HSC Chemistry Question Thread

[jakesilove]

hey so in the atarnotes by you guys it says this about preparing for titration:
" First test small quantities of each solution to find their pH. This tells you which indicator to use; if they are a strong acid and a strong base, you want an indicator with an equivalence point around 7(bromothymol blue )"
 or close too, and i don't really get. Testing their pH how? As well as how will you know if it’s a strong acid or strong base? What if it’s just a really concentrated weak acid, and how does this affect the equivalence point, because it will be an acidic salt formed? Or basic salt, what if both are weak? Then use 7? But how do we know? Confused ahahah.
like what do you treat as the equivalence point, can't you just make it 7, and then just add acid to force the equilibrium of the weak base across enough so that its completely neutralised - i am very confused. sorry if it doesnt make sense, im not great at this part.

Hey! To test the pH of a substance (roughly) you would just use some sort of indicator eg. Universal indicator, Litmus paper etc. However, the general rule is: Strong acid + Strong base = end point around neutral. Strong acid + Weak base = end point towards the acidic end. Weak acid + Strong base = end point towards the basic end. You just need to learn that. Also, you need to be able to identify whether an acid/base is strong or weak. In the HSC, Hydrochloric acid, Nitric acid and Sulfuric acid are the strong acids, and NaOH is the strong base. Everything else is weak.

So yeah, this is sort of stuff you just need to memorise, rather than really internalise and understand. I certainly didn't really think when performing titrations; just did what I knew was true.

[beau77bro]

Ohhh ok so u just memorise the string acids - not really much to know other than that. Ok well that helps a lot, thought u had to work it out from testing or something

[beau77bro]

hey so this is the molar heat of combustion prac? i was wondering if i covered everything or if there was any issue with my procedure. Any and all advice appreciated

i hope this helps anyone else doing pracs right now - i went pretty in depth (i think), but i didnt do discussion as my assessment wont have any of that end stuff.

[jakesilove]

hey so this is the molar heat of combustion prac? i was wondering if i covered everything or if there was any issue with my procedure. Any and all advice appreciated

i hope this helps anyone else doing pracs right now - i went pretty in depth (i think), but i didnt do discussion as my assessment wont have any of that end stuff.

Bloody brilliant. Literally the only thing I would change is the wording of your assessment of validity etc. Instead of saying 'not valid', say something along the lines of 'validity is severely limited due to...'. It is still, to a certain extent, valid; however, you're just indicating that there is much to be improved.

Again, brilliant, in-depth summary. Great job!

[beau77bro]

Bloody brilliant. Literally the only thing I would change is the wording of your assessment of validity etc. Instead of saying 'not valid', say something along the lines of 'validity is severely limited due to...'. It is still, to a certain extent, valid; however, you're just indicating that there is much to be improved.

Again, brilliant, in-depth summary. Great job!

ok yay, duly noted i will change that now.

im annoyed though because the next prac, galvanic cells - i already did this research task on it and i cant find it so now gotta start from scratch. will post if i find it

[bsdfjnlkasn]

Hey there!

Does anyone have any resource they're willing to share for processing second hand data relating to AAS? I can't seem to find any practice questions and I have an assessment with this in it- any help would be really appreciated! 

[jakesilove]

Hey there!

Does anyone have any resource they're willing to share for processing second hand data relating to AAS? I can't seem to find any practice questions and I have an assessment with this in it- any help would be really appreciated! 

What do you mean re processing second hand data relating to AAS? Can you like 'make up' the sort of question you mean, or just give me a better indication of what information you'd like? Happy to help!

[beau77bro]

this is a video from my teacher, its abit slow, but i think it has what you want.  https://www.youtube.com/watch?v=b4H0QPwWeDI
theres also some questions she posted (she said not to share them because copyright ahahha), so here yah go.

again im not really sure what your asking for, and i didnt do this, but hopefully this helps even a little? even in helping jake or the moderators (aka legends) find more questions for you.

[bsdfjnlkasn]

What do you mean re processing second hand data relating to AAS? Can you like 'make up' the sort of question you mean, or just give me a better indication of what information you'd like? Happy to help!

