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AS Chemistry help - Hess's Law

I'm very confused with Hess's Law. I need to answer the following question:

Calculate the enthalpy change for the following reactions given the values of enthalpy change of formation.

a) 2Al + Fe2O3 --> 2Fe + Al2O3

b) 2CO + O2 --> 2CO2

Values given:

Al2O3 = -1669
Fe2O3 = -822
CO2 = -393
CO = -111

For b, I came up with an answer of -564kJmol^-1 but I'm unsure whether this is correct.

Any help would be greatly appreciated!
For (b) your answer is correct :smile:
You don't actually need to learn how to draw the Hess' cycles (though I personally think it's better to do so) you can just remember that if you are give the enthalpies of combustion the enthalpy change is given by reactants - products (CRaP) and if you are given the enthalpies of formation (like in the questions you've provided) then you do the opposite of that -> products - reactants.

For example (b):
Products - Reactants
2(-393)-2(-111)=-564 kJmol^-1 (take care with your signs).

If you do want to learn how to draw the actual Hess' cycles then whenever you're given enthalpies of formation draw arrows from elements (at the bottom) UP to the chemical equation. When you are given enthalpies of combustion draw arrows pointing DOWN towards the combustion products. Then you go around from the top - if you're going against the arrow and + if you're going with the arrow. I'm really bad at explaining things so I'm sorry if you don't understand what I'm trying to say. If you have any other questions do ask, and I'll try to explain.
Reply 2
Original post by victoria98
For (b) your answer is correct :smile:
You don't actually need to learn how to draw the Hess' cycles (though I personally think it's better to do so) you can just remember that if you are give the enthalpies of combustion the enthalpy change is given by reactants - products (CRaP) and if you are given the enthalpies of formation (like in the questions you've provided) then you do the opposite of that -> products - reactants.

For example (b):
Products - Reactants
2(-393)-2(-111)=-564 kJmol^-1 (take care with your signs).

If you do want to learn how to draw the actual Hess' cycles then whenever you're given enthalpies of formation draw arrows from elements (at the bottom) UP to the chemical equation. When you are given enthalpies of combustion draw arrows pointing DOWN towards the combustion products. Then you go around from the top - if you're going against the arrow and + if you're going with the arrow. I'm really bad at explaining things so I'm sorry if you don't understand what I'm trying to say. If you have any other questions do ask, and I'll try to explain.


Thank you so much for your reply, you explained it so well! That really helped a lot.

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