Chemical reactions reversible/irreversible

This type of reaction sequence is common for most multielectron processes [e.g., Cu(II) - Cu(s) 02 - HOOH flCH(O) ACH2OH]. If both electron transfers are reversible and the chemical reaction is irreversible, the first peak should indicate an nrelectron process for small k values and an (n, + n2)-electron process for large k values. Therefore, an increase of the scan rate should decrease the apparent number of electrons involved in the overall process. Often the second reduction step occurs at a less negative potential than the first, which means that only a single irreversible peak is observed. Potential-scan reversal can provide anodic peaks and data for red and redx. 

Reversibility and irreversibility are central concepts of classical thermodynamics. Rigorously speaking, in physics and chemistry all processes are reversible. However, for many chemical processes the probability of the reverse process is negligible. For instance, processes such as the combustion of coal or hydrocarbons and enzyme reactions are in fact irreversible. On the other hand, even if the overall chemical reaction is irreversible, it always comprises at least one reversible step. For example... 

The Thiele moduli for first-order reactions in various geometries as well as the generalized moduli applicable for other reaction orders all assume that the chemical reaction is irreversible. Studies on the first-order reversible reaction A <-> B have shown that the same q>L-ri function as in the irreversible case can be used, when the Thiele modulus is defined using the characteristic size factor L of Equation 2.65. The reaction equilibrium constant K is used, if the D, value for A and B is more or less the same [13, 29] ... 

The ec scheme, which is a very common mechanism in organic electrochemistry, is described by Equations (6.17) and (6.18). The cyclic voltammogram observed depends on the relative rates of the two steps. The simplest situation is where the electron transfer is totally irreversible the presence of the chemical reaction has no effect on the voltammogram obtained and no kinetic data related to the chemical reaction can be derived. This situation leads to the properties in Table 6.2. Similar properties can also arise when the rate of the electron transfer step is relatively fast if the rate constant for the chemical reaction is very large. The full range of other possibilities where the chemical reaction can be reversible or irreversible and the electron transfer either reversible or quasi-reversible has been considered in detail by Nadjo Saveant [7], and the various kinetic zones have been identified. In this chapter the only case to be discussed in detail is that where the electron transfer is reversible and the chemical reaction is irreversible. 

This is another specific type of following reaction where the initial reaction product reacts chemically to yield a species O, which is itself at least as readily reduced as O. This type of reaction sequence is fairly common in multi-electron transfer processes in organic electrochemistry. It was discussed in some detail in an earlier chapter (Chapter 2) on pulse techniques, and the possibility of competing disproportionation reactions was considered. We will only consider here the case where homogeneous electron transfer can be ignored, the electron transfer are reversible, and the chemical reaction is irreversible. Other cases are discussed in the literature [7, 9-11]. 

Subscripts are used to provide additional information. The subscript for reversible (meaning that both forward and reverse processes are fast enough to maintain equilibrium or Nemstian conditions at the surface), and i represents irreversible (only the forward reaction is significant) i and r are limiting cases of q, or quasi-reversible (meaning that both the forward and reverse processes take place but are not fast enough to be considered at equilibrium). Thus, in an E Cj mechanism the electrode reaction is fast and reversible and the chemical reaction is irreversible. 

First of all, it must be pointed out that a homopolymer which undergoes a chemical reaction (reversible or irreversible) gives rise to a series of copolymers of progressively changing composition as the reaction advances. 

The cases of first order chemical reaction for irreversible and reversible processes are especially important in practice. 

Renewable carbon resources is a misnomer the earth s carbon is in a perpetual state of flux. Carbon is not consumed such that it is no longer available in any form. Reversible and irreversible chemical reactions occur in such a manner that the carbon cycle makes all forms of carbon, including fossil resources, renewable. It is simply a matter of time that makes one carbon from more renewable than another. If it is presumed that replacement does in fact occur, natural processes eventually will replenish depleted petroleum or natural gas deposits in several million years. Eixed carbon-containing materials that renew themselves often enough to make them continuously available in large quantities are needed to maintain and supplement energy suppHes biomass is a principal source of such carbon. 

Some chemical reactions are reversible and, no matter how fast a reaction takes place, it cannot proceed beyond the point of chemical equilibrium in the reaction mixture at the specified temperature and pressure. Thus, for any given conditions, the principle of chemical equilibrium expressed as the equilibrium constant, K, determines how far the reaction can proceed if adequate time is allowed for equilibrium to be attained. Alternatively, the principle of chemical kinetics determines at what rate the reaction will proceed towards attaining the maximum. If the equilibrium constant K is very large, for all practical purposes the reaction is irreversible. In the case where a reaction is irreversible, it is unnecessary to calculate the equilibrium constant and check the position of equilibrium when high conversions are needed. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Reversible electron transfer followed by an irreversible chemical reaction, ErC l mechanism ... 

