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ocr a f325 revision thread

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Reply 3340

Myocardium

Does anyone else feel really tired? I feel really sleepy and exam is in 2/3 hrs -_-

Reply 3341

ofudge

What do we do when it's diprotonic?

Write an expression of kc for:
2HCOOH + ba(oh)2 --> (hcoo-)2 ba2+ + 2h20

The (hcoo-)2 ba2+ is the salt.
Anyone know what kc would be? The expression that is

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Reply 3342

Namod

Original post by georgiaaaxo

haha thanks! so what exactly are you dividing by two? are you dividing the (q/moles) by 2?

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Yes

Reply 3343

Janjua

2.c NO2 reacts with CO emitted from car exhausts in the following rxn:

NO2 + CO -----> NO + CO2

Rate equation for this reaction is rate={NO2}2

Suggest 2 step rxn mechanism for this rxn that is consistent with the kinetic data and overall equation.

P.S june 2006 paper at http://pastpapers.org/A2/chemistry/unifying/2006_June_2816_01.pdf

thnx

Reply 3344

georgiaaaxo

Original post by Namod

Yes

ok thanks! I think I got it finally

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Reply 3345

georgiaaaxo

having a blonde moment: would there ever be h2o in a Kc expression???
I know you don't include it in kstab, is there anywhere where you would include h2o?

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Reply 3346

RandomMedic

Original post by Janjua

I would do:
2NO2 -> 2NO + O2
O2 + 2CO --> 2CO2

Reply 3347

chignesh10

Original post by MathsNerd1

Lets do this!!! :biggrin:

Come on!!

Reply 3348

RandomMedic

Original post by georgiaaaxo

having a blonde moment: would there ever be h2o in a Kc expression???
I know you don't include it in kstab, is there anywhere where you would include h2o?

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Yep, it's in Kc if it's a reactant or product!
The thing in Kstab is that because the whole thing is in solution, the conc of water is always going to stay relatively constant.

Reply 3349

Janjua

Original post by Myocardium

Does anyone else feel really tired? I feel really sleepy and exam is in 2/3 hrs -_-

take a little nap :lazy:

so you will be refreshed for the exams.

Reply 3350

MathsNerd1

Original post by Holz888

I would do:
2NO2 -> 2NO + O2
O2 + 2CO --> 2CO2

I did 2NO2 ---> NO + NO3
NO3 + CO ---> NO2 + CO2

Just to show another alternative really :smile:

Reply 3351

georgiaaaxo

Original post by Holz888

Yep, it's in Kc if it's a reactant or product!
The thing in Kstab is that because the whole thing is in solution, the conc of water is always going to stay relatively constant.

okay thankyou for clarifying!!!

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Reply 3352

v :)

ImageUploadedByStudent Room1371030526.816223.jpg

How do I do c i? I get so confused when working out the second half equation! Can anyone suggest a method to approach these questions with? Thanks x

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Reply 3353

MathsNerd1

Original post by Holz888

Yep, it's in Kc if it's a reactant or product!
The thing in Kstab is that because the whole thing is in solution, the conc of water is always going to stay relatively constant.

I thought we didn't include it if it was the solvent being used in the reaction?

Reply 3354

RandomMedic

Original post by MathsNerd1

I thought we didn't include it if it was the solvent being used in the reaction?

I was never taught this :/
I guess my best example is a esterification - the H2O is always in the Kc expression. But yeah, that does make sense

Reply 3355

eggfriedrice

I got 0.08 mol but why is it finally 0.8 mol in the answer as well as commentary? Another error?

Reply 3356

_HabibaH_

Original post by v :)

How do I do c i? I get so confused when working out the second half equation! Can anyone suggest a method to approach these questions with? Thanks x

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Hey,

Work backwards from the overall equation. Everything in that half equation, cancel it from the overall equation. Then form the second half equation from what you have left.

Reply 3357

Janjua

Original post by Holz888

I would do:
2NO2 -> 2NO + O2
O2 + 2CO --> 2CO2

Doesn't the rate determining step supposed to lead to the original equation in this case, the rxn btw NO2 + CO ----> NO + CO2 ?

Sorry for bothering you just get stuck at rate determining step. :angry:

Reply 3358

_sparks

Okay quick question!
January 2011, q7b)ii)

They got the formula: 2H2O + 2I- + 2NO2- = 2NO + I2 + 4OH-

But using the method given in the book I get:
H20 + 2I- + NO2- = I2 + 2OH- + NO

Both equations are balanced(or are they?), but the mark scheme does not even talk about the one I got. The strange thing is that theyre not even multiples of each other!

Some please help me, I'm really confused.