Energy of reaction: per moles of what?

[Danny-Boy]

Disclaimer: I've never had a chemistry class in my life. I'm reading a journal article that states the following:

S2O32- + 2O2 + H2O → 2SO42- + 2H+

has the standard Gibbs energy of −766 kJ/mol at 25ºC. So, this is energy produced per moles of what?! Reactant? Product? If so, which reactant or product and how do I figure that out? Or does it just mean that if I reacted 1 mole of thiosulfate with 2 moles of oxygen, I'd end up with 766 kJ of energy? Any help would be appreciated!

 

[Bystander]

1 mole of thiosulfate

Generally per mole of the principal reactant. You winkled it out.

 

[Danny-Boy]

Generally per mole of the principal reactant. You winkled it out.

Thanks for the reply! How do I determined which is the principal reactant if I'm not familiar with the reaction?

 

[Bystander]

which is the principal reactant if

Hopefully there's some context to point it out. Look for key phrases hidden in obscure places in the text, "per mole of _____" produced/consumed.

 

[Borek]

If there is no other information - "per mole of reaction as written". Say you are told ΔH for

2Al + 3Cl₂ → 2AlCl₃

is x kJ. That would mean x/2 kJ per mole of Al consumed or AlCl₃produced, x/3 kJ per mole of Cl₂.