VCE Chemistry Question Thread

[hums_student]

SLC does mean 1 molar concentration tho?

To my knowledge, standard conditions and standard lab conditions are different.

SLC (25 °C and 100 kPa)

It does not mention anything about concentrations.
Meanwhile equations on the electrochemical series are under standard conditions, aka 1 Molar.

EDIT - You beat me to it

[zenith101]

can someone please explain q3 aii from the NHT 2018 exam please? Are we required to talk about changes to K, because i thought the ratio of products to reactants only changes with temp changes? Thanks

[Bri MT]

can someone please explain q3 aii from the NHT 2018 exam please? Are we required to talk about changes to K, because i thought the ratio of products to reactants only changes with temp changes? Thanks

- K only changes with temperature
- however,  that doesn't mean that the concentrations stay constant.
- let's stay that when the concentrations are equal,  there's .25 M each. You can then find the value of K
- let's now double the concentrations of each to .5M and find the value of K.
- you would see that the two values of K are not identical even though [product] still equals [reactant]
- what change in relative concentration would have to occur for K to stay the same when more mols are added?


Hope this helps

[hums_student]

can someone please explain q3 aii from the NHT 2018 exam please? Are we required to talk about changes to K, because i thought the ratio of products to reactants only changes with temp changes? Thanks

It's the value of K that changes when the temperature changes. Ratio of products to reactants can change with the addition of products/reactants etc. For this question, imagine you initially started off with 2 mol of NOCl (aka experiment 1), and then you added another 2 mol of NOCl to get to experiment 2. That would be an addition of products, which shifts the reaction to the left, however remember that it's always a partial opposition, meaning that you'll still end up with a higher concentration of NOCl than NO.

You can also talk about it through Kc values - the expression for the Kc value of experiment one is [NOCl]^2/([NO]^2 x [Cl2]). As the Kc value of experiment two is still the same (because the temperature doesn't change, therefore Kc remains constant), experiment two should also be Kc = 1/[Cl2]. However you have a higher concentration of NOCl, meaning both [NO] and [Cl2] also have to increase - but to a lesser extent.

Hence [NOCl] > [NO] in experiment two.

Personally I think the Kc method is ridiculously confusing and it's better to just use the concept of partial opposition to explain it. Just remember that K does NOT change.

EDIT - Ooops, beaten by miniturtle

[zenith101]

Thanks Lsjnzy13 & miniturtle, that clears it up a fair bit.

Also in q7c) from the same exam, the examiner's report uses the molar heat of combustion of hydrogen gas from the data book despite the gas not at SLC conditions (100kPa and 18 degrees). Is this just a mistake or are we expected to ignore the SLC condition for the data book. Thanks

[marina_t]

Hi question about chromatography:

What are some factors that could affect the retardation factor? (i.e if you wanted to try and increase or decrease the retardation factor, what would you do?)


Thanks

[Azim.m]

Are all amino acids found as zwitterions in a neutral pH?
Two of the same questions, but they give different answers (answer is B for the MCQ)

[Lear]

Anyone else find the 2018 NHT exam significantly harder than any of the past exams from 2013? I typically finish my exams 40-50 minutes early but I barely finished 2018 NHT. Not sure wtf happened

[hums_student]

Anyone else find the 2018 NHT exam significantly harder than any of the past exams from 2013? I typically finish my exams 40-50 minutes early but I barely finished 2018 NHT. Not sure wtf happened

Yeah I found 2018 NHT to be difficult too. Though tbh I won't be surprised if our exam on Tuesday would be even harder - there was no question similar to the 2017 Mars question on the NHT exam, which I expect would be on ours.

Also have 2 questions - first, can anyone confirm if questions 4 and 6 in the 2017 NHT exam (short answer) are still on the Study Design? Second, on the 2018 NHT examiners' reports Vitamin D is classified as an essential vitamin, even though it can be synthesised by the body when you go in the sun. Is that just another mistake on VCAA's part or is Vitamin D now considered an essential vitamin? Thanks.

[ThomasJordan]

Hi, in the required answer of this question it suggests you cant look at the chemical shift in the data book to answer it, which is true but you need it to distinguish between Propanoic acid and Ethyl methanoate as the final answer. I understand what they've done but how would i arrive to the answer without knowing specific chemical shifts of the functional groups of these molecules, thanks.

