can't understand Hess' Law!! please help with the attached questions

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jazz_xox_
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We covered Hess' Law months ago now, but the teacher who taught it didn't understand it himself I understand the basics but don't understand what going at the bottom when you draw the triangular diagrams. It's particularly the enthalpy of combustion but I don't really understand any of it

I've attached 2 questions I'm stuck with, please could someone give me steps how to work them out? Would be very appreciated, thank you!
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h3rmit
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At the bottom of the triangular diagram, you have either the reactants (in their standard states) that form your products for formation reactions (usually C(s) and H2(g)), and for combustion reactions you have the products of combustion - usually CO2(g) and H2O(l).

Hence, you have to go up to the reagents of your reaction of unknown enthalpy from the reagents that form it with formation reactions (form and grow), and go down to the products of combustion with combustion reactions (burn in hell).

For the bottom question, you can add chemical reactions, cancel what appears on both sides until you get to the desired reaction. If you then do the same to the enthalpies as you did to the reactions, you get the enthalpy change of your desired reaction
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h3rmit
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(Original post by jazz_xox_)
We covered Hess' Law months ago now, but the teacher who taught it didn't understand it himself I understand the basics but don't understand what going at the bottom when you draw the triangular diagrams. It's particularly the enthalpy of combustion but I don't really understand any of it

I've attached 2 questions I'm stuck with, please could someone give me steps how to work them out? Would be very appreciated, thank you!
Also, the exact nature of the reagents required for formation/products of combustion and their proportions doesn't really matter unless you have to write a reaction for the formation/combustion.

Also, and don't feel patronised if you already know this, the enthalpy change of whatever you want to get to goes on the arrows (just \Delta H_r for the "main" reaction), e.g for the combustion of ethanamide, your arrow going up from the reactants to form ethanamide should have the enthalpy change of formation for ethanamide on it (-320). And, clockwise=anticlockwise.
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jazz_xox_
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(Original post by h3rmit)
At the bottom of the triangular diagram, you have either the reactants (in their standard states) that form your products for formation reactions (usually C(s) and H2(g)), and for combustion reactions you have the products of combustion - usually CO2(g) and H2O(l).

Hence, you have to go up to the reagents of your reaction of unknown enthalpy from the reagents that form it with formation reactions (form and grow), and go down to the products of combustion with combustion reactions (burn in hell).

For the bottom question, you can add chemical reactions, cancel what appears on both sides until you get to the desired reaction. If you then do the same to the enthalpies as you did to the reactions, you get the enthalpy change of your desired reaction
Thank you so much, I'm beginning to understand it more now. When you say at the bottom of the diagram you have the products of combustion.. which are combusting? I'm trying the first question again but I'm still struggling what to write there. Sorry for so many questions, hopefully it will all click in my brain soon thanks again!!
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h3rmit
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(Original post by jazz_xox_)
Thank you so much, I'm beginning to understand it more now. When you say at the bottom of the diagram you have the products of combustion.. which are combusting? I'm trying the first question again but I'm still struggling what to write there. Sorry for so many questions, hopefully it will all click in my brain soon thanks again!!
Whatever is in the main reaction is being burnt. If you'd been given the enthalpy changes of combustion instead of formation for the first question, then it'd be ethanamide being combusted to give the products of combustion
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jazz_xox_
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(Original post by h3rmit)
Whatever is in the main reaction is being burnt. If you'd been given the enthalpy changes of combustion instead of formation for the first question, then it'd be ethanamide being combusted to give the products of combustion
Okay thank you, I understand that For the first question, do you write the original equation with ethanamide reacting with O2 or not? Because you get given the enthalpies of formation in the box but then the question asks for the enthalpy of combustion?
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h3rmit
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(Original post by jazz_xox_)
Okay thank you, I understand that For the first question, do you write the original equation with ethanamide reacting with O2 or not? Because you get given the enthalpies of formation in the box but then the question asks for the enthalpy of combustion?
The type of enthalpy values you're given don't affect the "main" reaction, so you still write the equation for the combustion of ethanamide in oxygen. Your arrows, however, will be going up from the required forming reactants to the ethanamide on one side, and CO2 and H2O on the other side.
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