Which of the following enthalpies of formation cannot be measured directly?

Explanation

The enthalpy of formation of an ionic compound cannot be measured directly. This is because it is not possible to create an ionic compound from its constituent elements in their standard states. The ions of an ionic compound are held together by very strong electrostatic forces, and it is not possible to break these forces without adding a lot of energy. The enthalpies of formation of CO(g) and B2O3 can be measured directly.This is because CO(g) and B2O3 can be formed from their constituent elements in their standard states. The enthalpy of formation of an ionic compound can be calculated indirectly using the Born-Haber cycle. The Born-Haber cycle is a thermodynamic cycle that can be used to calculate the lattice energy of an ionic compound. The lattice energy is the energy required to convert the gaseous ions of the ionic compound into the solid ionic compound. The enthalpy of formation of CO(g) can be calculated using the following equation: ΔH°f (CO) = ΔH°f (C) + ΔH°at (O2) - ΔH°b (CO2) (i) Heat of formation of Al2O3 and B2O3 cannot be measured directly. It is difficult to burn these substances completely in oxygen, as protective layer of their oxides is formed on the surface. So, for the determination of heat of formation of Al2O3 and B2O3 Hess's law is used.(i) Heat of formation of CO cannot be measured directly due to the formation of CO2 with it.