HSC Chemistry Question Thread

[kiwiberry]

I know it sounds stupid but I can't get this question right-

A student was investigating the heat of combustion of ethanol. She used an ethanol
burner that had an initial mass of 68.0 g. She then lit the burner and placed it under a
beaker containing 500 g of water. After a few minutes, she noticed that the water
temperature had risen from 24°C to 38°C and the burner now weighed 66.5 g.
She made the assumption that only the water was heated.
What would be the student’s value for the heat of combustion for ethanol?

It is from a past half yearly paper at my school and it says the answer is 859 kJ mol–1 but I'm getting a different answer.

I got 899 kJ/mol haha this is weird

ΔH = mcΔt = (500)(4.18)(38-24)= 29260 J
m(C₂H₅OH) = 68 - 66.5 = 1.5g
n(C₂H₅OH) = 1.5/[2(12.01) + 6(1.008) + 16]= 0.0325... mol
ΔH per mol = 29260/0.0325... = 898633.12 = 899 kJ/mol

[1tankengine]

I got 899 kJ/mol haha this is weird

Oh I see where I went wrong, I wrote 68-66.5=2.5 ...
But I got 897 as well. It was a multiple choice question so maybe the working out was done wrong.
Anyway thanks for your help!

[dux99.95]

Hey
My question is, if you had to quickly study for Production of Materials and The Acidic Environment, how would you do it?
Like, whats most important in each e.g focus on pracs more in POM, focus on calculations more in acidic environment (these are just examples, idk if you should actually do that).
Obviously I should study EVERYTHING but half yearlies are soon - so what should I do?

[Aaron12038488]

So i have a chemistry practical tommorow, any tips. The week after I have to hand in my prac report. Also what parts do I need for a prac report.

[jakesilove]

Hey
My question is, if you had to quickly study for Production of Materials and The Acidic Environment, how would you do it?
Like, whats most important in each e.g focus on pracs more in POM, focus on calculations more in acidic environment (these are just examples, idk if you should actually do that).
Obviously I should study EVERYTHING but half yearlies are soon - so what should I do?

Hey! Unfortunately, you're going to have to focus on EVERYTHING in both modules. However, you can get a good idea of what the HSC assesses more regularly by doing HSC past papers! So, the best method of study from here on in is doing past papers, and this will get you prepared for the actual exam. Do any past papers you can get your hands on (HSCs, Trials, whatever) and do the paper in timed conditions. Refer to your notes if absolutely necessary, but try to avoid this. Make sure to mark your work, and write out any mistakes that you make in a separate book. Then, revise those mistakes to try and avoid making them again.

Jake

[jakesilove]

So i have a chemistry practical tommorow, any tips. The week after I have to hand in my prac report. Also what parts do I need for a prac report.

Hey! Clearly a bit late to help you in the prac today; let's talk about the prac report.

In the report, I would recommend you have

- Aim
- Hypothesis
- Materials/Equipment
- Method
- Results
- Discussion
- Conclusion

By providing all of the above information, you'll have gone into sufficient depth (and sufficiently shown your scientific prowess) to get a really good mark! Whilst we can't do an assignment for you, let us know if you're really struggling with anything or not sure how to interpret some of your data, and we'd be happy to help you out

Jake

[J.B]

I think it's A! All Bronsted-Lowry acids must contain a H⁺ to donate. When this proton is donated, the acid's conjugate base is formed, which will have an overall negative charge since a positive charge (H⁺) was removed from it. eg. HCl/Cl^-, HNO₃/NO₃^- etc.

Yeah that's what I thought as well! , but the answer said it was D?

[RuiAce]

I think it's A! All Bronsted-Lowry acids must contain a H⁺ to donate. When this proton is donated, the acid's conjugate base is formed, which will have an overall negative charge since a positive charge (H⁺) was removed from it. eg. HCl/Cl^-, HNO₃/NO₃^- etc.

Yeah that's what I thought as well! , but the answer said it was D?

It's not A.

NH₄⁺ is a B-L acid but clearly its conjugate base NH₃ is neutral, not negatively charged. Donating a proton does NOT necessarily imply that it will become negatively charged. Hence A is wrong.

B is obviously wrong (consider NO₃^-) and the above example also contradicts C. I haven't figured out why D is correct yet but I may come back to it later.

[Aaron12038488]

what does it mean by priniciple in a scientific report? Also for the method, can I say like what the compound looked like before heating? Or should I put this in my results section?

[bholenath125]

I have a lot of trouble understanding the structural formation of condensation polymers. Anyone know a site which explains this concept well?

[jakesilove]

what does it mean by priniciple in a scientific report? Also for the method, can I say like what the compound looked like before heating? Or should I put this in my results section?

Not entirely sure what a 'principle' of a scientific report is; maybe formal manner? Succinct? Scientific language? Including the right subsections and analysis?

Observations should be put in the results section, not the method. The method should just be numbered, past tense, exactly what you did to carry out the experiment!

[jakesilove]

I have a lot of trouble understanding the structural formation of condensation polymers. Anyone know a site which explains this concept well?

Which part of condensation polymers are you confused about? I've pasted an example below



You can think of condensation polymers as like two molecules, being smashed together, but leaving out some small parts from each molecule. In the above example, the OH is 'removed' from the acid, and H is 'removed' from the Amine (whatever that is!). The final molecule is just the connection of the two molecules, with the OH+H=H2O. Condensation polymers are just polymers that, when formed, have a small molecule removed

[parthie]

Hey can someone please try and explain what buffers are to me...I am really confused?
Thanks

[jakesilove]

Hey can someone please try and explain what buffers are to me...I am really confused?
Thanks

Hey! A buffer is any solution that resists small changes in pH. That's the definition, and always the first thing to say in any answer relating to buffers. However, the chemical properties of a buffer are a bit more complicated!

Buffers operate using Le Chatelier's principle. Le Chatelier's principle states that when an equilibrium is introduced to a change, the equilibrium shifts to minimise that change. We've just said that a buffer resists small changes in pH; the only way it could do this is by shifting AWAY from an acidic solution when an acid is added, and TOWARDS an acidic solution when a base is added (to neutralise the added acid).

Generally, a buffer is made up of a weak acid and it's conjugate base. You don't need to understand why; it just helps if you get a multiple choice question asking you to identify a buffer. Look for a conjugate acid/base pair, and then discard any strong Acids.

What does a buffer look like? Well, the most important buffer is the one in our blood.




So, what does this look like? Well, when the blood becomes too acidic, the equilibrium will shift to the LEFT (ie. away from the Hydrogen ion). This will decrease the acidity of the solution (ie. resists small changes in pH!). WHen the buffer becomes too basic, the equilibrium will shift to the RIGHT (ie. towards the Hydrogen ion). This will increase acidity of the solution (ie. again, resisting small changes in pH!). This buffer stops our blood from becoming too acidic, killing us from the inside.

Hope that all makes sense!

[bananna]

hi!
how do you work out the concentration of an acid given the pH of a substance?
for example,
A solution of HNO3 has a pH of 2.05. Calculate the concentration of the acid.

ALSO, (sorry)
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)

thank you!