Cyclohexane freezing point

In photoirradiated solid cyclohexane (freezing point 6.5°C), much higher fluorescence quantum yields and longer fluorescence lifetimes were observed than in the liquid phase [89]. In solid Ar matrices, the fluorescence characteristics, energy dependence of the lifetime and intensity, were found to be very similar to these characteristics in the gas phase. This points to the importance of cyclohexane-cyclohexane interactions to determine the excited-state characteristics in the liquid phase [76]. 

It is often the solvent effect fliat is flie only method of radical change of relative contents of different conformer forms. Thus, with flie help of the isochore equation of chemical reaction, flie data on equilibrium constants and enthalpies of dichloroacetaldehyde conformer transformation allow us to calculate that, to reach the equilibrium constant of axial rotamer formation in cyclohexane as solvent (it is equal to 0.79) to magnitude K=0.075 (as it is reached in DMSO as solvent), it is necessary to cool the cyclohexane solution to 64K (-209"C). At the same time, it is not possible because cyclohexane freezing point is -l-6.5"C. By analogy, to reach flie dimefliylsulfoxide constant to value of cyclohexane , DMSO solution must be heated to 435K (162"C). 

Cyclohexane. An excellent sohent for many determinations, particularly as, owing to the high value of K, a large fall in the freezing-point is obtained, and the accuracy of the determination is therefore correspondingly increased. Care should be taken to avoid super-cooling, however, as it has a marked effect on the true freezing-point of cyclohexane solutions. 

In carrying out a molar mass determination by freezing point depression, we must choose a solvent in which the solute is readily soluble. Usually, several such solvents are available. Of these, we tend to pick one that has the largest kf. This makes ATf large and thus reduces the percent error in the freezing point measurement From this point of view, cyclohexane or other organic solvents are better choices than water, because their kf values are larger. 

How many grams of the following nonelectrolytes would have to be dissolved in 100.0 g of cyclohexane (see Table 10.2) to increase the boiling point by 2.0°C To decrease the freezing point by 1.0°C ... 

For a detailed discussion of the calculation of activities (and excess Gibbs free energies) from freezing point measurements, see R. L. Snow. J. B. Ott. J. R. Goates. K. N. Marsh, S. O Shea, and R. N. Stokes. "(Solid + Liquid) and (Vapor + Liquid) Phase Equilibria and Excess Enthalpies for (Benzene + //-Tetradecane), (Benzene + //-Hexadecane). (Cyclohexane + //-Tetradecane), and (Cyclohexane +//-Hexadecane) at 293.15, 298.15, and... 

Liquid thermal conductivity data for cyclopcntane at 37.JTC and cyclohexane at 20 C and 37.8 C are available.13 Tabulated data are available.1 7 The effect of pressure on the thermal conductivity of cyclohexane is shown in Figure 40-11 API11 provides data on cyclohexane from the freezing point to the boiling point. The data lor cyclopropane and cy-rlobuianc were calculated by the method of Robbins and Kin grea." The data for cyclopcntane were extended by the method of Riedel ... 

The final remarks concern the B emission (1) compared to that of the TICT state it is only slightly dependent on the solvent polarity (the peak position varies from 29,300 cm-1 in cyclohexane to 28,200cm-1 in methanol at a temperature near to the freezing point) (2) it does not present any wavelength-dependent decay curves. This essentially proceeds from the fact that the dipole moments in the ground and in the excited B states are in the same direction and are not very different. 

Example 2. The freezing point of pure benzene is 5.45°C. The addition of 0.52 g of cyclohexane to 100.0 g of benzene forms a solution with freezing point 5.13°C. If 0.81 g of an unknown compound is added to 100.0 g of benzene, the resulting freezing point is 5.21°C. What is the molecular weight of the unknown compound ... 

In Table 1 are given values of the pertinent constants for two common solvents cyclohexane and water. It will be seen that, for freezing-point depressions of about 2 K, omission of the correction term kfATf eads to errors of the order of 1 percent in m or M. 

In this experiment the freezing point of a solution containing a known weight of an unknown solute in a known weight of cyclohexane is determined from cooling curves. From the result at each of two concentrations, the molar mass of the unknown is determined. 

Special thermometers are made for calorimetric and cryoscopic work, where it is desired to measure very accurately (to 0.01 or even 0.001 K) a temperatiue difference of the order of a few kelvin. For these thermometers the fineness of scale graduation has little to do with the accuracy with which the thermometer measures a single temperature. The scale may be in error by several tenths of a kelvin, but this error cancels out in taking differences. A typical thermometer for bomb caloiimetiy has a range of 19 to 35°C, with graduations of 0.02°C. For measuring freezing-point depressions with water or cyclohexane... 

The results clearly demonstrate that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes, e.g., when the distance comes down to 4 nm, even at 8.4 C, which is above the bulk freezing point of 6.4X3. Further, the degree of elevation was in good accord with the prediction by our model [3] based on the attractive potential energy of the wall. Though the extent of the elevation itself might look rather small, we believe that the finding of the definite existence of the elevation, and its accord with the prediction of our model would be of much importance in the research field of the phase behavior in nanopores. 

Three kinds of liquids of analytical grade (Wako Pure Chemicals) were used. All liquids were desiccated with zeolite 4A molecular sieve for one week before experimental use. Cyclohexane, and octamethylcyclotetrasiloxane (OMCTS), whose bulk freezing point were 6.4°C and ISTl respectively were used for the examination of freezing behavior. The reasons of the choice were i)affinity to carbonaceous surface, ii)freezmg point near ambient temperature, and iii)almost spherical molecular shape. Methanol, with bulk freezing point of -98 C was also used as a reference. All the force measurement were done after confirming the stability of the temperature under control for at least 10 minutes The temperature was set at various values above the bulk freezing point. 

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