Ratio of the Specific Heat Capacities at Constant Pressure and Volume for Real Gases
The ratioof 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 ofis 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 ratioto decrease for large molecules.