Opposing Reversible Reactions

Examples of reversible reacting systems, the reaction networks of which involve opposing reactions, are  

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Consecutive reactions are those in which the product of one reaction is the reactant in the next reaction. These are also called series reactions. Reversible (opposing) reactions, autocatalytic reactions, and chain reactions can be viewed as special types of consecutive reactions. 

It is found that after the elapse of a sufficient time interval, all reversible reactions reach a state of chemical equilibrium. In this state the composition of the equilibrium mixture remains constant, provided that the temperature (and for some gaseous reactions, the pressure also) remains constant. Furthermore, provided that the conditions (temperature and pressure) are maintained constant, the same state of equilibrium may be obtained from either direction of a given reversible reaction. In the equilibrium state, the two opposing reactions are taking place at the same rate so that the system is in a state of dynamic equilibrium. 

To this point we have focused on reactions with rates that depend upon one concentration only. They may or may not be elementary reactions indeed, we have seen reactions that have a simple rate law but a complex mechanism. The form of the rate law, not the complexity of the mechanism, is the key issue for the analysis of the concentration-time curves. We turn now to the consideration of rate laws with additional complications. Most of them describe more complicated reactions and we can anticipate the finding that most real chemical reactions are composites, composed of two or more elementary reactions. Three classifications of composite reactions can be recognized (1) reversible or opposing reactions that attain an equilibrium (2) parallel reactions that produce either the same or different products from one or several reactants and (3) consecutive, multistep processes that involve intermediates. In this chapter we shall consider the first two. Chapter 4 treats the third. 

Opposing reactions. D. S. Martin has considered the following reversible reaction ... 

Opposing reactions. The reversible interconversion of d.y-[Cr(cn)2(OH)2 + to trans is characterized by an equilibrium constant of 0.16 and a forward rate constant of 3.30 X 10 4 s"1.In an experiment starting with the pure cis isomer, how long does it take to form half the equilibrium concentration of trans Starting with pure trans, how long does it take for half the equilibrium concentration of trans to form ... 

C19-0094. The alkaline diy cell is not rechargeable. The solid products separate from the electrodes, so the reverse reactions cannot occur. What reactions may take place if an opposing potential sufficient to reverse the reactions is applied to a dry cell ... 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

Reaction complexities include reversible or opposing reactions, reactions occurring in parallel, and reactions occurring in series. The description of a reacting system in terms of steps representing these complexities is called a reaction network. The steps involve only species that can be measured experimentally. 

Reversible reactions are also termed equilibrium or opposing reactions. 

The only reactions considered so far have been those that proceed to all intents and purposes (>95%) to completion. The treatment of revers/We reactions is analogous to that given above, although now it is even more important to establish the stoichiometry and the thermodynamic characteristics of the reaction. A number of reversible reactions are reduced to pseudo first-order opposing reactions when reactants or products or both are used in excess... 

For exothermic reversible reactions the situation is different, for here two opposing factors are at work when the temperature is raised—the rate of forward reaction speeds up but the maximum attainable conversion decreases. Thus, in general, a reversible exothermic reaction starts at a high temperature which decreases as conversion rises. Figure 9.5 shows this progression, and its precise values are found by connecting the maxima of the different rate curves. We call this line the locus of maximum rates. 

Note that when we write a dissociation, we use a forward-and-backward double arrow. to indicate that the reaction takes place simultaneously in both directions. That is, a dissociation is a dynamic process in which an equilibrium is established between the forward and reverse reactions. The balance between the two opposing reactions defines the exact concentrations of the various species in solution. We ll learn much more about chemical equilibria in subsequent chapters. 

It means that in a reversible reaction the catalyst accelerates the forward and the reverse reactions equally. Thus, the ratio of the rates of two opposing reactions, i.e., the equilibrium constant, remains unchanged. 

Now a reversible reaction is at a point of equilibrium when no further apparent change is taking place. The two opposing reactions are without doubt taking place just the same, but they exactly undo the effect of each other, making the total change zero. Therefore... 

Let us consider reactions for which reactants and products coexist at equilibrium with no complete conversion. The simplest case of these reactions is that the forward unimolecular reaction is opposed by the reverse unimolecular reaction as ... 

Kinetics of Opposing or Reversible Reactions The kinetics of such reactions are usually studied in the initial stages of the process when the products are at too low a concentration to set up the opposing reaction at a noticeable rate. However, when the opposing reaction also takes place at a comparable rate, the problem becomes complicated and the rate constant obtained is not quite reliable. 

Consider the simplest case of an opposing reaction in which the forward as well as the reverse reactions are both first-order ... 

When two reactions oppose each other, they will eventually reach a point where the amount of product formed is equal to the amount of reactant formed. This situation of an equal give and take is called a state oi equilibrium. Equilibrium is defined as a state of balance between two opposing reactions that are occurring at the same rate. Notice that the definition says nothing about the amounts or concentrations of any reactants or products. The only factors that are equal at equilibrium are the rates of the forward and reverse reactions. 

A substrate cycle is produced when a non-equilibrium reaction in the forward direction of a pathway is opposed by another non-equUibrium reaction in the reverse direction of the pathway. The two opposing reactions must be catalyzed by separate enzymes. 

This and similar reactions have been treated as the irreversible ones however, the reversible process carmot be excluded at least for some MtX which are strong enough electrophEes. The mechanism of this opposing reaction can be presented as follows for the polymerization of THF ... 

In the previous case we found a pressure that finally stops and reverses the reaction, but it is not the mere mechanical pressure that is effective. An equal pressure of air—say 18 atmospheres—would not, in the case discussed, stop the action of zinc on sulphuric acid. What does oppose the reaction, and shows itself as pressure, is a definite concentration of hydrogen, like the definite concentration of water vapour in hydrates. So we must in the third place turn to cases more specially suited to measure affinity, in which a pressure stops and reverses the reaction indifferently, whether it be exerted by hydrogen or by a piston. These are the transformations taking place without evolution of gas in so-called condensed systems, such as that described (p. 26) for sulphur the latter consists in complete conversion in one direction or the other according to temperature,... 

A thermodynamically reversible half-reaction can be defined as one that can be made to proceed in either of two opposing directions by an infinitesimal shift in the potential from its equilibrium value. Contrary to general opinion, such reactions are rare. This definition should not necessarily be used as a basis for an experimental test for two reasons (i) a finite potential shift must be made to produce a finite net current, and (2) the point of zero current is not always the equilibrium potential. While irreversibility can be revealed in many systems, proof of thermodynamic reversibility at the molecular level in others is virtually impossible. 

Opposing reactions suggested as causing the lowering in periodate uptake were (a) reversion, in which a potential aldehyde group of one chain interacts with a primary alcohol group of another chain, as in (9),... 

Note that the opposing reactions are not the exact reverse of one another. If we add the two opposing reactions together, we obtain the net reaction... 

Initially, if only A and B are present, the forward rate of reaction will proceed at a finite rate while there will be no reverse reaction because no C and D are present. However, as soon as the reaction of A and B produces C and D, they will combine, and by the reverse reaction produce A and B. The reaction will proceed until the opposing reaction rates are equal, and... 

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