Reaction tendency

It has already been seen that whole oxidation-reduction reactions can be constructed from half-reactions. The direction in which a reaction goes is a function of the relative tendencies of its constituent half-reactions to go to the right or left These tendencies, in turn, depend upon the concentrations of the half-reaction reactants and products and their relative tendencies to gain or lose electrons. The latter is expressed by a standard electrode potential The tendency of the whole reaction to proceed to the right as written is calculated from the Nernst equation, which contains both EP and the concentrations of the reaction participants. These concepts are explained further in this section and the following section. 

2Ag+ + H2 2H+ + 2Ag This reaction is composed of the following two half-reactions  

It is called the standard hydrogen electrode (SHE), and has been assigned a value of exactly 0 V by convention its half-reaction is written as 

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The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

The size of the EDTA ring structure complex (not too big or too small), plus the multiple points of attachment, provides a very stable complex (high stability constant) that overcomes reaction tendencies to reach equilibrium via the lowest possible energy state or maximum state of disorder (entropy). 

It is now clear that the fate of chemicals in the environment is controlled by a combination of three groups of factors. First are the prevailing environmental conditions such as temperatures, flows and accumulations of air, water and solid matter and the composition of these media. Second are the properties of the chemicals which influence partitioning and reaction tendencies, i.e., the extent to which the chemical evaporates or associates with sediments, and how fast the chemical is eventually destroyed by conversion to other chemical species. Third are the patterns of use, into which compartments the substance is introduced, whether introduction is episodic or continuous and in the case of pesticides how and with which additives the active ingredient is applied. 

Reaction tendencies and by-products Film wetting behavior, film thickness, and... 

Both half- and overall reaction tendencies change with temperature, pressure (if gases are involved), and concentrations of the ions involved. Thus far, we have only been concerned with standard conditions. Standard conditions, as stated previously, are 25°C, 1 atm pressure, and 1 M ion concentrations. An equation has been derived to calculate the cell potential when conditions other than standard conditions are present. This equation is called the Nernst equation and is used to calculate the true E (cell potential)... 

To quantify the reaction tendencies of oxidizing and reducing agents To determine the effect of concentration on potential... 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

Why is it useful to know AG° for a reaction As we will see in more detail later, knowing the AG° values for several reactions allows us to compare the relative tendency of these reactions to occur. The more negative the value of AG°, the further a reaction will go to the right to reach equilibrium. We must use standard state free energies to make this comparison because free energy depends on pressure or concentration. Thus, to get an accurate comparison of reaction tendencies, we must compare all reactions under the same pressure or concentration conditions. We will have more to say about the significance of A G° later. 

However, the 1-benzyl derivative is rapidly isomerized in base, though the equilibrium favors the ring-substituted isomers under neutral conditions (70JCS(C)230,72JCS(Pl)46l). These reaction tendencies are generalized in Scheme 6. 

The conversion of NO(g) to N20(g) plus NOiig) is spontaneous under standard conditions. The forward reaction under these conditions is scarcely observed because its rate is so slow. Nonetheless, its equilibrium constant can be calculated Such calculations often have enormous impact in evaluating proposed solutions to practical problems. For example, the calculation shows that this reaction could be used to reduce the amount of NO in cooled exhaust gases from automobiles. The fundamental reaction tendency is there, but successful application requires finding a route to increasing the reaction rate at standard conditions. Had the equilibrium constant calculated from thermodynamics been small, this proposed application would be doomed at the outset and investment in it would not be justified. 

As an alternative, the tendency for a reduction to occur may also be expressed in terms of a hypothetical electron activity. Even though free electrons do not occur in aqueous solutions, it is convenient to relate the reaction tendency to this hypothetical activity. The electron activity for a reduction electrode X may be defined relative to the electron activity for a standard hydrogen electrode by the following relationship ... 

Thermodynamics can be used to predict the spontaneous direction of an electrochemical reaction (tendency for reaction to occur), but not the rate of reaction. Consider the electrochemical cell in Fig. 1, which shows a twosemipermeable membrane. One side contains a Zn electrode in a solution containing Zn " " ions at a concentration of 1 M, and the other side contains a Cu electrode in a solution containing Cu + ions at a concentration of 1 M. Every species is in its standard state. The cell is represented by the following cell reaction ... 

The reaction tendency is less. Or we can say that the corrosion tendency of copper in aerated water is not as pronounced as that of magnesium. 

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