Clausius–Clapeyron relation#Meteorology and climatology

{{Short description|Relation between vapour pressure and temperature}}

{{redirect2|Clapeyron equation|Clapeyron's equation|a state equation|ideal gas law}}

The Clausius–Clapeyron relation, in chemical thermodynamics, specifies the temperature dependence of pressure, most importantly vapor pressure, at a discontinuous phase transition between two phases of matter of a single constituent. It is named after Rudolf Clausius{{cite journal |last=Clausius |first=R. |year=1850 |title=Ueber die bewegende Kraft der Wärme und die Gesetze, welche sich daraus für die Wärmelehre selbst ableiten lassen |trans-title=On the motive power of heat and the laws which can be deduced therefrom regarding the theory of heat |url=http://gallica.bnf.fr/ark:/12148/bpt6k15164w/f518.image |journal=Annalen der Physik |language=de |volume=155 |issue=4 |pages=500–524 |bibcode=1850AnP...155..500C |doi=10.1002/andp.18501550403 |hdl-access=free |hdl=2027/uc1.$b242250}} and Benoît Paul Émile Clapeyron.{{cite journal |last=Clapeyron |first=M. C. |year=1834 |title=Mémoire sur la puissance motrice de la chaleur |url=http://gallica.bnf.fr/ark:/12148/bpt6k4336791/f157 |journal={{ill|Journal de l'École polytechnique|fr}} |language=fr |volume=23 |pages=153–190 |id=ark:/12148/bpt6k4336791/f157}} However, this relation was in fact originally derived by Sadi Carnot in his Reflections on the Motive Power of Fire, which was published in 1824 but largely ignored until it was rediscovered by Clausius, Clapeyron, and Lord Kelvin decades later.{{cite web |url=https://www.feynmanlectures.caltech.edu/I_45.html |title=Illustrations of Thermodynamics |last=Feynman |first=Richard |author-link=Richard Feynman |date=1963 |website=The Feynman Lectures on Physics |publisher=California Institute of Technology |access-date=13 December 2023 |quote=This relationship was deduced by Carnot, but it is called the Clausius-Clapeyron equation.}} Kelvin said of Carnot's argument that "nothing in the whole range of Natural Philosophy is more remarkable than the establishment of general laws by such a process of reasoning."{{cite journal |last=Thomson |first=William |date=1849 |title=An Account of Carnot's Theory of the Motive Power of Heat; with Numerical Results deduced from Regnault's Experiments on Steam |journal=Transactions of the Edinburgh Royal Society |volume=16 |issue=5 |pages=541–574 |doi=10.1017/S0080456800022481 |url=https://zenodo.org/record/2133961 |author-link=Lord Kelvin}}

Kelvin and his brother James Thomson confirmed the relation experimentally in 1849–50, and it was historically important as a very early successful application of theoretical thermodynamics.{{Cite book |last=Pippard |first=Alfred B. |title=Elements of classical thermodynamics: for advanced students of physics |date=1981 |publisher=Univ. Pr |isbn=978-0-521-09101-5 |edition=Repr |location=Cambridge |pages=116}} Its relevance to meteorology and climatology is the increase of the water-holding capacity of the atmosphere by about 7% for every 1 °C (1.8 °F) rise in temperature.

Definition

=Exact Clapeyron equation=

On a pressuretemperature (PT) diagram, for any phase change the line separating the two phases is known as the coexistence curve. The Clapeyron relation{{cite web |last1=Koziol |first1=Andrea |last2=Perkins |first2=Dexter |title=Teaching Phase Equilibria |url=https://serc.carleton.edu/research_education/equilibria/clapeyronequation.html |website=serc.carleton.edu |publisher=Carleton University |access-date=1 February 2023}}{{cite web | title=Clausius-Clapeyron Equation | website=Chemistry LibreTexts | date=2014-06-01 | url=https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_%28Physical_and_Theoretical_Chemistry%29/Physical_Properties_of_Matter/States_of_Matter/Phase_Transitions/Clausius-Clapeyron_Equation | access-date=2024-10-21}} gives the slope of the tangents to this curve. Mathematically,

