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Equilibria, Energetics and Elements (F325) - June 2011 Exam.

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Original post by terry_2006

omg when i looked into the paper again it is so much easier=.= .. anyways i have chosen benenecarboxylic acid does anyone think i will get a method mark for it?because i have worked out and the only thing being wrong is that chosen acid is not lactic acid or ethanoic acid.
=.=
i have commented on the temperature in the tounge is different from 25degree C and that these pH which these works might be different hence not able to produce the sweet tang .

Uhm that is a very interesting point about temperature affecting Ka of the acid! I think it is the correct answer too! I wish I wrote that xD

Reply 2921

Killmepls

Original post by Jsmalley1

anybody else feel like they have worked their arses of all year to try and get an A, and it has all been wasted because of OCR and MAGIC TANG?!

Defineatly, I feel like I wasted time revising imo. I truly believe had I done this exam 2 months ago I would have exactly the same result.

Reply 2922

Jtking3000

Original post by terry_2006

why would it not be lactic acid?

Reply 2923

Sereni

Original post by Swagger_Buck

i know what your talking about, thats only when the ration of acid to conjugate base is 1:1 that pka=ph, they never said it was...

Yea pka only equals ph when 1:1 but it does give you the relative strengths regardless of what the actual ratio is

Reply 2924

Sereni

Original post by Dim0nIX

Uhm that is a very interesting point about temperature affecting Ka of the acid! I think it is the correct answer too! I wish I wrote that xD

It will be AVP, anything that makes sense will get the mark imo, like i just wrote low lhs cause sour taste therefore it is valid

Reply 2925

LilacTweet

Original post by Dim0nIX

Interesting... I am a physics student as well and that is the first time I hear about variable half-life. In the chemistry book it explicitly said constant half-life implies first order.

I don't want to start a arguement here but I thought I should add this...
The curve was a line of best fit, so didn't go perfectly through all the points marked. Also there may be errors from taking the measurements, which is why although it may not be constant, It's close enough so +/- a few units isn't significant, so it was roughly 200 or something each time, so 1st order I reckon. I wouldn't say it had a 'variable' half life, It's just down to experiments not giving textbook results...

I think that the fact people even after the exam have conflicting views over the 'correct' answer shows a lot about how badly written/ridiculous this exam was. It's not black and white as to what is correct, so how OCR expect us to give a valid and accurate answers in such tight time I really don't know!

(edited 12 years ago)

Reply 2926

Jtking3000

Even if you thought that a percentage error of +/-5% meant that the half life wasn't constant it still says in the text book that the half life of a second order conc/time graph increases with time, this did not occur as the half life went from 190 to 210 to 200.

Reply 2927

Rob16

Original post by Dim0nIX

Hey

we are just discussing it. No need to get too emotional :wink:

I drew tangents at regular time intervals and plotted a new graph of rate-concentration (wasted lots of time but I wanted to double check). The graph was a curve. That clearly implies second (or third) order. HOWEVER as you said the half-life was almost constant and my curve was only slopping up a little bit. This as many people are saying might be due to experimental error. We were probably expected to think outside the box and assume that half-life is constant (which I think is unfair). I hope that we atleast get some method marks if second order is wrong.

I drew tangents as well (never even thought about half life :/) but i didnt draw a graph, I just looked at the gradients I got and they were more or less proportional to concentration, so I went for first order and put the difference down to error :/ Doesn't rate have to be proprtional to [X]^2 for second order?

It's weird how there's such a divided opinion on this question :frown:

Maybe OCR will have both options on the MS depending on how we reason it :s-smilie:

Tbf though, I went first order, so that means its probably second order. I ballsed that test up big-style

Reply 2928

Limesasquatch

Original post by Dim0nIX

So it was second order?

I think it was first because if it was second order the half lifebincreases with time, however this one went from 190 to 210 to 200

Reply 2929

RodrigoB

Guys are the concentrations of the weak acid and salt in the magic tang thing :

Acid = 5.62x10-4
salt= 2.8x10-4

that's what I got, using lactic acid.

Reply 2930

gids

Original post by Jtking3000

why would it not be lactic acid?

why is not the answer

Reply 2931

RodrigoB

2nd order always increases with time guys
so, personally, I think it was first order.
May be wrong, may be right. lol

Reply 2932

Lucy23

has anyone got any idea how many raw marks i'd need for 146/150??

Reply 2933

RodrigoB

Original post by Rob16

Maybe OCR will have both options on the MS depending on how we reason it :s-smilie:

Tbf though, I went first order, so that means its probably second order. I ballsed that test up big-style

WELL I drew a tangent too. I did that to calculate initial rate ?
and then I used it to calculate K. Isn't that rright :frown:

(((
Cos we could take from the graph the initial conc and the time . ( mine was 255s)

so I did ( Initial rate = 0.01/225 )

GOD I think I screwed it up