INSTRUCTOR: We've talked about how both the enthalpy
and the internal energy change as the temperature changes.
In this screencast,
we'll discuss how a change of pressure
in an isothermal system affects the energy of a system.
It has been shown that the internal energy,
as well as the specific volume of a solid
or liquid is approximately equal to zero.
So it's pretty much independent of pressure.
So what we can do now in terms of the enthalpy,
we can write the Delta U, which is approximately zero.
Therefore, Delta H,
which is equal to Delta U plus Delta PV hat,
which is the specific volume.
And so that's equal to approximately the Delta P.
This seems like a simple expression, and it is.
However, it's the units that generally trip people up.
For our enthalpy,
we want the units to be in kilojoules per mole.
However, our numbers are generally given in different units,
like atmospheres and centimeter, cube per gram, et cetera.
So I'll show you how to convert them.
So we want our V Delta P to be in units
of kilojoules per mole.
So if we write this out,
our specific volume is equal to one over the density.
So that's centimeters cubed per gram,
and then one divided by the specific gravity.
And then we have our Delta P
and that's gonna be in atmosphere.
And now let's start doing some conversions.
One liter is equal to ten to the third centimeters cube,
and now we're gonna do a ratio of gas constant.
And this is one of the best ways
to go from our liter atmospheres to our joules.
So we multiply this by 8.314 joules per mole kelvin,
divided by 0.08206 liter atmospheres per mole kelvin.
And then we multiply that by the molecular weight,
and that's gonna be in grams per mole.
And then one kilojoule is gonna be equal to 1000 joules.
So let's do some canceling.
We know that centimeters cubed and centimeters cubed,
and we have grams and grams,
and we have atmospheres liters, liters atmosphere,
and we have joules, joules, kelvin, kelvin, mole, mole,
and I think that's everything.
So this is gonna be equal, in kilojoules per mole to 1.013,
times 10 to the minus fourth,
our molecular weight,
times our Delta P divided by our specific gravity.
So for ideal gases,
in order for any energy change due to a change in pressure
under isothermal conditions,
the temperature has to be very much less than zero degrees C
and or the pressure has to be very much greater
than one atmosphere.
So generally you can discount any energy
due to a change in pressure under isothermal conditions.
So if we summarize it,
we'll start with solids and liquids.
Here, our Delta U is approximately equal to zero
and our Delta H is approximately equal
to our specific volume, times our Delta P.
If you have an ideal gas,
as I showed you before,
both our Delta U and our Delta H
are approximately equal to zero.