Energy of reaction: per moles of what?

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Discussion Overview

The discussion revolves around the interpretation of standard Gibbs energy values in chemical reactions, specifically focusing on the reaction involving thiosulfate and oxygen. Participants explore how to determine the energy produced per mole of reactants or products in the context of a given reaction.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether the standard Gibbs energy of −766 kJ/mol refers to energy produced per mole of the reactant, product, or both, and seeks clarification on how to interpret this value.
  • Another participant suggests that it generally refers to the principal reactant, indicating that the energy is typically calculated per mole of the main reactant involved in the reaction.
  • A follow-up inquiry addresses how to identify the principal reactant when the participant is unfamiliar with the reaction.
  • Further clarification is provided that if no additional information is available, the energy value should be interpreted as "per mole of reaction as written," with examples given to illustrate how to calculate energy per mole of each reactant or product based on the stoichiometry of the reaction.

Areas of Agreement / Disagreement

Participants generally agree that the Gibbs energy value is typically per mole of the principal reactant, but there is some uncertainty regarding how to identify this principal reactant in unfamiliar reactions. The discussion remains unresolved regarding the best method for determining the principal reactant without additional context.

Contextual Notes

Participants note the importance of context in interpreting energy values, including the need to look for specific phrases in the text that indicate how the energy is distributed among reactants and products. There is an acknowledgment that without clear definitions or additional information, assumptions may vary.

Danny-Boy
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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!
 
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Danny-Boy said:
1 mole of thiosulfate
Generally per mole of the principal reactant. You winkled it out.
 
Bystander said:
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?
 
Danny-Boy said:
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.
 
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.
 

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