Volume variation expansion, contraction

Volume variations with conversion are large for constant-pressure gas-phase reactions with change in mole number. Here, as a rule, operation at constant volume poses no difficulties. Liquid-phase reactions may also entail volume contraction or expansion. However, these are not related to changes in mole number and can be predicted only if information on partial molar volumes is at hand. Because liquids are essentially incompressible, even at elevated temperature, it is unsafe to conduct liquid-phase reactions without a gas cap in a closed reactor. Some variation of liquid-phase volume with conversion therefore is apt to occur. Fortunately, the variation at constant temperature is usually so small that it can be neglected in the evaluation or accounted for by a minor correction. 

Two main factors that cause retention-volume variations with column temperature are assumed an expansion or a contraction of the mobile phase in the column and the secondary effects of the solute to the stationary phase. When the column temperature is... 

The influence of volume variation during a reaction, where there is contraction or expansion of fluid, on the CSTR and PFR volumes may be expressed as a function of the parameter ba, for first- or second-order reactions. The effect of sa on the CSTR or PFR volume is depicted in Figure 16.2 (curve 4 with M= 1). 

Two main factors that cause retention-volume variations with column temperature are assumed an expansion or a contraction of the mobile phase in the column and the secondary effects of the solute to the stationary phase. When the column temperature is 10°C higher than room temperature, the mobile phase (temperature of the mobile phase is supposed to be the same as room temperature in this case) will expand about 1 % from when it entered the columns, resulting in an increase in the real flow rate in the column due to the expansion of the mobile phase and the decrease in the retention volume. The magnitude of the retention-volume dependence on the solvent expansion is evaluated to be about one-half of the total change in the retention volume. The residual contribution to the... 

The contraction of solids on heating seems anomalous because it offends the intuitive concept that atoms will need more room to move as the vibrational amplitudes of the atoms increase. However, this argument is incomplete. Figure 11.9 plots schematically the variation of A with V at two temperatures, for both positive and negative thermal expansion. The volumes marked explicitly on the E-axis give the minima of each A vs. V isotherm. These are the equilibrium volumes at temperatures T and T2 respectively (J2 > 7j) and zero pressure. 

A reaction system at constant pressure can be open or closed and can occur in liquid, gas, or vapor phase. When the reactions are carried out in gas or vapor phase in an open system and there is variation of the number of moles, there will be volume expansion or contraction. In a closed system, consider a piston moving without friction, according to the schemes ... 

In a closed gas system, one assumes a piston that keeps the pressure constant but allows variation in the gas volume (expansion or contraction). In this case. Equation 14.15 becomes broader, i.e. ... 

The characteristics of the solvent can also be affected by temperature variations that can induce an expansion or a contraction of its volume, a change in its refraction index and/or influence chemical or physical equilibria. It is necessary, therefore, to use a thermostated cell all the times that these variables can significantly influence the measurement. 

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