AS OCR New syllabus chemistry evaluative task - Enthalpy

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 :frown:

Anyone who has done/can help please can you suggest hints and tips on what to revise?? thanks A LOT!!! xx

Reply 1

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.``

OP

??

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 :biggrin:

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 :biggrin:

Reply 4

shazz92

what is the second evaluative task?

Reply 5

aishu1997

are the questions difficult?

Reply 6

MidnightDream

16

It's not hard at all if you just learn enthalpy as a whole you'll be fab - good luck :smile:

Reply 7

Claire_62

2

Hey, have you done it yet? Was it hard? Got mine tomorrow- sooo nervous :frown:

Reply 8

AsianBeauty

2

Has anyone done the eval task on enthalpy change of combustion ?? I'm really confused :frown:

Reply 9

Pigster

20

Original post by partysunil

delta h = -q/n

It is negative only if the reaction is exothermic, i.e. you get a temperature rise.

Reply 10

Borek

4

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.

(edited 10 years ago)

Reply 11

AsianBeauty

2

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.