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# OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION! watch

1. (Original post by Killjoy-)
Put a negative sign in front of your value for Q (since its an exothermic reaction so heat is evolved) and divide by the number of moles of glucose that you calculated earlier.
hey yh thts what i tried to do but im getting a completely different answer.. hav a look at the mark scheme
pg 20

the q is 7.23 and amout in moles in 4.62 x 10 ^ -3

how do they get
1560 :s
2. (Original post by touran22)
you know for hess's law....if you don't draw the cycle but just have it as like products - reactants (for formation) and you get the correct answer..full marks?

(Original post by bright)
someone please teach me hess cycles i've forgotten it!!!

i know how to draw it like formation pointing up and combustion down with products co2 and h20 at the bottom, and i know the labels a b c, but how do you label when each one is?

(Original post by killjoy-)
finding enthalpy of reaction using enthalpy of formation:

Sum of enthalpies of formation for products - sum of enthalpy of formation for reactants. (you go against the arrows for reactants.)

finding enthalpy change of reaction using enthalpy of combustion:

Sum of enthalpies of combustion for reactants - sum of enthalpy of combustion for products. (again, you go against the arrows but for products)

for enthalpy change of reaction (as opposed to enthalpy change of formation and combustion) remember it is associated with the molar quantities in the equation, not for one particular molecule. So if the enthalpy change you have calculated is for a substance that appears as two moles, double your answer etc.

If you think of the elements forming and then reacting to make the rhs of the equation for formation. (a way to think about it.)

and in the same way think of the combustion products forming and then reacting 'backwards' to create the products in the equation.

Its easier to explain with diagrams, if you are still not sure ask your teacher.

(Original post by smithythedrummer)
it's weird. In the exams they tend to give you the enthalpy change of combustion and ask you to calculate formation or vice versa. The only way i have found for that to work is if you reverse the signs on the enthalpy changes and proceed as normal :s

i really hope that the free radical and electrophilic mechanisms come up a lot...

(Original post by godknowsprerna)
hey can someone plz explain to me how does ch3ch2ch2ch2oh change to ch3ch3chch2 when reacted with an acid/heat :s question 3 from june 2010 paper

(Original post by lolo-x)
is it true to say that in an exothermic reaction, more energy is required to make bonds than to break bonds?
do we draw dots on our radicals? Why does the mark scheme say "ignore dots" when that's exactly what the book says?!
3. (Original post by godknowsprerna)
THANK YOU SOO MUCH DEAR life saver!

hey if u r not busy could u plz explain to me how to find the enthalpy change of combustion when enthalpy change of formations are given :s
q -3 c) jan 2010 paper
You use Hess' Law.

4. (Original post by sixthformer)
do we draw dots on our radicals? Why does the mark scheme say "ignore dots" when that's exactly what the book says?!
You can draw them, you won't gain or lose marks as they will simply 'ignore' it.
5. (Original post by sixthformer)
["]do we draw dots on our radicals? Why does the mark scheme say "ignore dots" when that's exactly what the book says?![/SIZE]
Whoa.

Radicals are meant to have dots on them but I assume the mark scheme means that it doesn't matter if you forget to put them in.
6. 14 JANUARY 2010 QUESTION 3c)

PLEASE SOMEONE HELP ME WITH ENTHALPY Hess cycle drawing of this!

So far I have only got C02 + H20 at the bottom, where do I go from here?

Thanks.
7. (Original post by godknowsprerna)
hey yh thts what i tried to do but im getting a completely different answer.. hav a look at the mark scheme
pg 20

the q is 7.23 and amout in moles in 4.62 x 10 ^ -3

how do they get
1560 :s
I've divided 7.23 by 4.62x10^3 and I've got that answer.
8. (Original post by Killjoy-)
I've divided 7.23 by 4.62x10^3 and I've got that answer.
I got that too.
9. Can someone explain to me what an Octane number is and the benefits/disadvantages of a higher/lower Octane number?
10. (Original post by medusa cascade)
whoa.

Radicals are meant to have dots on them but i assume the mark scheme means that it doesn't matter if you forget to put them in.
my teacher took away marks because i drew dots!
11. (Original post by Killjoy-)
I've divided 7.23 by 4.62x10^3 and I've got that answer.
OMG im so sorry :/ it is 10 ^ -3 i was doin 10 ^ 3 ughhhh these r the sme sort of mistakes im going to mke in the exam as well >.<
12. (Original post by godknowsprerna)
THANK YOU SOO MUCH DEAR life saver!

hey if u r not busy could u plz explain to me how to find the enthalpy change of combustion when enthalpy change of formations are given :s
q -3 c) jan 2010 paper
The way I see that, after like half an hour of looking through the chapter, is that if you have the enthalpy changes for formation, you use the total enthalpy of the products-total enthalpy of the reactants. In this case it is (6(-394)+6(-286))-(-1250)=2830. So it doesn't matter if they are asking for the enthalpy change of combustion, you do it with enthalpy change of formation values. This is the same for the reverse situation.
13. (Original post by Bright)
14 JANUARY 2010 QUESTION 3c)

PLEASE SOMEONE HELP ME WITH ENTHALPY Hess cycle drawing of this!

So far I have only got C02 + H20 at the bottom, where do I go from here?

Thanks.
You are using enthalpy changes of formation to calculate enthalpy changes of combustion, I don't know how you got CO2 and H2O at the bottom.
14. (Original post by Bright)
14 JANUARY 2010 QUESTION 3c)

PLEASE SOMEONE HELP ME WITH ENTHALPY Hess cycle drawing of this!

So far I have only got C02 + H20 at the bottom, where do I go from here?

Thanks.
YES I NEED HELP WITH THIS AS WELL plzzzzz help guys.
15. Im ****ed
16. (Original post by sixthformer)
do we draw dots on our radicals? Why does the mark scheme say "ignore dots" when that's exactly what the book says?!
the specification says: "equations should show which species are radicals using a single 'dot' to represent the unpaired electron"

if the mark scheme says to ignore them, then it doesn't matter if you draw them or not

if the mark scheme doesn't say to ignore them then you will need to have drawn them, so always draw them!
17. (Original post by lolo-x)
the specification says: "equations should show which species are radicals using a single 'dot' to represent the unpaired electron"

if the mark scheme says to ignore them, then it doesn't matter if you draw them or not

if the mark scheme doesn't say to ignore them then you will need to have drawn them, so always draw them!
YOU LEGEND

one more question
thank you

enthalphy change = bonds broken - bonds made
enthalphy of cobustion = bonds broken - bonds made
enthalphy of formation = bonds made - bonds broken

18. (Original post by godknowsprerna)
YES I NEED HELP WITH THIS AS WELL plzzzzz help guys.
C6H12O6 + 602 -----------------------> 6CO2 + 6H20

.........................6C + 9O2 + 6H2

and the arrows from the bottom line need to go up to the top equation..
then you just use the formation value of C6H12O6 on the left arrow going up
and the formation value for 6CO2s and 6H2Os on the right arrow going up

and subtract the left arrow from the right arrow
19. CH3CH2CH2Cl + H20--------> CH3CH2CH2OH + H+ +?
20. (Original post by Moody)
Can someone explain to me what an Octane number is and the benefits/disadvantages of a higher/lower Octane number?
The Octane Number is how efficient a fuel burns. The longer and less branched the hydrocarbon chain is, the lower the octane number and the lower the efficiency. This is why we use fuels with higher octane numbers so that less waste is produced and less energy is used to overcome the activation energy(I think).

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