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,

<math display="block">\frac{\mathrm{d}P}{\mathrm{d}T} = \frac{L}{T\,\Delta v} = \frac{\Delta s}{\Delta v},</math>

where <math>\mathrm{d}P/\mathrm{d}T</math> is the slope of the tangent to the coexistence curve at any point, <math>L</math> is the molar change in enthalpy (latent heat, the amount of energy absorbed in the transformation), <math>T</math> is the temperature, <math>\Delta v </math> is the molar volume change of the phase transition, and <math>\Delta s </math> 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 <math>R</math> and liquid volume is neglected as being much smaller than vapor volume V. It is often used to calculate vapor pressure of a liquid.

<math display="block">\frac{\mathrm{d} P}{\mathrm{d} T} = \frac {P L}{T^2 R},</math>

where N is the amount of substance and n is the number density.

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

Derivations

thumb|300px|A typical [[phase diagram. 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 Gibbs–Duhem relation

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

<math display="block">\mu_\alpha = \mu_\beta.</math>

Furthermore, along the coexistence curve,

<math display="block">\mathrm{d}\mu_\alpha = \mathrm{d}\mu_\beta.</math>

One may therefore use the Gibbs–Duhem relation

<math display="block">\mathrm{d}\mu = M(-s \, \mathrm{d}T + v \, \mathrm{d}P)</math>

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

<math display="block">-(s_\beta - s_\alpha) \, \mathrm{d}T + (v_\beta - v_\alpha) \, \mathrm{d}P = 0.</math>

Rearrangement gives

<math display="block">\frac{\mathrm{d}P}{\mathrm{d}T} = \frac{s_\beta - s_\alpha}{v_\beta - v_\alpha} = \frac{\Delta s}{\Delta v},</math>

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 <math>v_\text{g}</math> greatly exceeds that of the condensed phase <math>v_\text{c}</math>. Therefore, one may approximate

<math display="block">\Delta v = v_\text{g} \left(1 - \frac{v_\text{c{v_\text{g\right) \approx v_\text{g}</math>

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

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

<math display="block">\frac{\mathrm{d} P}{\mathrm{d} T} = \frac{L}{T\,\Delta v},</math>

we can obtain the Clausius–Clapeyron equation

<math display="block">\ln \frac{P_2}{P_1} \cong -\frac{L}{R} \left( \frac{1}{T_2} - \frac {1}{T_1} \right).</math>

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),

<math display="block">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].</math>

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, <math>\mathrm{d}x</math>, and an infinitesimal difference in temperature <math>\mathrm{d}T</math>, the heat absorbed is

<math display="block">Q = L\,\mathrm{d}x,</math>

and the corresponding work is

<math display="block">W = \frac{\mathrm{d}p}{\mathrm{d}T}\,\mathrm{d}T(V - V'),</math>

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

The ratio <math>W/Q</math> is the efficiency of the Carnot engine, <math>\mathrm{d}T/T</math>. Substituting and rearranging gives

<math display="block">\frac{\mathrm{d}p}{\mathrm{d}T} = \frac{L}{T(v - v')},</math>

where lowercase <math>v - v'</math> 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

<math display="block">e_\text{s}(T) = 6.1094 \exp \left( \frac{17.625 T}{T + 243.04} \right),</math>

where <math display="inline">e_\text{s}</math> is in hPa and <math>T</math> is in degrees Celsius (whereas everywhere else on this page, <math>T</math> is an absolute temperature in units of kelvin).

The above formula is also sometimes called the Magnus or Magnus–Tetens approximation, though this attribution is historically inaccurate; see also the discussion of the accuracy of different approximating formulae for saturation vapour pressure of water.

Under typical atmospheric conditions, the denominator in the exponent <math display="inline">T + 243.04</math> depends weakly on <math display="inline">T</math>. Therefore, the August–Roche–Magnus formula implies that saturation 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 <math>{\Delta T}</math> below 0 °C. Note that water is unusual in that its change in volume upon melting is negative. We can assume

<math display="block">\Delta P = \frac{L}{T\,\Delta v} \, \Delta T,</math>

and substituting in

we obtain

<math display="block">\frac{\Delta P}{\Delta T} = -13.5~\text{MPa}/\text{K}.</math>

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 cm<sup>2</sup>). 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

<math display="block">\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}

</math>

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