\frac{\mathrm{d}P}{\mathrm{d}T} = \frac{L}{T\,\Delta v} = \frac{\Delta s}{\Delta v},

where \mathrm{d}P/\mathrm{d}T is the slope of the tangent to the coexistence curve at any point, L is the molar change in enthalpy (latent heat, the amount of energy absorbed in the transformation), T is the temperature, \Delta v is the molar volume change of the phase transition, and \Delta s is the molar entropy change of the phase transition. Alternatively, the specific values may be used instead of the molar ones.

=Clausius–Clapeyron equation=

The Clausius–Clapeyron equation{{rp|p=509}} applies to vaporization of liquids where vapor follows ideal gas law using the ideal gas constant R and liquid volume is neglected as being much smaller than vapor volume V. It is often used to calculate vapor pressure of a liquid.{{cite web |last1=Clausius |last2=Clapeyron |title=The Clausius-Clapeyron Equation |url=https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php |website=Bodner Research Web |publisher=Purdue University |access-date=1 February 2023}}

\frac{\mathrm{d} P}{\mathrm{d} T} = \frac {P L}{T^2 R},

v = \frac{V}{n} = \frac{RT}{P}.

The equation expresses this in a more convenient form just in terms of the latent heat, for moderate temperatures and pressures.

Derivations

File:Phase-diag2.svg. The dotted green line gives the anomalous behavior of water. The Clausius–Clapeyron relation can be used to find the relationship between pressure and temperature along phase boundaries.]]

= Derivation from state postulate =

Using the state postulate, take the molar entropy s for a homogeneous substance to be a function of molar volume v and temperature T.{{cite book |last=Wark |first=Kenneth |title=Thermodynamics |orig-year=1966 |edition=5th |year=1988 |publisher=McGraw-Hill, Inc. |location=New York, NY |isbn=978-0-07-068286-3 |chapter=Generalized Thermodynamic Relationships}}{{rp|p=508}}

\mathrm{d} s = \left(\frac{\partial s}{\partial v}\right)_T \, \mathrm{d} v + \left(\frac{\partial s}{\partial T}\right)_v \, \mathrm{d} T.

The Clausius–Clapeyron relation describes a Phase transition in a closed system composed of two contiguous phases, condensed matter and ideal gas, of a single substance, in mutual thermodynamic equilibrium, at constant temperature and pressure. Therefore,{{rp|p=508}}

\mathrm{d} s = \left(\frac{\partial s}{\partial v}\right)_T \,\mathrm{d} v.

Using the appropriate Maxwell relation gives{{rp|p=508}}

\mathrm{d} s = \left(\frac{\partial P}{\partial T}\right)_v \,\mathrm{d} v,

where P is the pressure. Since pressure and temperature are constant, the derivative of pressure with respect to temperature does not change.{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/pvtsur.html |title=PvT Surface for a Substance which Contracts Upon Freezing |access-date=2007-10-16 |author=Carl Rod Nave |year=2006 |work=HyperPhysics |publisher=Georgia State University }}{{cite book |last=Çengel |first=Yunus A. |author2=Boles, Michael A. |title=Thermodynamics – An Engineering Approach |orig-year=1989 |edition=3rd |series=McGraw-Hill Series in Mechanical Engineering |year=1998 |publisher=McGraw-Hill |location=Boston, MA. |isbn=978-0-07-011927-7}}{{rp|pp=57, 62, 671}} Therefore, the partial derivative of molar entropy may be changed into a total derivative

\mathrm{d} s = \frac{\mathrm{d} P}{\mathrm{d} T} \, \mathrm{d} v,

and the total derivative of pressure with respect to temperature may be factored out when integrating from an initial phase \alpha to a final phase \beta,{{rp|p=508}} to obtain

