Using simple adiabatic expansion, the ratio of the constant pressure to constant volume heat capacities of gases will be measured. This property will be correlated to the internal motion and molecular structure of the gases studied. Several gases will be compared and the contributions of the the different types internal molecular motions to the total thermal energy of the gas molecules will be gleaned.
Theory
For a perfect (ideal) gas, C_{p} = C_{V} + R , where C_{p} and C_{V} are the molar heat capacities at constant pressure and volume, respectively (see on-line lecture notes for a derivation of this and related formulae). For an arbitrary real gas a slightly more complicated relationship between these heat capacities may be derived from the equation of state. Essentially, however, the difference between heating a gas at constant volume and constant pressure is expansion work. Thus, the ratio C_{p} / C_{V} is related to the capacity of the system to do work upon expansion. This ratio is usually given the symbol [lower case greek gamma].
Properties of C_{V}
The heat capacity of a molecule is clearly related to the way that the molecule can
accomidate energy at a given temperature. The energy of the molecule is
partitioned among the types of motion the molecule can exhibit. It is important in
thermodynamics to count and categorize such molecular motion in a systematic way.
The number of degrees of freedom (DOF) for a given molecule is the number of
independent coordinates needed to specify all its nuclear positions (what do I ignore in this statement?).
A molecule of n atoms therefore has 3n DOF. These could be assigned to the
coordinates of the individual n atoms, or alternatively they can be classified
as follows:
From these considerations it is clear that for a monotonic gas (like He) no vibrational or rotational energy terms exist, but, like all gaseous molecules, energy may be contained in translational motion. Thus the molar energy of a monoatomic gas is simply 3RT/2. The constant volume heat capacity of a monotonic gas is therefore
eq.1
For diatomic or polyatomic molecules,
eq.2
Contributions from electronic states to the total internal energy have been neglected under the assumption that at room temperature electronic transitions out of the ground state are unlikely. On the other hand, the population of excited vibrational states depends strongly on temperature and thus the various vibrational modes can be at least partially active. In general, if a vibration involves a heavy atom or possesses a smaller force constant, then the normal mode will be more active and make a greater individual contribution to the heat capacity. For example, the frequencies of bending modes tend to be much lower than those of stretching modes. Since in the case of most diatomics there are only stretching modes, the vibrational contribution to C_{V} will be very small. Indeed, N_{2} would have its equipartition value for C_{V} only above about 4000 K. In contrast many polyatomic molecules, especially those containing heavy atoms,will at room temperature have significant partial vibrational contributions to the heat capacity.
At ordinary temperatures many of the excited state rotational levels are thermally accessible and hence the rotational contribution is in accord with the equipartition theorem of classical mechanics.
Thus, it is possible to calculate definite values for C_{V} and hence, through the ideal gas equation of state, the ratio for ideal monatomic and polyatomic gases using the above expressions. Statistical thermodynamics provides even better results, for which the partially 'frozen' vibrational contribution to the heat capacity may be evaluated exactly (you guessed it, I have some lecture notes on this, too).
Thermodynamics
A two step process can be used to experimentally determine the ratio of the heat capacities. These are:
I. An adiabatic reversible (isentropic) expansion from
the initial pressure p_{1},
to the intermediate pressure, p_{2}
eq.3
II. Restoration of the temperature to its initial value T_{1}, at constant
volume.
eq.4
Recall that for any adiabatic process,
eq.5
The First Law states:
eq.6
So for the first step of the experiment
eq.7
At constant volume the heat capacity relates
the change in temperature to the change in internal energy,
eq.8
Equating our two expressions for dU,
eq.9
Inserting the ideal gas law and integrating each side,
eq.10
Equation (9) predicts the decrease in temperature accompanying the reversible
adiabatic expansion of a perfect gas. This expression can be cast in terms of
the initial and final pressures and the ratio of the heat capacities by noting
that for an ideal gas,
eq.11
yielding,
eq.12
or after rearranging and using C_{p} = C_{V} + R
eq.13
For step II, the temperature is restored to T_{1}, and
eq.14
Thus,
eq.15
and after a final rearrangement, we have the desired expression for the ratio
of the molar heat capacities:
eq.16
This simple and classical method is a slightly modified version of that attributed to due to Clement and Desormes [ref to be found] that has even older origins [1] and uses the apparatus shown in the schematic below. A gas is initially contained in the sample vessel at a pressure (p_{1}) slightly higher than atmospheric pressure (p_{2}). The adiabatic reversible expansion (step I) is carried out by quickly removing and replacing the stopper in the top of the sample container. Step II occurs by simply waiting for the gas remaining in the carboy to return to its initial temperature (T_{1}) and final pressure (p_{3}) by heat transfer from the surroundings. The initial pressure p_{1}, and the final pressure p_{3} are read using a capacitance manometer pressure gauge.
The thermodynamic expressions used to derive the heat capacity ratio apply only
to the part of the gas that remains in the carboy after the stopper is replaced
since molar volumes and molar heat capacities were used. Physically, the
reversibility of the expansion can be justifed as follows: One can imagine an
invisible surface separating the gas that remains within the carboy and the gas
that escapes when the stopper is removed. The gas below this surface expands
in an approximately reversible way against the surface. Work is done as the
upper gas is pushed out. The change is approximately adiabatic only because it
is rapid copared to heat flow but slow on the scale of the mechanical work.
No appreciable amount of heat is transferred between the reservoir
and the gas in the carboy during the short period of the expansion, but after the
stopper is replaced, the temperature is seen to increase as thermal equilibrium is
reestablished for the gas remaining in the vessel.
Experimental
The lab is equipped with several apparati as shown above, each filled with a different sample gas. Student teams shall rotate through the stations so that each team may obtain data on all the sample gases. For each gas, the following procedure is suggested:
The choice of sample gases varies with availability but usually includes He, Ar, CO_{2}, N_{2}, butane, etc.
Other useful references
D. Shoemaker, C. Garland, and J. Nibler, 'Experiments in
Physical Chemistry", McGraw-Hill, New York
P. Atkins, "Physical Chemistry", 5th ed., W. H. Freeman, New York (1994)