# How are DeltaH_"reaction"^(@) assessed?

$\Delta H$ is not accessed by calculation..........
$\Delta {H}_{\text{rxn}}^{\circ}$ is accessed by measurement. Look in your text for $\Delta {H}_{f}^{\circ}$ for $M g O$.
$\Delta {H}_{f}^{\circ}$ values are extensively tabulated, and represents the enthalpy change associated with $1$ $m o l$ of substance from its elements in their standard states at $298 \cdot K$.