What is the significance of enthalpy?
The internal energy $U$ is the sum of all the energies stored in the chemical bonds. If a chemical reaction happens, the bonds are losing or getting energy. Heat is getting in or out of the system, and this heat $\Delta Q$ can be measured. $\Delta U$ =$\Delta Q$, if the volume is constant.  But if the transformation is carried out in contact with the atmosphere, at constant pressure, the volume may change. A fraction of this energy is converted into work $\Delta w$ to repel the atmosphere, because the volume of the system may change. Usually this volume change is small (except if a gas is produced or consumed). And this volume change may be difficult to know with precision. But it must be taken into account to calculate $\Delta U$. So if this small amount of work is neglected, a correction must be added to the internal energy $U$, that depends on pressure and volume. Neglecting this volume correction gives a sort of “apparent internal energy”, which is called enthalpy $H$, with $H = U + PV$. And then, in all transformations made at ordinary pressure, the following expression is valid : $\Delta H$ = $\Delta Q_p $.
Enthalpy is a sort of &quot;apparent internal energy&quot;. There is a similar difference between the weight and the &quot;apparent weight&quot;, which is measured in air. The weight of an object should be measured in a vacuum. The weight, as measured with a dynamometer in the usual atmosphere, is a little bit smaller than in a vacuum, because of Archimedes.