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OCR Chemistry F322~ 4th June 2013~ AS Chemistry

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Hi!!! I'd find it sooooo helpful if someone could please tell me how you get the answer on 4 (ii), (iii) and (iv)???
http://www.ocr.org.uk/Images/131287-question-paper-unit-f322-chains-energy-and-resources.pdf
Thank you very much for your time :smile:

*bows*

Seeing that part ii) and iii) were answered. The answer to iv) I think is 250+92=342, because for the molecules to react they need to reach have the sufficient amount of energy. Since the products of the first reaction, so the ammonia has 92 KJ less, and the initial activation energy was 250. Therefore in order to acquire sufficient energy to react it, you have to add both of these values. Sorry its a poor explanation but I didnt know how else to word it.

Reply 841

Blashnet

Original post by reneetaylor

Haha, thank you so far!

I hope you don't mind if I ask you further
But why half on 92?
And why is (III) anything under 250? Thanks :smile:

It's because, the enthalpy change for the reaction is -92, and that gives you 2 moles of ammonia!

so one mole of NH3 is half, -46 :biggrin:

Reply 842

Kat12395

Original post by motivatedshroom

Thanks for setting up the forum! I just scraped an A on f321 exam with 72/90 UMS points. Very happy it's an A but I wish I scored more UMS points.

Does anyone know the definitions for:
-E/Z isomerism.
-Cis/trans isomerism.

Also I'd be really grateful if someone could please explain the difference. I'm a bit confused. Thanks. :smile:

Both E/Z and Cis/Trans are defined as being 'the same structural formula, but a different arrangement of atoms in space'.
Cis/Trans is the simple one- you have an alkene which has four groups attached- each carbon has one non-hydrogen group attached to it. Cis = both are on the same side; Trans = they are on different sides.
E/Z is similar, but it means that it can be a bit more complicated. It means that there aren't always hydrogens- there could be, for a really simple example:

I Cl

\ /
C=C
/ \
Br F
Z

I F
\ /
C=C
/ \
Br Cl
E

It is, therefore, impossible to tell which atom chlorine, for example, would be compared against, because all the atoms around the carbons are different. In E/Z isomers, there is an order you can follow (called a priority order), but if you're on OCR A, you don't need to know about that:L It would tell you whether a molecule is an E/Z isomer- as ~~poorly~~ shown above!
You won't need to know much about E/Z- mostly only Cis/Trans; but you should probably know that E corresponds to Trans; and Z corresponds to Cis! If in doubt- write E/Z!

Reply 843

reneetaylor

Original post by theCreator

Thanks a lot! I get it now :smile:

Reply 844

reneetaylor

Original post by Blashnet

It's because, the enthalpy change for the reaction is -92, and that gives you 2 moles of ammonia!

so one mole of NH3 is half, -46 :biggrin:

This makes sense, thank you! :smile:

Reply 845

needtosucceed=)

Original post by .raiden.

Compound I was given in the question and because chlorine radicalisation means hydrogens are replaced by chlorine atoms the number of structural isomers only depend on the number of carbons. No new branches are formed etc. And the reason that the Cl can be on the first and last carbon and be a different structural isomer is because compound I is not symmetrical. This picture might help. Sneaky question really lol

ahh that makes sense, thanks a lot!

Reply 846

Dejir

How much do you need for an A

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Reply 847

Gotzz

Original post by Dejir

How much do you need for an A

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It varies, usually just under 80.

Reply 848

wndms

Original post by Farringtonn

ImageUploadedByStudent Room1369914434.130546.jpg

ImageUploadedByStudent Room1369914446.107529.jpg

For this question the mark scheme shows the answer like that but would I gain all marks if I drew the stereoisomers like this

ImageUploadedByStudent Room1369914495.877401.jpg

ImageUploadedByStudent Room1369914495.877401.jpg

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My teacher says yes

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Reply 849

chigyy

Does anyone have notes of some sort of conditions and catalysts for mechanisms?

Reply 850

SheriB

Hi guys. How would you draw a compound with a structural formula of (ch3)2chch2br ??

Reply 851

Gotzz

Original post by chigyy

Does anyone have notes of some sort of conditions and catalysts for mechanisms?

http://www.thestudentroom.co.uk/attachment.php?attachmentid=193431&d=1358260985

Reply 852

.raiden.

Original post by SheriB

Hi guys. How would you draw a compound with a structural formula of (ch3)2chch2br ??

Not sure how there can be 2 lots of CH3 at the start of a chain

Reply 853

Adil96m

Does anyone have the list of conditions we need to know for each reaction

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Reply 854

Sam_1996

Original post by SheriB

Hi guys. How would you draw a compound with a structural formula of (ch3)2chch2br ??

It 1-bromo-2-methylpropane

Reply 855

Gotzz

Original post by Adil96m

Does anyone have the list of conditions we need to know for each reaction

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http://www.thestudentroom.co.uk/attachment.php?attachmentid=193431&d=1358260985

Reply 856

Adil96m

Original post by needtosucceed=)

I tried drawing them all out but its too time consuming. Best to just guess a reasonable number then come back to it if you've got time at the end. Not worth spending 5 minutes on a 1 mark question.

I just did this paper and I struggled with that question aswell, I think its 8 as you can add the I to any of the 8 carbons.

Here's my teachers answer

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1369928780298.jpg39.6KB

Reply 857

needtosucceed=)

Original post by SheriB

Hi guys. How would you draw a compound with a structural formula of (ch3)2chch2br ??

Original post by .raiden.

Not sure how there can be 2 lots of CH3 at the start of a chain

They're not both at the start of the chain as such, it's just that's how you write structual formula when the two methyl groups are on one carbon. I've attached a drawing :smile:

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(edited 10 years ago)

1369929089675.jpg12.7KB

Reply 858

Farringtonn