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OFFICIAL THREAD OCR A AS Chemistry F322 2nd JUNE 2015

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Free radical substitution of alkanes with a halogen gas
Nucleophilic substitution of a halogenoalkane with hydroxide ions
Electrophillic addition of an alkene with a halogen or hydrogen halide!
There's a page in the textbook with them all together
p176

thanks so much :smile:

Reply 81

yoda123

can someone explain why it is +679 and how do you get +431.5

Reply 82

_Aqsxo

Add the values for H-H and Cl-Cl, then add the enthalpy change value and divide it by two cos you have two moles and you only need to know it for one mole.

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Reply 83

al hayyul qayyum

Original post by yoda123

can someone explain why it is +679 and how do you get +431.5

Just use maths algebra

Set up an equation

Enthalpy change = bonds broken - bonds made (2HCl)

Solve for HCl

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Reply 84

victoria98

Original post by yoda123

can someone explain why it is +679 and how do you get +431.5

ΔH=Σ bonds broken - Σ bonds formed
So 436 + 243 - 2H-Cl = -184
Rearrange to get 2H-Cl=863 so H-Cl is 431.5

Reply 85

iBall

Original post by SheLikeTheMango

June 14 definitely. Other hard paper was Jan 2011 i think. And June 12 wasn't hard but had some weird compounds in it (epoxyethane wtf) so required a bit of thinking :smile:

Lol ok, going to save that paper for the weekend then. :smile:

Reply 86

jas29031997

Can someone please explain why on the mark scheme the answer is C4H10O and it says do not allow C4H9OH? Surely they are the same thing? Thanks

Reply 87

SteelManatee

Original post by jas29031997

Can someone please explain why on the mark scheme the answer is C4H10O and it says do not allow C4H9OH? Surely they are the same thing? Thanks

I also made the same mistake. I think it's because the question asks for the molecular formula so all the H's needed to be grouped together.

Someone correct me if I am wrong :smile:

Reply 88

jas29031997

Original post by SheLikeTheMango

I also made the same mistake. I think it's because the question asks for the molecular formula so all the H's needed to be grouped together.

Someone correct me if I am wrong :smile:

Ah ok thank you :smile:

Reply 89

rosiemayx

Guys does anyone know why long chained hydrocarbons are not directly used as plastics? Instead of cracking the long chained hydrocarbons from fractional distillation to alkenes and then making them undergo polymerisation and making a long saturated hydrocarbon chain?

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Reply 90

9836vicky

Anybody got a page of notes with all the reagents and equations ( including state symbols) for the reactions we need to know?

Reply 91

_Aqsxo

Original post by GetOverHere

Is it true that you don't actually need to learn Hess's cycle, because the enthalpy formulae are enough to work out the various standard enthalpy changes?

You should know it as it is part of the syllabus although it doesn't come up much but who knows, maybe we will get it this year

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Reply 92

chuckster111

Does anyone have any good notes on Green Chemistry (and the bit on polymers in Alkenes section?)

Thanks

Reply 93

Messier31

Does anyone know what it usually is for full ums?

Reply 94

PenToPaper

Original post by GetOverHere

Is it true that you don't actually need to learn Hess's cycle, because the enthalpy formulae are enough to work out the various standard enthalpy changes?

It's in the spec to know the Hess' cycle, however they have NEVER asked for it to be used in a calculation or anything, so in theory you should be fine if you don't learn it... But you never know.

Reply 95

SteelManatee

Original post by Messier31

Does anyone know what it usually is for full ums?

Usually around 95%, but will fluctuate depending on the A boundary that year :smile:

Reply 96

SteelManatee

Original post by GetOverHere

Is it true that you don't actually need to learn Hess's cycle, because the enthalpy formulae are enough to work out the various standard enthalpy changes?

Enthalpy formulae? Is that the 'reactants - products' and 'products - reactants' formulas?

Reply 97

lukejoshjames

Original post by Messier31

Does anyone know what it usually is for full ums?

OCR Chemistry uses a linear UMS system, and I'm pretty sure that no matter what the A grade boundary is, full UMS is always 100%. You can still get really high UMS WITH 90-95% though, as the grade boundaries are quite low.

Reply 98

lukejoshjames

Original post by chuckster111

Does anyone have any good notes on Green Chemistry (and the bit on polymers in Alkenes section?)

Thanks

Can't help with polymers, but this guy has a summary of green chemistry which gets straight to the parts you need to know:
https://www.youtube.com/watch?v=anUFwxitWsE

Reply 99

xsashax

do we need to know anything about the Haber process? or will they just ask us why certain conditions are what they are?