An Isobaric Process is a thermodynamic process, in which the pressure of the system remains constant (P = Constant). The heat transfer into or out of the system does work, but also changes the internal energy of the system.
Since there are changes in internal energy (dU) and changes in system volume (ΔV) engineers often use the enthalpy of the system, which is defined as:
H = U + PV
In many thermodynamic analyses it is convenient to use the enthalpy instead of the internal energy. Especially in case of the first law of thermodynamics.
The enthalpy is the preferred expression of system energy that changes in many chemical, biological, and physical measurements at constant pressure. It is so useful that it is tabulated in the steam tables along with specific volume and specific internal energy. It is due to the fact, it simplifies the description of energy transfer. At constant pressure, the enthalpy change equals the energy transferred from the environment through heating (Q = H₂ – H₁) or work other than expansion work. For a variable-pressure process, the difference in enthalpy is not quite as obvious.
There are expressions in terms of more familiar variables such as temperature and pressure:
dH = Cp dT + V (1 – αT) dp
Where Cp = Heat capacity at constant pressure.
α = coefficient of (cubic) thermal expansion.
For ideal gas αT = 1 and therefore:
dH = Cp dT.