[Eternal Student (OP)]

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.

[paul cotter]

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)

[Eternal Student (OP)]

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.

[Eternal Student (OP)]

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.

[paul cotter]

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.