Boiling points pure liquids

In addition to the orthodox method, just described, for the determination of the boiling points of liquids, the student should determine the boiling points of small volumes (ca. 0 5 ml.) by Siwolobofifs method. Full details are given iri Section 11,12. Determine the boiling points of the pure liquids listed in the previous paragraph. Observe the atmospheric pressure and if this differs by more than 5 mm. from 760 mm., correct the boiling point with the aid of Table II,9,B. Compare the observed boiling points with the accepted values, and draw a calibration curve for the thermometer. 

The boiling point is defined as "the temperature at which a liquid s vapor pressure equals the pressure of the atmosphere on the liquid." If the pressure is exactly 1 atmosphere (101,325 Pa), the temperature is referred to as "the normal boiling point." Pure chemicals have a unique boiling point, and this fact can be used in some laboratory investigations to check on the identity and / or purity of a material. Mixtures of two or more compounds have a boiling point range. This chapter focuses on the estimation of boiling points for pure compounds only. 

A method has been developed for calculating equilibrium vapor compositions, based on boiling point vs. liquid composition data, for systems saturated with a salt. Such ternary systems in effect have been treated as binaries (26) in which the standard state of each of the two liquid components is that of being saturated with salt instead of being pure and with the pure-component vapor pressures being so adjusted. For example, in the ethanol-water-salt ternary systems tested, they have been considered as binaries composed of water saturated with salt as one component and ethanol saturated with salt as the other component. In the testing to which it has been subjected so far, the method seems encouraging. 

The mercury metal is then purified by distillation. Distillation is the process of heating two or more liquids to their boiling points. Different liquids boil at different temperatures. The liquid that is wanted (such as mercury) can be collected at its boiling point. Mercury that is more than 99 percent pure can be collected by distillation. 

A student holds a beaker of pure liquid A in one hand and pure liquid B in the other. Liquid A has a higher boiling point than liquid B. When the student pours a small amount of liquid B into liquid A, the temperature of the solution increases. Which of the following statements is true ... 

The very high purity of hydrogen from a liquid source arises from the fact that at the normal boiling point of liquid hydrogen, all materials (except helium) are frozen solid, have very low vapor pressures, and are essentially insoluble in the liquid hydrogen. Liquid hydrogen, therefore, when vaporized, is exceptionally pure if no recontamination has occurred. 

Molecular nitrogen is obtained by fractional distillation of air (the boiling points of liquid nitrogen and liquid oxygen are -196°C and -183°C, respectively). In the laboratory, very pure nitrogen gas can be prepared by the thermal decomposition of ammonium nitrite ... 

As we saw in Section 11.5, the normal boiling point is the temperature at which a pure liquid is in equilibrium with its vapor at a pressure of 1 atm. (a) Write the chemical equation that defines the normal boiling point of liquid carbon tetrachloride, CChfl). (b) What is the value of AG° for the equilibrium in part (a) (c) Use thermodynamic data in Appendix C and Equation 19.20 to estimate the normal boiling point of CCI4. 

Kikuti has carefully studied the viscosities of sodium-ammonia solutions with concentrations ranging from infinite dilution to about 7.5M (supersaturated) at temperatures between —30 and -1-30°C. At all temperatures the coefficient of viscosity decreased with increase in concentration, the decrease being quite steep up to AM and then slowing down. At the highest concentrations, the coefficient of viscosity seemed to approach a constant value characteristic of the temperature. At — 30° C, close to the boiling point of liquid ammonia, the coefficient of viscosity at infinite dilution was 0.25 centipoises and at 7.5M it was 0.16 centipoises. At all concentrations, the viscosity decreased with increase in temperature similar to the behavior observed for pure ammonia. 

Density and Volume Expansion. Kraus, Johnson, and collaborators have performed careful measurements of the densities of alkali-metal—ammonia solutions. All the measurements were carried out near the boiling point of liquid ammonia and for the concentration range from IM to saturation. They found the densities of the solutions for all three metals to be less than that of pure ammonia. It is more meaningful to consider the apparent expansion in volume per gram atom of metal dissolved which is obtained from the equation... 

Pure hydrazine is a colourless liquid, melting point 275 K, and boiling point 387 K. It is surprisingly stable for an endothermic compound = -i- 50.6 kJ mol ). Each nitrogen atom has a lone pair of electrons and either one or both nitrogen atoms are able to accept protons to give and the less stable... 

Pure hydrogen peroxide is a colourless, viscous liquid, m.p. 272.5 K, density l,4gcm . On heating at atmospheric pressure it decomposes before the boiling point is reached and a sudden increase of temperature may produce explosive decomposition, since the decomposition reaction is strongly exothermic ... 

Pure sulphuric acid is a colourless, viscous and rather heavy liquid (density 1.84 g cm ). On heating, it decomposes near its boiling point, forming sulphur trioxide and a constant boiling (603 K) mixture of water and sulphuric acid containing 98% of the latter. This is concentrated sulphuric acid, which is usually used. Further heating gives complete dissociation into water and sulphur trioxide. 

The most important source of helium is the natural gas from certain petroleum wells in the United States and Canada. This gas may contain as much as 8 % of helium. Because helium has a lower boiling point Table 12.1) than any other gas, it is readily obtained by cooling natural gas to a temperature at which all the other gases are liquid (77 K) almost pure helium can then be pumped off. The yearly production in this way may be many millions of m of gas. but something like 10 m per year is still wasted. 

If the thermometer is to be used to determine the elevation of the boiling-point of a liquid on the addition of a solute, it must be remembered that at the boiling-point of the pure solvent the mercury must now be about 1-2 above the bottom of the scale S, and hence for adjustment purposes the temperature of the beaker of water should be 6—7 above the boiling-point of the liquid itself, instead of 1-2 as before. 

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

The composition of the vapour can easily be calculated as follows — Assuming that the gas laws are applicable, it follows that the number of molecules of each component in the vapour wdll be proportional to its partial pressure, i.e., to the vapour pressure of the pure liquid at that temperature. If and p are the vapour pressures of the two liquids A and B at the boiling point of the mixture, then the total pressure P is given by ... 

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystalhsation does not alter it, the compound may be regarded as pure. For a hquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boUing point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

Separations based upon differences in the physical properties of the components. When procedures (1) or (2) are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Sections 11,15 and 11,17) the degree of separation may be determined by the range of boiling points and/or the refractive indices and densities of the different fractions that are collected. A mixture of non-volatile sohds may frequently be separated by making use of the differences in solubilities in inert solvents the separation is usually controlled by m.p. determinations. Sometimes one of the components of the mixture is volatile and can be separated by sublimation (see Section 11,45). 

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