Reversible chemical reaction preceding an irreversible electron transfer, CrEi mechanism ... 

The double arrows indicate reversibifity, an intrinsic property of all chemical reactions. Thus, for reaction (1), if A and B can form P and Q, then P and Q can also form A and B. Designation of a particular reactant as a substrate or product is therefore somewhat arbitrary since the products for a reaction written in one direction are the substrates for the reverse reaction. The term products is, however, often used to designate the reactants whose formation is thermodynamically favored. Reactions for which thermodynamic factors strongly favor formation of the products to which the arrow points often are represented with a single arrow as if they were irreversible ... 

Depending on the nature of the system, the adsorption process can be either reversible or irreversible. In the first case an adsorption equilibrium exists between the particles adsorbed on the adsorbent s surface and the particles in the electrolyte (or in any other phase contacting with the adsorbent). After removing the substance from the electrolyte, adsorbed particles leave the surface and reenter into the electrolyte. In the case of an irreversible adsorption, the adsorbed particles remain at the surface even if their concentration in the bulk phase drops to zero. In this case the adsorbed particles can be removed from the surface only by means of a chemical reaction... 

Theoretically, all chemical reactions are reversible. There are, however, many reactions in which the extent of the reverse reaction (i.e., combination of the products to produce the reactants) is very small as to be considered negligible. Such reactions which are ordinarily found to proceed to completion in one direction are said to be irreversible reactions. The decomposition of potassium chlorate... 

Irreversibility versus reversibility inpolarography. Previously in this chapter we dealt only with reversible redox systems, i.e., with truly Nemstian behaviour and merely diffusion control. This also applies to combined processess of electron transfer and chemical reaction (e.g., complexation) provided that both take place instantly. For instance, in EC such as... 

Separation in absorption is sometimes enhanced by adding a component to the liquid that reacts with the solute. The discussion regarding absorption has so far been restricted to physical absorption. In chemical absorption, chemical reactions are used to enhance absorption. Both irreversible and reversible reactions can be used. An example of an irreversible reaction is the removal of S02... 

In an irreversible metal hydride, hydrogen is generated through a chemical reaction, which is not easily reversible onboard a vehicle. Common reactant is water or an alcohol. For example, sodium borohydride can produce hydrogen as... 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Chemical "affinity" remained part of the tool kit of the chemist, however badly defined and understood. Affinity cannot simply be explained away as heat, insisted Wurtz, a leading advocate of chemical and physical atomism in France in the generation following Dumas.58 As we will see in chapter 5, "energy" replaced "affinity" in the late 1800s as the driving force of chemical reactions. In addition, the concepts of spontaneity and irreversibility entered the domain of physics, undermining the classical mechanics of matter and force in which processes are, in principle, reversible. Conceptually, the notions of spontaneity and irreversibility were more closely allied with experimental results in classical chemistry than in classical physics. 

No current ratio /pr//pf exists. In this connection, it must be taken into account that the lack of any reverse response is not sufficient to diagnose an electrochemically irreversible step. As we will see below, the presence of chemical reactions involving the electrogenerated species can make the reverse response disappear. 

For a preceding chemical reaction two mechanisms are possible, depending on whether the electron transfer is reversible or irreversible. 

At least from a theoretical viewpoint, several situations are possible depending on the extent of electrochemical reversibility of the electron transfer and on the reversibility or irreversibility of the chemical reaction following the electron transfer. 

First-order irreversible chemical reaction following a reversible electron transfer. The general scheme of the ErQ mechanism is ... 

Second-order irreversible chemical reaction following a reversible electron transfer dimerization. It is quite common in chemical reactions that newly formed radicals couple to each other. This also often happens in the electrochemical generation of radicals according to a dimerization process that can be written as ... 

Diagnostic criteria to identify an irreversible dimerization reaction following a reversible electron transfer. In the presence of a chemical reaction following an electron transfer, the dependence of the cyclic voltammetric parameters from the concentration of the redox active species are sufficient by themselves to reveal preliminarily a second-order complication (a ten-fold change in concentration from = 2 10-4 mol dm-3 to 2 10-3 mol dm-3 represents a typical path). 

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