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Enthalpy questions??

I'm really stuck on enthalpy diagrams. I understand the concept of 1 mole of reactant in ΔHc equations and 1 mole of product in formation equations, however when it comes to the diagrams I get completely lost.
For example, I'm trying to do a question that's znco3 ----> zno + co2, calculate the ΔHfor the following reactions given that the ΔHf(kjmol-1) of znco3 is -812, zno is 348, co2 is -398.
Problem 1: I know as I've done this q before and got it right that the reactants in the enthalpy diagram are zn + c + O2 but why are they this, as zn + c + 02 don't form znco3?
Problem 2: how am I meant to know in any enthalpy diagrams which way the arrows go? Up or down, from reactants to products or products to reactants?
Im sure all of this sounds v stupid but I've got myself in to a state and can't see any possible way of figuring it out:confused:
Reply 1
Avatar for Pigster
Pigster
20
Problem 1, you need 3/2O2.

Problem 2, formation: arrows up (elements at bottom), combustion: down (combustion products at bottom).
Reply 2
Avatar for Flask
Flask
OP
1
Original post by Pigster
Problem 1, you need 3/2O2.

Problem 2, formation: arrows up (elements at bottom), combustion: down (combustion products at bottom).


Thank you😊

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