Enthalpy refers to the heat energy absorbed or released during a chemical reaction at constant pressure. It represents the change in internal energy of a system.
A principle stating that if a reaction can be expressed as the sum of two or more reactions, then its enthalpy change will be equal to the sum of their individual enthalpy changes.
Standard Enthalpy Change: The enthalpy change that occurs when reactants in their standard states are converted into products in their standard states, under standard conditions.