Enthalpy changes of vaporisation

Differences in the size of the van der Waals forces can be used to explain the trend in the enthalpy change of vaporisation and boiling points of the noble gases. Figure 4.35 shows how these vary with the number of electrons present. (The enthalpy change of vaporisation is the energy required to convert a mole of liquid into a mole of gas.)... 

Figure 4.35 a Enthalpy changes of vaporisation and b boiling points of the noble gases plotted against the number of electrons present. 

Water has a much higher enthalpy change of vaporisation and boiling point than expected. 

This is due to its extensive hydrogen bonding. Figure 4.44 shows the enthalpy changes of vaporisation of water and other Group 16 hydrides. 

Figure 4.44 Enthalpy changes of vaporisation for Group 16 hydrides plotted against number of electrons present.

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

This change from the liquid state to the gas state is called vaporisation. The energy required to change one mole of liquid to one mole of gas is called the enthalpy change of vaporisation. 

Figure 10. The enthalpy change for water vapor sorption determined as a function of RH using a RH perfusion device coupled with an isothermal microcalorimeter. Assuming a second order BET model, the enthalpy of moisture sorption was found to be 53.33 kJ mol" of water from 0 to 40% RH and 43.77 kJ mol" of water from 40 to 80% RH (close to the enthalpy change of vaporisation). The available surface area changed from 0.68 mg m to 0.72 mg m". The data suggest binding to the drug surface up to 40% RH then formation of multiple hydrate spheres above 40% RH.

Enthalpy of vaporisation The enthalpy change that takes place when one mole of liquid is changed to one mole of vapour at the same temperature. 

When a liquid the enthalpy change taking place is called the enthalpy of vaporisation, AvapH°. Thus the corresponding change is ... 

Enthalpy of Vaporisation It is the enthalpy change that accompanies the change of 1 mole of a liquid into vapour state completely at its boiling point, e.g.,... 

Entropy of Vaporisation (DSvap) The vaporisation of a substance occurs at a definite temp, (i.e., its b.p) and is accompanied by absorption of heat (enthalpy of vaporisation). The change in entropy known as entropy of vaporisation is given by... 

The Clapeyron equation gives the rate of change of pressure p with absolute temperature T during the phase changes fusion and vaporisation. If the phase change is accompanied by a change in enthalpy A// and a change in volume AT, then... 

Enthalpy changes accompany such processes as the dissolution of a solute, the formation of micelles, chemical reaction, adsorption onto solids, vaporisation of a solvent, hydration of a solute, neutralisation of acids and bases, and the melting or freezing of solutes. 

The enthalpy data of [62MCD/SIN] above 1036 K has not been used in the present review since as indicated by the authors the higher temperature data is effected by melting of the solid Zrp4 as well as the enthalpy change associated with the heat of vaporisation. McDonald et al. [62MCD/S1N] attempted to correct for these effects but the data has not been used since there is additional uncertainty in using the corrected data. 

If the liquid enthalpy change is very small from plate to plate, and if the molar enthalpy of vaporisation of components is close, then we may safely assume that ... 

At the critical point the difference between liquid and vapour volumes approaches zero, suggesting that the energy needed for phase transition, in other words the enthalpy of vaporisation, should diminish to zero. The above observation illustrates in a simple manner the link between the PVT behaviour of a fluid and the energy implied in its physical changes. 

In the above example it was not necessary to know how the specific heats varied with temperature, at what temperature the phase change occurred and the latent heat of vaporisation at that temperature. By working in enthalpies it was sufficient to know the enthalpy of the feed stream and the enthalpy of the product stream. Thus, whenever possible, engineers extract enthalpy data from reference sources or computer databases in preference to specific heat capacity and latent heat data. 

Pressure (bar) Saturation temperature (°Q Specific volume of vapour (m kg ) Specific enthalpy of liquid (kJ kg- ) Enthalpy due to change of phase latent heat of vaporisation) kJ kg ) Specific enthalpy vapour kJ kg- )... 

The RH perfusion device lends itself to such studies and the performance of the Thermometric RH perfusion device has been found to be 1% RH from 0.5 to 95% RH (see Figure 6). The removal of the hydrate water has a corresponding enthalpy change that is proportional to the hydration state of the molecule. For this calculation, the enthalpy of vaporisation for water of 2.44kJ g" [30] can be used. 

Feed enthalpy feedforward is, rather fortunately, not often of great benefit. Changes in enthalpy are usually small compared to reboiler duty. If feedforward is justified then measuring enthalpy may present a problem. Provided the feed is below its bubble point or above its dew point then, provided its composition and pressure are reasonably constant, it is sufficient to use temperature as the DV. However if the feed is partially vaporised a measurement of wetness is not possible. Since change in wemess represents a large change in enthalpy it is probably not realistic to make any assumption about its value. 

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