The change of internal energy of formation can usually be neglected in energy balance, if

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Question: The change of internal energy of formation can usually be neglected in energy balance, if

Option given: d. there are no chemical reactions in the system

Since there are no provided alternative options in the data, I will focus on evaluating the stated option.

- The internal energy of formation refers to the energy change associated with forming a substance from its elements in standard states. In an energy balance, this term is relevant when chemical reactions are occurring, because the formation or consumption of compounds changes the internal energy content of the system through chemical bonds being formed or broken.
- The stated condition for neglecting the change of internal energy of formation is that there are no chemical reactions in the system. In that case, the composition of the system is constant and no new bonds are formed or broken, so the formation energetics do not contribute to the energy balance. Consequently, ΔU_formation ≈ 0 under this scenario.
- This reasoning relies on the assumption that the system is closed with fixed chemical species and no phase transitions or chemical reactions; physical changes like heating, cooling, phase changes, or mechanical work may still occur, but they do not involve changes in chemical formation energies.
- It is important to contrast with systems where reactions do occur: in such cases, the sum of formation energies for products minus reactants would contribute to the energy balance, and neglecting it would lead to errors. Also, if reactions are reversible or involve catalytic pathways, their thermodynamic contributions still arise from formation energy terms.
- Potential misconceptions to watch for: one might think “no reactions” implies no energy changes at all. While ΔU_formation may be negligible, other energy terms (sensible heat, latent heat, PV-work, etc.) can still be relevant. Therefore, the blanket statement that all internal energy changes can be neglected is not generally true; only the formation-energy portion can be neglected when there are truly no chemical reactions.
- In summary, the option is correct in the narrow sense that if there are no chemical reactions in the system, the internal energy change associated with formation from elements can be neglected in the energy balance, since formation processes are not taking place. If reactions do occur, this simplification does not hold and formation energies must be accounted for.

The change of internal energy of formation can usually be neglected in energy balance, if

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