Why about 1 kJ/kg/K for so many solids?

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Discussion Overview

The discussion revolves around the observed specific heat capacity of many solid materials, which is approximately 1 kJ/kg/K. Participants explore the underlying reasons for this phenomenon, seeking explanations that are comparable to those for perfect gases.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that many solid components have a specific heat of about 1 kJ/kg/K and seeks a simple explanation.
  • Another participant suggests that the explanation involves mode-counting, similar to perfect gases, where the vibrational modes of atoms contribute to the specific heat.
  • A different participant discusses specific examples of solids, indicating that heavier molecules may allow for more vibrational modes, thus affecting the specific heat capacity.
  • One participant references the Law of Dulong and Petit, stating that most solids have a specific heat of about 25 J/mol/K, linking it to the previous explanations regarding vibrational modes.

Areas of Agreement / Disagreement

Participants express various viewpoints on the reasons behind the specific heat capacity of solids, with some agreeing on the mode-counting explanation while others introduce additional factors related to molecular weight and structure. No consensus is reached on a singular explanation.

Contextual Notes

Some assumptions regarding the nature of molecular rigidity and the specific contributions of different atoms to specific heat are not fully explored, leaving room for further discussion.

lalbatros
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Most solid components I need to consider at work have a specific heat of about 1 kJ/kg/K.
Why is that so?
Any explanation as simple as for perfect gases?

Thanks
 
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Yes, it's mode-counting, similar to perfect gasses. Counting the vibrational modes of each atom in the lattice it's 3/2 kT for the kinetic modes and another 3/2 kT for the potential energies.
 
Thanks conway.

One of the solids in my long list is 3(CaO).SiO2 and another is 3(CaO)(MgO)2(SiO2).
This would mean that "heavier" molecules are not very rigid, therefore making room for more modes and more specific heat per molecule. The end result around 1 kJ/kg/K depends also on the atoms involved.
 
This makes more sense when you express it as 25 J/mol/K. Most solids have that as their specific heat. This is called the Law of Dulong and Petit. It works for exactly the reason Conway said.
 

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