[AhmadAkkad]

DO we need to know how to do combustion analysis? Do you guys thing it's going to be in the exam?

[Freddie Hg]

Hi, in the required answer of this question it suggests you cant look at the chemical shift in the data book to answer it, which is true but you need it to distinguish between Propanoic acid and Ethyl methanoate as the final answer. I understand what they've done but how would i arrive to the answer without knowing specific chemical shifts of the functional groups of these molecules, thanks.

it says in the answers that one will have shift thats greater than the others. the only way to know this is looking at the data book.
which part of the question says you cant use the data book?

Anyone else find the 2018 NHT exam significantly harder than any of the past exams from 2013? I typically finish my exams 40-50 minutes early but I barely finished 2018 NHT. Not sure wtf happened

same experience man. im hoping our exam isnt that challenging. for some reason 2013 was known to be the hardest exam in the past study design. i found it a breeze compared to this one.

Also have 2 questions - first, can anyone confirm if questions 4 and 6 in the 2017 NHT exam (short answer) are still on the Study Design? Second, on the 2018 NHT examiners' reports Vitamin D is classified as an essential vitamin, even though it can be synthesised by the body when you go in the sun. Is that just another mistake on VCAA's part or is Vitamin D now considered an essential vitamin? Thanks.

for 2017 nht exam 1 or 2?
i only found exam 2 so no for both questions
again i just assumed that was wrong since vitamin D is an essential vitamin.
Nearly all vitamins are considered essential. However, some vitamins like vitamin D can be synthesized by the body, so they are not technically considered "essential

Are all amino acids found as zwitterions in a neutral pH?
Two of the same questions, but they give different answers (answer is B for the MCQ)

yeah from my experience with question all are zwitterions, it exists as a dipole of the amino acid.
check out 2017 vce chemistry mcq 10 for vcaa confirmation

[ThomasJordan]

it says in the answers that one will have shift thats greater than the others. the only way to know this is looking at the data book.
which part of the question says you cant use the data book?

Some functional groups in both molecules arent in the data book, this is my problem lol. For example the ppm for carboxylic acid is 10-12 ppm but it doesnt tell you this in the data book. It points this out in the answer aswell.

[Freddie Hg]

Some functional groups in both molecules arent in the data book, this is my problem lol. For example the ppm for carboxylic acid is 10-12 ppm but it doesnt tell you this in the data book. It points this out in the answer aswell.

it says that the data book offers limited help to get the answer as ethyl ethanoate. but the data book can be used to eliminate the possibility of propanoic acid, because the data book says that a proton (the last one that is listed in the data book) in the hydroxyl group of a carboxylic acid has a shift from 9-13 then we can assume that the proton NMR given is not of a hydroxyl group belonging to a carboxylic acid. as its lone hydrogen has a shift less than the predicted on of an acid. by eliminating propanoic acid we can safely confirm that the answer is ethyl methanoate
so your right the data book does not directly help in finding the answer, as the functional groups of the answer are not listed. but it does confirm that its not an acid. confirming whats not the answer. which essentially gives us the answer. (sorry for a late reply my laptop ran out of charge)

[ThomasJordan]

it says that the data book offers limited help to get the answer as ethyl ethanoate. but the data book can be used to eliminate the possibility of propanoic acid, because the data book says that a proton (the last one that is listed in the data book) in the hydroxyl group of a carboxylic acid has a shift from 9-13 then we can assume that the proton NMR given is not of a hydroxyl group belonging to a carboxylic acid. as its lone hydrogen has a shift less than the predicted on of an acid. by eliminating propanoic acid we can safely confirm that the answer is ethyl methanoate
so your right the data book does not directly help in finding the answer, but it helps in confirming whats not the answer. which essentially gives us the answer. (sorry for a late reply my laptop ran out of charge)

Thankyou! I thought this was the case when i looked at the VCAA data booklet but the insight data book (i thought) didnt provide R-COOH. Having another look at it, it seems that on the next page it continued the H-NMR data and had the R-COOH information. My bad!