Table of Contents
What Is Enthalpy?
Enthalpy (H) is a thermodynamic quantity representing the total heat content of a system at constant pressure. The enthalpy change (ΔH) tells us how much heat is absorbed or released during a process like heating, cooling, phase transitions, or chemical reactions. Positive ΔH means heat is absorbed (endothermic); negative means heat is released (exothermic).
At constant pressure, which is the standard condition for most everyday processes, the enthalpy change equals the heat transferred. This makes enthalpy particularly practical for analyzing cooking, HVAC systems, chemical manufacturing, and atmospheric science.
Formula
Where m is mass (kg), c_p is specific heat at constant pressure (J/kg·K), and ΔT is temperature change (K or °C). For chemical reactions, ΔH = ΣH(products) - ΣH(reactants).
Specific Heat Values
| Substance | c_p (J/kg·K) |
|---|---|
| Water | 4,186 |
| Air (dry) | 1,006 |
| Aluminum | 897 |
| Iron | 449 |
| Copper | 385 |
| Ethanol | 2,440 |
Frequently Asked Questions
What is the difference between enthalpy and heat?
Heat (q) is the energy transferred between systems due to temperature difference. Enthalpy change (ΔH) equals heat only at constant pressure. At constant volume, the heat equals the change in internal energy (ΔU) instead. For gases, ΔH = ΔU + PΔV.
Why does water have high specific heat?
Water molecules form extensive hydrogen bond networks. Breaking and reforming these bonds absorbs significant energy without raising temperature proportionally. This makes water an excellent thermal buffer, moderating coastal climates and serving as coolant in engines and power plants.
How is enthalpy used in HVAC?
HVAC engineers use enthalpy to calculate total heat content of moist air, combining sensible heat (temperature) and latent heat (humidity). Air conditioning must remove both components to achieve comfort, so enthalpy-based calculations determine equipment sizing and energy costs.