A Molar heat capacity of CO2 is too high. Why?

Philip Koeck
Gold Member
Messages
801
Reaction score
229
At very high temperatures CO2 should have Cp = 15/2 R, since there are 3 translational, 2 rotational and 4 vibrational degrees of freedom.
Experimental values are a bit higher than that, at least according to a figure I found on the internet.
Is that correct? And what is the explanation?
A student suggested that when the molecule vibrates in the bending mode it gets an additional rotational degree of freedom, since it is bent most of the time.
Is that a sensible explanation?
 
Physics news on Phys.org
Philip Koeck said:
Experimental values are a bit higher than that, at least according to a figure I found on the internet.
Is that correct?
See
https://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Units=SI&Mask=1&Type=JANAFG&Plot=on#JANAFG

At 3000 K, it is pretty much 15/2 R. It goes to higher values at higher temperatures.

Philip Koeck said:
A student suggested that when the molecule vibrates in the bending mode it gets an additional rotational degree of freedom, since it is bent most of the time.
Is that a sensible explanation?
A molecule cannot gain degrees of freedom. A linear triatomic has 2 rotational degrees of freedom, but gains one has an additional vibrational d.o.f. because bending is degenerate. If the molecule is bent, then you would loose a vibrational d.o.f. and gain a rotational one, which would actually reduce Cp, since the vibrational mode contributes 2 quadratic d.o.f. to the 1 rotational d.o.f. (see the Cp values for water).

Treating vibrations has harmonic oscillators is an approximation. The anharmonic character increases as vibrational excitation goes up, hence the approximation ##C_p = (f/2 + 1) R## is less good at higher T.

[Edit: Changed the language a bit to make it more consistent.]
 
Last edited:
  • Like
Likes Spinnor, Twigg and Philip Koeck
Not an expert in QM. AFAIK, Schrödinger's equation is quite different from the classical wave equation. The former is an equation for the dynamics of the state of a (quantum?) system, the latter is an equation for the dynamics of a (classical) degree of freedom. As a matter of fact, Schrödinger's equation is first order in time derivatives, while the classical wave equation is second order. But, AFAIK, Schrödinger's equation is a wave equation; only its interpretation makes it non-classical...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. Towards the end of the first lecture for the Qiskit Global Summer School 2025, Foundations of Quantum Mechanics, Olivia Lanes (Global Lead, Content and Education IBM) stated... Source: https://www.physicsforums.com/insights/quantum-entanglement-is-a-kinematic-fact-not-a-dynamical-effect/ by @RUTA
Is it possible, and fruitful, to use certain conceptual and technical tools from effective field theory (coarse-graining/integrating-out, power-counting, matching, RG) to think about the relationship between the fundamental (quantum) and the emergent (classical), both to account for the quasi-autonomy of the classical level and to quantify residual quantum corrections? By “emergent,” I mean the following: after integrating out fast/irrelevant quantum degrees of freedom (high-energy modes...
Back
Top