Reaction changes caused

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

Several other changes that are supposed to slow down the reaction can cause runaway. In the case of ethylene oxidation, chlorinated hydrocarbons are used as inhibitors. These slow down both the total and the epoxidation, although the latter somewhat less. When the reaction is running too high and the inhibitor feed is suddenly increased in an attempt to control it, a runaway may occur. The reason is similar to that for the feed temperature cut situation. Here the inhibitor that is in the ppm region reacts with the front of the catalytic bed and slowly moves down stream. The unconverted reactants reach the hotter zone before the increased inhibitor concentration does. 

If there is a decrease in the order of a reaction with respect to a particular substance as the concentration of that species increases, the dominant form of that species in solution may be undergoing a change caused by the change in concentration. 

Kinetic Isotope Effect. The change in reaction rate caused by isotopic substitution. 

This ease with which we can control and vary the concentrations of H+(aq) and OH (aq) would be only a curiosity but for one fact. The ions H+(aq) and OH (aq) take part in many important reactions that occur in aqueous solution. Thus, if H+(aq) is a reactant or a product in a reaction, the variation of the concentration of hydrogen ion by a factor of 1012 can have an enormous effect. At equilibrium such a change causes reaction to occur, altering the concentrations of all of the other reactants and products until the equilibrium law relation again equals the equilibrium constant. Furthermore, there are many reactions for which either the hydrogen ion or the hydroxide ion is a catalyst. An example was discussed in Chapter 8, the catalysis of the decomposition of formic acid by sulfuric acid. Formic acid is reasonably stable until the hydrogen ion concentration is raised, then the rate of the decomposition reaction becomes very rapid. 

Fig. 8.4 Absorption spectrum of dinoflagellate luciferin, and the spectral changes caused by luminescence reaction after the addition of luciferase, in 0.2 M phosphate buffer, pH 6.3, containing 0.1 mM EDTA and BSA (O.lmg/ml) (Nakamura et al., 1989). Reproduced from Hastings, 1989, with permission from the American Chemical Society and John Wiley Sons Ltd.

Increasing RQ with very small changes causes the constants a and b to change in die reaction. 

A mechanical system, typified by a pendulum, can oscillate around a position of final equilibrium. Chemical systems cannot do so, because of the fundamental law of thermodynamics that at all times AG > 0 when the system is not at equilibrium. There is nonetheless the occasional chemical system in which intermediates oscillate in concentration during the course of the reaction. Products, too, are formed at oscillating rates. This striking phenomenon of oscillatory behavior can be shown to occur when there are dual sets of solutions to the steady-state equations. The full mathematical treatment of this phenomenon and of instability will not be given, but a simplified version will be presented. With two sets of steady-state concentrations for the intermediates, no sooner is one set established than the consequent other changes cause the system to pass quickly to the other set, and vice versa. In effect, this establishes a chemical feedback loop. 

In order for an experimental test of the kinetic behaviour to be as informative as possible, the system investigated should fulfil various specific requirements. From the experimental point of view, the reaction should cause a minimum of change in the reaction medium and be without side-reactions as far as possible, in order for accurate and well-defined rate measurements to be feasible. For the same reason an accOTate physical method which can be applied without distiubing the reacting system is to be preferred. From the theoretical point of view, it is desirable that the steric effects play as important a role in the reaction as possible, because only then is a sizeable effect to be expected. Finally, a transition state of well-known conformation is a necessary prerequisite for the quantitative application of the theory. 

A knowledge of the concentrations of all reactants and products is necessary for a description of the equilibrium state. However, calculation of the concentrations can be a complex task because many compounds may be Imked by chemical reactions. Changes in a variable such as pH or oxidation potential or light intensity can cause large shifts in the concentrations of these linked species. Aggregate variables may provide a means of simplifying the description of these complex systems. Here we look at two cases that involve acid-base reactions. 

Internal heat exchange is realized by heat conduction from the microstructured reaction zone to a mini channel heat exchanger, positioned in the rear of the reaction zone [1,3,4], The falling film micro reactor can be equipped, additionally, with an inspection window. This allows a visually check of the quality of film formation and identification of flow misdistribution. Furthermore, photochemical gas/liquid contacting can be carried out, given transparency of the window material for the band range of interest [6], In some cases an inspection window made of silicon was used to allow observation of temperature changes caused by chemical reactions or physical interactions by an IR camera [4, 5]. 

In gas-phase reactors, the volume and volumetric flow rate frequently vary, owing to the molar changes caused by reaction and the effects of temperature and pressure on gas phase volume. These influences must be taken into account when formulating the mass and energy balance equations. 

Electrode reactions are intrinsically surface reactions, and cause changes in the electrolyte composition near the surface. A thin layer, impoverished in the reacting species, develops at the electrode surface, and the ions or molecules move across this layer by diffusion down the concentration gradients. Ions move also under the influence of the applied electric field that is, they migrate. 

Structural changes in the polymer, which will accompany the formation of small molecule products from the polymer, or may be produced by other reactions, can cause significant changes to the material properties. Development of colour, e.g. in polyacrylonitrile by ladder formation, and in poly(vinyl chloride) through conjugated unsaturation, is a common form of degradation. 

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