# Sackur–Tetrode equation

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The Sackur–Tetrode equation is an expression for the entropy of a monatomic classical ideal gas which incorporates quantum considerations which give a more detailed description of its regime of validity.

The Sackur–Tetrode equation is named for Hugo Martin Tetrode[1] (1895–1931) and Otto Sackur[2] (1880–1914), who developed it independently as a solution of Boltzmann's gas statistics and entropy equations, at about the same time in 1912.

## Formula

The Sackur–Tetrode equation is written:

$S = k N \left(\ln \left[ \frac VN \left(\frac{4\pi m}{3h^2}\frac UN\right)^{3/2}\right]+ {\frac 52}\right)$

where V is the volume of the gas, N is the number of particles in the gas, U is the internal energy of the gas, k is Boltzmann's constant, m is the mass of a gas particle, h is Planck's constant and ln() is the natural logarithm. See Gibbs paradox for a derivation of the Sackur–Tetrode equation. See also the ideal gas article for the constraints placed upon the entropy of an ideal gas by thermodynamics alone.

The Sackur–Tetrode equation can also be conveniently expressed in terms of the thermal wavelength $\Lambda$.

$\frac{S}{kN} = \ln\left[\frac{V}{N\Lambda^3}\right]+\frac{5}{2}.$

Note that the assumption was made that the gas is in the classical regime, and is described by Maxwell–Boltzmann statistics (with "correct Boltzmann counting"). From the definition of the thermal wavelength, this means the Sackur–Tetrode equation is only valid for

$\frac{V}{N\Lambda^3}\gg 1$

and in fact, the entropy predicted by the Sackur–Tetrode equation approaches negative infinity as the temperature approaches zero.

## Sackur–Tetrode constant

The Sackur–Tetrode constant, written S0/R, is equal to S/kN evaluated at a temperature of T = 1 kelvin, at standard pressure (100 kPa or 101.325 kPa, to be specified), for one mole of an ideal gas composed of particles of mass equal to one atomic mass unit (mu = 1.660 538 782(83)×10−27 kg). Its 2010 CODATA recommended value is:[3]

S0/R = −1.151 7078(23) for po = 100 kPa
S0/R = −1.164 8708(23) for po = 101.325 kPa.

## References

1. ^ H. Tetrode (1912) "Die chemische Konstante der Gase und das elementare Wirkungsquantum" (The chemical constant of gases and the elementary quantum of action), Annalen der Physik 38: 434 - 442. See also: H. Tetrode (1912) "Berichtigung zu meiner Arbeit: "Die chemische Konstante der Gase und das elementare Wirkungsquantum" " (Correction to my work: "The chemical constant of gases and the elementary quantum of action"), Annalen der Physik 39: 255 - 256.
2. ^ Sackur published his findings in the following series of papers:
1. O. Sackur (1911) "Die Anwendung der kinetischen Theorie der Gase auf chemische Probleme" (The application of the kinetic theory of gases to chemical problems), Annalen der Physik, 36: 958 - 980.
2. O. Sackur, "Die Bedeutung des elementaren Wirkungsquantums für die Gastheorie und die Berechnung der chemischen Konstanten" (The significance of the elementary quantum of action to gas theory and the calculation of the chemical constant), Festschrift W. Nernst zu seinem 25jährigen Doktorjubiläum gewidmet von seinen Schülern (Halle an der Salle, Germany: Wilhelm Knapp, 1912), pages 405 - 423.
3. O. Sackur (1913) "Die universelle Bedeutung des sog. elementaren Wirkungsquantums" (The universal significance of the so-called elementary quantum of action), Annalen der Physik 40: 67 - 86.
3. ^ CODATA2010