Chemical equilibrium concentration effect

The kinetic factor is proportional to the energetic state of the system and (for heterogeneous catalytic systems) the number of active sites per unit volume (mass) of catalyst. The driving-force group includes the influence of concentration and distance from chemical equilibrium on the reaction rate, and the hindering group describes the hindering effect of components of the reaction mixture on the reaction rate. The kinetic factor is expressed as the rate constant, possibly multiplied by an equilibrium constant(s) as will be shown later. 

In the preceding chapter, the choice of reactor type was made on the basis of the most appropriate concentration profile as the reaction progressed, in order to minimize reactor volume for single reactions or maximize selectivity (or yield) for multiple reactions for a given conversion. However, there are still important effects regarding reaction conditions to be considered. Before considering reaction conditions, some basic principles of chemical equilibrium need to be reviewed. 

When a solid acts as a catalyst for a reaction, reactant molecules are converted into product molecules at the fluid-solid interface. To use the catalyst efficiently, we must ensure that fresh reactant molecules are supplied and product molecules removed continuously. Otherwise, chemical equilibrium would be established in the fluid adjacent to the surface, and the desired reaction would proceed no further. Ordinarily, supply and removal of the species in question depend on two physical rate processes in series. These processes involve mass transfer between the bulk fluid and the external surface of the catalyst and transport from the external surface to the internal surfaces of the solid. The concept of effectiveness factors developed in Section 12.3 permits one to average the reaction rate over the pore structure to obtain an expression for the rate in terms of the reactant concentrations and temperatures prevailing at the exterior surface of the catalyst. In some instances, the external surface concentrations do not differ appreciably from those prevailing in the bulk fluid. In other cases, a significant concentration difference arises as a consequence of physical limitations on the rate at which reactant molecules can be transported from the bulk fluid to the exterior surface of the catalyst particle. Here, we discuss... 

Concentration (1), pressure (1) and temperature (1) may, if changed, alter the position of a chemical equilibrium. These factors often, but not always, have an effect on the position of equilibrium. 

The chapter starts with a brief review of thermodynamic principles as they apply to the concept of the chemical equilibrium. That section is followed by a short review of the use of statistical thermodynamics for the numerical calculation of thermodynamic equilibrium constants in terms of the chemical potential (often designated as (i). Lastly, this statistical mechanical development is applied to the calculation of isotope effects on equilibrium constants, and then extended to treat kinetic isotope effects using the transition state model. These applications will concentrate on equilibrium constants in the ideal gas phase with the molecules considered in the rigid rotor, harmonic oscillator approximation. 

In this section, you compared strong and weak acids and bases using your understanding of chemical equilibrium, and you solved problems involving their concentrations and pH. Then you considered the effect on pH of buffer solutions solutions that contain a mixture of acid ions and base ions. In the next section, you will compare pH changes that occur when solutions of acids and bases with different strengths react together. 

It should be noted that the condition of a dilute solution was introduced into the considerations for two reasons primarily, in order that it would be possible to replace the activities by concentrations and thus determine the equilibrium concentrations on the basis of (2.3.3) and, secondarily, in order for it to be possible to neglect the effect of pressure on the chemical potentials of the components whose electrochemical potentials appear in (2.3.2). Because of the differing ionic concentrations in solutions 1 and 2, the osmotic pressures in these solutions are not identical and this difference must be compensated by external pressure. A derivation considering the effect of pressure can be found, for example in [9] or p. 191 of [18]. 

The equilibrium of the reaction strongly favors lactate formation. At high concentrations of lactate and NAD"", however, oxidation of lactate to pyruvate is also possible (see p. 18). LDH catalyzes the reaction in both directions, but—like all enzymes—it has no effect on chemical equilibrium. 

Figure 19.12. Batch parametric processing of solid-liquid interactions such as adsorption or ion exchange. The bottom reservoir and the bed interstices are filled with the initial concentration before pumping is started, (a) Arrangement of adsorbent bed and upper and lower reservoirs for batch separation, (b) Synchronization of temperature levels and directions of flow (positive upward), (c) Experimental separation of a toluene and n-hcptane liquid mixture with silica gel adsorbent using a batch parametric pump. (Reprinted from Wilhelm, 1968, with permission of the American Chemical Society), (d) Effect of cycle time t on reservoir concentrations of a closed system for an NaCl-H20 solution with an ion retardation resin adsorbent. The column is initially at equilibrium with 0.05M NaCl at 25°C and a = 0.8. The system operates at 5° and 55°C. [Sweed and Gregory, AIChE J. 17, 171 (1971)J.

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. 

The effect ol change ol temperature nn a system in chemical equilibrium is thin the equilibrium point is shifted ll) toward the side itiii/v from that which evolves heat when the temperature is wised, and (2) toward the side which evolves heal when the temperature is lowered. It is tis if the amount of heal were a muterinl reactant and its concentration (temperature or intensity of heal) increased, in respect to the tUrttiitm of the sltilt of tile equilibrium point. The amount of die shill at constant pressure can he calculated in eases where one possesses the proper data. 

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