Intermediate reaction steps

In Chapter 6 we considered the basic mles obeyed by simple electrode reactions occurring without the formation of intermediates. However, electrochemical reactions in which two or more electrons are transferred more often than not follow a path involving a number of consecutive, simpler steps producing stable or unstable intermediates (i.e., they are multistep reactions). 

The set of all intermediate steps is called the reaction pathway. A given reaction (involving the same reactants and products) may occur by a single pathway or by several parallel pathways. In the case of invertible reactions, the pathway followed in the reverse direction (e.g., the cathodic) may or may not coincide with that of the forward direction (in this example, the anodic). For instance, the relatively simple anodic oxidation of divalent manganese ions which in acidic solutions yields tetrava-lent manganese ions Mn +— Mn -l-2e , can follow these two pathways  

The second pathway includes a step in which the trivalent manganese ions formed as intermediates disproportionate. 

It is convenient to represent multistep electrode reactions involving one or more pathways in the form of tables such as this  

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

---

An influence on the reaction pathway (i.e., on the nature and sequence of intermediate reaction steps)... 

Hess s law states that the value of an energy obtained is independent of the number of intermediate reaction steps taken. 

The above value of n does not represent any imaginable kinetic process. Furthermore, no satisfactory relationships could be obtained when the data were analyzed in terms of a variety of diffusion models. A satisfactory explanation may involve a more complex reaction sequence than that of Equation 1, perhaps including some intermediate reaction steps. 

The simultaneous measurement of the impedance and mass/potential transfer function leads to new information on the kinetics of the processes involved. It may lead to chemical identification of the species involved in the intermediate reaction steps by allowing the atomic masses of the adsorbed intermediates of the multistep reaction mechanisms to be estimated. 

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

Write the thermochemical equation for the reaction whose reaction enthalpy is unknown. Write thermochemical equations for intermediate reaction steps whose reaction enthalpies are known. These steps must sum up to the overall reaction whose enthalpy is unknown. 

Sum the reaction enthalpies of the intermediate reaction steps to obtain the unknown reaction enthalpy (see Toolbox 6.1 in the text). 

Isotope Exchange If a particular atom of the substrate or product is labeled with an isotope other than the natural one, its location at various intermediate reaction steps can be followed, providing information on what is happening to that part of the molecule. 

Fig. 39. Proposed parallel electron transport chains feeding the different steps of nitrogen reduction in P. denitrificans when it respires with nitrate. Each intermediate reaction step has its own nitrogen compound reductase (or cytochrome oxidase. After references 406-408.

When a reaction proceeds by way of intermediate steps, thermodynamics can be expected to place a ceiling on the allowable concentration of intermediates that may exist in the gas phase, may be adsorbed on the finite number of catalyst sites, or both. In this case the over-all rate of reaction depends upon the activity of the catalyst for the intermediate reaction steps, in a characteristic manner which can be illustrated by the reaction... 

This is analogous to the electrical current in a set of resistors subject to a voltage. The lowest resistance among a number of parallel resistors takes the largest current and determines the overall circuit resistance if the others have much higher resistances. If the resistors are in series, the highest resistance in the set is the dominant one and determines the overall resistance if the others are low in comparison. In a set of intermediate reaction steps the rate coefficient k varies with... 

Many chemical reactions, including the ozone-oxygen reaction above, do not occur in a single step as written. Instead, they proceed by a number of smaller intermediate reaction steps. These are known as the elementary reaction steps, and it is these individual elementary reaction steps which determine the rate law for the overall chemical reaction. 

In general, the principles and various electrochemical techniques described in this chapter can be used to study all the important electrochemical aspects of a battery or fuel cell. These include the rate of electrode reaction, the existence of intermediate reaction steps, the stability of the electrolyte, the current collector, the electrode materials, the mass-transfer conditions, the value of the limiting current, the formation of resistive films on the electrode surface, the impedance characteristics of the electrode or cell, and the existence of the rate-limiting species. 

Case study 2. A process chemist generated a problematic by-product in an intermediate reaction step. The chemist was able to purify this impurity to an 80% level from chromatography. SFC is an... 

In addition to charge transfer and diffusion limitation, chemical reactions can also take place at intermediate reaction steps. For instance, in Ni/Ce02 cermet anodes, a possible intermediate reaction product can be NiO. These processes seem not to be rate limiting, and they have not been uniquely identified and thoroughly investigated. 

Write thermochemical equations for intermediate reaction steps whose reaction enthalpies are known. 

High electron transfer number of 12 for complete oxidation (methanol is 6 and hydrogen is 2), resulting in reduced theoretical fuel requirement. However, this does also indicate a more complex molecule requiring a greater number of intermediate reaction steps and pathways and generally worse kinetics. 

Both reaction pathways to CH, and CH3OH, formation show that most of the intermediate reaction steps are endothermic. However, both CH, and CH3OH, formation from CO + are exothermic. Heat evolution occurs from adsorption and bond disso-... 

---