Enthalpy is so confusing (chemistry question)

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Helena200189
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I'm in year 12 studying AQA chem and the way my teacher has taught enthalpy to us basically makes no sense.
So with Hess's Law, I just find the triangle thing confusing, why can't we just do combustion = r - p and formation = p - r?
Do you think that I will need to understand the triangle route 1= route 2 thing or can I get away with just using basic maths?

If someone is able to outline what I need to know, that would be much appreciated! Thanks
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holly263
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You are actually right. I'm in year 13 right now doing OCR chemistry, but Hess' law is the same in most exam boards. You can use combustion = r - p and formation = p - r !! It is a lot simpler to understand and quicker when working out the asnwer(especially for me anyway). Hope this helped!
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zarasheikh003
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(Original post by Helena200189)
I'm in year 12 studying AQA chem and the way my teacher has taught enthalpy to us basically makes no sense.
So with Hess's Law, I just find the triangle thing confusing, why can't we just do combustion = r - p and formation = p - r?
Do you think that I will need to understand the triangle route 1= route 2 thing or can I get away with just using basic maths?

If someone is able to outline what I need to know, that would be much appreciated! Thanks
Hi- so the Hess' Law cycle is to help you understand the theory behind Hess' law. As Hess' Law states the value of enthalpy is independent to the route taken, so no matter which way you go, you will always get the same value.
In terms of the combustion and formation equations- you can do them in your exam, however they don't really display your knowledge of the cycle. This is particularly useful when you move onto A level content in year 13- when you will study Thermodynamics and Born Haber. So its good to know.
In terms of what you need to know- you should know the definition of Hess' law and enthalpy of combustion and formation and be able to draw the cycles for both. You can use the equations to help you calculate the enthalpy values, but might not be of help when you cross the two- for example when you need to calculate the enthalpy of formation but are given combustion data.
I recommend looking at these two videos- perhaps they can give a clearer explanation.

https://www.youtube.com/watch?v=Fq9mqz1TbSw

https://www.youtube.com/watch?v=pQAND6QlLNQ

Hope they help
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Helena200189
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(Original post by zarasheikh003)
Hi- so the Hess' Law cycle is to help you understand the theory behind Hess' law. As Hess' Law states the value of enthalpy is independent to the route taken, so no matter which way you go, you will always get the same value.
In terms of the combustion and formation equations- you can do them in your exam, however they don't really display your knowledge of the cycle. This is particularly useful when you move onto A level content in year 13- when you will study Thermodynamics and Born Haber. So its good to know.
In terms of what you need to know- you should know the definition of Hess' law and enthalpy of combustion and formation and be able to draw the cycles for both. You can use the equations to help you calculate the enthalpy values, but might not be of help when you cross the two- for example when you need to calculate the enthalpy of formation but are given combustion data.
I recommend looking at these two videos- perhaps they can give a clearer explanation.

https://www.youtube.com/watch?v=Fq9mqz1TbSw

https://www.youtube.com/watch?v=pQAND6QlLNQ

Hope they help
Ok thank you! I just don't get why the triangles of combustion and formation have arrows pointing in different directions. I guess I can't really wrap my head around it but I'll check out the videos
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holly263
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(Original post by Helena200189)
Ok thank you! I just don't get why the triangles of combustion and formation have arrows pointing in different directions. I guess I can't really wrap my head around it but I'll check out the videos
Yes, I completely understand where your coming from because i was in the exact same position as you this time last year! If you want, i can try and explain it to you if you want???
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Helena200189
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(Original post by holly263)
Yes, I completely understand where your coming from because i was in the exact same position as you this time last year! If you want, i can try and explain it to you if you want???
You would be a lifesaver if you could, thanks!!
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charco
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(Original post by Helena200189)
You would be a lifesaver if you could, thanks!!
Here's a video specifically to address this issue:

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zarasheikh003
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(Original post by Helena200189)
Ok thank you! I just don't get why the triangles of combustion and formation have arrows pointing in different directions. I guess I can't really wrap my head around it but I'll check out the videos
Okay So, for formation
the arrow points down all of them point up to the reactants in the middle because that is what they are made from
combustion the arrows point down from reactants and products to the water and CO2 at the bottom because that is what they are going to make.
is that any better ?
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DSKE
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(Original post by holly263)
You are actually right. I'm in year 13 right now doing OCR chemistry, but Hess' law is the same in most exam boards. You can use combustion = r - p and formation = p - r !! It is a lot simpler to understand and quicker when working out the asnwer(especially for me anyway). Hope this helped!
I’m also doing OCR :five: are you doing OCR A or OCR B?
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xsowmix
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(Original post by holly263)
You are actually right. I'm in year 13 right now doing OCR chemistry, but Hess' law is the same in most exam boards. You can use combustion = r - p and formation = p - r !! It is a lot simpler to understand and quicker when working out the asnwer(especially for me anyway). Hope this helped!
completely agree and it will help a lot when you come on to do enthalpy in A2.
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charco
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(Original post by holly263)
You are actually right. I'm in year 13 right now doing OCR chemistry, but Hess' law is the same in most exam boards. You can use combustion = r - p and formation = p - r !! It is a lot simpler to understand and quicker when working out the answer (especially for me anyway). Hope this helped!
The problem with doing this is that you may simply "remember" it the wrong way round. It really is better to understand the whole process that way you can extend your knowledge to unfamiliar situations.

However, if you are going to simply try and remember the "formulae" I propose an easy acronym.

For example:
Energetics is CRaP (Combustion = R - P)
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holly263
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(Original post by DSKE)
I’m also doing OCR :five: are you doing OCR A or OCR B?
Hi - yeh i do OCR A wbu?
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holly263
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(Original post by xsowmix)
completely agree and it will help a lot when you come on to do enthalpy in A2.
Yes! I agree because when doing A2 enthalpy it really helps doing AS enthalpy as quickly as possible - but getting the answers correct!
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Helena200189
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(Original post by charco)
Here's a video specifically to address this issue:

Thank you so much
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Helena200189
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(Original post by zarasheikh003)
Okay So, for formation
the arrow points down all of them point up to the reactants in the middle because that is what they are made from
combustion the arrows point down from reactants and products to the water and CO2 at the bottom because that is what they are going to make.
is that any better ?
Yes kind of... But I don't understand why there's an arrow going from products to products on complete combustion (on my sheet anyway).
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Helena200189
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(Original post by charco)
The problem with doing this is that you may simply "remember" it the wrong way round. It really is better to understand the whole process that way you can extend your knowledge to unfamiliar situations.

However, if you are going to simply try and remember the "formulae" I propose an easy acronym.

For example:
Energetics is CRaP (Combustion = R - P)
Yeah we were actually taught to remember "crap" and "flipper"! I will try and actually learn the process though if have the motivation one day
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