Heat effect/change

The same general considerations relative to the constancy of flow rates apply to multicomponent and binary mixtures. Thus the latent heat of vaporization at various positions in the column should serve as the principal criterion, although in multicomponent systems there is more possibility of large sensible heat effects changing the overflow and vapor rates. 

At the outset it will be profitable to deal with an ideal solution possessing the following properties (i) there is no heat effect when the components are mixed (ii) there is no change in volume when the solution is formed from its components (iii) the vapour pressure of each component is equal to the vapour pressure of the pure substances multiplied by its mol fraction in the solution. The last-named property is merely an expression of Raoult s law, the vapour pressure of a substance is pro-... 

The standard Gibbs-energy change of reaction AG° is used in the calculation of equilibrium compositions. The standard heat of reaclion AH° is used in the calculation of the heat effects of chemical reaction, and the standard heat-capacity change of reaction is used for extrapolating AH° and AG° with T. Numerical values for AH° and AG° are computed from tabulated formation data, and AC° is determined from empirical expressions for the T dependence of the C° (see, e.g., Eq. [4-142]). 

In concentrated wstems the change in gas aud liquid flow rates within the tower and the heat effects accompanying the absorption of all the components must be considered. A trial-aud-error calculation from one theoretical stage to the next usually is required if accurate results are to be obtained, aud in such cases calculation procedures similar to those described in Sec. 13 normally are employed. A computer procedure for multicomponent adiabatic absorber design has been described by Feiutnch aud Treybal [Jnd. Eng. Chem. Process Des. Dev., 17, 505 (1978)]. Also see Holland, Fundamentals and Modeling of Separation Processes, Prentice Hall, Englewood Cliffs, N.J., 1975. 

Changes in the composition of a specimen over the analyzed depth can be caused by beam heating or by beam charging of the specimen. Beam heating can lead to selective vaporization of some elements or diffrisional redistribution of the elements. If the surflice charges up to some potential, then electric-field enhanced diffusion can selectively redistribute certain elements. Beam-heating effects usually... 

Warme-theorie, /. theory of heat, -tisch, m. warming table, -tonung, /. heat effect (of a reaction), heat of reaction, heat change, heat tone. -Ubergang, m. passage of heat, heat transmission, heat transfer, -ubertragimg, /. heat transfer. 

Since the instrument responds to a heating effect, it is slow to respond to changes in the measured quantity. 

Why is the heat effect so much larger in the chemical reaction than in the phase change you studied ... 

Because of these differences, chemists differentiate these two kinds of change. We have already named the solidification of wax—in Section 1-1.2 we called this type of change a phase change. A change like combustion, with its much larger heat effects, is called a chemical change or a chemical reaction. 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

This heat effect can be used in predicting how K changes with temperature. Le Chatelier s Principle indi-... 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

Corollary. dVJ is a perfect differential when the pressure is constant, and Qi is independent of the path. The independence of the heat effect on the path requires that the change shall occur either at constant volume or at constant pressure. If the volume is maintained constant (dv = 6) the pressure may be changed in any way if the pressure is maintained constant (dp = o) the volume may be altered in any manner so that the limiting conditions are satisfied but if both pressure and volume change... 

Energy balances are needed whenever temperature changes are important, as caused by reaction heating effects or by cooling and heating for temperature control. For example, such a balance is needed when the heat of reaction causes a change in reactor temperature. This is seen in the information flow diagram for a non-isothermal continuous reactor as shown in Fig. 1.19. 

The specific application of the first law of thermodynamics to study chemical reactions is referred to as thermochemistry. Thermochemistry is concerned with the measurement or calculation of the heat absorbed or given out in chemical reactions. Precisely therefore, thermochemistry is the part of thermodynamics dealing with enthalpy (i.e., heat content) changes accompanying chemical reactions. In this context, it will be useful to refer to some of the important terms associated with thermal effects. 

If the no-phase-change restriction does not rigorously apply, a simple design procedure can be formulated based on the results discussed in Section III, where it is shown that thermal equilibrium is quickly achieved in gas-liquid systems because of the large heat effects associated with evaporation or condensation. Although the total mass transfer between the phases may be small, it is not unrealistic to assume that the gas and liquid phases have the same temperature at each axial position. 

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