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

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The question asks under what condition the change of internal energy of formation can usually be neglected in an energy balance.
Option analysis (based on the provided correct choice):
- The statement suggests that neglect is valid when there are no chemical reactions in the system. In a system where no chemical reactions occur, the formation of compounds from their elements does not take place, so there is no change in the internal energy associated with chemical formation processes. Consequently, the term for energy of formation change would be zero or constant, making it negligible in an energy balance, provided other sources of energy exchange (sensible, latent, etc.) are accounted for separately.
- If there are chemical reactions in the system, even if they are small or limited, there will generally be changes in the internal energy associated with the formation or breaking of chemical bonds. In such cases, neglecting the energy of formation would typically introduce error, because formation enthalpies or internal energy changes contribute to the overall energy balance through reaction energetics.
- A potential misconception would be to think that the energy of formation is always negligible due to the small magnitudes of some formation energies; however, in reactions involving large changes in bond energies or significant amounts of reactants/products, the formation energy term can be substantial and should not be neglected.
- Another pitfall is confusing formation energy with other energy terms like sensible heat or phase-change energies. Even when no phase change occurs, chemical formation/rupture events introduce distinct internal-energy contributions that must be considered if reactions are present.
- If the system contains only inert species or if each species is already in its standard state and no net formation from elements occurs during the process, the internal energy of formation change remains constant or zero, supporting neglect in the energy balance under those specific circumstances.
- In summary, neglecting the internal energy of formation in an energy balance is appropriate primarily when there are no chemical reactions occurring within the system, so that no formation or decomposition energetics contribute to the energy accounting. If reactions can occur, this term generally cannot be ignored without justification and quantification.

BH20A0720 Engineering Thermodynamics - Blended teaching 2.9.2024-15.12.2024 Quiz 3

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

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