Final answer:
The energy transferred as heat is equal to the enthalpy change because during constant pressure processes, enthalpy change represents the heat absorbed or released. This equivalence is based on the definition of enthalpy and upheld by Hess's law, which states that enthalpy change depends only on the initial and final states of the system.
Step-by-step explanation:
The reason why the energy transferred (heat) is equal to the enthalpy change (ΔH) in a system is fundamentally related to the laws of thermodynamics, specifically when processes are carried out at constant pressure. When a chemical or physical process occurs at constant pressure with the only work being done due to expansion or contraction, the heat flow (qp) and the enthalpy change are identical. This is because the enthalpy is defined as the internal energy plus the product of the system's pressure and volume (H = U + PV). So, when pressure is constant, the enthalpy change is just the heat absorbed or released by the system.
Additionally, according to Hess's law, the ΔH of a reaction is equal to the sum of the enthalpy changes of the individual steps of the reaction, regardless of the pathway. This is another expression of how enthalpy is a state function, dependent only on the initial and final states and not on the specific process.
Experiments support this as well, showing that the energy transferred as heat in a system causes temperature changes directly tied to ΔH. For cyclic processes, the net work done by the system also equals the net heat transferred.