Ed excel Chemistry A2 12th June, Wednesday

[Jan 2013]
Methods for investigating reaction rates include
A colorimetry
B collecting and measuring the volume of a gas
C quenching, followed by titration with acid
D quenching, followed by titration with iodine solution.
Which method would be most suitable to investigate the rate of the following
reactions?
(b) C4H9Br(l) + OH−(aq)  C4H9OH(l) + Br−(aq)

The answer is C
Why is colorimetry not used for this? I thought Br- was coloured?
And what does the acid react with in the titration?

9. This question concerns four solutions, A to D. They were prepared by mixing equal
volumes of 0.2 mol dm–3 solutions of two different substances. The substances were
A HCl(aq) and NaOH(aq)
B HCl(aq) and NaCl(aq)
C NH3(aq) and NH4Cl(aq)
D CH3COOH(aq) and CH3CO2Na(aq)
Select, from A to D, the mixture which would:
(a) have the lowest concentration of hydrogen ions ??

The answer is C.. but why ?

[Jan 2013]
Methods for investigating reaction rates include
A colorimetry
B collecting and measuring the volume of a gas
C quenching, followed by titration with acid
D quenching, followed by titration with iodine solution.
Which method would be most suitable to investigate the rate of the following
reactions?
(b) C4H9Br(l) + OH−(aq)  C4H9OH(l) + Br−(aq)

The answer is C
Why is colorimetry not used for this? I thought Br- was coloured?
And what does the acid react with in the titration?

[Jan 2013]
Methods for investigating reaction rates include
A colorimetry
B collecting and measuring the volume of a gas
C quenching, followed by titration with acid
D quenching, followed by titration with iodine solution.
Which method would be most suitable to investigate the rate of the following
reactions?
(b) C4H9Br(l) + OH−(aq) C4H9OH(l) + Br−(aq)

The answer is C
Why is colorimetry not used for this? I thought Br- was coloured?
And what does the acid react with in the titration?

the answer to that Cl- concentration is C .... i have no idea how it is true

Really? I'm sure it's B!

Pretty sure it is B too, C makes no sense... I think AS01 asked the same question yesterday & said it was B in the MS

I just realised it's asking for the lowest concentration of H+, it's not the same as the one yesterday

Br- is not coloured, Br2 is coloured
say if OH- is in excess, and reaction is completed
using acid would be a good idea to titrate the unreacted alkali
i think, thats how i make myself understood :P it may not be right

Br- is colourless... & the acid reacts with the OH- ... to form water so it stops the reaction

Br- is a colourless ion in solution . Acid reacts with OH- ions.

How did it go guys?

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I thought it went fairly well, answered all the questions, feeling confident that I will at least improve my U from last time. I'm not too good at maths so it took me ages to work out how to convert pKa back to Ka so I could calculate the pH, but I got there eventually.
Imagine my horror when I got to the last question and was missing my ruler!
Luckily the data booklets provided were backed with card so I used that instead!

Hope everyone else found it ok, it seemed a lot nicer than the January exam!

I thought the exam was pretty difficult..

I wouldnt say that the paper was easy and it was weird type! That ethanedioic acid question! Took me a while to figure out what the hell am I supposed to write lol

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I think once you realised it was just a weak acid but both ends could dissociate to give 2 H+ like sulfuric does, but when the sodium is added on one end it only dissociates one H+ i think, thats what I went on about anyway lol

Yeah that's what it is but took me a while. Can u remember all of the question?
I wanted more organic chem!

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