What is the definition of enthalpy?

Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system.

Enthalpy, symbolised as H, is a fundamental concept in thermodynamics, the branch of physical science that deals with the relations between heat and other forms of energy. It is defined as the total heat content or the total energy of a system. It's a state function, meaning its value depends only on the state of the system and not on the path taken to reach that state.

Enthalpy is usually measured in Joules (J) in the International System of Units (SI). However, it can also be measured in other units like calories (cal), British thermal units (BTU), or kilowatt-hours (kWh). It's important to note that we cannot measure the absolute value of the enthalpy of a system, but we can measure changes in enthalpy (∆H) during a process, such as a chemical reaction.

The change in enthalpy is a crucial concept in chemistry, particularly in the study of energy changes in chemical reactions. It is used to calculate the heat absorbed or released in a chemical reaction, which is vital in predicting whether a reaction is endothermic (absorbs heat) or exothermic (releases heat).

In an exothermic reaction, the enthalpy of the products is less than the enthalpy of the reactants, resulting in a negative ∆H and the release of heat. Conversely, in an endothermic reaction, the enthalpy of the products is greater than the enthalpy of the reactants, resulting in a positive ∆H and the absorption of heat.

Understanding enthalpy and its changes is fundamental to the study of energy transformations in chemical reactions, and it plays a significant role in fields such as physical chemistry, chemical engineering, and material science.