HSC Chemistry Exam Solutions / Thoughts (1 Viewer)

[Arrowshaft]

OH CRAP I FORGOT REFLUX FOR NUCLEOPHILLIC SUBSTITUTION FAAAAAAAAA

 

[worldno17]

You dont need conditions for NaOH right? At least thats what I was taught

yeah, you don't. dang, umm, i really hope they don't penalise you for that because it's a legitimate and more effective way of substitution.
i also hope they won't penalise me for not being explicit in the reagents part. i can't rlly remember my answer though.

are you procrastinating on phys like i am?

 

[sharky564]

There was a question which you had to determine the pH of a solution given the Keq was something of the order 10^-7, and the pH was like 10.smth. To do that q accurately, you had to have the initial concentration of [OH-] in pure water to be 10^-7, otherwise your quadratic is a bit off. I don't think it affected the final answer, but only because it was 1 place out of the required number of significant figures but you may still be marked down for not including it.

 

[Jendeukie]

Do you reckon I’d lose marks for saying NaOH..? Damn it, shouldve gone with water, had way more conditions!

If this is for turning it from chloroethane into ethanol, I said refluxed with NaOH (at high temperatures, this should be a given right) for a substitution reaction to occur.

 

[Jendeukie]

There was a question which you had to determine the pH of a solution given the Keq was something of the order 10^-7, and the pH was like 10.smth. To do that q accurately, you had to have the initial concentration of [OH-] in pure water to be 10^-7, otherwise your quadratic is a bit off. I don't think it affected the final answer, but only because it was 1 place out of the required number of significant figures but you may still be marked down for not including it.

omg i don't remember this question i don't know if i got it or not!?
can you describe it in more detail

 

[Arrowshaft]

If this is for turning it from chloroethane into ethanol, I said refluxed with NaOH (at high temperatures, this should be a given right) for a substitution reaction to occur.

Yea i cannot believe I frickin forgot reflux!!!!!!!!!!!!!! Screwed myself over my gosh!!!! i havent seen textbooks actually include reflux though for NaOH, you reckon they could penalise me?

 

[worldno17]

OH CRAP I FORGOT REFLUX FOR NUCLEOPHILLIC SUBSTITUTION FAAAAAAAAA

oh that's a thing? doesn't say in nelsonnet nor pearson, i just checked. how weird.
they also don't mention heating for water substitution either - just requires a dilute acid catalyst.

 

[Arrowshaft]

oh that's a thing? doesn't say in nelsonnet nor pearson, i just checked. how weird.
they also don't mention heating for water substitution either - just requires a dilute acid catalyst.

Ik but yeah Ive seen GCSE and A Level tbooks mention it, hopefully its not required for hsc tho and i escape with it

 

[worldno17]

There was a question which you had to determine the pH of a solution given the Keq was something of the order 10^-7, and the pH was like 10.smth. To do that q accurately, you had to have the initial concentration of [OH-] in pure water to be 10^-7, otherwise your quadratic is a bit off. I don't think it affected the final answer, but only because it was 1 place out of the required number of significant figures but you may still be marked down for not including it.

i just made an ICE table with Kb = x^2/(0.20-x) and solved the quadratic accordingly. most people i think just took x to be negligible.

 

[sharky564]

i just made an ICE table with Kb = x^2/(0.20-x) and solved the quadratic accordingly. most people i think just took x to be negligible.

I mean same, but x(x+10^-7)/(0.20-x), and the resulting quadratic has coeff for x 4.3E-7 x as opposed to 3.3E-7 x

 

[Arrowshaft]

Damn this is so demoralising lmao, just watching my marks dissolve away. I feel my potential b6 has been washed away to hahaha. Is it just me or does everyone else also have a mini heart attack when smth is brought up?

