AQA CHEM4 - 13th June 2012

OP

Just done the whole acids and bases section, HATE it lol :tongue:

Well at least it's done and over with. Need to get a move on the other bits :h:

Reply 221

OP

Original post by Casshern1456

Hi I'm resitting this exam in June i have done it twice before and didn't get the grade I wanted :sigh:

I'm actually quite terrified for this exam, if I don't score well again i'm fcuked :frown:

Same, I really need an amazing grade in this exam :frown:

Reply 222

Original post by Doctor.

Same, I really need an amazing grade in this exam :frown:

can someone explain to me on how to identify the rate determine step???

Reply 223

OP

Original post by shuaib786

can someone explain to me on how to identify the rate determine step???

Simply put, it's the SLOWEST step In the entire reaction. The slowest step determines the rate, also including the reactions that occur before the slowest step mat also be included in the rate equation...

That's all there is to it, as far as Im aware :smile:

Reply 224

Original post by Doctor.

Simply put, it's the SLOWEST step In the entire reaction. The slowest step determines the rate, also including the reactions that occur before the slowest step mat also be included in the rate equation...

That's all there is to it, as far as Im aware :smile:

Thanks, but how do you know which one is the slowest step???

Reply 225

OP

Original post by shuaib786

Thanks, but how do you know which one is the slowest step???

I assume they'll practically tell us in the exam which is the slowest, all we have to do is pick It out of the steps which are obviously fast.

Eg C-Br bond breaking, it's obviously going to be slow! Where as a carbocstion and nucleophile is going to be rapid!

Reply 226

Original post by Doctor.

I assume they'll practically tell us in the exam which is the slowest, all we have to do is pick It out of the steps which are obviously fast.

Eg C-Br bond breaking, it's obviously going to be slow! Where as a carbocstion and nucleophile is going to be rapid!

oh rite so basically reacting with intermediate species, will more likely to be a fast reaction??? and reacting with normal molecules will more likely be a slow reaction???

Reply 227

OP

Original post by shuaib786

oh rite so basically reacting with intermediate species, will more likely to be a fast reaction??? and reacting with normal molecules will more likely be a slow reaction???

Intermediates are always going to be fast! So yes :yep:

Yeah thats right :smile:

If you need any help, just post it in this thread and I'll try my best :h:

Reply 228

Original post by shuaib786

oh rite so basically reacting with intermediate species, will more likely to be a fast reaction??? and reacting with normal molecules will more likely be a slow reaction???

They will most likely give a rate equation. The species in the rate equation will appear in the slowest step, or the one before. They will not appear in any steps after the slowest step.

Reply 229

Original post by jamiepango

They will most likely give a rate equation. The species in the rate equation will appear in the slowest step, or the one before. They will not appear in any steps after the slowest step.

but do you remember the question they gave in june 11 paper???

Reply 230

OP

Original post by shuaib786

but do you remember the question they gave in june 11 paper???

Yeah, they're trying to be less repetitive with questions so make sure you understand it :smile:

Reply 231

Sheo

5

Hello guys,

Just trying to do this Chem 4 question and I can't seem to get the answer for the concentration of Q in exp. 4

The answer for initial rate of reaction for experiment 3 is 8.1x10-5 and the answer for exp. 4 is 6.7x10-5

I seriously cannot remember how I did this last time so any help would be appreciated! :smile:

Reply 232

Original post by shuaib786

but do you remember the question they gave in june 11 paper???

Yes I did that one as a mock. I think the same principles apply though. Steps 1 and 2 contained the same amount of species as in the rate equation, so it does allow you to work it out. Just make sure you use the information they give you and it should be okay.

Reply 233

Original post by Sheo

Hello guys,

Just trying to do this Chem 4 question and I can't seem to get the answer for the concentration of Q in exp. 4

The answer for initial rate of reaction for experiment 3 is 8.1x10-5 and the answer for exp. 4 is 6.7x10-5

I seriously cannot remember how I did this last time so any help would be appreciated! :smile:

Have you worked out the value of the rate constant?

Reply 234

Sheo

5

Original post by jamiepango

Have you worked out the value of the rate constant?

yes but that is the next question. I guess I could use the rate constant to work out the answer, but how do you do it without the rate constant?

The rate constant is 3750 mol-2dm6s-1

Reply 235

MuffinMonster

Original post by Sheo

Hello guys,

Just trying to do this Chem 4 question and I can't seem to get the answer for the concentration of Q in exp. 4

The answer for initial rate of reaction for experiment 3 is 8.1x10-5 and the answer for exp. 4 is 6.7x10-5

I seriously cannot remember how I did this last time so any help would be appreciated! :smile:

Is the answer 6.0x10-3?
Compared to the first one, the rate of 4 is 1/6 of the rate.
But P is 1.5 times bigger
So if exp 1 has a conc of 1.8 for P, it's rate would be 2.7.
2.7/0.3 would be 9.
Square root 9 (second order) is 3.
So the conc of Q in exp4 is 3 times the one in exp 1.

Reply 236

Original post by Sheo

yes but that is the next question. I guess I could use the rate constant to work out the answer, but how do you do it without the rate constant?

The rate constant is 3750 mol-2dm6s-1

Ah okay. I worked it out using the rate constant, but you can do answer it without finding k as well.

If you use the first and 4th steps, you can write rate equations in terms of k. If you rearrange both equations so that K is the subject, you can equate the 2 equations. Then you should be left with [Q]^2, which you can find because you have the values for everything else.

Let me know if you get the answer :smile:

Reply 237

undertaker1

11

right people 2.5 weeks left till this exam!!!

anyone have the jan 12 paper?

(edited 11 years ago)

Reply 238

OP