Heat capacity lowering

The upper solid line represents the measured heat capacity, lower lines show the deconvolution with solid lines for the individual melting... 

The advantages of 50% slush hydrogen over atmospheric pressure liquid hydrogen are due to its 36 Btu/lb higher heat capacity (lower enthalpy) and/or its 13 % smaller tankage requirements (lower specific volume). Both nonvented and vented storage are discussed because either might be used for an actual space mission. 

The integral under the heat capacity curve is an energy (or enthalpy as the case may be) and is more or less independent of the details of the model. The quasi-chemical treatment improved the heat capacity curve, making it sharper and narrower than the mean-field result, but it still remained finite at the critical point. Further improvements were made by Bethe with a second approximation, and by Kirkwood (1938). Figure A2.5.21 compares the various theoretical calculations [6]. These modifications lead to somewhat lower values of the critical temperature, which could be related to a flattening of the coexistence curve. Moreover, and perhaps more important, they show that a short-range order persists to higher temperatures, as it must because of the preference for unlike pairs the excess heat capacity shows a discontinuity, but it does not drop to zero as mean-field theories predict. Unfortunately these improvements are still analytic and in the vicinity of the critical point still yield a parabolic coexistence curve and a finite heat capacity just as the mean-field treatments do. 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

Physical Dilution. The flame retardant can also act as a thermal sink, increasing the heat capacity of the polymer or reducing the fuel content to a level below the lower limit of flammabiHty. Inert fillers such as glass fibers and microspheres and minerals such as talc act by this mechanism. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

In plasma chemical vapor deposition (PCVD), the starting materials are typically SiCl, O2, 2 6 GeCl (see Plasma technology). Plasma chemical vapor deposition is similar to MCVD in that the reactants are carried into a hoUow siUca tube, but PCVD uses a moving microwave cavity rather than a torch. The plasma formed inside the microwave cavity results in the deposition of a compact glass layer along the inner wall of the tube. The temperatures involved in PCVD are lower than those in MCVD, and no oxide soots are formed. Also, the PCVD method is not affected by the heat capacities or thermal conductivities of the deposits. 

When vapor stream has lower heat capacity than liquid stream (AT, > ATl), use ... 

Figure 4.7 Heat capacity and phase transitions in phosphine. The same entropy at point (a) is obtained by going the stable (lower) route or by going the metastable (upper) route. Point (b) is the normal boiling temperature.

Figure 4.8 Heat capacity of glycerol as a function of temperature. The solid line indicates Cp,m for the liquid and glassy phase. The dashed line represents Cp m for the solid. The dotted line at the melting temperature of 291.05 K. indicates the change in heat capacity upon melting. A glass transition occurs in the supercooled liquid at approximately 185 K. The heat capacities of the solid and the glass approach one another as the temperature is lowered they are almost identical below 140 K.

The ABC cereal company is developing a new type of breakfast cereal to compete with a rival product that they call Brand X. You are asked to compare the energy content of the two cereals to see if the new ABC product is lower in calories so you burn 1.00-g samples of the cereals in oxygen in a calorimeter with a heat capacity of 600. J-(°C). When the Brand X cereal sample burned, the temperature rose from 300.2 K to 309.0 K. When the ABC cereal sample burned, the temperature rose from 299.0 K to 307.5 K. (a) What is the heat output of each sample (b) One serving of each cereal is 30.0 g. How would you label the packages of the two cereals to indicate the fuel value per 30.0-g serving in joules in nutritional Calories (kilocalories) ... 

Assuming that the heat capacity of an ideal gas is independent of temperature, calculate the entropy change associated with lowering the temperature of 2.92 mol of ideal gas atoms from 107.35°C to —52.39°C at (a) constant pressure and (b) constant volume. 

The discovery of a transition which we identify with this has been reported by Simon, Mendelssohn, and Ruhemann,16 who measured the heat capacity of hydrogen with nA = 1/2 down to 3°K. They found that the heat capacity, after following the Debye curve down to about 11°K, rose at lower temperatures, having the value 0.4 cal/deg., 25 times that of the Debye function, at 3°K. The observed entropy of transition down to 3°K, at which the transition is not completed, was found to be about 0.5 E.U. That predicted by Eq. (15) for the transition is 2.47 E.U. 

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