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(edited 1 year ago)
Hey, don't know if this helps.

Hess' law is essentially the idea that no matter what route a reaction takes it will always have the same enthalpy change.
This means you can break both products and reactants down to a common set of compounds and work from there. You often use formation enthalpies (so both products and reactants can be broken back into common elements) or combustion enthalpies (so you imagine you combusted both products and reactants into a common set of compounds) .
You can then just work out the enthalpy changes of your two new reactions and use these to work out the original reaction. You will be given the data needed to work out these enthalpy changes.

No idea is that makes any sense. Basically you're just finding an alternate route for the reaction that you have the data to work out enthalpy changes for then applying it back to the original reaction.
Original post by y5d
I have no idea what to do, I don't understand how to do these calculations at all. This may partly be my fault but I wasn't in for a few lessons but the teacher doesn't explain anything, she just expects us to know how to do stuff. I've sent a few emails and had no response.

Can someone help explain Hess' law and how to calculate enthalpy change pls?


These will help:

https://m.youtube.com/watch?v=xIsyfHtja9M

https://m.youtube.com/watch?v=WiHRt-mrk7g

https://m.youtube.com/watch?v=uaeMVD46MH0

https://m.youtube.com/watch?v=x80BgrVgy7g
Hi @y5d, a good place to start is by reading the following: https://www.chemguide.co.uk/physical/energetics/sums.html

"The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs." This means that the starting materials and product both have a defined enthalpy. The enthalpy difference between the two will therefore be a defined value, and the route to get from reaction to product doesn't affect this.

A (non-chemical) example of this could be heating of water from 25C to 50C. The energy difference will be the same whether we heat water from 25C to 50C, or if we cool from 25C to 0C, then heat from 0C to 50C. The energy difference between starting (25) & ending (50) states will be the same, although a different route was taken. Now, this example can be applied to chemical reactions, but enthalpy is used instead of energy.

Hess cycles can be used to calculate reaction enthalpies. This is a cycle constructed that shows different routes to get from one state to another. However, taking into account that the reaction route doesn't make a difference we can calculate missing 'arrows' on the cycle because we know enough information from the other reaction routes and Hess' law.

Have a read and let me know if you have any other questions :smile:
Rob
PhD Chemistry Student
Student Ambassador
Original post by y5d
THANK U!!! this rlly helped. I rlly need videos to help understand something, reading a bunch of words did not help

Which exam board are you doing?

Allery chemistry is good for all exam boards, but if you are doing OCR A, MaChemGuy is best and if you are doing AQA, Eliot Rintoul is best.
Original post by y5d
thanks! i think im starting to get it now. do u know where i could find some good practice questions on this? for aqa pls

Physics and Maths tutor

Should be under ‘Physical I’ https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/
(edited 1 year ago)
I know its a bunch of words- but it walks you htrough some examples- might help you get it!!!

https://www.science-revision.co.uk/A-Level_Hess's_law.html

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