Concentration reaction direction prediction

Commonly, the electrode reaction is assumed to be reversible , which simplifies the mathematics considerably because a direct prediction of surface concentrations is possible from the potential alone, as in Fig. 6. However, kinetic information is entirely lacking from experiments conducted under reversible conditions since the electrode reaction is then an equilibrium. 

The value of an equilibrium constant is independent of the concentrations of the reactants and products. However, the position of equilibrium is very definitely influenced by the concentrations. The.direction of change is readily predictable from Le Chatelier s principle. Consider "tfie reaction of ironODDO with iodide ... 

The experimental data of Lamb and Elder (96), however, are not in agreement with this predicted rate expression for they find the initial rate proportional to the square of the ferrous ion concentration and directly proportional to the oxygen pressure. This has recently been confirmed by the author (97), and it would appear that either the autoxidation is subject to a true catalysis by trace impurities (an induced reaction is excluded by the total ferrous ion oxidized being large, about M/20) or the actual mechanism is different from that suggested by Weiss. 

At low pH values, when additional protons are present, the separation step becomes reversible and one observes homogeneous proton recombination. The reaction under these conditions undergoes a transition from unimolecular (correlated pairs) to a bimolecular (or pseudo-unimolecular) reaction. The rate of this recombination reaction is expected to diminish with increasing concentration of inert salt, which screens the Coulombic attraction between the proton and the anion. In fact, the classical Bronsted-Bjerrum theory of salt effects puts all of the effect in the recombination reaction while predicting zero salt effect on the dissociation direction [7]. 

Concepts presented in this and the two preceding chapters give students an integrated insight into some central issues in chemistry Why and in which direction does the reaction proceed How fast does it proceed and How far does it proceed The critical use of the equilibrium constant summarizes these critical interpretations. Important concepts in prior chapters are utilized to aid student use of LeChatelier s Principle— predicting and calculating the effects on reaction direction and extent when temperature, pressures, concentrations, and other reaction conditions are altered. 

Sample Problem 17.4 relies on molecular scenes to predict reaction direction and Sample Problem 17.5 relies on concentration data. 

Predicting Reaction Direction and Calculating Equilibrium Concentrations... 

Using the reaction quotient Given the concentrations of substances in a reaction mixture, predict the direction of reaction. (EXAMPLE 15.5)... 

The question just asked can be stated more generally Given the concentrations of substances in a reaction mixture, will the reaction go in the forward or the reverse direction To answer this, you evaluate the reaction quotient and compare it with the equilibrium constant K. The reaction quotient has the same form as the equilibrium-constant expression, but the concentrations of substances are not necessarily equilibrium values. Rather, they are concentrations at the start of a reaction. To predict the direction of reaction, you compare with K. ... 

High-Pressure Concern (see Table 6.21. For gas phase reactions, the concentration of reactants is proportional to the pressure. For a situation where the reaction rate is directly proportional to the concentration, operation at 25 bar rather than at 1 bar would increase the reaction rate by a factor of 25 (assuming ideal gas behavior). Although we do not know that the rate is directly proportional to the concentration, we can predict that the effect of pressure is likely to be substantial, and the reactor size will be substantially reduced. 

Use the initial concentrations to calculate the reaction quotient (0 for the initial concentrations. Compare QtoKto predict the direction in which the reaction will proceed. 

CH3I should approach the enolate from the direction that simultaneously allows its optimum overlap with the electron-donor orbital on the enolate (this is the highest-occupied molecular orbital or HOMO), and minimizes its steric repulsion with the enolate. Examine the HOMO of enolate A. Is it more heavily concentrated on the same side of the six-membered ring as the bridgehead methyl group, on the opposite side, or is it equally concentrated on the two sides A map of the HOMO on the electron density surface (a HOMO map ) provides a clearer indication, as this also provides a measure of steric requirements. Identify the direction of attack that maximizes orbital overlap and minimizes steric repulsion, and predict the major product of each reaction. Do your predictions agree with the thermodynamic preferences Repeat your analysis for enolate B, leading to product B1 nd product B2. 

Kgp values can be used to make predictions as to whether or not a precipitate will form when two solutions are mixed. To do this, we follow an approach very similar to that used in Chapter 12, to determine the direction in which a system will move to reach equilibrium. We work with a quantity Q, which has the same mathematical form as K. The difference is that the concentrations that appear in Q are those that apply at a particular moment. Those that appear in are equilibrium concentrations. Putting it another way, the value of Q is expected to change as a precipitation reaction proceeds, approaching Ksp and eventually becoming equal to it. 

