Forward reaction definition

This definition for reaction order is directly meaningful only for irreversible or forward reactions that have rate expressions in the form of Equation (1.20). Components A, B,... are consumed by the reaction and have negative stoichiometric coefficients so that m = —va, n = —vb,. .. are positive. For elementary reactions, m and n must be integers of 2 or less and must sum to 2 or less. 

It is not possible at present to evaluate this function theoretically. The system is too complicated. The complexity of theoretical calculations can best be explained by considering the definition of AGg and processes involved in the activation process. AGf is defined in Figure 6.1. Free energy of activation for the forward reaction is the free-energy difference between the free energy of the activated state, G, and the free energy of the initial state, G ... 

Since all of the above-mentioned interconversion reactions are reversible, any kinetic analysis is difficult. In particular, this holds for the reaction Sg - Sy since the backward reaction Sy -+ Sg is much faster and, therefore, cannot be neglected even in the early stages of the forward reaction. The observation that the equilibrium is reached by first order kinetics (the half-life is independent of the initial Sg concentration) does not necessarily indicate that the single steps Sg Sy and Sg Sg are first order reactions. In fact, no definite conclusions about the reaction order of these elementary steps are possible at the present time. The reaction order of 1.5 of the Sy decomposition supports this view. Furthermore, the measured overall activation energy of 95 kJ/mol, obtained with the assumption of first order kinetics, must be a function of the true activation energies of the forward and backward reactions. The value found should therefore be interpreted with caution. 

It is easy to see some of the factors affecting the equilibrium composition. From the definition in Eq. 9.42, large values of Kp dictate that the products of the reaction are favored over the reactants that is, thermodynamics pushes the forward reaction toward completion. From Eq. 9.44, a reaction that is very exothermic, AH° << 0, favors product formation that is, Kp will be very large. 

Proton transfers in solution reach equilibrium very rapidly and for all weak acids and bases, we have to consider the reverse proton transfer reaction as well as the forward reaction. For example, the CN ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN- is a base it is called the conjugate base of HCN. In general, a conjugate base is the species left when an acid donates a proton ... 

The correct answer is (C). Bases in this reaction would be considered the substances that are accepting protons (Bronsted-Lowry definition). In the forward reaction, HzO receives a hydrogen to become a hydronium ion. In the reverse reaction, CN receives a proton to become HCN. HCN donates a proton, which makes it an acid. 

When K has a value much greater than 1, the product concentrations are relatively large and the reactant concentrations are relatively small. In both cases, however, the rate of the forward reaction equals the rate of the reverse reaction at equilibrium (this is a definition of equilibrium). 13. No, it doesn t matter in which direction the equilibrium position is reached. Both experiments will give the same equilibrium position since both experiments started with stoichiometric amounts of reactants or products. 15. When equilibrium is reached, there is no net change in the amount of reactants and products present since the rates of the forward and reverse reactions are equal. The first diagram has 4 A2B molecules,... 

Figure 18-2a shows a mixture of nitrogen and hydrogen just as the reaction begins at a definite, initial rate. No ammonia is present so only the forward reaction can occur. 

The rate of the forward reaction is RatCf = f[A] [B] the rate of the reverse reaction is RatCj. = j.[A2B]. In these expressions, and k. are the specific rate constants of the forward and reverse reactions, respectively. By definition, the two rates are equal at equilibrium (Ratef = Ratej.). So we write... 

The simplified analysis of kinetics given here is only valid if the back reaction can be neglected. For example, as reaction 1.98 proceeds, the product C accumulates and may begin to dissociate back to A and B. (Eventually, once the back reaction rate equals that of the forward reaction, steady-state or equilibrium is achieved.) For this reason, kinetic studies are typically done in the early stages of a reaction before back reactions begin to invalidate the definition of reaction rate as given by equation 1.92. 

Tabulated values of equilibrium constants for weak acids and bases, by definition, correspond to the equation with the un-ionized molecules on the left side of the equation. That is not to say that the reverse reactions cannot proceed indeed they proceed to a far greater extent than the forward reaction, but the tabulated values are for the un-ionized molecules reacting to form ions. 

Ammonia—a Bronsted-Lowry base All of the acids and bases that fit the Arrhenius definition of acids and bases also fit the Bronsted-Lowry definition. But some other substances that lack a hydroxide group and, therefore, cannot be considered bases according to the Arrhenius definition can be classified as acids according to the Bronsted-Lowry model. One example is ammonia (NHsj.When ammonia dissolves in water, water is a Bronsted-Lowry acid in the forward reaction. Because the NH3 molecule accepts a H+ ion to form the ammonium ion (NH4+), ammonia is a Bronsted-Lowry base in the forward reaction. 

This idea can be expressed in another way the rate of proton transfer for the forward reaction is diffusion-controlled if the acceptor binds the proton more tightly than the donor, that is, pK pK. As long as this inequality prevails, /f is constant. By definition, log/f - log/, = pK - pK y = A(p/C) so that log i, is linearly related to A pK), This can be expressed in still another way as... 

The nature of catalysis in homogeneous systems has been the subject of a considerable amount of research. A catalyst is any substance which affects the rate of reaction but is not consumed in the overall reaction. From thermodynamic principles we know that the equilibrium constant for the overall reaction must be independent of the mechanism, so that a catalyst for the forward reaction must also be one for the reverse reaction. In aqueous solution, a large number of reactions are catalyzed by acids and bases for our purposes we shall employ the Bronsted definition of acids and bases as proton donors and acceptors, respectively. Catalysis by acids and bases involves proton transfer either to or from the substrate. For example, the dehydration of acetaldehyde hydrate is subject to acid catalysis [20], probably by the mechanism (II). 

Given this definition of an equilibrium constant, what does it tell us For example, what does a large equilibrium constant (Kgq 1) imply about a reaction It means that the forward reaction is largely favored and that there will be more products than reactants when equilibritun is reached. For example, consider the reaction ... 

By our definition of a chemical reaction, reactants are consumed and products are produced. This is known as forward reaction. A reversible process is one in which the products can also he consumed to produce reactants, a process known as the reverse reaction. 

This observation is the first part of the cancellation puzzle [20, 21, 27, 29]. We know from Section lll.B that we should be able to solve it directly by applying Eq. (19), which will separate out the contributions to the DCS made by the 1-TS and 2-TS reaction paths. That this is true is shown by Fig. 9(b). It is apparent that the main backward concentration of the scattering comes entirely from the 1-TS paths. This is not a surprise, since, by definition, the direct abstraction mechanism mentioned only involves one TS. What is perhaps surprising is that the small lumps in the forward direction, which might have been mistaken for numerical noise, are in fact the products of the 2-TS paths. Since the 1-TS and 2-TS paths scatter their products into completely different regions of space, there is no interference between the amplitudes f (0) and hence no GP effects. 

TOF spectra of the H atom products have been measured at 18 laboratory angles (from 117.5° to —50° at about 10° intervals). Figure 19 shows a typical TOF spectrum at the laboratory (LAB) angle of —50° (forward direction). By definition, the forwardness and backwardness of the OH product is defined here relative to the 0(7D) beam direction. The TOF spectrum in Fig. 19 consists of a lot of sharp structures. All these sharp structures clearly correspond to individual rotational states of the OH product, indicating that these TOF spectra have indeed achieved rotational state resolution for the 0(1D)+H2 — OH+H reaction. By converting these TOF spectra from the laboratory (LAB) frame to the center-of-mass (CM) frame... 

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