Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Eternal Student on 04/06/2022 12:14:21
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Hi.
Please can I also ask a minor question?
Reactions are often given a ΔH value in KJ/mol. Now if the reaction is something like this:
3A2 + 3B2 → 2A3B3 ΔH = -100 KJ/mol.
With A , B = arbitrary atoms.
Then what is that ΔH value actually telling us? There's nothing in that reaction that has a stoichiometric value of 1. Is there actually 100 KJ of heat released for every 3 moles of A2 consumed? Or equivalently 100 KJ for every 2 moles of A3B3 produced? Or are you supposed to divide by 3 and say there was 33.3 KJ of heat released per 1 mole of A2 consumed, or 50 KJ per 1 mol of A3B3 produced? Anyway... you get the idea... the quoted enthalpy value is supposed to be per mole of what exactly?
Thanks and Best Wishes.
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It's 50 years since I studied this stuff and I really should not be hazarding a "guess" especially since physical chemistry was my worst performing subject. The enthalpy is for the formation of the product, 2 moles at 50kj each. You cannot ascribe it to the reactants without going through the full cycle of initial activation energy to produce the reactive species that participate in forming the new molecule. A lot of ionic reactions derive much of their net negative enthalpy from lattice energy of the formed product.(I shouldn't have said anything-i'm sure I have confused you as much as myself)
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Hi.
Thanks @paul cotter .
Now, if the reaction actually had two products and different stoichiometric values all over the place, then what product do you use?
3A2 + 3B2 → 1 A4B2 + 2 AB2
I don't know. Is there a standard rule or policy, or do you just have freedom to state the enthalpy value per any chosen molecule? ΔH = -100 KJ per mole of A4B2 produced OR ΔH = -50 KJ per mole of AB2 produced?
Best Wishes.
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Hi again.
I decided to be less lazy and found a reference:
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Energies_and_Potentials/Enthalpy/Heat_of_Reaction
There are mis-prints in that article but I think what they're trying to say is that the quoted ΔH values for the reaction are based on the stoichiometric coefficients as they appear in the way the reaction was written.
For example, the reaction and enthalpy of the reaction should be written as:
3A2 + 3B2 → 2A3B3 with ΔH = -100 KJ
.... and just leave the enthalpy of the reaction in KJ - do not put the enthalpy into KJ/mol.
So it says exactly the same as this:
6A2 + 6B2 → 4A3B3 with ΔH = -200 KJ
where everything was just multiplied by 2.
I don't know if that's right, I still would appreciate someone else's expertise. If I've understood this correctly, then there is no established convention for writing the enthalpy change of a reaction in units of KJ/mol, instead the quoted enthalpy change is specific to the way the reaction was expressed.
Best Wishes.
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If your reaction produces two products the total delta h will be the sum of the individual delta h of each product multiplied by the molar concentration of each final product. Often delta h values are not easily determined by direct experimental method and are computed by known values associated with the initial reactants. Google the born-haber cycle.