Enthalpy
Enthalpy is the sum of internal energy and pressure-volume work.
Enthalpy is a thermodynamic property defined as the sum of a system's internal energy and the product of its pressure and volume. It is a state function used extensively in chemical, biological, and physical systems at constant external pressure, such as Earth's ambient atmosphere. Enthalpy simplifies the description of energy transfer, particularly in chemical reactions, where the change in enthalpy at constant pressure equals the heat exchanged with the environment.
- field
- Thermodynamics
- known_for
- Definition as H = U + pV; state function; standard enthalpy of reaction
- unit
- Joule (SI); also calorie and British thermal unit
Lore & Background
Enthalpy is defined as H = U + pV, where U is internal energy, p is pressure, and V is volume. The pressure-volume term represents the work done against constant external pressure to establish the system's volume from zero to its final size. For solids and liquids at common conditions, this term is very small; for gases, however, it is not negligible—for an ideal gas, pV is comparable to the internal energy and can be of the same order of magnitude. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken. For an ideal gas, enthalpy depends only on temperature, which is consistent with pV being significant relative to U.
Reader's Guide
Enthalpy is a central concept in thermodynamics because it allows practical measurement of energy changes in chemical systems at constant pressure, which is the typical condition under Earth's atmosphere. The total enthalpy of a system cannot be measured directly due to unknown internal energy components, but changes in enthalpy are readily measured. This quantity is the standard heat of reaction at constant pressure and temperature, and can be measured by calorimetric methods even if temperature varies, provided initial and final conditions correspond to the standard state. Enthalpy changes are positive for endothermic processes and negative for exothermic ones. For ideal gases, enthalpy depends only on temperature, not on pressure or volume, a behavior closely approximated by real gases at common conditions. The word 'enthalpy' derives from the Greek word enthalpein, meaning 'to heat'.
Did You Know?
- Enthalpy is derived from the Greek word enthalpein, which means 'to heat'.
- The pressure-volume term in enthalpy is very small for solids and liquids at common conditions.
- For an ideal gas, enthalpy is independent of pressure or volume and depends only on temperature.
Frequently Asked Questions
Who is Enthalpy?
Enthalpy is a thermodynamic state function that bundles a system's internal energy together with the pressure-times-volume work term into one convenient quantity. In the Thermodynamics And Statistical Mechanics 1-22 arc it serves as the default bookkeeping tool whenever a system sits under constant external pressure, like Earth's ambient atmosphere.
What are Enthalpy's powers/role?
Enthalpy's signature ability is to fold heat exchange and pressure-volume work into a single measurable number, so you no longer have to track them separately. At constant pressure the shift in enthalpy is exactly the heat the system absorbs from or dumps into its surroundings, which is why it dominates calorimetry and reaction energetics.
How does Enthalpy's story end?
Enthalpy doesn't so much 'end' as settle: once a reaction reaches completion, the net enthalpy change is locked in by the identities of reactants and products. That final, tabulated value—the standard enthalpy of reaction—is the closing number fans pull from data tables to predict whether a process is exothermic or endothermic.
Why is Enthalpy important?
Because virtually every everyday and industrial process—digestion, combustion, metallurgy, drug synthesis—runs at roughly constant atmospheric pressure, enthalpy is the natural quantity for describing how much energy those processes release or absorb. It is expressed in joules (or, in older units, calories and BTUs) and underpins thermochemistry, biological energetics, and engineering design alike.
What is Enthalpy's defining equation?
The core identity is H = U + pV: enthalpy equals internal energy plus the product of pressure and volume. Because both U and the pV term are state-dependent, this sum inherits state-function status, meaning the change in enthalpy depends only on the starting and ending conditions, never on the particular path the system took between them.
More in Thermodynamics And Statistical Mechanics 1-22
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record