Equilibrium position factors that affect

This shows that the difference between two irreversible dissolution reactions can be written as a reversible oxidation/reduction reaction. An important implication of this is that the addition of a dopant is, by itself, not sufficient to enhance the conductivity of a metal oxide photoelectrode one also needs to ensure that the equilibrium of reaction (2.17) lies at the right-hand side. The factors that affect the equilibrium position are oxygen partial pressure, dopant concentration, and temperature. The equihbrium constant of (2.17) is given by... 

The reactions catalysed by enzymes are equilibrium reactions. When the ratio of substrate to product has reached a constant value, the velocities of the forward reaction and the back reaction are equal. It does not mean tliat the substrate and product are present in the same concentration. Usually one or other predominates by a factor of several hundred. An enzyme may induce a reaction that cannot be appreciably detected in its absence, but it is unable to affect the equilibrium position. All that the enzyme does is to increase the rate at which the reaction proceeds to equilibrium. It is generally considered that enzymes combine with their substrates at three or more points. There is probably a simultaneous attack on the substrate by two groups of the enzyme, one withdrawing an electron from one position, whilst the other is donating an electron to a different atom of the substrate. This would explain why enzymes are much more effective then mono-functional catalysts like acids or bases. 

Factors That Affect Chemical Equilibrium Changes in concentration can affect the position of an equilibrium state—that is, the relative amounts of reactants and products. Changes in pressure and volume may have the same effect for gaseous systems at equilibrium. Only a change in temperature can alter the value of equilibrium constant. A catalyst can establish the equilibrium state faster by speeding the forward and reverse reactions, but it can change neither the equilibrium position nor the equilibrium constant. 

If one or more factors that affect an equilibrium is changed, the position of equilibrium shifts in the direction that reduces (opposes) the change. 

A catalyst is a substance that increases the rate of a reaction without affecting the position of equilibrium. It follows that the rate in the reverse direction must be increased by the same factor as that in the forward direction. This is a consequence of the principle of microscopic reversibility (Section 3.3), which applies at equilibrium, and rates are often studied far from equilibrium. 

Another factor that can possibly affect the redox potential in biological systems is the presence of secondary chelating agents that can participate in coupled equilibria (3). When other chelators are present, coupled equilibria involving iron-siderophore redox occur and a secondary ligand will cause the siderophore complex effective redox potential to shift. The decrease in stability of the iron-siderophore complex upon reduction results in a more facile release of the iron. Upon release, the iron(II) is available for complexation by the secondary ligand, which results in a corresponding shift in the redox equilibrium toward production of iron(II). In cases where iron(II) is stabilized by the secondary chelators, there is a shift in the redox potential to more positive values, as shown in Eqs. (42)—(45). 

Factors which affect the oxepin-benzene oxide equilibrium positions are similarly expected to influence the thiepin-benzene episulfide distribution at equilibrium. Since however the thianorcaradiene tautomer has not to date been detected, the main evidence for this form is based upon the thermal instability and reactions of the thiepin system. Thus it is assumed that where the thianorcaradiene isomer is present, a spontaneous thermal decomposition involving extrusion of a sulfur atom will occur. Substitution at the 2,7-positions in the oxepin-arene oxide system leads to a preference for the seven-membered ring form and this effect was further enhanced by bulky substituents (e.g. Bu ). A similar effect was observed in thiepins and thus the remarkable thermal stability of (49) (2,7-r-butyl groups) and (51) (2,7-hydroxyisopropyl groups) contrasts with the behavior of thiepin (55)(2,7-isopropyl groups), which was thermally unstable even at -70 °C (78CL723). The stability of thiepin (49) results from the 2,7-steric (eclipsed) interactions which obtain in the thianorcaradiene form but which are diminished in the thiepin tautomeric form (relative to the episulfide tautomer). 

It is important to understand the factors that control the position of a chemical equilibrium. For example, when a chemical is manufactured, the chemists and chemical engineers in charge of production want to choose conditions that favor the desired product as much as possible. In other words, they want the equilibrium to lie far to the right. When Fritz Haber was developing the process for the synthesis of ammonia, he did extensive studies on how the temperature and pressure affect the equilibrium concentration of ammonia. Some of his results are given in Table 6.2. Note that the amount of NH3 at equilibrium increases with an increase in pressure but decreases with an increase in temperature. Thus the amount of NH3 present at equilibrium is favored by conditions of low temperature and high pressure. 

SUMMARY OF FACTORS THAT MAY AFFECT THE EQUILIBRIUM POSITION ... 

The Efifect of a Catalyst Summary of Factors That May Affect the Equilibrium Position... 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

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