Vaporization Of Evaporation Essay

Deductions from Figure 5.1: If temperature, T and pressure, P corresponds to a point on the vapor-liquid (VL) equilibrium curve for a substance, P is the vapor pressure of the substance at temperature, T and T is the boiling point of the substance at pressure, P. The boiling point of a substance at P=1 atm is the normal boiling point of the substance. If (T, P) falls on the solid-vapor equilibrium curve (AB), then P is the vapor pressure of the solid at temperature, T, and T is the Sublimation Point at pressure, P. If (T, P) falls in the solid-liquid equilibrium curve (BC), then T is the Melting Point or Freezing Point at pressure, P. The point (T, P) at which solid, liquid and vapor phases can all coexist is called the Triple Point of

There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon. Evaporation

Evaporation is a phase transition from the liquid phase to vapor that occurs at temperatures below the boiling temperature at a given pressure. Evaporation occurs on the surface. Evaporation only occurs when the partial pressure of vapor of a substance is less than the equilibrium vapor pressure. Boiling

Boiling is also a phase transition from the liquid phase to gas phase, but boiling is the formation of vapor as bubbles of vapor below the surface of the liquid. Boiling occurs when the equilibrium vapor pressure of the substance is greater than or equal to the environmental pressure. The temperature at which boiling occurs is the boiling temperature, or boiling point. The boiling point varies with the pressure of the environment.

5.2.3c

A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer. Whereas, the commonly used empirical relations for estimating vapor pressures are:

(i) The Clausius-Clapeyron Equation.

(ii) Cox Charts.

(iii) Antoine Equation.

5.2.4a The Clausius-Clapeyron equation

This equation can be applied to estimate the vapor pressure at any temperature and to estimate the heat of phase transition from the vapor pressures measured at two temperatures. Experiments showed that the vapor pressure p*, enthalpy of vaporization, Hvap, and temperature T are related by:

p^*=A exp⁡〖(-(∆H_vap)/RT)〗 (5.23)

where R = 8.3145 J mol-1 K-1 and A are the gas constant and unknown constant. This is known as the Clausius- Clapeyron equation. If p_1^* and p_2^* are the vapor pressures at two temperatures T1 and T2, the equation has the form:

In((p_1^*)/(p_2^* ))=-〖∆H〗_vap/R (1/T_2 -1/T_1 ) (5.24)

The Clausius-Clapeyron equation allows us to estimate the vapor pressure at another temperature, if the vapor pressure is known at some temperature, and if the enthalpy of vaporization is known. If the data of p* versus T is available, then ln p* versus 1/T can be plotted to determine Hvap/R and A graphically or using the least square