 

[Jendeukie]

Yea i cannot believe I frickin forgot reflux!!!!!!!!!!!!!! Screwed myself over my gosh!!!! i havent seen textbooks actually include reflux though for NaOH, you reckon they could penalise me?

don't worry! i went tutoring for chem, i've never used a chem textbook in my life (this is more because my school never uses/buys us textbooks, they only use powerpoint slides).
i'd assume that if you purely react it with NaOH you're still missing reaction conditions? i don't believe you'll lose more than one mark - it's a 7 mark question, it's difficult to tell where marks are allocated exactly - so if you wrote heaps for everything else they might give you full marks, and who knows, they might be nice enough to let you just mention the reagent - they might just let it slip depending on everything else you wrote.

so yeah, have no fear, it's barely a mark, even if your standards are super high it'll be fine.

 

[worldno17]

I mean same, but x(x+10^-7)/(0.20-x), and the resulting quadratic has coeff for x 4.3E-7 x as opposed to 3.3E-7 x

oh, uh, i thought i was already overcomplicating it with the quadratic. tbh, i think hsc usually accepts x to be negligible, so i think it's fine lol. 10^(-7) isn't a major difference tbh. in all the pH questions i've done at school and tutoring, i haven't encountered a scenario where u gotta take into the self-ionisation of water. maybe this will be the first? i doubt it though, since we gotta account for the rest of the state.

 

[Arrowshaft]

i just made an ICE table with Kb = x^2/(0.20-x) and solved the quadratic accordingly. most people i think just took x to be negligible.

I mean same, but x(x+10^-7)/(0.20-x), and the resulting quadratic has coeff for x 4.3E-7 x as opposed to 3.3E-7 x

Don’t think nesa will be that picky. Also they only judge sig figs for certain questions, usually 1 or 2 in the whole exam mention penalty for incorrect sig figs

 

[worldno17]

Damn this is so demoralising lmao, just watching my marks dissolve away. I feel my potential b6 has been washed away to hahaha. Is it just me or does everyone else also have a mini heart attack when smth is brought up?

bro you've barely lost 5 marks? calm haha it'll all be fine, u defs haven't gone to low 80s raw so far.

 

[Jendeukie]

oh, uh, i thought i was already overcomplicating it with the quadratic. tbh, i think hsc usually accepts x to be negligible, so i think it's fine lol. 10^(-7) isn't a major difference tbh.

omgomgoomg seeing you write this is a lot more comforting, i do remember having a basic quadratic and just simpling it down through an assumption, i used the Kb that we calculated from the previous question.
think i got [OH-], then found pOH and subtracted that from 14 to get the final pH.


now that i think about it, my sig figs were an absolute mess. perks of cramming chem the literal day before after 3u and eco, i didn't know what the hell i was doing or studying and if they gave us an actual difficult exam with heaps of mod 8, as hard as my school trials i would've bombed. came in expecting 60, walked out expecting 90+ aksaksks

MC is what gets me good though, have already lost 2 marks from there - my 2 most unsure questions so it was expected but still frustrating. don't know if i made silly mistakes elsewhere, and i BET i made a calculation error or some kind of mistake in one of the numerous calculation questions. UGH

 

[Drdusk]

Just as a note:

You can do sig figs on your calculator. Some people actually don't know this.

 

[worldno17]

omgomgoomg seeing you write this is a lot more comforting, i do remember having a basic quadratic and just simpling it down through an assumption, i used the Kb that we calculated from the previous question.
think i got [OH-], then found pOH and subtracted that from 14 to get the final pH.

mhm it's that question with the naocl salt thingie. all good! u sound like u did great. now...i better go study phys, bye yall. let's keep this thread inactive tmr so we don't keep on dredging up the past, haha.

 

[Jendeukie]

mhm it's that question with the naocl salt thingie. all good! u sound like u did great. now...i better go study phys, bye yall. let's keep this thread inactive tmr so we don't keep on dredging up the past, haha.

fingers crossed! haha i didn't know this thread existed before, it's cathartic to discuss a few questions with everyone the day of the exam, by the next day it's indeed better to not dwell on it. i'll keep an eye out if anyone has any other questions to discuss.

good luck to you for physics and everyone doing it!

 

[CarbonMini]

Yea i cannot believe I frickin forgot reflux!!!!!!!!!!!!!! Screwed myself over my gosh!!!! i havent seen textbooks actually include reflux though for NaOH, you reckon they could penalise me?

I just did NaOH + heat for substitution of the chloroethane; thats how Ive learnt it.