Why does specific heat vary from molecule to molecule?
The specific heat of a molecule refers to the amount of heat required to raise the temperature of one gram of the substance by one degree Celsius. The specific heat varies from molecule to molecule due to two primary reasons: molecular mass and degrees of freedom to store energy.
For example:
- H2 has a specific heat of 14.3 J/mol°C.
- He has a specific heat of 5.19 J/mol°C.- H2O has a specific heat of 4.18 J/g°C.
Degrees of freedom [Briefly]
Molecules are always in motion in three x.y and z axes in space. The need for kinetic energy for these motions which is supplied by the surroundings. Molecules store energy in how they move. For example, a mono-atomic molecule like He has only a freedom in each axis totaling three. A di-atomic molecule is more complex than a monoatomic molecule. A diatomic molecule has five degrees of freedom. A triatomic molecule like H2O is more complex in its molecular structure. H2O has about nine degrees of freedom to store energy.
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More details
Collisions don't produce enough energy at low temperatures to propel particles out of their ground states and cause vibration or rotation.
Only translation causes an increase in average energy. The average kinetic energy of the molecules rises as the material gets hotter. There is enough energy imparted by the collisions for rotation to happen. Next, rotation increases the specific heat and adds to the internal energy. The vibrational energy states are farther apart than the rotation energy states. Vibration does not jump out of the ground state until high temperatures (1000k). It then contributes to the specific heat.
The differences in specific heat values among these molecules can be attributed to their molecular masses and the number of degrees of freedom each molecule has to store energy. H2, being a diatomic molecule with a relatively low molecular mass, has a higher specific heat compared to He, which is a monatomic molecule. Similarly, H2O, being a larger and more complex molecule, has a higher specific heat compared to both H2 and He.
Understanding the specific heat of different molecules is crucial for various scientific and engineering applications, as it can impact heat transfer and temperature changes in different materials.
Vice President - Strategic Business Development at Montana Technologies LLC
1 年Also, there is a change in specific heat as to the phase of the molecule. For example, liquid water has 4.187 kJ / kg K; water vapor has 1.996 kJ / kg K and ice has 2.108 kJ / kg K.