Enthalpy Calculator

Enthalpy (H) is a thermodynamic quantity equal to a system's internal energy plus the product of its pressure and volume, H=U+PV. Physically, it represents the total heat content of a system at constant pressure: internal energy plus the extra 'flow work' needed to make room for the system against the surrounding pressure.

By definition, H=U+PV, where U is internal energy, P is pressure, and V is volume. For a constant-pressure heating or cooling process, the more practical formula dH=m*Cp*dT is used instead, where m is mass, Cp is specific heat capacity at constant pressure, and dT is the temperature change.

Use H=U+PV (Definition mode) when you know a system's internal energy, pressure, and volume directly and want its total enthalpy. Use dH=m*Cp*dT (Heating Process mode) for the much more common practical case of finding how much heat is needed to change a known mass of material by a given temperature at constant pressure.

A pascal is one joule per cubic meter (Pa = J/m3), so kPa*m3 = 1000 Pa * m3 = 1000 J = 1 kJ automatically, no conversion factor needed. This calculator's Definition mode relies on exactly this unit consistency: entering P in kPa and V in m3 gives PV directly in kJ.

At constant pressure, the heat added to a system equals its change in enthalpy directly (dH=Q), with no separate bookkeeping needed for the PV expansion work the system does on its surroundings. That expansion work is already built into the definition of H, which is exactly why chemists and engineers track enthalpy, not raw internal energy, for reactions and processes that occur at constant (usually atmospheric) pressure.

Yes, for a temperature change (not an absolute temperature), the size of one Kelvin degree equals the size of one Celsius degree exactly, so dT=50 K and dT=50 degrees C represent the identical temperature change. This is different from converting an absolute temperature, where you must add or subtract 273.15.

Water has an unusually high Cp of 4.186 kJ/(kg*K), which is why water resists temperature change and is used as a coolant. Air is about 1.005 kJ/(kg*K), aluminum 0.897, copper 0.385 (metals generally have low Cp), ice 2.108, and steam 1.996 kJ/(kg*K). This calculator includes all of these as presets.

Yes, since H depends on internal energy U, which has no absolute zero reference point in classical thermodynamics, only enthalpy differences (dH) are usually physically meaningful. A negative dH means heat was released (an exothermic process at constant pressure), while a positive dH means heat was absorbed (endothermic).

Chemists report reaction enthalpies (dH) to indicate whether a reaction releases heat (exothermic, dH less than 0) or absorbs heat (endothermic, dH greater than 0) at constant pressure, the standard condition for most laboratory and industrial chemistry. Enthalpy also appears directly in the Gibbs free energy formula, G=H-TS, which determines reaction spontaneity.

Steam power plant cycles like the Rankine cycle compute turbine work and boiler heat input directly from enthalpy differences between process states, since these systems operate essentially at constant pressure within each stage. Enthalpy lets engineers track energy flow through a turbine or boiler without separately accounting for flow work at every step.

Internal energy (U) is the total kinetic and potential energy of a system's particles. Enthalpy (H=U+PV) adds the flow work term PV on top of that, accounting for the energy needed to 'push' the system into existence against the ambient pressure. The two are identical only when PV is zero or constant and ignored, which is why enthalpy, not internal energy, is used for constant-pressure heat accounting.