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

Original post by asinghj

Ok fair enough but for AS we only need to know about the M+ peak due to carbon 13 hence what I said. Also don't think we need to be able to identify a halo alkane from mass spectrometry

Think it's more A2 but I think the worst they could ask is for the M+2 as you get M+4 etc and have to normally identify type of dihaloalkane, or suggest, due to the isotopic ratio

But doubt they would ask a question like that and as far as I'm aware they haven't in past papers

Reply 783

Original post by Cherx

Why is the boiling point for silicon high? Due to the breaking of covalent bonds? And what's the difference between graphite and graphene?

Yeah it forms a giant covalent lattice and covalent bonds are much stronger than London forces

Graphite is an allotrope of carbon with parallel layers of hexagonal carbon atoms with weak London forces

Graphene is similar just a single layer

Both have delocalised electrons, conduct electricity etc

Reply 784

Original post by Cherx

Wait guys I've just gotten even more confused about the UV process.

By the way do we need to know how the catalyst works eg. the adsorption bit? There's a tiny bit of it in our textbook. Also have you guys bothered learning definitions?

I assume you have the Oxford textbook so there's a bit on ozone I think somewhere

Not in particular but the old spec used to ask about catalytic converters a lot so I've kinda learnt the process passively

I think it's firstly adsorped (weakly bonded) to the catalyst, this causes a chemical reaction and weakens the chemical bonds and lowers the activation energy then the products are desorbed

Not 100% sure on that

Reply 785

Cherx

20

Original post by Jitesh

Yeah it forms a giant covalent lattice and covalent bonds are much stronger than London forces

Graphite is an allotrope of carbon with parallel layers of hexagonal carbon atoms with weak London forces

Graphene is similar just a single layer

Both have delocalised electrons, conduct electricity etc

Ah ok so both are trigonal planar including graphite.

Original post by Jitesh

I assume you have the Oxford textbook so there's a bit on ozone I think somewhere

Not in particular but the old spec used to ask about catalytic converters a lot so I've kinda learnt the process passively

I think it's firstly adsorped (weakly bonded) to the catalyst, this causes a chemical reaction and weakens the chemical bonds and lowers the activation energy then the products are desorbed

Not 100% sure on that

Yeah it's the second propogations step which I don't understand. How comes it reacts with 0? Where does that come from.

Ah ok so I found it. HETEROGENEOUS (make sure you spell it right they can deduct marks lol) has a different physical state from the reactant. Reactant molecules adsorbed onto the surface of the catalyst. Products of the reaction leave the surface of the catalyst by desorption.

(edited 7 years ago)

Reply 786

Original post by Cherx

Ah ok so both are trigonal planar including graphite.

Yeah it's the second propogations step which I don't understand. How comes it reacts with 0? Where does that come from.

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

Reply 787

Original post by adeninesbff

Thanks! Welp, my brain is mush now trying to remember it all :smile:

Hey, I made a mistake with my previous reply for question 2. The correct ionic equation is:
Cl2(g) + 2OH-(aq) ---> Cl-(aq) + ClO-(aq) + H2O(l)
I use a piece of paper and realised I made a mistake sorry about that.
In my defence it was 7 AM and I had just woken up

Reply 788

Original post by Jitesh

Think it's more A2 but I think the worst they could ask is for the M+2 as you get M+4 etc and have to normally identify type of dihaloalkane, or suggest, due to the isotopic ratio

But doubt they would ask a question like that and as far as I'm aware they haven't in past papers

Well the spec only mentions knowledge of the M+1 peak..

Reply 789

Original post by asinghj

Well the spec only mentions knowledge of the M+1 peak..

In the spec for biology a it only mentioned G1, S G2, M and C for the cell cycle and G0 came up...

Reply 790

Original post by Cherx

Wait guys I've just gotten even more confused about the UV process.

By the way do we need to know how the catalyst works eg. the adsorption bit? There's a tiny bit of it in our textbook. Also have you guys bothered learning definitions?

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

Reply 791

Keira44556

3

Original post by Cherx

Why is the boiling point for silicon high? Due to the breaking of covalent bonds? And what's the difference between graphite and graphene?

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)

Reply 792

Original post by Jitesh

I assume you have the Oxford textbook so there's a bit on ozone I think somewhere

Not in particular but the old spec used to ask about catalytic converters a lot so I've kinda learnt the process passively

I think it's firstly adsorped (weakly bonded) to the catalyst, this causes a chemical reaction and weakens the chemical bonds and lowers the activation energy then the products are desorbed

Not 100% sure on that

That's right, as KnockHardy notes mention it, heterogeneous = HARD (heterogeneous, adsorption, reaction, desorption)

Reply 793

Original post by Jitesh

In the spec for biology a it only mentioned G1, S G2, M and C for the cell cycle and G0 came up...

Yeah fair enough, and chi squared also came up on that paper. Let's hope the chemists at OCR stay in the AS spec

Reply 794

Original post by asinghj

Yeah fair enough, and chi squared also came up on that paper. Let's hope the chemists at OCR stay in the AS spec

Chi was on the required calculations bit but I thought it was referring to the full a level course so kind skipped it lol

Yeah hopefully, breadth was a nice paper (compared to biology)

Reply 795

4nonymous

11

Original post by Bahcaci

No, that is wrong. For the overall equation you have: O3 + O --> 2O2

You cant have that equation you said, check your book(s), trust me.

Thats the equation it says in my textbook.

Reply 796

4nonymous

11

Can someone please explain Electronegativity and Periodicity
Also, the formation of Pi bonds?

Reply 797

Original post by Jitesh

Chi was on the required calculations bit but I thought it was referring to the full a level course so kind skipped it lol

Yeah hopefully, breadth was a nice paper (compared to biology)

In the main part of spec, chi squared is somewhere in module 5 (A2 stuff). Them stupid idiots at OCR

Reply 798