# Clausius-Clapeyron Equation

## Clausius-Clapeyron equation

[klȯz·ē·əs kla·pā‚rōn i‚kwā·zhən]
(thermodynamics)
An equation governing phase transitions of a substance, dp/dT = Δ H /(T Δ V), in which p is the pressure, T is the temperature at which the phase transition occurs, Δ H is the change in heat content (enthalpy), and Δ V is the change in volume during the transition. Also known as Clapeyron-Clausius equation; Clapeyron equation.

## Clausius-Clapeyron Equation

a thermodynamic equation relating to the transition processes of a substance from one phase to another (such as vaporization, fusion, sublimation, polymorphic transformation). According to the Clausius-Clapeyron equation, the heat of a phase transition (for example, heat of vaporization, heat of fusion) under equilibrium conditions is defined by the equation where T is the temperature of transition (isothermal process), dp/dT is the value of the derivative of pressure with respect to temperature at a given temperature of the transition, and (V2V1) is the change in the volume of the substance during its transition from the first phase to the second.

The equation was initally derived in 1834 by B. P. E. Clapeyron from analysis of the Carnot cycle for the condensation of steam in thermal equilibrium with the liquid. R. Clausius refined the equation in 1850 and extended it to other phase transitions. The Clausius-Clapeyron equation is applicable to any phase transition accompanied by either absorption or evolution of heat (the phase transition of the first order) and is a direct result of the conditions of phase equilibrium from which it is derived.

The Clausius-Clapeyron equation may be used for calculating any of the quantities entering into the equation if the remaining quantities are known. In particular, the equation is used for calculating the heats of evaporation, which are difficult to determine experimentally.

The Clausius-Clapeyron equation may be formulated in terms of the derivatives dp/dT or dT/dp: For evaporation and sublimation processes, dp/dT expresses the change in the saturation vapor pressure p with the temperature T, whereas for the processes of fusion and polymorphic transformation, dT/dp determines the change in the transition temperature with the pressure. In other words, the Clausius-Clapeyron equation is a differential equation of the curve of phase equilibrium in terms of the variables p and T.

In order to solve the Clausius-Clapeyron equation it is necessary to know how the quantities L, V1, and V2 change with temperature and pressure, which is a difficult task. This relationship is usually determined empirically and the Clausius-Clapeyron equation is solved numerically.

The Clausius-Clapeyron equation may be applied to pure substances and to solutions and the separate components of solutions. In the latter cases, the Clausius-Clapeyron equation relates the partial pressure of unsaturated vapor of a given component to its partial heat of vaporization.

### REFERENCE

Kurs fizicheskoi khimii, 2nd ed., vol. 1. Edited by la. I. Gerasimov. Moscow, 1969.

IU. I. POLIAKOV

References in periodicals archive ?
Scientists call this the Clausius-Clapeyron equation.
upsilon]]) in the Clausius-Clapeyron equation, and the second method uses a previously published temperature-dependent linear function for [l.
The Clausius-Clapeyron equation is often used to establish the isosteric heat of sorption because it provides a rapid computational procedure.
Briefly, the method is based on the generation of known vapor concentrations, calculated from the Clausius-Clapeyron equation, from a solid at different temperatures.
Thus, for deriving a cooling-heating cycle vapor pressure equation an arithmetic average of the corresponding integrated Clausius-Clapeyron equation constants of the cool-down and heat-up steps (described as the intercept, which is proportional to vaporization entropy, and slope, which is proportional to vaporization enthalpy or heats of vaporization) was taken.
To calculate the isosteric heat (QST) a linear regression of data was run between the natural logarithm of water activity and the reciprocal of temperature at different equilibrium moisture content, and then the Clausius-Clapeyron equation (equation 8) was applied to it.
The Clausius-Clapeyron equation was used to calculate isosteric heat ( H ) of adsorption.
In contrary to the temperature dependence of diffusion described generally by Clausius-Clapeyron equation (16), the diffusion coefficients for [SO.
i] at each temperature was calculated using Clausius-Clapeyron equation.
Then by using the Clausius-Clapeyron equation, the vapour pressure corresponding to this temperature is calculated.
It consists of (i) determining water content of samples after equilibration at several relative humidities (usually 15-20 or more) and at two temperatures, (ii) drawing isotherms by a computer program which provides best fits to the experimental data, and (iii) calculating thermodynamic parameters by means of the Clausius-Clapeyron equation for heat of vaporization at different tissue water contents.

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