Pressure change effect equilibrium

For the liquid-phase mass-transfer coefficient /cl, the effects of total system pressure can be ignored for all practical purposes. Thus, when using Kq and /cl for the design of gas absorbers or strippers, the primary pressure effects to consider will be those which affect the equilibrium curves and the values of m. If the pressure changes affect the hydrodynamics, then Icq, and a can all change significantly. 

H,0(1) - C6Hl206(aq) + 6 02(g), and AH° = +2802 kj. Suppose that the reaction is at equilibrium. State the effect that each of the following changes will have on the equilibrium composition (tends to shift toward the formation of reactants, tends to shift toward the formation of products, or has no effect), (a) The partial pressure of 02 is increased, (b) The system is compressed, (c) The amount of C02 is increased. 

The expression for K involving the concentrations of the species involved is found to be independent of volume. This implies that any change of pressure is not going to change the final state of equilibrium. The same result can be obtained by taking into consideration the alternative expression involving the partial pressures. If the pressure on the system is increased to n times its original value then all the partial pressures will be increased in the same proportion. This obviously implies that the equilibrium is independent of the pressure. The effect of some other factors on this reaction may now be considered. One such factor can be the addition of substances. For example, on addition of more A2, the partial pressure of A2 in the reactor would increase momentarily from pAl to some value, p A/. It has already been seen that... 

Le Chatelier s principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift in a tendency to reduce that stress. A stress is something done to the system (not by the equilibrium reaction). The stresses that we consider are change of temperature, change of pressure, change of concentration(s), and addition of a catalyst. Let us consider the effect on a typical equilibrium by each of these stresses. 

EFFECTS OF TEMPERATURE AND PRESSURE CHANGES ON THE EQUILIBRIUM CONSTANT FOR A REACTION... 

The equilibrium constant Ka is independent of pressure for those cases where the standard states are taken as the pure components at 1 atm. This case is the one used as the basis for deriving equation 2.6.9. Tjie effect of pressure changes then appears in the terms KfjP and ps + t+ b c . The influence of pressure on KfjP is quite small. However, for cases where there is no change in the total number of gaseous moles during the reaction, this is the only term by which pressure changes affect the equilibrium yield. For these... 

Gas production and subsequent pressure-time histories can be investigated successfully only in pressure vessels such as the VSP. If the gaseous product dissolves partly in the reaction mixture (i.e., the vapor-liquid equilibrium is changed), careful investigations of the pressure effect within the possible variations of the operating conditions are necessary. Pressurized vessels are also useful to investigate any mass transfer improvement for gas-liquid or gas-dissolved (suspended) solid reactions. 

Since a higher desorption temperature is usually related to a higher equilibrium pressure via Van t Hoff equation, there is a tendency to attribute the kinetic effects to the difference in temperatures when desorption temperature is being changed from a relatively low value to a high value. However, one must remember that the underlying factor is still the difference between the experimental desorption pressure and the equilibrium dissociation pressure. 

It is most important to know in this connection the compressibility of the substances concerned, at various temperatures, and in both the liquid and the crystalline state, with its dependent constants such as change of. melting-point with pressure, and effect of pressure upon solubility. Other important data are the existence of new pol3miorphic forms of substances the effect of pressure upon rigidity and its related elastic moduli the effect of pressure upon diathermancy, thermal conductivity, specific heat capacity, and magnetic susceptibility and the effect of pressure in modif dng equilibrium in homogeneous as well as heterogeneous systems. 

We should note that in most cases in environmental organic chemistry, we can neglect the effect of pressure changes on equilibrium partitioning. Exceptions might... 

CHEMICAL EQUILIBRIUM. The fundamental law of chemical equilibrium was enunciated by Le Chalclier (I884i. and may be stated as follows If any stress or force is brought to bear upon a system in equilibrium, the equilibrium is displaced in a direction which lends to diminish the intensity ol the stress or force. This is equivalent to the principle of least aclion. Its great value to the chemist is that it enahles him to predict the effect upon systems in equilibrium ol changes in temperature, pressure, and concentration. 

In applying Le Chatelier s principle to a heterogeneous equilibrium, the effect of pressure changes on solids and liquids can be ignored because the volume (and concentration) of a solid or a liquid is nearly independent of pressure. Consider, for example, the high-temperature reaction of carbon with steam, the first step in converting coal to gaseous fuels ... 

Related Calculations. While nothing is said above about kinetics, increasing the temperature very frequently changes the reaction rate favorably. Accordingly, in some exothermic-reaction situations, it may be worthwhile to sacrifice some degree of equilibrium conversion in favor of shorter reactor residence time by raising reaction temperature. Similarly, a pressure change may have an effect on kinetics that is contrary to its effect on equilibrium. 

Equations (XV.5.8) and (XV.5.9) predict a change of rate constant with pressure which will depend logarithmically on the partial molar volume change for the transition-state reaction. An exactly similar equation, the Kelvin equation, can be written for the change of equilibrium constant with pressure. Since AY /RT is of the order of magnitude of 10 atm " for solution reactions, it is evident that the effect of these pressure changes will be of importance only at pressures in excess of 10 atm, and indeed this is verified experimentally. 

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