\frac{\mathrm{d} P}{\mathrm{d} T} = \frac {\Delta s}{\Delta v},

where \Delta s \equiv s_\beta - s_\alpha and \Delta v \equiv v_\beta - v_\alpha are respectively the change in molar entropy and molar volume. Given that a phase change is an internally reversible process, and that our system is closed, the first law of thermodynamics holds:

\mathrm{d} u = \delta q + \delta w = T\,\mathrm{d} s - P\,\mathrm{d} v,

where u is the internal energy of the system. Given constant pressure and temperature (during a phase change) and the definition of molar enthalpy h, we obtain

\mathrm{d} h = T \,\mathrm{d} s + v \,\mathrm{d} P,

\mathrm{d} h = T\,\mathrm{d}s,

\mathrm{d}s = \frac{\mathrm{d} h}{T}.

Given constant pressure and temperature (during a phase change), we obtain{{rp|p=508}}

\Delta s = \frac {\Delta h}{T}.

Substituting the definition of molar latent heat L = \Delta h gives

\Delta s = \frac{L}{T}.

Substituting this result into the pressure derivative given above (\mathrm{d}P/\mathrm{d}T = \Delta s / \Delta v), we obtain{{rp|p=508}}{{cite web |url=http://www.chem.arizona.edu/~salzmanr/480a/480ants/clapeyro/clapeyro.html |title=Clapeyron and Clausius–Clapeyron Equations |access-date=2007-10-11 |last=Salzman |first=William R. |date=2001-08-21 |work=Chemical Thermodynamics |publisher=University of Arizona |archive-url=https://web.archive.org/web/20070607143600/http://www.chem.arizona.edu/~salzmanr/480a/480ants/clapeyro/clapeyro.html |archive-date=2007-06-07 |url-status=dead }}

\frac{\mathrm{d} P}{\mathrm{d} T} = \frac {L}{T \, \Delta v}.

This result (also known as the Clapeyron equation) equates the slope \mathrm{d}P/\mathrm{d}T of the coexistence curve P(T) to the function L/(T \, \Delta v) of the molar latent heat L, the temperature T, and the change in molar volume \Delta v. Instead of the molar values, corresponding specific values may also be used.

= Derivation from Gibbs–Duhem relation =

Suppose two phases, \alpha and \beta, are in contact and at equilibrium with each other. Their chemical potentials are related by

\mu_\alpha = \mu_\beta.

Furthermore, along the coexistence curve,

\mathrm{d}\mu_\alpha = \mathrm{d}\mu_\beta.

One may therefore use the Gibbs–Duhem relation

\mathrm{d}\mu = M(-s \, \mathrm{d}T + v \, \mathrm{d}P)

(where s is the specific entropy, v is the specific volume, and M is the molar mass) to obtain

-(s_\beta - s_\alpha) \, \mathrm{d}T + (v_\beta - v_\alpha) \, \mathrm{d}P = 0.

Rearrangement gives

\frac{\mathrm{d}P}{\mathrm{d}T} = \frac{s_\beta - s_\alpha}{v_\beta - v_\alpha} = \frac{\Delta s}{\Delta v},

from which the derivation of the Clapeyron equation continues as in the previous section.

=Ideal gas approximation at low temperatures=

When the phase transition of a substance is between a gas phase and a condensed phase (liquid or solid), and occurs at temperatures much lower than the critical temperature of that substance, the specific volume of the gas phase v_\text{g} greatly exceeds that of the condensed phase v_\text{c}. Therefore, one may approximate

\Delta v = v_\text{g} \left(1 - \frac{v_\text{c}}{v_\text{g}}\right) \approx v_\text{g}

at low temperatures. If pressure is also low, the gas may be approximated by the ideal gas law, so that

v_\text{g} = \frac{RT}{P},

where P is the pressure, R is the specific gas constant, and T is the temperature. Substituting into the Clapeyron equation

\frac{\mathrm{d} P}{\mathrm{d} T} = \frac{L}{T\,\Delta v},

we can obtain the Clausius–Clapeyron equation{{rp|p=509}}

\frac{\mathrm{d} P}{\mathrm{d} T} = \frac {P L}{T^2 R}

for low temperatures and pressures,{{rp|p=509}} where L is the specific latent heat of the substance. Instead of the specific, corresponding molar values (i.e. L in kJ/mol and {{mvar|R}} = 8.31 J/(mol⋅K)) may also be used.

