Clausius–Clapeyron relation. 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 and Benoît Paul Émile Clapeyron. 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.  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."

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.  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 pressure–temperature (P–T) diagram, for any phase change the line separating the two phases is known as the coexistence curve. The Clapeyron relation gives the slope of the tangents to this curve. Mathematically,

where  is the slope of the tangent to the coexistence curve at any point,  is the molar change in enthalpy (latent heat, the amount of energy absorbed in the transformation),  is the temperature,  is the molar volume change of the phase transition, and  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 applies to vaporization of liquids where vapor follows ideal gas law using the ideal gas constant  and liquid volume is neglected as being much smaller than vapor volume V. It is often used to calculate vapor pressure of a liquid.

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

Derivations

Derivation from state postulate 
Using the state postulate, take the molar entropy  for a homogeneous substance to be a function of molar volume  and temperature .

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,

Using the appropriate Maxwell relation gives

where  is the pressure. Since pressure and temperature are constant, the derivative of pressure with respect to temperature does not change. Therefore, the partial derivative of molar entropy may be changed into a total derivative

and the total derivative of pressure with respect to temperature may be factored out when integrating from an initial phase  to a final phase , to obtain

where  and  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:

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

Given constant pressure and temperature (during a phase change), we obtain

Substituting the definition of molar latent heat  gives

Substituting this result into the pressure derivative given above (), we obtain

This result (also known as the Clapeyron equation) equates the slope  of the coexistence curve  to the function  of the molar latent heat , the temperature , and the change in molar volume . Instead of the molar values, corresponding specific values may also be used.

Derivation from Gibbs–Duhem relation 

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

Furthermore, along the coexistence curve,

One may therefore use the Gibbs–Duhem relation

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

Rearrangement gives

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  greatly exceeds that of the condensed phase . Therefore, one may approximate

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

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

we can obtain the Clausius–Clapeyron equation

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

Let  and  be any two points along the coexistence curve between two phases  and . In general,  varies between any two such points, as a function of temperature. But if  is approximated as constant,

or

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  = 8.31 J/(mol⋅K),

Clapeyron's derivation
In the original work by Clapeyron, the following argument is advanced.
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, , and an infinitesimal difference in temperature , the heat absorbed is

and the corresponding work is

where  is the difference between the volumes of  in the liquid phase and vapor phases.
The ratio  is the efficiency of the Carnot engine, . Substituting and rearranging gives

where lowercase  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 

where  are the pressures at temperatures  respectively and  is the ideal gas constant. For a liquid–gas transition,  is the molar latent heat (or molar enthalpy) of vaporization; for a solid–gas transition,  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  and  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

where

The temperature dependence of the latent heat  can be neglected in this application. The August–Roche–Magnus formula provides a solution under that approximation:

where  is in hPa, and  is in degrees Celsius (whereas everywhere else on this page,  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. 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  (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.

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  below 0 °C. Note that water is unusual in that its change in volume upon melting is negative. We can assume

and substituting in

we obtain

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) 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.

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

where subscripts 1 and 2 denote the different phases,  is the specific heat capacity at constant pressure,  is the thermal expansion coefficient, and  is the isothermal compressibility.

See also
 Van 't Hoff equation
 Antoine equation
 Lee–Kesler method

References

Bibliography

Notes

1849 in science
1850 in science
Thermodynamic equations
Atmospheric thermodynamics
Engineering thermodynamics