The ratio
of the specific heat capacity at constant pressure,
to that at constant volume,
is found experimentally by some method and the results shown below.
|
Gas |
Temp./°C |
γ |
|
Monatomic gases |
|
|
|
Helium |
0 |
1.63 |
|
Argon |
0 |
1.67 |
|
Neon |
19 |
1.64 |
|
Krypton |
19 |
1.69 |
|
Xenon |
19 |
1.67 |
|
Mercury vapour |
310 |
1.67 |
|
|
|
|
|
Diatomic gases |
|
|
|
Air (dry) |
−79.3 |
1.41 |
|
Air (dry) |
0–17 |
1.401/2 |
|
Air (dry) |
500 |
1.36 |
|
Air (dry) |
900 |
1.32 |
|
Hydrogen |
4–17 |
1.407/8 |
|
Nitrogen |
20 |
1.4 |
|
Oxygen |
5–14 |
1.4 |
|
Carbon monoxide |
1 800 |
1.3 |
|
Nitric oxide |
— |
1.39 |
|
|
|
|
|
Triatomic gases |
|
|
|
Ozone |
— |
1.29† |
|
Water vapour |
100 |
1.33 |
|
Carbon dioxide |
4–11 |
1.3 |
|
Carbon dioxide |
300 |
1.22 |
|
Carbon dioxide |
500 |
1.2 |
|
Ammonia, NH-3 |
50 |
1.31 |
|
Nitrous oxide, N-2 O |
— |
1.32 |
|
Nitrogen peroxide N-2 O-4 |
20 |
1.17 |
|
Sulphur dioxide S0-2 |
16–34 |
1.26 |
|
|
|
|
|
Polyatomic gases |
|
|
|
Methane, CH4 |
20 |
1.31 |
|
Ethane, C2H6 |
20 |
1.2 |
|
Propane, C3H8 |
20 |
1.14 |
|
Acetylene, C2H2 |
20 |
1.24 |
|
Ethylene, C2H4 |
20 |
1.25 |
|
Benzene C6H6 |
20 |
1.4 |
|
Benzene C6H6 |
99.7 |
1.11 |
|
Chloroform CHCl3 |
24–42 |
1.11 |
|
CCl4 |
— |
1.13 |
|
Methyl alcohol |
99.7 |
1.26 |
|
Methyl bromide |
— |
1.27 |
|
Methyl chloride |
19–30 |
1.28 |
|
Methyl iodide |
— |
1.29 |
|
Ethyl alcohol |
53 |
1.13 |
|
Ethyl alcohol |
99.8 |
1.13 |
|
Ethyl bromide |
— |
1.19 |
|
Ethyl chloride |
22.7 |
1.19 |
|
Ethyl ether |
12–20 |
1.02 |
|
Ethyl ether |
99.7 |
1.11 |
|
Acetic acid |
136.5 |
1.15 |
The value of
is a result of many factors. Theory says that the energy is distributed over all the energy states equally – vibrational, translational, rotational, and for large molecules there may be many vibrational energy states. Since
(this equation only applies to ideal gases) we expect the ratio
to decrease for large molecules.