OCR Chemistry A Exam Thread (Breadth - May 27 2016 and Depth - June 10 2016)

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Reply 820

Rubberbandit

OMG OMG Help what is an ion?!?!

Reply 821

Cherx

Original post by Jitesh

Naturally uv radiation from th sun breaks down ozone and oxygen through homolytic fission
Normally the formation of ozone has been equal to the depletion of ozone (equilibrium) but CFCs have screwed things up

We can think of ozone being formed and depleted as O2 + O (equilibrium sign) O3

And O2 -> 2O breaking via homolytic fission due to presence of UV light.. Not sure why we don't show the radical dot but we got told not to.. As far as I'm aware it is just a radical

I was just looking up on this. So the steps are always the same. The 03 and 0 bit in the second propagation. I guess I should just remember that. And substitute. Since they give us an unfamiliar CFC and ask us to show the free radical? And it's literally just putting in the numbers! So thank you :smile:

Original post by asinghj

I know the definitions of enthalpy changes and homologous series and isomerism but my teacher said the examiner won't ask for definitions, instead it will be about applying your knowledge.

Looking at the spec it says "explanation of the role of a catalyst. Details of processes are not required"
So I don't think we need to learn about adsorption but we need to know the difference between homogeneous and heterogeneous catalysts

Ah yeah that's true this paper is more on application. I might just memorise a couple before I go into the exam. Ah so in the UV process Cl would be a homogenous catalyst? The HARD mneumic you gave was really good. I'll be able to remember that if it does come up.

Original post by Keira44556

Silicon is a giant covalent structure, like carbon! So yes. And Graphite is layers of hexagonally arrange carbon atoms, bonded covalently, each layer is bonded together by London forces, graphene is a single layer! Also, in graphite and graphene, only 3/4 of the carbon atoms are bonded and so there is delocalised electrons, therefore they can conduct electricity! (In diamond, all four of the C atoms are bonded so can't conduct)

Thank you as well! I thought graphite was like a prefix or something lol. Like in bio you have atria/atrium.

Reply 822

asinghj

Original post by maxross

nice.

what you recon'll come up today???

Hopefully a nice 6 marker on mass and IR spectroscopy. And definitely mechanisms. I wouldn't mind PV=nRT and assigning CIP priorities for E/Z isomerism.
Another definite is bonding and structure and periodicity (trends).

How do you feel about today

Reply 823

maxross

Original post by asinghj

feeling quite good. would like a 6 marker on trends like periodicity. also explaining a titration would be a great 6 marker!

Reply 824

asinghj

Original post by 4nonymous

Thanks.

I just had look and pi bonds are formed after a sigma bond has been formed and is formed by the p-orbitals overlapping above and below the carbon atoms

Right, thanks 👻

Reply 825

asinghj

Original post by maxross

feeling quite good. would like a 6 marker on trends like periodicity. also explaining a titration would be a great 6 marker!

Please no titrations.. I hate it

Reply 826

maxross

Original post by asinghj

Please no titrations.. I hate it

what?! really, id love a titration hahah

Reply 827

4nonymous

Can someone explain what we need to know for titrations. As in planning one.

Reply 828

asinghj

Original post by maxross

what?! really, id love a titration hahah

It's just way too much to remember. In chemistry, I like application question and calculating stuff

Reply 829

maxross

Original post by asinghj

It's just way too much to remember. In chemistry, I like application question and calculating stuff

HATE application. much better at recall

Reply 830

asinghj

Original post by snickercell

Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which

Combustion and bond enthalpies is reactants - products
Formation is the odd one out so products - reactants

(edited 7 years ago)

Reply 831

asinghj

Original post by maxross

HATE application. much better at recall

Wow ok, so I assume you don't do maths or physics right?

Reply 832

Jitesh

Original post by Cherx

Yeah just write it out a couple times

For ozone depletion, Cl and NO radicals are the main ones they ask about

CFCs split by homolytic fission to form a Cl radical (depends what they give etc)... it's the Cl itself that's mainly responsible rather than the other radical formed

E.g. CF3Cl -> CF3 Radical and Cl Radical (under UV)

Reply 833

username1530463

OK, so:
-Any of the reaction mechanisms likely to come up.
-Reactivity trends likely to come up
-Spectroscopy likely to come up.

Reply 834

asinghj

Original post by Jitesh

Do we need to know the termination stages? If so could you write them here? Thanks

Reply 835

asinghj

Original post by asmuse123

OK, so:
-Any of the reaction mechanisms likely to come up.
-Reactivity trends likely to come up
-Spectroscopy likely to come up.

How did you write that so big?

Reply 836

maxross

Original post by asinghj

Wow ok, so I assume you don't do maths or physics right?

I do Physics and HATE it, and am not very good at it.

I am a Biology, Chemistry person. Not a maths, Physics person

Reply 837

username1530463

Original post by asinghj

How did you write that so big?

Click advanced, then choose number.

Also, was I right about these predictions?

Reply 838

Jitesh

Original post by asinghj

Do we need to know the termination stages? If so could you write them here? Thanks

Depends on what they give you
Cl radical + Cl radical -> Cl2 is one for Cl catalysis

Could be CF2Cl radical + CF2Cl radical -> C2F4CL2 as well for the example earlier