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Unformatted text preview: r text.) The
graph in section 3-2 of your text displays graphical information about the constant volume molar
heat capacity of some pure substances in the gas phase by showing plots of cV/R as a function of
Temperature. In this FNT we will make sense of some of those plots: particularly the monatomic
gases, hydrogen gas (H2) and nitrogen gas (N2). Remember that cV/R = (number of modes per
particle/2). (See Activity 3.4.1, “The Microscopic Basis of Heat Capacity”)
Interesting info on modes: NOTE: These are interesting facts about modes in polyatomic molecules, but
ARE NOT fundamental to understanding the particle model of thermal energy.
• Polyatomic molecules in the gaseous state almost always have rotational kinetic energy (KErot) modes,
i.e., the rotational modes do not usually “freeze out” in a gas. Linear molecules have two KErot modes,
and non-linear molecules have three KErot modes.
• In general, the two vibrational modes of a diatomic gas molecule are frozen out at room temperature.
• A mode gradually begins to “freeze out” when the average energy per mode (1/2kBT) begins to get as
small as the energy gap between the ground state and the first excited state for that mode.
Use the facts stated above plus the predictions of the particle model of matter for specific values of heat
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