OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

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Can Someone please explain how to determine the rate determining step I don't understand it

Firstly the rate determining step is the slowest step in the mechanism.

The reactants for the rate determining step are those (and only those) that feature in the rate equation. If a reactant is 2nd order, then two molecules of it will be on the left, if 1st order then one.

These combine together to make an intermediate (just make the left balance with the right, marking schemes are normally very flexible about what your products are on this step) - This is your rate determining step.

Then whatever reactants from your chemical equation were not used above combine in a second (faster) step with your intermediate to make the product.

See the below attached example.

I hope that helps clear it up a bit

Reply 21

silver0

Does anyone have any tips on how to remember all the equations to work out things like ph, ka etc.. .

Reply 22

clownfish

Original post by silver0

Does anyone have any tips on how to remember all the equations to work out things like ph, ka etc.. .

Afraid you just need to learn them, but Ka is just like Kc, it's (concentration of products) over (concentration of reactants) for the acid dissociation equilibrium.

I recommend you summarise them on one page, they will seem a lot less!

Reply 23

moh.alt

2CH3OH(l) + 3O2(g) goes to 2CO2(g) + 4H2O(l) - This question was in the June 2011 paper. I dont understand why there is a decrease in entropy? Any help?. Thanks.

Reply 24

clownfish

Original post by moh.alt

2CH3OH(l) + 3O2(g) goes to 2CO2(g) + 4H2O(l) - This question was in the June 2011 paper. I dont understand why there is a decrease in entropy? Any help?. Thanks.

As the no of moles of gas is decreasing. This is more important than the overall increase in moles.

Reply 25

moh.alt

Original post by clownfish

As the no of moles of gas is decreasing. This is more important than the overall increase in moles.

Oh yess! Ah didnt think of that, i just looked at the increase in the moles and not the decrease in gas moles. Thanks a lot.

Reply 26

ManPowa

If Anyone wants all the JAN 12 papers for biology and chemistry OCR A and B, heres the link:

http://www.thestudentroom.co.uk/showthread.php?t=1943168

Reply 27

splendidsun

Born haber cycle calculations practice?

Reply 28

Tom172

Not particularly enjoying this section so far. Much preferred F324. Anyone got any good revision materials/practice questions?

Reply 29

splendidsun

Original post by Tom172

Not particularly enjoying this section so far. Much preferred F324. Anyone got any good revision materials/practice questions?

http://www.a-levelchemistry.co.uk/

Reply 30

1234girl

Hey how is everyone revising for this?? Im thinking past exam papers obviously and learning the content, but that did not work for f324, so anoyone got any advice? Much appreciated :smile:

Subscribing :smile:

Needing 125 in this unit, but I much prefer organic chemistry.. Ah well, I have Easter to try write some notes.

Reply 33

SpringNicht

I'm starting to panic about finishing the spec, we're about halfway through now...

Reply 34

LifeIsGood

Original post by SpringNicht

I'm starting to panic about finishing the spec, we're about halfway through now...

If you need help I don't mind :smile:

I've done a lot of it but need to brush up on Transition Metals and start papers. Oh and cover Electrodes :eek:

Need to finish this unit very quickly myself.

Reply 35

flowerpower1410

hey wondering what resources (in terms of books not the internet) to use?

can anyone reccommend the jimclark calculations book as i think both the ocr textbooks are dismal for calculations?

thanks

Reply 36

LifeIsGood

Original post by flowerpower1410

hey wondering what resources (in terms of books not the internet) to use?

can anyone reccommend the jimclark calculations book as i think both the ocr textbooks are dismal for calculations?

thanks

Jim Clarke Calculations book are fantastic! It's the same guy who created ChemGuide.

Reply 37

swaggiee

Original post by ManPowa

If Anyone wants all the JAN 12 papers for biology and chemistry OCR A and B, heres the link:

http://www.thestudentroom.co.uk/showthread.php?t=1943168

you're awesome!

Reply 38

pcorbett94

I really hate this unit. Just like I hated ABG (unit 1) which I'm resitting in May. A summer filled of (mainly) inorganic chemistry topics is not my idea of fun. Want to just sit RPA 4 times :P