In thermodynamics, internal energy and enthalpy are two fundamental concepts. While they're related, they describe different aspects of a system's energy. Think of internal energy as the total energy *inside* a system, and enthalpy as the internal energy *plus* the energy related to pressure and volume. Let's break it down further!
Internal energy (U) represents the total energy of a system. This includes the kinetic energy of the molecules (their movement), the potential energy of the molecules (due to intermolecular forces), and the energy within the atoms themselves (electronic, nuclear).
Enthalpy (H) is defined as the sum of the internal energy (U) and the product of the pressure (P) and volume (V) of the system. It's particularly useful for reactions occurring at constant pressure, which is common in many chemical processes.
| Feature | Internal Energy (U) | Enthalpy (H) |
|---|---|---|
| Definition | Total energy of a system's molecules. | Total energy of a system plus the product of its pressure and volume ($U + PV$). |
| Symbol | $U$ | $H$ |
| Usefulness | Fundamental property, useful in all thermodynamic processes. | Especially useful for processes at constant pressure. |
| Change at Constant Pressure | $\Delta U = q_p - P\Delta V$ | $\Delta H = q_p$ (heat transferred at constant pressure) |
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