How come Enthalpy of Formation is calculated differently from the typical chemical reaction under non-standard conditions (DeltaH)? For the latter, all one needs to do in order to find a reaction's change in enthalpy is re-arrange the additional (intermediate) chemical reactions (if needed) (altering their Delta H too) and then simply add their enthalpies. The way Enthalpy of Formation is calculated also seems to, in a way, not follow Hess's law and the fact that energy is a state function. Maybe, because I learned how to solve for enthalpy change of reactions under non-standard conditions first I haven't considered the entire meaning of Enthalpy. But, how come it is not correct to just add all the enthalpy of formations (with their potential alterations) to determine the main reaction's change in enthalpy of formation?
The enthalpy of formation is calculated differently because it is specifically defined for the formation of one mole of a compound from its elements in their standard states under standard conditions (298 K, 1 atm). This standardization provides a consistent reference point in thermodynamic calculations. While Hess's Law still applies, the enthalpies of formation are already pre-determined values under these specific conditions. Simply adding these values directly for reactions under non-standard conditions is incorrect because changes in temperature, pressure, or stoichiometry can alter the enthalpy values. Enthalpy is a state function, but the state must match the defined standard conditions for ΔfH° to be valid. Thus, the enthalpy of formation acts as a fixed baseline, and any deviations from standard conditions require recalibrating the enthalpy changes accordingly.
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How come Enthalpy of Formation is calculated differently from the typical chemical reaction under non-standard conditions (DeltaH)? For the latter, all one needs to do in order to find a reaction's change in enthalpy is re-arrange the additional (intermediate) chemical reactions (if needed) (altering their Delta H too) and then simply add their enthalpies.
The way Enthalpy of Formation is calculated also seems to, in a way, not follow Hess's law and the fact that energy is a state function. Maybe, because I learned how to solve for enthalpy change of reactions under non-standard conditions first I haven't considered the entire meaning of Enthalpy. But, how come it is not correct to just add all the enthalpy of formations (with their potential alterations) to determine the main reaction's change in enthalpy of formation?
The enthalpy of formation is calculated differently because it is specifically defined for the formation of one mole of a compound from its elements in their standard states under standard conditions (298 K, 1 atm). This standardization provides a consistent reference point in thermodynamic calculations. While Hess's Law still applies, the enthalpies of formation are already pre-determined values under these specific conditions. Simply adding these values directly for reactions under non-standard conditions is incorrect because changes in temperature, pressure, or stoichiometry can alter the enthalpy values. Enthalpy is a state function, but the state must match the defined standard conditions for ΔfH° to be valid. Thus, the enthalpy of formation acts as a fixed baseline, and any deviations from standard conditions require recalibrating the enthalpy changes accordingly.
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