Hey sorry about the lack of clarity!
On my notification one of the task outcomes is: process second hand data relating to AAS and sulfate content in fertiliser. Some examples would probably be the questions given in conquering chemistry as I find them quite difficult but any general questions about sketching or interpreting AAS graphs would be really helpful. I'm sort of getting the hang of them now but would like to expose myself to all the question types/harder versions of them. Also any ideas what they could ask in regards to the sulfate content in fertiliser apart from calculating the percentage in a sample?

Thanks again

[beau77bro]

Hey sorry about the lack of clarity!
On my notification one of the task outcomes is: process second hand data relating to AAS and sulfate content in fertiliser. Some examples would probably be the questions given in conquering chemistry as I find them quite difficult but any general questions about sketching or interpreting AAS graphs would be really helpful. I'm sort of getting the hang of them now but would like to expose myself to all the question types/harder versions of them. Also any ideas what they could ask in regards to the sulfate content in fertiliser apart from calculating the percentage in a sample?

Thanks again

Hey bsdfjnlkasn did the things I posted help?

[bsdfjnlkasn]

this is a video from my teacher, its abit slow, but i think it has what you want.  https://www.youtube.com/watch?v=b4H0QPwWeDI
theres also some questions she posted (she said not to share them because copyright ahahha), so here yah go.

again im not really sure what your asking for, and i didnt do this, but hopefully this helps even a little? even in helping jake or the moderators (aka legends) find more questions for you.

Hey thank you so much, these are the sorts of questions I was looking for ! I'll definitely give these a crack, sorry to be really annoying but do you think you could possibly send the answers to these questions? I'm still not confident enough to be sure of my working at this point, but if you don't have any then I can just post them on here and get some confirmation. Thanks again, I really appreciate it

[beau77bro]

Oh man I'm really sorry but I don't have any answers? I don't even know where my teacher got it from. Maybe the moderators could check it or work out some answers? - I would offer to help but I don't think I'm a very credible source haha

[beau77bro]

hey so i was wondering why are galvanic cell voltages so often different to the ones worked out from the data sheet? like a list of reasons would be nice - is it just that the standard conditions were used or it wasn't in 1 molar solutions. we found in class that increasing the size of the salt bridge would increase the voltage dramatically, because of course the metal wires wont be the limiting factor in the movement of charge, it would be the salt bridge right? 

also how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow?

thankyou, sorry for squeezing soo many questions in was kind of a train of thought thing

[bsdfjnlkasn]

hey so i was wondering why are galvanic cell voltages so often different to the ones worked out from the data sheet? like a list of reasons would be nice - is it just that the standard conditions were used or it wasn't in 1 molar solutions. we found in class that increasing the size of the salt bridge would increase the voltage dramatically, because of course the metal wires wont be the limiting factor in the movement of charge, it would be the salt bridge right? 

also how do u deal with molar concentration in more complicated overall reactions? like for a reaction where it takes 2 moles of one metal to displace 1 mole of the other, does that mean you use 2 molar solution of the first metal ions, or what? - how does that affect charge flow?

thankyou, sorry for squeezing soo many questions in was kind of a train of thought thing

Hey there!

Here are some reasons for the discrepancies in read voltage and the theoretical value obtained through the standard potentials:

1. Not maintaining SLC is a major reason as we can't exactly ensure that the concentrations are all 1M (except always annotate this in your diagrams)
- Higher concentrations will increase the rate of reaction/current flow
- Lab conditions and concentrations are linked as if we were to increase the temperature of the room, then we make the solutions more concentrated (as water molecules would have evaporated)

2. Current flow is inhibited by resistance in wires (from external circuit)

3. There may be impurities on the electrodes so cleaning them will make it easier for the respective redox reactions to occur

4. Difficult to regulate the dispersion of ions in the salt bridge which also adds resistance
- If the ions haven't moved into their respective beakers, then we don't have a complete circuit preventing charge from flowing. But this doesn't stop the redox reactions from occurring, so the reactions go without powering anything because of the slow dispersion in the salt bridge. By the time there is a full circuit, the voltage would have decreased as there is less metal than what we started off with (as some have already reacted when we couldn't get an accurate reading)
NB: This is a really hazy explanation so would appreciate it if someone could step in and clarify things

Hopefully these helped

[beau77bro]

also, for titrations my teacher says to always put the base in the flask - im not sure why though?

is it to do with the base reacting with the glass - wont that still happen in the flask? or maybe its to do with clogging the tap somehow? i really dont know