Let (P_1, T_1) and (P_2, T_2) be any two points along the coexistence curve between two phases \alpha and \beta. In general, L varies between any two such points, as a function of temperature. But if L is approximated as constant,

\frac{\mathrm{d} P}{P} \cong \frac {L}{R} \frac {\mathrm{d}T}{T^2},

\int_{P_1}^{P_2} \frac{\mathrm{d}P}{P} \cong \frac{L}{R} \int_{T_1}^{T_2} \frac{\mathrm{d} T}{T^2},

\ln P\Big|_{P=P_1}^{P_2} \cong -\frac{L}{R} \cdot \left.\frac{1}{T}\right|_{T=T_1}^{T_2},

or{{rp|p=672}}{{cite book |last1=Masterton |first1=William L. |last2=Hurley |first2=Cecile N. |title=Chemistry : principles and reactions |date=2008 |publisher=Cengage Learning |isbn=9780495126713 |page=230 |edition=6th |url=https://books.google.com/books?id=teubNK-b2bsC&q=clapeyron%20equation%20boiling |access-date=3 April 2020}}

\ln \frac{P_2}{P_1} \cong -\frac{L}{R} \left( \frac{1}{T_2} - \frac {1}{T_1} \right).

These last equations are useful because they relate equilibrium or saturation vapor pressure and temperature to the latent heat of the phase change without requiring specific-volume data. For instance, for water near its normal boiling point, with a molar enthalpy of vaporization of 40.7 kJ/mol and {{mvar|R}} = 8.31 J/(mol⋅K),

P_\text{vap}(T) \cong 1~\text{bar} \cdot \exp\left[-\frac{40\,700~\text{K}}{8.31} \left( \frac{1}{T} - \frac{1}{373~\text{K}} \right) \right].

=Clapeyron's derivation=

In the original work by Clapeyron, the following argument is advanced.{{Cite journal |last=Clapeyron |first=E |date=1834 |title=Mémoire sur la puissance motrice de la chaleur |journal= Journal de l ́École Polytechnique |volume=XIV |pages=153–190}}

Clapeyron considered a Carnot process of saturated water vapor with horizontal isobars. As the pressure is a function of temperature alone, the isobars are also isotherms. If the process involves an infinitesimal amount of water, \mathrm{d}x, and an infinitesimal difference in temperature \mathrm{d}T, the heat absorbed is

Q = L\,\mathrm{d}x,

and the corresponding work is

W = \frac{\mathrm{d}p}{\mathrm{d}T}\,\mathrm{d}T(V'' - V'),

where V'' - V' is the difference between the volumes of \mathrm{d}x in the liquid phase and vapor phases.

The ratio W/Q is the efficiency of the Carnot engine, \mathrm{d}T/T.{{efn|In the original work, 1/T was simply called the Carnot function and was not known in this form. Clausius determined the form 30 years later and added his name to the eponymous Clausius–Clapeyron relation.}} Substituting and rearranging gives

\frac{\mathrm{d}p}{\mathrm{d}T} = \frac{L}{T(v'' - v')},

where lowercase v - v' denotes the change in specific volume'' during the transition.

Applications

=Chemistry and chemical engineering=

For transitions between a gas and a condensed phase with the approximations described above, the expression may be rewritten as

\ln \left( \frac{P_1}{P_0} \right) = \frac{L}{R} \left( \frac{1}{T_0}-\frac{1}{T_1} \right)

where P_0, P_1 are the pressures at temperatures T_0, T_1 respectively and R is the ideal gas constant. For a liquid–gas transition, L is the molar latent heat (or molar enthalpy) of vaporization; for a solid–gas transition, L is the molar latent heat of sublimation. If the latent heat is known, then knowledge of one point on the coexistence curve, for instance (1 bar, 373 K) for water, determines the rest of the curve. Conversely, the relationship between \ln P and 1/T is linear, and so linear regression is used to estimate the latent heat.

