Why is Hess's Law Generally Not Obeyed?

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Discussion Overview

The discussion revolves around Hess's Law and its perceived limitations in practical applications, particularly in the context of chemical reactions. Participants explore the theoretical underpinnings of Hess's Law, its implications as a state function, and scenarios where it may not appear to hold true in real-world situations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the general obedience of Hess's Law and seeks clarification on the concept.
  • Another participant defines Hess's Law as relating to enthalpy being a state function, suggesting that it accounts only for energy in chemical bonds and not for heat or work exchanged with surroundings.
  • A participant requests examples of violations of Hess's Law, expressing confusion about why it would not be obeyed given that enthalpy is a state function.
  • One participant speculates that the professor's question might refer to real-world scenarios where varying conditions and paths affect the application of Hess's Law.
  • Another participant argues that Hess's Law is obeyed under conditions where state functions hold, prompting inquiry into what aspects of chemical reactions might lead to perceived violations.
  • A later reply suggests that the quantities of reactants must be in correct molar proportions for the ΔHs to be valid, implying that deviations in these proportions could lead to misunderstandings of Hess's Law.
  • One participant counters that even with insufficient reagents, the change in enthalpy should be proportional to the moles that reacted, questioning whether this truly invalidates Hess's Law.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Hess's Law in practical scenarios, with some suggesting it is always obeyed under certain conditions, while others highlight potential limitations based on real-world factors. No consensus is reached regarding specific examples of violations or the implications of varying reactant quantities.

Contextual Notes

Participants acknowledge the importance of molar proportions in reactions and the conditions under which Hess's Law applies, but there are unresolved questions about how these factors influence the law's applicability in practice.

BrianC12
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Had a recent midterm and got this question wrong: why is Hess's Law generally not obeyed? Any ideas?

Thanks.
 
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What do you understand by Hess' law?
 
That enthalpy is a state function and when chemical reactions are added, their enthalpies must be added as well. Is the answer the fact that it only accounts for the energy in the chemical bonds and not the heat or work exchanged with the surroundings?
 
Can you give an example where it is violated? Since enthalpy is a state function I don't see why it would not be obeyed.
 
mishrashubham said:
Can you give an example where it is violated? Since enthalpy is a state function I don't see why it would not be obeyed.

If what I said above is right, then I'm guessing the professor meant why is Hess's Law not obeyed in real world examples where conditions and paths can vary. I feel like he should have included that in the question though...oh well. If what I said isn't right then I wouldn't know under what circumstances Hess's law wouldn't be obeyed either.
 
Well, as has been said, Hess's law relates to the difference of state functions so is obeyed under any conditions which these hold.

So you need to enquire what is there about most chemical reactions that disobeys these conditons.

I suggest the answer is not in the reactions themselves or the sequence of reactions that make up a Hess chain, but the quantities of reactants themselves.

The ΔHs only hold good for reactions where the reagents are in the correct molar proportions.
 
But even if you don't have enough of one type of reagent, it's sort of intuitive that the resulting change in enthalpy would just be a fraction of the number of moles that reacted, which wouldn't make Hess's Law wrong...right?
 

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