Why enthalpy is a state function . please explain it .
Enthalpy h is a state function because it is defined solely in terms of other state functions:
h≡u+pv
Where u, p, and v are the specific internal energy, the pressure, and the specific volume, respectively.
The more difficult question is, why are u, p and v state functions? Let’s stick with pure component systems for now, and as a first example, we’ll consider the two most commonly encountered state functions: pressure and temperature.
Imagine you have a glass of water at 10 ∘
C, and another at 30 ∘C, both of them at atmospheric pressure. You allow them to go to room temperature, which happens to be 20 ∘ C. Do you expect the density of the water in the two glasses to be the same? How about the internal energy? Or the entropy?
It would be very weird if these properties of the water depended on the history of the system. It would essentially mean doing an experiment today will have different results than doing one tomorrow. We know this isn’t true (barring obvious factors like the weather). This means that systems can be described by a set of properties which only depend on their current state, not their history. We call these state functions. Pressure and specific volume are two examples, as is internal energy, thus, by definition, enthalpy.
Enthalpy h is a state function because it is defined solely in terms of other state functions:
h≡u+pv
Where u, p, and v are the specific internal energy, the pressure, and the specific volume, respectively.
The more difficult question is, why are u, p and v state functions? Let’s stick with pure component systems for now, and as a first example, we’ll consider the two most commonly encountered state functions: pressure and temperature.
Imagine you have a glass of water at 10 ∘
C, and another at 30 ∘C, both of them at atmospheric pressure. You allow them to go to room temperature, which happens to be 20 ∘C. Do you expect the density of the water in the two glasses to be the same? How about the internal energy? Or the entropy?
It would be very weird if these properties of the water depended on the history of the system. It would essentially mean doing an experiment today will have different results than doing one tomorrow. We know this isn’t true (barring obvious factors like the weather). This means that systems can be described by a set of properties which only depend on their current state, not their history. We call these state functions. Pressure and specific volume are two examples, as is internal energy, thus, by definition, enthalpy.
