∆Hf of which of the following cannot be measured directly?

Explanation

Standard enthalpies of formation (∆Hf) for certain compounds, such as Al2O3 (aluminum oxide), B2O3 (boron oxide), and CO (carbon monoxide), cannot be measured directly. Instead, these values are typically calculated indirectly using other thermodynamic data and principles. The standard enthalpy of formation, ∆Hf, represents the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. However, for some compounds like Al2O3, B2O3, and CO, it is challenging or impractical to perform direct calorimetric measurements to determine their ∆Hf values due to various reasons, such as high stability, limited availability, or extreme conditions.