In thermodynamics we use the term enthalpy quite a bit. What we want to discuss in this
screencast is exactly what is enthalpy? Why would we use it? What are a few of its properties?
So the simplest answer is enthalpy is just the sum of the internal energy and the product
of the pressure times the volume. We could stop there, because that is really all
enthalpy is. It is a convenient term, because it shows up in calculations. So we define
a new term, but let's identify each of these terms. So I have just written out in words,
what each of these terms represent, and I have also indicated their units. So if we
look at the pressure times volume units we have Newtons per square meter,
and we have cubic meters per mole. So that's Newtons, meters, per mole.
So this is just the units for PV. A Newton meter is a joule. So it is joules per mole, and that
is the unit for pressure times volume. So we are adding energy units to energy units.
So an important property of enthalpy is that it is a state function. It's a sum of state functions; internal energy,
pressure, and volume. The state function means if we are going from point A to point B. It
does not matter what pathway we pick we can calculate delta H if we know the enthalpy at
the final point, and we know the enthalpy at the initial point. If we were to look at
see how enthalpy might arise in for example applying the first law. Let's look at the first
law for closed system. So no mass is being added or removed, and we will look at a reversible
system, and if it is reversible I can write dW as the pressure of my system times dV.
So I am just going to rearrange this slightly. PdV equals dQ. So now let's put an additional
restriction on this system, namely constant pressure. Constant pressure means I can
take this pressure term and put inside the differential since it is not changing. So
I have dU plus d(PV) equals dQ. But U plus PV is H, so dH equals dQ. Remember restriction
is that this was constant pressure. So on a constant pressure system the heat transfer
is equal to dH for a closed system. So this might be one reason why we'd want to use enthalpy
in calculations. Let's look at some other properties. Let's look at an ideal gas, so again I will look
at my definition in differential form dU plus d(PV) and now I am going to take advantage
of the ideal gas law PV=RT. Remember this is volume per mole. In addition to the ideal
gas law I am going to write, so dH now, for dU, for an ideal gas, dU is CvdT
in the ideal gas, and the d(PV) term is d(RT), and of course R is a constant. So I can write
it as RdT. So Cv plus R times dT, that is dH, but dH is also CpdT, that means Cp for an
ideal gas is Cv plus R. Heat capacity constant pressure is the heat capacity constant volume
plus the gas constant. Important to remember for an ideal gas that enthalpy and internal
energy only depend on temperature. They don't depend on pressure. The other instance
where enthalpy shows up is that if we have a flow system and so imagine we are flowing some
packet of gas or liquid. It has some internal energy coming in, and now to push this gas
through the system we do work on it. We do (PV) in is energy it takes to move it through. So we
could look at the total energy and of course similar terms for leaving the system and so
we can look at energy then in terms of the enthalpy in and then, at the outlet, the enthalpy out. So again
enthalpy is a convenient term, but remember it is convenient term, and enthalpy is just
U plus PV.