=Meteorology and climatology=

Atmospheric water vapor drives many important meteorologic phenomena (notably, precipitation), motivating interest in its dynamics. The Clausius–Clapeyron equation for water vapor under typical atmospheric conditions (near standard temperature and pressure) is

\frac{\mathrm{d}e_s}{\mathrm{d}T} = \frac{L_v(T) e_s}{R_v T^2},

where

{{unbulleted list | style = padding-left: 1.5em

| e_s is saturation vapor pressure,

| T is temperature,

| L_v is the specific latent heat of evaporation of water,

| R_v is the gas constant of water vapor.

}}

The temperature dependence of the latent heat L_v(T) can be neglected in this application. The {{Anchor|August–Roche–Magnus formula|August–Roche–Magnus approximation}}AugustRocheMagnus formula provides a solution under that approximation:{{Citation

|last1 = Alduchov |first1 = Oleg

|last2 = Eskridge |first2 = Robert

|date = 1997-11-01

|title = Improved Magnus' Form Approximation of Saturation Vapor Pressure

|url = http://www.osti.gov/scitech/servlets/purl/548871

|publisher = NOAA

| doi = 10.2172/548871

| doi-access = free

}} Equation 21 provides these coefficients.{{cite journal |last1=Alduchov |first1=Oleg A. |last2=Eskridge |first2=Robert E. |title=Improved Magnus Form Approximation of Saturation Vapor Pressure |journal=Journal of Applied Meteorology |date=1996 |volume=35 |issue=4 |pages=601–609 |doi=10.1175/1520-0450(1996)035<0601:IMFAOS>2.0.CO;2 |bibcode=1996JApMe..35..601A |url=https://digital.library.unt.edu/ark:/67531/metadc693874/ |doi-access=free}} Equation 25 provides these coefficients.

e_s(T) = 6.1094 \exp \left( \frac{17.625T}{T + 243.04} \right),

where e_s is in hPa, and T is in degrees Celsius (whereas everywhere else on this page, T is an absolute temperature, e.g. in kelvins).

This is also sometimes called the Magnus or Magnus–Tetens approximation, though this attribution is historically inaccurate.{{cite journal |author1-link=Mark G. Lawrence |last1=Lawrence |first1=M. G. |title=The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications |journal=Bulletin of the American Meteorological Society |date=2005 |volume=86 |issue=2 |pages=225–233 |doi=10.1175/BAMS-86-2-225 |url=http://climate.envsci.rutgers.edu/pdf/LawrenceRHdewpointBAMS.pdf |bibcode=2005BAMS...86..225L}} But see also the discussion of the accuracy of different approximating formulae for saturation vapour pressure of water.

Under typical atmospheric conditions, the denominator of the exponent depends weakly on T (for which the unit is degree Celsius). Therefore, the August–Roche–Magnus equation implies that saturation water vapor pressure changes approximately exponentially with temperature under typical atmospheric conditions, and hence the water-holding capacity of the atmosphere increases by about 7% for every 1 °C rise in temperature.IPCC, Climate Change 2007: Working Group I: The Physical Science Basis, [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-3-2.html "FAQ 3.2 How is Precipitation Changing?"]. {{Webarchive|url=https://web.archive.org/web/20181102212322/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-3-2.html |date=2018-11-02 }}.

Example

One of the uses of this equation is to determine if a phase transition will occur in a given situation. Consider the question of how much pressure is needed to melt ice at a temperature {\Delta T} below 0 °C. Note that water is unusual in that its change in volume upon melting is negative. We can assume

\Delta P = \frac{L}{T\,\Delta v} \, \Delta T,

and substituting in

{{unbulleted list | style = padding-left: 1.6em

| L = 3.34 \times 10^5~\mathrm{J/kg} (latent heat of fusion for water),

| T = 273 \, \mathrm{K} (absolute temperature in kelvins),

| \Delta v = -9.05 \times 10^{-5}~\mathrm{m^3 / kg} (change in specific volume from solid to liquid),

}}

we obtain

\frac{\Delta P}{\Delta T} = -13.5~\text{MPa}/\text{K}.

