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

In the exam it asked us for something, and all the arrows were pointing downwards. So you had to reverse the enthalpy change of solution so its arrow was pointing upwards I think, I can't remember the paper.

But yeah everything below 'Breakdown of the ionic lattice' and the next page kinda suggests the relationship between all three enthalpy changes, so in the exam they were asking you to apply this.

Ok, tbf i actually agree with u with the fact it covered most topics

But where on the syllabus does it teach you to apply chemistry in the way they applied it in the exam paper?

E.g negative enthalpy change of solution? Recharging cells? Buffer ratios?

Theres not a single question, spread, paragraph or even a sentence that teaches you to apply these topics in the way they asked in the exam

Its not long before you add up the marks for these sort of question and find youve lost 20+ marks

Completely agree with you... revised so hard for this exam did loadsa pass papers revised everything on spec and know without a doubt ive dropped over 20marks .. OCR better drop the boundaries

Anybody resitting F324 on Friday like me? Any predictions

To all those complaining about the magic tang question, it really wasn't that difficult.

There's an example in the OCR textbook (question 9c on page 159) about calculating the ratio of acid/salt needed to form a buffer at a particular pH.

And on page 153 of the same textbook, the stretch/challenge box explains how the pH of a buffer is equal to the pKa of the acid when the acid exists in a 1:1 ratio with its salt. And then it explains that by altering the ratio from 1:10 to 10:1 you can change the pH +1 or -1. Hence you should initially choose an acid with a pKa close to the required pH and then fine-tune the ratio to get the required pH.

Once again, the majority of candidates have failed to learn the whole syllabus and then complain at questions they can't answer. Jeez.

true, but even if they threw just one example in the book, just to give me a heads up, then I wouldn't be bothered

my main concern was that the paper was predominantly application/stretch and challenge, rather than just a couple of questions

we'll have to wait until august I guess...hopefully OCR haven't sabotaged my university chances

To all those complaining about the magic tang question, it really wasn't that difficult.

There's an example in the OCR textbook (question 9c on page 159) about calculating the ratio of acid/salt needed to form a buffer at a particular pH.

And on page 153 of the same textbook, the stretch/challenge box explains how the pH of a buffer is equal to the pKa of the acid when the acid exists in a 1:1 ratio with its salt. And then it explains that by altering the ratio from 1:10 to 10:1 you can change the pH +1 or -1. Hence you should initially choose an acid with a pKa close to the required pH and then fine-tune the ratio to get the required pH.

Once again, the majority of candidates have failed to learn the whole syllabus and then complain at questions they can't answer. Jeez.

I am! My whole class is lol we ****ed up
How you feeling about it?

Ah im really scared and we just need to accept the fact that it will be a WIERD paper with WIERD questions.. Im scared 'cos i tried really hard in Jan, did well in all past papers then got a D.. so im worries it may happen again! In terms of revision, im just going through NMR right now then got pile of papers to get through. Welcome all nighter.
How about you?

To all those complaining about the magic tang question, it really wasn't that difficult.

There's an example in the OCR textbook (question 9c on page 159) about calculating the ratio of acid/salt needed to form a buffer at a particular pH.

And on page 153 of the same textbook, the stretch/challenge box explains how the pH of a buffer is equal to the pKa of the acid when the acid exists in a 1:1 ratio with its salt. And then it explains that by altering the ratio from 1:10 to 10:1 you can change the pH +1 or -1. Hence you should initially choose an acid with a pKa close to the required pH and then fine-tune the ratio to get the required pH.

Once again, the majority of candidates have failed to learn the whole syllabus and then complain at questions they can't answer. Jeez.

I could do the stupid buffer calculation, it's the irrelevant trivia part at the end that annoyed me about that question.

how did you do it ?
I chose the weak acid
and from it's pka
I calculated Ka.
then I did

concentration of acid = (ph required)^2 / ka ( of the weak acid I chose)

then I used the concentration of the acid
to find the conc of salt.

IS that right ?

I can't remember the full details of the question, but I got 0.49 for my ratio. I think it was something like use the full expression for Ka (not the simplified one you use for pH calculations) and the ratio is equal to Ka/required concentration of H+.

god I knew how to calculate the ratio ratio but
I ended up calculating the ACID and SALT conc !
I thought that this was what they were asking me to do no ?

true, but even if they threw just one example in the book, just to give me a heads up, then I wouldn't be bothered