# Why do we need enthalpy?

In thermodynamics, the enthalpy of a system is defined as the sum of the internal energy of the system and the product of its pressure and volume. Since it is just a combination of other state properties of the system, why need we define it at all?

We define everything in physics because it's useful. In this case it's useful when a fluid flows in a steady state system and you want to look at energy flows. If a fluid flows through a box, and has a change in specific enthalpy $$\Delta h$$, with a flow rate of $$\dot m$$, then the power transferred to the fluid is $$\dot m \, \Delta h$$

Sometimes all you know is the energy change, which can tell you the change in enthalpy, but without more information, you don't know how much is heat and how much is pressure. So by using enthalpy you can simplify the problem.

We don't need it.

Some very good examples of applications of the concept (flow through pipes; obtaining a generalized Bernoulli equation; etc.) can be found in the answers to the related question What exactly is enthalpy?, which illustrate why we use it.

But the real answer is that we don't need enthalpy. It is, however, convenient. It's the same with other definitions in physics, e.g., linear momentum in mechanics: we can live without it, but it's easier to talk about the conservation of momentum instead of the conservation of the product of mass and velocity; the same way it's easier to refer to the velocity instead of the time derivative of the position, and so on.

To be precise, in the context of axiomatization of theories, one can say that (e-print)

every deﬁnable concept is eliminable, in the sense that the deﬁnendum can be replaced by the deﬁniens,

so it's really only a matter a convenience.