Thermodynamics: Cp Variation with Temperature Limit

[asdf1]

Why does variation of Cp with temperature tends to a limit of ~3R?

 

[Astronuc]

Read these -

Sp. heat of gases - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/shegas.html - especially the last plate

Equipartition of energy - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/eqpar.html

Solids - http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/dulong.html

 

[asdf1]

thank you very much!