Rates of Reversible Reactions

Any complex reaction can be separated into elementary steps. The rate of the slowest elementary step determines the rate of the overall reaction and is called the rate determining step. If the slow step is the first step, the rate law can be derived directly from this step and no other. If the slow step is other than the first step, the slow step is still the rate determining step, but steps prior to the slow step will contribute to the rate law. Steps after the slow step will make no contribution to the rate law. 

Remember that the slow step determines the rate. 

Notice that if we add these two equations together, we arrive at the original equation. Elementary steps must add to give the complex reaction. Since the first step is the slow step, the rate law for the overall reaction is given by this step and is  

Wc know that the exponent for [N02] is 2 because we derived the rate law from an elementary equation, and in the elementary equation two N02 molecules collide to create a reaction. Automatically using the coefficient of the balanced equation for the exponent in the rate law works only if the equation is elementary. Don t forget, the rate law above assumes negligible contribution from the reverse reaction, and it also assumes a sufficient concentration of CO for the fast step to occur. 

When the first step of a reaction series is the fast step, things can be a little tricky. If the first reaction is the fast reaction, the rate of the overall reaction is still equal to the rate of the slowest step. However, now one of the products of the fast step is a reactant in die slow step. Such a species is called an intermediate. The concentration of the intermediate is tricky to predict. An intermediate is usually not stable. If we 

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A large fraction of the chemical reactions known are used to form heterocyclic compounds. Displacement reactions and cycloadditions are particularly important, and their rates are therefore of great practical interest. The same is true for the rates of reverse reactions — ring opening by displacements or retrocycloadditions. It was realized over the last 40 years that... 

Reactivity increases with greater stability of the leaving group as an anion and with its lower basicity and lower nucleophihcity. These properties produce a low rate of reverse reaction of the product with the departed leaving group. 

I. The rate of forward reaction is equal to the rate of reverse reaction. 

The presentation of the rate of reversible reaction, r, as a difference of two values, r+ and r, helps in the derivation of kinetic equations instead of deriving the equation for r, one may derive the equation for r+ by means of (71), which is simpler, and then obtain at once the equation for r, applying (73). 

Substituent or solvent effects may be similar for concerted and stepwise processes. It has been shown that provided the rates of reverse reactions are almost independent of changes in oxidation potential, plots of E°, the standard reduction potential for the half cell (8) against log kf for a series of acceptors, Ox +, reacting with a hydride donor must have a slope of 30 mV/ log unit whether the rate-limiting step is hydride transfer, or hydrogen-atom transfer, or electron transfer (Kurz and Kurz, 1978). 

These expressions, with known initial concentrations of A and B, allow the time course of the approach to equilibrium to be described. (We consider this simple reversible reaction later when we take up rates of reversible reactions in Section 2.13.)... 

Vacuum Volatile products released from the reactant are lost most easily during reactions in vacuum. No corrections are required for sample buoyancy or atmosphere flow. However, the absence of convective heating reduces the rates of heat movement and may introduce uncertainties into measured reactant temperatures. The effect of product availability on rates of reversible reactions is reduced, but, at least in some of these reactions,... 

Exponent in general order of reaction expression Partial pressure Universal gas constant Component R, S also moles of component R, S Reaction rate Initial reaction rate Rate of reverse reaction Frequency factor in Arrhenius expression... 

Rate of forward reaction = 1(03] Rate of reverse reaction = i[02][0]... 

As discussed in Section 1.3.1, the first exponent in Eq. (2.4) describes the direct (oxygen reduction) reaction, while the second exponent represents the reverse reaction of water electrolysis. At 77 > RT/ aF) the second exponent in Eq. (2.4) can be neglected and (pBV reduces to t (Section 1.3.3). Physically, far from equilibrium the rate of reverse reaction is negligibly small. 

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