Boiling enthalpy

Hydride formulae Normal E-H bond DCf and names boiling enthalpy (kJmoL1) point (°C) (kJ mol -1) at 298 K)... 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

Enthalpy of Vaporization (or Sublimation) When the pressure of the vapor in equilibrium with a liquid reaches 1 atm, the liquid boils and is completely converted to vapor on absorption of the enthalpy of vaporization ISHv at the normal boiling point T. A rough empirical relationship between the normal boiling point and the enthalpy of vaporization (Trouton s rule) is ... 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

Correlations for Enthalpy of Vaporization. Enthalpy or heat of vaporization, which is an important engineering parameter for Hquids, can be predicted by a variety of methods which focus on either prediction of the heat of vaporization at the normal boiling point, or estimation of the heat of vaporization at any temperature from a known value at a reference temperature (5). 

Enthalpies of vaporization, at the normal boiling point, T, can be estimated roughly with the simple rule of Pictet and Trouton (19) which... 

The heat requirements in batch evaporation are the same as those in continuous evaporation except that the temperature (and sometimes pressure) of the vapor changes during the course of the cycle. Since the enthalpy of water vapor changes but little relative to temperature, the difference between continuous and batch heat requirements is almost always negligible. More important usually is the effect of variation of fluid properties, such as viscosity and boiling-point rise, on heat transfer. These can only be estimated by a step-by-step calculation. 

Many materials are suitable for refrigerant purposes, and each usually has some special characteristics that allow it to serve a particular application better than some of the others. Before selecting a refrigerant, it is important to evaluate its flammability and toxicity data, pressure-temperature-volume relationships, enthalpy, density, molecular weight, boiling and freezing points, and various effects on gaskets, metals, oils, etc. ... 

Pressure, temperature, and enthalpy or total heat values may be obtained from tables or diagrams covering each particular refrigerant. Table 11-12 presents a few comparative values of boiling points (evaporator temperature) and corresponding pressures as taken from such data. 

The boiling point of water is 100 °C (212 °F) at atmospheric pressure, and at this same point the enthalpy of evaporation (latent heat of vaporization) is 970 Btu/lb. 

In other words, at its boiling point water requires the further addition of 970 Btu of energy to convert one pound of water to steam, which is 5.39 times as much energy as required to raise the water temperature from 0 °C (32 °F) to 100 °C. The temperature of the hot water remains constant until it has been entirely vaporized, at which point the steam is fully saturated with heat and the enthalpy of the steam (heat content as calculated from a starting point ofO °C) is 1,150 Btu/lb. 

Figure 8 shows that increasing the heat flux at constant mass velocity causes the peak in wall temperature to increase and to move towards lower enthalpy or steam quality values. The increase in peak temperature is thus due not only to a higher heat flux, which demands a higher temperature difference across the vapor film at the wall, but to a lower flow velocity in the tube as the peaks move into regions of reduced quality. The latter effect of lower flow velocity is probably the dominant factor in giving fast burn-out its characteristically rapid and often destructive temperature rise, for, as stated earlier, fast burn-out is usually observed at conditions of subcooled or low quality boiling. 

Table 2.2 Enthalpies and temperatures of fusion and vaporization. Normal melting and boiling points and enthalpies of fusion and vaporization are tabulated by type of...

P8.1 The molar enthalpy of vaporization of liquid mercury is 59.229 kJ-mol l at its normal boiling point of 630.0 K. The heat capacities of the liquid and gaseous phases, valid over the temperature range from 250 to 630 K, are as follows ... 

Self-Test 6.9A A sample of benzene, C6H6, was heated to 80°C, its normal boiling point. The heating was continued until 15.4 kj had been supplied as a result, 39.1 g of boiling benzene was vaporized. What is the enthalpy of vaporization of benzene at its boiling point ... 

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