The Student Room Group

rates question?? isn’t the concentration tim

Isn’t the concentration time graph of a zero order reaction meant to be a straight line downwards??? why is it a curved line here?


IMG_1847.jpeg
It should be a straight line (in theory) for a zero-order reactant, provided you have used a massive excess of your other reagents (so that they have approximately constant concentrations and so the rate is limited only by the concentration of the limiting reagent).

I suspect the mixture used does not contain a massive excess of hydroxide or acetone and so that's probably why it forms the curve. I'd need to see what the mark scheme says as it does appear that they haven't given you the right information to deduce this.
Original post by UtterlyUseless69
It should be a straight line (in theory) for a zero-order reactant, provided you have used a massive excess of your other reagents (so that they have approximately constant concentrations and so the rate is limited only by the concentration of the limiting reagent).

I suspect the mixture used does not contain a massive excess of hydroxide or acetone and so that's probably why it forms the curve. I'd need to see what the mark scheme says as it does appear that they haven't given you the right information to deduce this.


In fact, that question is complete rubbish. Bromoacetone only ever forms if you brominate propanone in acidic conditions. If you throw in a base, it undergoes a bromoform reaction (the reasons why are not expected to be known at A level).

It's a badly written question with dubious chemistry and insufficient information given.

Which paper was it taken from?
Reply 3
Original post by UtterlyUseless69
In fact, that question is complete rubbish. Bromoacetone only ever forms if you brominate propanone in acidic conditions. If you throw in a base, it undergoes a bromoform reaction (the reasons why are not expected to be known at A level).
It's a badly written question with dubious chemistry and insufficient information given.
Which paper was it taken from?
Thanks for the help. I’ve lowkey just given up with this question because it’s too much to think about for one mark. it’s from the 2024 a level aqa paper, i’ve attached the markscheme

IMG_1848.jpeg
Original post by maryam06
Thanks for the help. I’ve lowkey just given up with this question because it’s too much to think about for one mark. it’s from the 2024 a level aqa paper, i’ve attached the markscheme

IMG_1848.jpeg

That's fair. It is a stupid question after all.

I'm surprised you have access to both the paper and mark scheme (unless of course you are a teacher), but on that note please refrain from sharing the paper and/or mark scheme in full on TSR. I think the sharing of how to score just one mark is fine, though.
Reply 5
Original post by maryam06
Isn’t the concentration time graph of a zero order reaction meant to be a straight line downwards??? why is it a curved line here?
IMG_1847.jpeg

The thing that is missing from the Q is some idea of the conc. of the three chemicals.

IF [CH3COCH3] & [OH-] >> [Br2] then [CH3COCH3] & [OH-] would effectively be constant so the Br2 would act like it were zero order and [Br2] would decrease linearly.

IF all three concs were the same, then it would be second order overall and all three concs would decrease together.

You could even have the conc of Br2 super high relative and effectively it wouldn't change.

Shame on you AQA.

And shame on you too maryam06 for sharing a 2024 paper Q.
Reply 6
Original post by UtterlyUseless69
That's fair. It is a stupid question after all.
I'm surprised you have access to both the paper and mark scheme (unless of course you are a teacher), but on that note please refrain from sharing the paper and/or mark scheme in full on TSR. I think the sharing of how to score just one mark is fine, though.

I’m retaking, this is my paper from this year which is why I have the paper 😞 I’ll refrain from sharing any more questions on that note
Original post by maryam06
I’m retaking, this is my paper from this year which is why I have the paper 😞 I’ll refrain from sharing any more questions on that note

Makes sense. Thank you for choosing not to share any more 2024 questions until the papers are released to the public. Until then, please do feel free to ask about past papers from 2023 and before.

Quick Reply