was only 4 units long, you would still have to draw the brackets and the 'n' subscript to show that those 4 units are repeating. Without them, your structure tells the reader that the polymer only has 4 units.

Hope this helps

Thanks! So does that mean every time you have a hydrogen environment that has to 'go through' an O to get to the neighbouring carbon, it doesn't split and its neighbouring carbon is considered to have 0 hydrogens?

[VanillaRice]

Thanks! So does that mean every time you have a hydrogen environment that has to 'go through' an O to get to the neighbouring carbon, it doesn't split and its neighbouring carbon is considered to have 0 hydrogens?

For VCE - generally, yes

[Butterflygirl]

For 3/4 chem, what topic are you guys up to and how many SAC's do u have left?

My school is still on spectroscopy and we still have two SAC's to do. I most likely will be having a SAC next term. Do you think that it would okay for me to learn the food chemistry stuff myself and then do the SAC soon after my spectroscopy SAC (this term)?

Not sure if I'm allowed to do that but I'm freaking out and don't know what else to do 

[noregret]

Hello, I have questions abouth Year 11 stoichemistry.

I do not get why you have to use 19.46ml for a dilution of HCL (Q23b) and when you are solving the concentration of undiluted HCL, why you have to divided 250/25? (Q23c)

Question 23
Some commercial brick cleaners are concentrated hydrochloric acid (HCl).
In an experiment to determine the concentration of the hydrochloric acid in the brick cleaner, the following steps were taken:
• A 25.00 mL sample of brick cleaner was diluted to 250.0 mL in a volumetric flask.
• A number of 20.00 mL samples of 0.450 mol L–1 sodium carbonate solution were placed in conical flasks and a few drops of methyl orange indicator added to each.
• Samples of the diluted brick cleaner (hydrochloric acid) were titrated against the sodium carbonate samples, and the average volume of the hydrochloric acid samples was found to be 19.46 mL.
The reaction between hydrochloric acid and sodium carbonate solution follows: 2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
(a) Show that there are 9.00 × 10–3 moles of sodium carbonate in the 20.00 mL samples. (2 marks)
... ... ... ...
(b) Determine the concentration of the diluted brick cleaner (hydrochloric acid). (2 marks) ... ... ... ...
(c) What was the concentration of the hydrochloric acid in the undiluted brick cleaner solution? (1 ma

[VanillaRice]

For 3/4 chem, what topic are you guys up to and how many SAC's do u have left?

My school is still on spectroscopy and we still have two SAC's to do. I most likely will be having a SAC next term. Do you think that it would okay for me to learn the food chemistry stuff myself and then do the SAC soon after my spectroscopy SAC (this term)?

Not sure if I'm allowed to do that but I'm freaking out and don't know what else to do 

No need to panic! At my high school, we had a chemistry SAC in term 4 as well This happened across a few other units as well. While we had a few complaints, I found it okay.
I'm not quite sure what you mean by learning the food chemistry content yourself and then doing the SAC? Do you have control of when you can do the SAC?

Hello, I have questions abouth Year 11 stoichemistry.

I do not get why you have to use 19.46ml for a dilution of HCL (Q23b) and when you are solving the concentration of undiluted HCL, why you have to divided 250/25? (Q23c)

Question 23
Some commercial brick cleaners are concentrated hydrochloric acid (HCl).
In an experiment to determine the concentration of the hydrochloric acid in the brick cleaner, the following steps were taken:
• A 25.00 mL sample of brick cleaner was diluted to 250.0 mL in a volumetric flask.
• A number of 20.00 mL samples of 0.450 mol L–1 sodium carbonate solution were placed in conical flasks and a few drops of methyl orange indicator added to each.
• Samples of the diluted brick cleaner (hydrochloric acid) were titrated against the sodium carbonate samples, and the average volume of the hydrochloric acid samples was found to be 19.46 mL.
The reaction between hydrochloric acid and sodium carbonate solution follows: 2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
(a) Show that there are 9.00 × 10–3 moles of sodium carbonate in the 20.00 mL samples. (2 marks)
... ... ... ...
(b) Determine the concentration of the diluted brick cleaner (hydrochloric acid). (2 marks) ... ... ... ...
(c) What was the concentration of the hydrochloric acid in the undiluted brick cleaner solution? (1 ma

23b) n = cV, and since we are looking for c(diluted HCl), we do c(diluted HCl) = n(diluted HCl)/V(diluted HCl)

23c) "A 25.00 mL sample of brick cleaner was diluted to 250.0 mL in a volumetric flask"
You have turned a 25ml solution into a 250ml by adding water. Hence, you are diluting by a factor of 5 (250/25=5 i.e. decreasing the concentration by a factor of 5). 250/25 is the dilution factor that we need in order to go back from the diluted concentration to the undiluted concentration.

Hope this helps

[sweetiepi]

For 3/4 chem, what topic are you guys up to and how many SAC's do u have left?

My school is still on spectroscopy and we still have two SAC's to do. I most likely will be having a SAC next term. Do you think that it would okay for me to learn the food chemistry stuff myself and then do the SAC soon after my spectroscopy SAC (this term)?

Not sure if I'm allowed to do that but I'm freaking out and don't know what else to do 

Try not to worry, last year my class had the last SAC in early term 4, due to time constraints (mainly due to the fact that we had 7 x 71 min classes per fortnight haha).

Self learning can be beneficial, but I wouldn't rush into doing the SAC early imo- you're most likely better off starting some exam revision early, especially reviewing what you covered in unit 3 and early unit 4.

[Butterflygirl]

No need to panic! At my high school, we had a chemistry SAC in term 4 as well This happened across a few other units as well. While we had a few complaints, I found it okay.
I'm not quite sure what you mean by learning the food chemistry content yourself and then doing the SAC? Do you have control of when you can do the SAC?

I was going to ask my teacher if I was able to do it early, not sure if I'm allowed. But its not like its an advantage because I'm doing it earlier not later (so less time to study compared to others). If others want they can do that too. Its just that I really don't trust my school atm cos I'm not even sure if they'll get the course done before exams.

Try not to worry, last year my class had the last SAC in early term 4, due to time constraints (mainly due to the fact that we had 7 x 71 min classes per fortnight haha).

Self learning can be beneficial, but I wouldn't rush into doing the SAC early imo- you're most likely better off starting some exam revision early, especially reviewing what you covered in unit 3 and early unit 4.

Okay thanks! Its just that I didn't want to study for a SAC over the term holidays but instead, focus fully on exam prep during the holidays and into term 4.

[kaii]

Hi, I was just wondering if there are resources on food chemistry? My teacher doesn't explain the concept and just skims through the book, but I kinda get it since we're so close to the exams so I would really love your help! Thanks

[Bri MT]

Hi, I was just wondering if there are resources on food chemistry? My teacher doesn't explain the concept and just skims through the book, but I kinda get it since we're so close to the exams so I would really love your help! Thanks

Because it is new to the chem study design, there aren't many VCE chemistry resources for it but:
-most of the content is covered in biology, so if you have friends doing bio they may be able to explain confusion
-there's content about calorimetery from previous study designs
-there's online content about all of it if you search up the terms


If you have any specific questions I'm happy to help; my class has finished content

[117922]

I have a sac on enzymes, more specifically rennin and its effect on the coagulation of milk.
There are two parts of this sac and the first part was studying the effect pH has on the coagulation of milk. The optimum pH level of Rennin is 5.8—6.2 but when we did the experiment the fastest to coagulate was the milk with a pH 3. From fastest to slowest to occur was pH 3, 5, 7 and 11. I asked my teacher and our results where correct. Why is this the case when the optimum pH level is not 3?