Chemistry 3/4 2013 Thread

[lzxnl]

So say we add HCl to the solution. The concentration of sorbic acid increases and the concentration of sorbate ions decreases. The concentration of hydroxide decreases. Would the equilibrium position shift to the left (to increase the concentration of sorbate ions) or to the right (to increase the concentration of hydroxide). That's where I'm confused. I mean I know it shifts to the right, but I'm not completely sure why

Wait. As soon as you add HCl to the solution, you reduce the concentration of OH-. The equilibrium shifts to the right to attempt to increase hydroxide concentration.
If you're considering the reaction with sorbate ions, then pretend to add a H+ to both sides; sorbate plus H+ <=> sorbic acid
Even so, adding H+ shifts the equilibrium to the right.

[Edward21]

Just confirming, I came across a Unit 3 practice paper, it had questions to do with fractional distillation and thermal cracking equations, they're not in the study design anymore, right? I don't remember learning this

[clıppy]

Just confirming, I came across a Unit 3 practice paper, it had questions to do with fractional distillation and thermal cracking equations, they're not in the study design anymore, right? I don't remember learning this

Pretty sure it's been taken out of the course but just in case I ran a ctrl-F on the study design
http://www.vcaa.vic.edu.au/Documents/vce/chemistry/ChemistrySD-2013.pdf
Only result i got was

Production of fuels from fractional distillation of crude oil

So I'd be safe in assuming it was taken out

[Alwin]

Just confirming, I came across a Unit 3 practice paper, it had questions to do with fractional distillation and thermal cracking equations, they're not in the study design anymore, right? I don't remember learning this

Hmm, I haven't seen it on a practice exam before, where was it from Edward21?

Pretty sure it's been taken out of the course but just in case I ran a ctrl-F on the study design
http://www.vcaa.vic.edu.au/Documents/vce/chemistry/ChemistrySD-2013.pdf
Only result i got was So I'd be safe in assuming it was taken out

@clippy, did you just search for fractional distillation in the context of crude oil separation, or fractional  distillation in general?
The 2011 VCAA Exam 1 Q7b has a nice q asking the principles behind fractional distillation,

The chemist decided to use fractional distillation to separate the final product from the reaction mixture. Describe the principles of fractional distillation
Assessment report in spoiler:

Spoiler

[thushan]

Hmm, I haven't seen it on a practice exam before, where was it from Edward21?
@clippy, did you just search for fractional distillation in the context of crude oil separation, or fractional  distillation in general?
The 2011 VCAA Exam 1 Q7b has a nice q asking the principles behind fractional distillation,

The chemist decided to use fractional distillation to separate the final product from the reaction mixture. Describe the principles of fractional distillation
Assessment report in spoiler:

Spoiler

Fractional distillation is no longer on the course.

[09Ti08]

Hello, do we need to care about significant figures in the exam? I have always been warned by my teacher about this... Thank you!

[Alwin]

Hello, do we need to care about significant figures in the exam? I have always been warned by my teacher about this... Thank you!

YES!!

well, thats if you want a decent-good score at least

[Edward21]

Hmm, I haven't seen it on a practice exam before, where was it from Edward21?
@clippy, did you just search for fractional distillation in the context of crude oil separation, or fractional  distillation in general?
The 2011 VCAA Exam 1 Q7b has a nice q asking the principles behind fractional distillation,

The chemist decided to use fractional distillation to separate the final product from the reaction mixture. Describe the principles of fractional distillation
Assessment report in spoiler:

Spoiler

Obviously and old one, you know the A+ books, the 9 practice exam booklet thing, it had a detachable examination, and I was looking at the questions, just doing some light revision, and had no idea about some of its contents! :O

[barydos]

k = [H3O+][OH-]/[H2O]^2
k_w = [H3O+][OH-]
so if temperature increases, K_w increases, but then consequently pH decreases.

why does pH decrease? i get that pH decreases when there is more H+, but since there is more OH- as well, shouldn't it "cancel out"?

[Aurelian]

k = [H3O+][OH-]/[H2O]^2
k_w = [H3O+][OH-]
so if temperature increases, K_w increases, but then consequently pH decreases.

why does pH decrease? i get that pH decreases when there is more H+, but since there is more OH- as well, shouldn't it "cancel out"?

pH is defined as pH = -log₁₀[H⁺]. As a result, the concentration of OH^- is irrelevant to the pH of the solution. Hence the pH will decrease because [H⁺] is higher. However, the solution will still be neutral, because [H⁺] and [OH^-] will remain equal.

The key point is not to confuse the concept of solution pH with the concept of solution neutrality.

Hope that helps! =)

[xenial]

Hello!

Would anyone be able to give me a list of what they think are the 'wordy' topics in chemistry units 3/4 that we pretty much need to rote learn for the exam?

A perfect example would be forensic analysis in Unit 3. Things like fossil fuels as well. An example of something that ISN'T rote memorisation would be gravimetric analysis, or organic chemistry. (Although reaction pathways would be borderline)

Just interested. Might have a rote cram day soon. It's not so bad in chemistry but in subjects like biology there are always topics that just don't come up on practice exams, and then VCA will throw in a two-marker about it on the day.

Thanks in advance

[thushan]

Forensic analysis (DNA profiling, gel electrophoresis) is no longer on the course.

Fossil fuels - you no longer need to remember the specifics, you just need to know which ones are renewable and how they are.

Chem doesn't need any rote learning. Any stuff you memorise will be stuff that you'll use - so in other words, do lots of questions

[Edward21]

Forensic analysis (DNA profiling, gel electrophoresis) is no longer on the course.

Fossil fuels - you no longer need to remember the specifics, you just need to know which ones are renewable and how they are.

Chem doesn't need any rote learning. Any stuff you memorise will be stuff that you'll use - so in other words, do lots of questions

Yes, I love chemistry questions!! Just a quick recap of unit 3, at pH 7, will an amino acid be as it appears in the data book, or as a zwitterion with both + and - charges cancelling out??? I can't remember which is which and the textbook doesn't specifically state it, I know what I zwitterion is! I just can't figure out when it would occur as opposed to the amino acid being completely non-charged at both ends?

[barydos]

pH is defined as pH = -log₁₀[H⁺]. As a result, the concentration of OH^- is irrelevant to the pH of the solution. Hence the pH will decrease because [H⁺] is higher. However, the solution will still be neutral, because [H⁺] and [OH^-] will remain equal.

The key point is not to confuse the concept of solution pH with the concept of solution neutrality.

Hope that helps! =)

Wow, that helped HEAPS! Haha, thanks, you're a legend

[brightsky]

Yes, I love chemistry questions!! Just a quick recap of unit 3, at pH 7, will an amino acid be as it appears in the data book, or as a zwitterion with both + and - charges cancelling out??? I can't remember which is which and the textbook doesn't specifically state it, I know what I zwitterion is! I just can't figure out when it would occur as opposed to the amino acid being completely non-charged at both ends?

in neutral environment, amino acids exist in their zwitterionic form. in acidic environment, amino acids exist mainly in their cationic form; most amino acid molecules will become protonated, due to abundance of H+ ions, which are incredibly strong acids. in basic environment, amino acids exist mainly in their anionic form; most amino acid molecules will become deprotonated due to abundance of OH- ions, which are incredibly strong bases.