We have seen in these three examples a reaction that is second-order but not bi-molecular, another whose rate varies directly with a species not involved in the stoichiometric process, and a third whose rate is independent of the concentration of one reactant. Not one of these findings could have been predicted from the stoichiometric equation, which can guide one a priori neither to the rate law nor to the mechanism. 

We have seen that the value of an equilibrium constant tells us whether we can expect a high or low concentration of product at equilibrium. The constant also allows us to predict the spontaneous direction of reaction in a reaction mixture of any composition. In the following three sections, we see how to express the equilibrium constant in terms of molar concentrations of gases as well as partial pressures and how to predict the equilibrium composition of a reaction mixture, given the value of the equilibrium constant for the reaction. Such information is critical to the success of many industrial processes and is fundamental to the discussion of acids and bases in the following chapters. 

Predict the direction of a reaction, given K and the concentrations of reactants and products (Example 9.5). 

All these steps can influence the overall reaction rate. The reactor models of Chapter 9 are used to predict the bulk, gas-phase concentrations of reactants and products at point (r, z) in the reactor. They directly model only Steps 1 and 9, and the effects of Steps 2 through 8 are lumped into the pseudohomoge-neous rate expression, a, b,. ..), where a,b,. .. are the bulk, gas-phase concentrations. The overall reaction mechanism is complex, and the rate expression is necessarily empirical. Heterogeneous catalysis remains an experimental science. The techniques of this chapter are useful to interpret experimental results. Their predictive value is limited. 

Particle phase reactions of pesticides in the atmosphere is are an area of great uncertainty [Atkinson et al (1999)], and no direct conclusions about possible impacts can be drawn from just the fact that they are not resolved in the model. High particle bound mass fractions are predicted in high latitudes (>80 %) in winter. Thus, degradation in air, as it is assumed to be limited to the gaseous phase, is reduced. An additional degradation process in the particle phase is assumed to reduce concentrations in the Arctic, consequently. On the other hand lifetimes of particle-bound DDT is limited by deposition, much more than in the gas-phase. 

So far in this chapter, you have worked with reactions that have reached equilihrium. What if a reaction has not yet reached equilihrium, however How can you predict the direction in which the reaction must proceed to reach equilihrium To do this, you substitute the concentrations of reactants and products into an expression that is identical to the equilihrium expression. Because these concentrations may not he the concentrations that the equilihrium system would have, the expression is given a different name the reaction quotient. The reaction quotient, Qc, is an expression that is identical to the equilihrium constant expression, but its value is calculated using concentrations that are not necessarily those at equilihrium. 

Case 1 appears to accurately predict the observed dependence on persulfate concentration. Furthermore, as [Q]+otal approaches [KX], the polymerization rate tends to become independent of quat salt concentration, thus qualitatively explaining the relative insensitivity to [Aliquat 336]. The major problem lies in explaining the observed dependency on [MMA]. There are a number of circumstances in free radical polymerizations under which the order in monomer concentration becomes >1 (18). This may occur, for example, if the rate of initiation is dependent upon monomer concentration. A particular case of this type occurs when the initiator efficiency varies directly with [M], leading to Rp a [M]. Such a situation may exist under our polymerization conditions. In earlier studies on the decomposition of aqueous solutions of potassium persulfate in the presence of 18-crown-6 we showed (19) that the crown entered into redox reactions with persulfate (Scheme 3). Crematy (16) has postulated similar reactions with quat salts. Competition between MMA and the quat salt thus could influence the initiation rate. In addition, increases in solution polarity with increasing [MMA] are expected to exert some, although perhaps minor, effect on Rp. Further studies are obviously necessary to fully understand these polymerization systems. 

As we saw with the steady-state water-column application of the one-dimensional advection-diffusion-reaction equation (Eq. 4.14), the basic shapes of the vertical concentration profiles can be predicted from the relative rates of the chemical and physical processes. Figure 4.21 provided examples of profiles that exhibit curvatures whose shapes reflected differences in the direction and relative rates of these processes. Some generalized scenarios for sedimentary pore water profiles are presented in Figure 12.7 for the most commonly observed shapes. 

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