To provide a rough example of how much pressure this is, to melt ice at −7 °C (the temperature many ice skating rinks are set at) would require balancing a small car (mass ~ 1000 kg{{cite web |url=http://hypertextbook.com/facts/2000/YanaZorina.shtml |title=Mass of a Car |last=Zorina |first=Yana |year=2000 |work=The Physics Factbook}}) on a thimble (area ~ 1 cm2). This shows that ice skating cannot be simply explained by pressure-caused melting point depression, and in fact the mechanism is quite complex.{{Cite journal |last1=Liefferink |first1=Rinse W. |last2=Hsia |first2=Feng-Chun |last3=Weber |first3=Bart |last4=Bonn |first4=Daniel |date=2021-02-08 |title=Friction on Ice: How Temperature, Pressure, and Speed Control the Slipperiness of Ice |journal=Physical Review X |volume=11 |issue=1 |pages=011025 |doi=10.1103/PhysRevX.11.011025|doi-access=free |hdl=11245.1/a901a712-0a68-42ca-a80f-732e541288d2 |hdl-access=free }}

Second derivative

While the Clausius–Clapeyron relation gives the slope of the coexistence curve, it does not provide any information about its curvature or second derivative. The second derivative of the coexistence curve of phases 1 and 2 is given by{{cite journal

| last1 = Krafcik | first1 = Matthew

| last2 = Sánchez Velasco | first2 = Eduardo

| title = Beyond Clausius–Clapeyron: Determining the second derivative of a first-order phase transition line

| journal = American Journal of Physics

| volume = 82

| issue = 4

| pages = 301–305

| date = 2014

| doi = 10.1119/1.4858403|bibcode = 2014AmJPh..82..301K }}

\begin{align}

\frac{\mathrm{d}^2 P}{\mathrm{d} T^2} &= \frac{1}{v_2 - v_1} \left[\frac{c_{p2} - c_{p1}}{T} - 2(v_2\alpha_2 - v_1\alpha_1) \frac{\mathrm{d}P}{\mathrm{d}T}\right] \\

{} &+ \frac{1}{v_2 - v_1}\left[(v_2 \kappa_{T2} - v_1 \kappa_{T1}) \left(\frac{\mathrm{d}P}{\mathrm{d}T}\right)^2\right],

\end{align}

where subscripts 1 and 2 denote the different phases, c_p is the specific heat capacity at constant pressure, \alpha = (1/v)(\mathrm{d}v/\mathrm{d}T)_P is the thermal expansion coefficient, and \kappa_T = -(1/v)(\mathrm{d}v/\mathrm{d}P)_T is the isothermal compressibility.

See also

References

{{reflist}}

Bibliography

{{refbegin}}

  • {{cite book |first1=M. K. |last1=Yau |first2=R. R. |last2=Rogers |title=Short Course in Cloud Physics |publisher=Butterworth–Heinemann |edition=3rd |year=1989 |isbn=978-0-7506-3215-7 }}
  • {{cite book |first1=J. V. |last1=Iribarne |first2=W. L. |last2=Godson |chapter=4. Water-Air systems § 4.8 Clausius–Clapeyron Equation |title=Atmospheric Thermodynamics |chapter-url=https://books.google.com/books?id=UHHyCAAAQBAJ&pg=PA60 |date=2013 |publisher=Springer |isbn=978-94-010-2642-0 |pages=60–}}
  • {{cite book |first=H. B. |last=Callen |title=Thermodynamics and an Introduction to Thermostatistics |publisher=Wiley |year=1985 |isbn=978-0-471-86256-7 }}

{{refend}}

Notes