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You could also run a reaction reversibly using a van't Hoff equilibrium box.DrDu said:
Why is ##dQ = dH## still valid for chemical reaction (Callen)?
- Context: Undergrad
- Thread starter EE18
- Start date
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- Tags
- Enthalphy Heat Thermodynamics
Click For Summary
Discussion Overview
The discussion revolves around the validity of the equation ##dH = dQ## in the context of chemical reactions, particularly as presented in Callen's work. Participants explore the implications of this equation when the number of moles of reactants and products changes during a reaction, questioning the assumptions underlying its application in such scenarios.
Discussion Character
- Debate/contested
- Technical explanation
- Conceptual clarification
Main Points Raised
- Some participants argue that the derivation of ##dH = dQ## relies on the assumption that the number of moles ##dN_i## is constant, which is not the case during chemical reactions.
- Others propose that if the system is treated as a black box in contact with a constant pressure environment, the first law implies that ##\Delta H = Q##, but this relationship may depend on changes in reactants and products.
- A participant questions how to reconcile the fundamental relation for ##dH## when ##dN_i## are not constant, suggesting that the derivation does not hold in such cases.
- Some responses emphasize the irreversibility of spontaneous chemical reactions and the impact of finite temperature gradients on heat transfer, complicating the relationship between ##Q## and ##\Delta S##.
- There is a discussion about the conditions under which chemical reactions can be considered quasistatic, with some participants suggesting that idealized situations are often assumed in introductory texts.
Areas of Agreement / Disagreement
Participants express differing views on the applicability of ##dH = dQ## in the context of chemical reactions, with no consensus reached on whether the assumptions made by Callen are valid in these scenarios. The discussion remains unresolved regarding the conditions necessary for the equation to hold true.
Contextual Notes
Participants note that the derivation of ##dH = dQ## may not account for changes in mole numbers during reactions, and the implications of irreversibility and temperature gradients are highlighted as significant factors that complicate the analysis.
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