Define enthalpy change under constant pressure.

Enthalpy change (ΔH) is defined as the heat energy change of a system at constant pressure. It represents the difference in enthalpy between the products and reactants of a chemical reaction. The formula for calculating enthalpy change is ΔH = H_products - H_reactants. This value can be positive or negative, indicating whether the reaction absorbs heat (endothermic, ΔH > 0) or releases heat (exothermic, ΔH < 0). Understanding enthalpy change is crucial for predicting the energy dynamics of chemical reactions.