Chemical reactions in solution

Most reactions in the general chemistry laboratory are carried out in solution. This is partly because mixing the reactants in solution helps to achieve the close contact between atoms, ions, or molecules necessary for a reaction to occur. The stoichiometry of reactions in solutions can be described in the same way as the stoichiometry of other reactions, as we saw in Example 4-6. A few new ideas that apply specifically to solution stoichiometry are also helpful. 

One component of a solution, called the solvent, determines whether the solution exists as a solid, liquid, or gas. In this discussion we will limit ourselves to aqtieous solutions— solutions in which liquid water is the solvent. The other components of a solution, called solutes, are dissolved in the solvent. We use the notation NaCl(aq), for example, to describe a solution in which liquid water is the solvent and NaCl is the solute. The term aqueous does not convey any information, however, about the relative proportions of NaCl and H2O in the solution. For this purpose, the property called molarity is commonly used. 

The composition of a solution may be specified by giving its molar concentration (or molarity), which is defined as the amount of solute, in moles, per liter of solution  

440 mol urea, CO(NH2)2, is dissolved in enough water to make 1.000 L of solution, the solution concentration, or molarity, is 

When calculating concentration, we must determine the amount of solute in moles from other quantities that can be readily measured, such as the mass of 

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As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Harris A L, Berg M and Harris C B 1986 Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution J. Chem. Phys. 84 788... 

Dne approach to the simulation of chemical reactions in solution is to use a combination t)f [uantum mechanics and molecular mechanics. The reacting parts of the system are treated [uantum mechanically, with the remainder being modelled using the force field. The total mergy Etot for the system can be written ... 

Chemical reactions are undoubtedly the most important issue in theoretical chemistry, where electronic structure plays an essential role. However, as will be demonstrated in this section, solvent effects also often play a crucial role in the mechanism of a chemical reaction in solution. 

Studies of chemical reactions in solution and in enzymes present an enormous challenge because of the enormous size and complexity of these systems. MM force fields have made a tremendous impact in certain areas, but they cannot... 

There are three main factors whose influence on chemical reactions in solution need to be considered (a) the nature of the solvent (b) temperature and (c) the presence of catalysts. 

To deduce the relation between rate constants and equilibrium constants, we note that the equilibrium constant for a chemical reaction in solution that has the form A + B C + D is... 

FIG. 5 Probing ionic reactions with a dual-pipette device, (a) Simple transfer of a cation (b) and (c) IT is followed by a chemical reaction in solution, (b) The reaction product is not transferred into the collector pipette, (c) Both the cation and the reaction product are collected. (Reprinted with permission from Ref. 10. Copyright 1998 American Chemical Society.)... 

In the absence of chemical reactions in solution, depends only on the normalized distance between the centers of the disks d/r, where d is the center-to-center separation). The theory developed for two identical coplanar disks gives the following approximation (valid at d/r 2) [23] ... 

When a compound that can form several modifications crystallizes, first a modification may form that is thermodynamically unstable under the given conditions afterwards it converts to the more stable form (Ostwald step rule). Selenium is an example when elemental selenium forms by a chemical reaction in solution, it precipitates in a red modification that consists of Se8 molecules this then converts slowly into the stable, gray form that consists of polymeric chain molecules. Potassium nitrate is another example at room temperature J3-KN03 is stable, but above 128 °C a-KNOs is stable. From an aqueous solution at room temperature a-KN03 crystallizes first, then, after a short while or when triggered by the slightest mechanical stress, it transforms to )3-KN03. 

Mo Y, Gao J, (2000) An ab initio molecular orbital-valence bond (MOVB) method for simulating chemical reactions in solution. J Phys Chem A 104(13) 3012-3020... 

Major DT, Gao J (2007) An integrated path integral and free-energy perturbation-umbrella sampling method for computing kinetic isotope effects of chemical reactions in solution and in enzymes. J Chem Theory Comput 3 949—960... 

Subsequent to a fire in a teaching laboratory, it was discovered that a mixture of equal weights of the three dry solids, itself stable, reacted violently when wetted with up to two parts of water and was capable of igniting paper. All components (which exhibit an oscillating chemical reaction in solution) were necessary for this effect. 

