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OCR Chemistry F322~ 4th June 2013~ AS Chemistry

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Reply 920

.raiden.

Original post by Gotzz

Umm, that sounds about right :redface:

Also more van der waal forces for longer chains is always a good mention :biggrin:

Reply 921

Sara_A

Guys, i need help with q4a in jan 2012, i got the first part and got +679 but dont understand how to do the rest

Reply 922

Kat12395

Original post by Liberty.

Does anyone else feel like they just get dumped in the deep end when it comes to Mass Spectrometry :tongue:

It just feels like a bunch of guessing, like a puzzle!

Does anyone have any tips?

YES! I hated mass spec- after A2, it makes WAY more sense.
Basically, if you're given any information about the molecule in question- use it. If not- start with your m/z peak.
Now you've gotta end up totalling to that. Try and get close (about ten or so) off using carbons and oxygens; then you've made a start!
When you've figured that out, doodle a molecule that fits what you think you'll need. They usually state whether its an alcohol, organic product or hydrogen in the question- so that tells you how many oxygens you're looking at (roughly). Any huge peaks? Look out for 15 (ch3+) or 29 (c2h5+), cause they'll knock points to fill off the all important m/z total- then its easier to puzzle out! If the question asks you to give an ion- for gods sake put a +! I've been caught out on that before:L
There really is no 'magic solution' but it gets easier with practise!

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Reply 923

.raiden.

Original post by Sara_A

Guys, i need help with q4a in jan 2012, i got the first part and got +679 but dont understand how to do the rest

Enthalpy change = bond enthalpy of reactants - bond enthalpy of products

So

-184 = 679 - bond enthalpy of product

Bond enthalpy of product = 679 + 184
= 863

BUT this is the bond enthalpy of 2HCL
They ask for HCL so divide your answer by two. 431.5... 432 (3sf)

Hope that helped :smile:

(edited 10 years ago)

Reply 924

Hello...

Original post by Sara_A

Guys, i need help with q4a in jan 2012, i got the first part and got +679 but dont understand how to do the rest

You need to use the equation
(Sum of Bonds Broken)-(Sum of Bonds made) = Delta H
Therefore:
We have an unknown (H-Cl) so we'll call it x
(436+243) - (2x)=-184
679 -2x =-184
-2x = -863
-x = -431.5
x = 431.5

Reply 925

OldSchoolTech

Original post by Kat12395

I try and think of them as equations.
Once you know the M+ value, set this equal to the general formula then just evaluate to find n.

If the compound was an alcohol with M+ of 60:
General formula for alcohol CnH2n+1OH

60 = CnH2n+1OH
43 = CnH2n+1 (Subtracted 17=OH)
42 = CnH2n

CH2 = 14
42/14 = 3 = n

The put this pack into the general formula to get
C3H7OH. (propanol)

Then use other m/z peaks to determine if its propan-1-ol or 2-ol.

Hope that made sense. Seems complicated but it works for me - just do loads of practice!!!

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(edited 10 years ago)

Reply 926

ray96

CH3
I
H3C-C-OH Hi guys,, can anyone name this alcohol..I thought it was 2 dimethyl-propanol,,, but is wrong
I
CH3

Reply 927

Tania2k9

hi guys i'm sitting this as well cna someone test me please

Reply 928

needtosucceed=)

Original post by ray96

CH3
I
H3C-C-OH Hi guys,, can anyone name this alcohol..I thought it was 2 dimethyl-propanol,,, but is wrong
I
CH3

the structure you've drawn is wrong, one of your carbons is making 5 bonds?

Reply 929

.raiden.

Original post by ray96

CH3
I
H3C-C-OH Hi guys,, can anyone name this alcohol..I thought it was 2 dimethyl-propanol,,, but is wrong
I
CH3

Could you write the structural formula or draw it? A bit unclear

Reply 930

supernova99

Original post by Sara_A

I dont know how to draw the cycle

I'll try and help :smile:

.

1) Right down the equation given e.g.
6C +7H2--------> C6H14

2) Look at the values given. If they are for enthalpy of combustion than then the bottom of the triangle will have CO2 and H2O as they are the products of combustion. If they are enthalpy of formation values then the bottom of the triangle will be for the constituent elements e.g. C and H2 in the example above.

3) If it is enthalpy of combustion the arrows go from the products and reactants DOWN to CO2 and H2O because CO2 and H2O are being produced. If it is enthalpy of formation the arrows will go UP to the reactants and products because the reactants are being formed from the constituent elements.

4) The next step is to put the enthalpy change values on the cycle from the table given. Remember to multiply the value according to the number of moles. e.g. 6 moles of C and 7 of H2 as seen above.

5) Then follow the arrows from the reactants to the products. If you are going in the direction of the arrows, add the values, if you are going against them then subtract the value.

I hope this helps! Here's an example for the equation above------->

enthalpy change cycle.jpg101.2KB

Reply 931

Tania2k9

Original post by ray96

CH3
I
H3C-C-OH Hi guys,, can anyone name this alcohol..I thought it was 2 dimethyl-propanol,,, but is wrong
I
CH3

can y actually paste the exact structur because in that form it's very hard to decide

Reply 932

Joloiyse

where do u guys go to get past papers? i went to pastpapers.org but theres nothing on energy or green chemistry in the pre 08 papers /: meaning there are actually only 3 papers!!

Reply 933

supernova99

Original post by Joloiyse

where do u guys go to get past papers? i went to pastpapers.org but theres nothing on energy or green chemistry in the pre 08 papers /: meaning there are actually only 3 papers!!

The ocr website----> http://www.ocr.org.uk/qualifications/as-a-level-gce-chemistry-a-h034-h434/

Reply 934

Jacksgap

I am actually scared for this exam hope it's easy!!

Good luck to all of you!

Reply 935

Hello...

Original post by ray96

CH3
I
H3C-C-OH Hi guys,, can anyone name this alcohol..I thought it was 2 dimethyl-propanol,,, but is wrong
I
CH3

do you have a picture of it?? It came out wierd in the post... I'm not sure it can even exist

Reply 936

Jacksgap

Original post by Tania2k9

can y actually paste the exact structur because in that form it's very hard to decide

Yh..true.. :smile:

Reply 937

needtosucceed=)

Original post by Joloiyse

where do u guys go to get past papers? i went to pastpapers.org but theres nothing on energy or green chemistry in the pre 08 papers /: meaning there are actually only 3 papers!!

http://www.freeexampapers.com/

go to alevel>chemistry>ocr

theres loads of different papers there, the ones relevant for this unit are 2813/01, 2812 and 2815/03.

2813/01 = all the energy stuff, module 3
2812 - organic stuff, modules 1 and 2
2815/03 = resources/green stuff, module 2. but half the paper has stuff not on our spec anymore so just pick and choose relevant questions