OCR Unifying Concepts in Chemistry...

I wouldn't go blahing all over that it's brilliant yellow, because I'm fairly certain you might be wrong. Weak acid + Strong base means use something with a range above 7. You follow where the change in pH is fastest. I can't remember the range for brilliant yellow, but thymol blue seemed the most logical choice because of this fact.
The end point of the reaction is irrelevant - neutralisation always occurs at ph 7. You want to find the point where change is fastest.

i also got Ca3 (PO)4 and it balanced etc.
first q- initial rate
= definition of rate of reaction with initial before it
i think the initial amount of moles of reactant used in given time

it was pretty simple exam- i was happy with it and nice and easy shame the boundary might go up .
NOW DOES ANYONE KNOW WHY THE LAST PART OF THE LAST QUESTION HAD
BOLD WRITING? i got the answer - 0.06/0.5 and 0.03/0.5 to get the moles or conc i cant remmber but what the f*** did it have to do with gaining 2 electrons?
and what were the optimum conditions for the reaction? hig temp/pressure
i got the rest of that q fine

i'm really scared. will someone who has done chemistry papers tell me or just try and calm my nerves please/

i just got scared not being able to define what the calcium phosphate was. i really need to get the A in this subject and i feel today i've wreck every possible chances


oh btw, the answers for the gases i got a similar figure, but its in dm cubed surely?


help. i got synoptic bio tommorow. i feel so terrible. awful.

I wouldn't go blahing all over that it's brilliant yellow, because I'm fairly certain you might be wrong. Weak acid + Strong base means use something with a range above 7. You follow where the change in pH is fastest. I can't remember the range for brilliant yellow, but thymol blue seemed the most logical choice because of this fact.
The end point of the reaction is irrelevant - neutralisation always occurs at ph 7. You want to find the point where change is fastest.

I'm having trouble remembering the question now, but I thought it stated that the half life was 3 seconds? Therefore after 3 seconds, the concentration would have halved?

i'm with you dude! didnt actually work out where the equiv point was, just drew the standard weak acid/strong base curve which has the vertical bit at roughly pH8. hence i chose thymol blue.
all in all not too bad a paper really. some odball questions but nothing toooo difficult.

i can honestly say that i thought that paper was hardddd!!!nuffin like ne of the past papers, a few easy qs but soo many to catch u out. like for the qs where it sed write the rate equation, did u put hcl in the rate equation or not. for the 2nd from last qs which acid did u say was the strongest? i cant beleive there was no bufffers i had got my head around them.

Thymol blue had a pH range maximum of 6.8

I said that H3PO4 was the strongest as it had the lowest pKa value.

I did put HCl in the rate equation, but not sure if it should have been there as it was only a Catalyst, but it was in 1st order, so I assumed it should be in the equation.

Buffers are cool, why weren't there any!

The values givern were Ka not pKa if the numbers were pKa they would all have been of the order 0-10, not x x 10^-whatever, thus you got it the wrong way around, if i remember the question right

nuh-uh, it was something like 7-8 or something. definitely had an 8 in there.

i put that the one with biggest Ka had biggest conc of H+, hence strongest (lowest pH)...is this right?

yes

yes it was definatly >7

Yeah, I know the numbers were Ka, but I converted them to pKa by doing -logKa.

Is a low pKa a strong acid like a low pH is? That is what I assumed.

smaller the pka weaker the acid

Bugger!

larger the pka weaker the acid