Most mechanistic work has focused on chemical reactions in solution or extremely simple processes in the gas phase. There is increasing interest in reactions in solids or on solid surfaces, such as the surfaces of solid catalysts in contact with reacting gases. Some such catalysts act inside pores of defined size, such as those in zeolites. In these cases only certain molecules can penetrate the pores to get to the reactive surface, and they are held in defined positions when they react. In fact, the Mobil process for converting methanol to gasoline depends on zeolite-catalyzed reactions. 

The most well-studied enzyme catalyzes the reaction S P. The kinetic question is how time influences the amount of S and P. In the absence of enzyme, the conversion of S to P is slow and uncontrolled. In the presence of a specific enzyme (S-to-Pase1), S is converted swiftly and specifically to product. S-to-Pase is specific it will not convert A to B or X to Y. Enzymes also provide a rate acceleration. If you compare the rate of a chemical reaction in solution with the rate of the same reaction with the reactants bound to the enzyme, the enzyme reaction will occur up to 1014 times faster. 

To make QM studies of chemical reactions in the condensed phase computationally more feasible combined quantum me-chanical/molecular mechanical (QM/MM) methods have been developed. The idea of combined QM/MM methods, introduced first by Levitt and Warshell [17] in 1976, is to divide the system into a part which is treated accurately by means of quantum mechanics and a part whose properties are approximated by use of QM methods (Fig. 5.1). Typically, QM methods are used to describe chemical processes in which bonds are broken and formed, or electron-transfer and excitation processes, which cannot be treated with MM methods. Combined QM and MM methods have been extensively used to study chemical reactions in solution and the mechanisms of enzyme-catalyzed reactions. When the system is partitioned into the QM and MM parts it is assumed that the process requiring QM treatment is localized in that region. The MM methods are then used to approximate the effects of the environment on the QM part of the system, which, via steric and electrostatic interactions, can be substantial. The... 

When QM/MM methods are used to study chemical reactions in solution and the reacting species are small enough to be treated completely with QM methods it is straightforward to separate... 

MONTE CARLO SIMULATIONS OF CHEMICAL REACTIONS IN SOLUTION... 

Beyond the clusters, to microscopically model a reaction in solution, we need to include a very big number of solvent molecules in the system to represent the bulk. The problem stems from the fact that it is computationally impossible, with our current capabilities, to locate the transition state structure of the reaction on the complete quantum mechanical potential energy hypersurface, if all the degrees of freedom are explicitly included. Moreover, the effect of thermal statistical averaging should be incorporated. Then, classical mechanical computer simulation techniques (Monte Carlo or Molecular Dynamics) appear to be the most suitable procedures to attack the above problems. In short, and applied to the computer simulation of chemical reactions in solution, the Monte Carlo [18-21] technique is a numerical method in the frame of the classical Statistical Mechanics, which allows to generate a set of system configurations... 

In order to apply the Monte Carlo method to a chemical reaction in solution, two general problems immediately appear. Firstly, how do the configurational space have to be sampled That is, which configurations are considered and what kind of chemical information can be extracted from them. Second, how is the potential energy of each configuration evaluated The discussion of this last point will be delayed until section 6. 

If only the solvation of the gas-phase stationary points are studied, we are working within the frame of the Conventional Transition State Theory, whose problems when used along with the solvent equilibrium hypothesis have already been explained above. Thus, the set of Monte Carlo solvent configurations generated around the gas-phase transition state structure does not probably contain the real saddle point of the whole system, this way not being a correct representation of the conventional transition state of the chemical reaction in solution. However, in spite of that this elemental treatment... 

Free energy is the key quantity that is required to determine the rate of a chemical reaction. Within the Conventional Transition State Theory, the rate constant depends on the free energy barrier imposed by the conventional transition state. On the other hand, in the frame of the Variational Transition State Theory, the free energy is the magnitude that allows the location of the variational transition state. Then, it is clear that the evaluation of the free energy is a cornerstone (and an important challenge) in the simulation of the chemical reactions in solution... 

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