AS OCR New syllabus chemistry evaluative task - Enthalpy

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The_wise
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#1
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#1
Hey!! Im currently doing the NEW OCR AS Syllabus and I have been told that we have are going to do "an enthalpy evaluation" task. I really need help now in this task because this is my final chance and I did really bad in the other two tasks Anyone who has done/can help please can you suggest hints and tips on what to revise?? thanks A LOT!!! xx
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partysunil
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#2
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#2
Hess's cycle. just know that q=mct and delta h = -q/n and u WILL be fine. iv done it and it's easy.``
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The_wise
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#3
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#3
??
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smflesh
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#4
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#4
Basically, just revise everything that you have been taught which is related to enthalpy changes.
This includes: Definitions e.g. enthalpy of formation, enthalpy of combustion, mean bond enthalpy and how to calculate enthalpy changes (q = mct) and the definition for Hess's Law and how to do a Hess's Law cycle and calculation. Also, looking over energy diagrams and how to draw and recognise the energy diagram for an endothermic and exothermic reaction won't hurt either Exothermic reactions have reactants with high energy at the start but the products have a low energy. Endothermic reactions have reactants that have low energy at the start but the products have a high energy.
Good luck
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shazz92
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#5
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#5
what is the second evaluative task?
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aishu1997
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#6
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#6
are the questions difficult?
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MidnightDream
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#7
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#7
It's not hard at all if you just learn enthalpy as a whole you'll be fab - good luck
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Claire_62
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#8
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#8
Hey, have you done it yet? Was it hard? Got mine tomorrow- sooo nervous
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AsianBeauty
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#9
Report 8 years ago
#9
Has anyone done the eval task on enthalpy change of combustion ?? I'm really confused
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Pigster
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#10
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#10
(Original post by partysunil)
delta h = -q/n
It is negative only if the reaction is exothermic, i.e. you get a temperature rise.
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Borek
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#11
Report 8 years ago
#11
(Original post by Pigster)
It is negative only if the reaction is exothermic, i.e. you get a temperature rise.
Sign of ΔH change tells whether the reaction is exo- or endothermic, but the general equation as written by partysunil was correct. q is a function of ΔT, which is always Tfinal-Tinitial - and it already takes care of the sign.

You can't change the sigh in the equation depending on what kind of reaction you deal with, you have to stick to either plus or minus (depending on what convention you follow). Once you choose the convention, you have to use it consistently.
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AsianBeauty
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#12
Report 8 years ago
#12
(Original post by Borek)
Sign of ΔH change tells whether the reaction is exo- or endothermic, but the general equation as written by partysunil was correct. q is a function of ΔT, which is always Tfinal-Tinitial - and it already takes care of the sign.

You can't change the sigh in the equation depending on what kind of reaction you deal with, you have to stick to either plus or minus (depending on what convention you follow). Once you choose the convention, you have to use it consistently.
So have you done the eval then ??
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Borek
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#13
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#13
Nope.

It doesn't stop me from knowing a thing or two about theory behind.
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Louise02
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#14
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#14
(Original post by AsianBeauty)
Has anyone done the eval task on enthalpy change of combustion ?? I'm really confused
Hey, I have the evaluative task next week and I completely failed my other tasks any tips or hints about what to revise would be so so so helpful! Thank you!
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Benjamin Zhang
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#15
Report 8 years ago
#15
(Original post by partysunil)
Hess's cycle. just know that q=mct and delta h = -q/n and u WILL be fine. iv done it and it's easy.``
Can u give us a little bit more details please!! really confused! just some tips about revision will be very helpful thank you!!!
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jerseyalevel
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#16
Report 7 years ago
#16
I have this task tomorrow, has anyone done it? Some tips on the questions would be really helpful, thanks!
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username1333513
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#17
Report 7 years ago
#17
hey guys I'm doing my practical/coursework whatever it's called for Ocr chemistry A on Thursday, any tips please?


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oceanbeauty
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#18
Report 7 years ago
#18
im doing this on friday can you give me any tips at all please, thanks
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Pigster
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#19
Report 7 years ago
#19
It says on the papers:

For assessment use between 1 June 2014 and 14 May 2015
You're not doing it on Friday.
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