Solutions molecular view

Stratt R M and Maroncelli M 1996 Nonreactive dynamics in solution the emerging molecular view of solvation dynamics and vibrational relaxation J. Phys. Chem. 100 12 981... 

Molecular view of an aqueous solution of ammonium nitrate. Ammonium ions separate from nitrate ions, but both these species remain intact as polyatomic clusters. 

Any acid that undergoes quantitative reaction with water to produce hydronium ions and the appropriate anion is called a strong acid. Table gives the structures and formulas of six common strong acids, all of which are supplied commercially as concentrated aqueous solutions. These solutions are corrosive and normally are diluted for routine use in acid-base chemistry. At the concentrations normally used in the laboratory, a solution of any strong acid in water contains H3 O and anions that result from the loss of a proton. Example shows a molecular view of the proton transfer reaction of a strong acid. 

In one type of titration, a solution of a strong base such as sodium hydroxide is added slowly to a solution that contains an unknown amount of an acid. Each hydroxide ion added to the acid solution accepts one proton from a molecule of acid. As the titration proceeds, fewer and fewer acid molecules remain in the acid solution, but the solution is still acidic. At the stoichiometric point, just enough hydroxide ions have been added to react with every acidic proton present in the acid solution before the titration was started. The hydroxide ions in the next drop of titrant do not react because acid molecules are no longer present in the solution. Before the stoichiometric point, the solution contains excess acid. After the stoichiometric point, the solution contains excess OH". Figure 4-11 shows a titration setup and molecular views illustrating titration of a strong acid by a strong base. 

When a strip of zinc metal is added to a solution of copper(II) sulfate, the blue color slowly fades, and the zinc metal is replaced by copper metal (Figure 4-13). As copper ions in the solution are reduced to copper metal, zinc atoms are oxidized to Zn cations. This is an example of a metal displacement reaction, in which a metal ion in solution (Cu ) is displaced by another metal (Zn) by means of a redox reaction. Figure 4-13 also shows molecular views of this displacement reaction. 

Figure 12-10 is a molecular view showing that the equilibrium concentration of a dissolved gas varies with the partial pressure of that gas. An increase in the partial pressure of gas results in an increase in the rate at which gas molecules enter the solution. This increases the concentration of gas in solution. The increased concentration in solution, in turn, results in an increase in the rate at which gas molecules escape from the solution. Equilibrium is reestablished when the solute concentration is high enough that the rate of escape equals the rate of capture. 

Molecular view of a gasnsolution equilibrium, (a) At equilibrium, the rate of escape of gas molecules from the solution equals the rate of capture of gas molecules by the solution, (b) An increase in gas pressure causes more gas molecules to dissolve, throwing the system out of equilibrium, (c) The concentration of solute increases until the rates of escape and capture once again balance. 

Figure 12-11 is a molecular view of how a solute changes this liquid-vapor equilibrium of the solvent. The presence of a solute means that there are fewer solvent molecules at the surface of the solution. As a result, the rate of solvent evaporation from a solution is slower than the rate of evaporation of pure solvent. At equilibrium, the rate of condensation must be correspondingly slower than the rate of condensation for the pure solvent at equilibrium with its vapor. In other words, the vapor pressure drops when a solute is added to a liquid. A solute decreases the concentration of solvent molecules in the gas phase by reducing the rates of both evaporation and condensation. 

This molecular view of Figure 12-11 suggests that the extent of vapor pressure lowering will depend on the fraction of solvent molecules that has been replaced. In other words, the vapor pressure should be proportional to the mole fraction of the solvent. The molecular view also suggests that this effect does not depend on the nature of the solute, but only on its mole fraction. Experiments show that this is often the case, particularly for dilute solutions. A simple equation, Raoult s law, expresses this proportionality between vapor pressure and mole fraction V V /Jpuj-g solvent Raoulfs law states that the vapor pressure of a solution is the... 

Molecular views of the rates of solid-liquid phase transfer of a pure liquid and a solution at the normal freezing point. The addition of solute does not change the rate of escape from the solid, but it decreases the rate at which the solid captures solvent molecules from the solution. This disrupts the dynamic equilibrium between escape and capture. 

A molecular view of the major species present in an aqueous solution of acetic acid (vinegar). ... 

A molecular view of the solubility equilibrium for a solution of sodium chloride in water. At equilibrium, ions dissolve from the crystal surface at the same rate they are captured, so the concentration of ions in the solution remains constant. 

In a solution of a weak acid, the major species are water molecules and the acid, HA. The products of the proton transfer reaction, H3 0+ and A, are present in smaller concentrations as minor species. Figure 17-5 provides a molecular view. 

The drawing shows a molecular view of a very small region of an aqueous solution of oxalic acid. For clarity, water molecules are not shown. Redraw this molecular picture to show the solution (a) after two hydroxide ions react with these molecules and (b) after four hydroxide ions react with these molecules. Include in your drawings the water molecules that form as products. 

C18-0017. The molecular view below represents a small portion of a solution that matches Point A on the corresponding titration curve. Redraw the molecular picture to show how the figure should look for each of the points B-D along the titration curve. Your drawings should show any water molecules formed as part of the titration process. 

When a strip of zinc metal is dipped in a solution of copper(II) sulfate, zinc is oxidized to 7n (a >5 )1 and q) is reduced to copper metal. The insoluble metal precipitates from the solution, hi the molecular views, water moiecuies and spectator anions have been omitted for clarity. 

The enhancement of kerosene dissolution occurs even at low humic acid content in the aqueous solution. In view of the fact that humic substances are relatively high molecular weight species containing nonpolar organic moieties, Chiou et al. (1986) assumed that a partition-like interaction between a solute of very low solubility in aqueous solution and a microscopic organic environment of dissolved humic molecules can explain solute solubility enhancement. 

We consider a molecular view of a portion of the fluid, shown schematically in Fig. 12.10. A test plane is shown at height z where the concentration of solute species is N/V. For convenience, we will denote this concentration, in number of molecules per unit volume, as C(z). The sheets of molecules separated by one mean-free path L above and below the test plane have concentrations... 

FIGURE 11.8 A molecular view of Henry s law. (a) At a given pressure, an equilibrium exists in which equal numbers of gas particles enter and leave the solution, (b) When pressure is increased by pushing on the piston, more gas particles are temporarily forced into solution than are able to leave, so solubility increases until a new equilibrium is reached (c). 

From a computational point of view, the effects of the surrounding medium on the NMR parameters can be divided into direct and indirect solvent effects [5], The direct effects arise from the interaction of the electronic distribution of the solute with the surrounding medium, assuming a fixed molecular geometry, while indirect (secondary) effects are caused by the changes in the solute molecular geometry by the solvent. Experimentally the total effect is observable, while in the computational models they can be separated. 

In this chapter, we will review some of the work that we have been doing in recent years in the context of solvation and dynamical properties in polar and non-polar supercritical solutions using molecular dynamics computer simulations. First we will discuss solvation of alkaloids in SC-CO2 and provide detailed molecular views of the main structural features of the local density augmentation around simple alkaloids... 

Although separation of amylose from amylopectin is the main purpose of the process described, it has been observed that subfractionation of at least one of the two components of starch (namely, amylose) likewise occurs. In this connection, it is to be noted here that the experimental results strongly indicate that fractionation of amylose into its different molecular-weight species is remarkably efficient in salt solutions. In view of the great disparity in solvent power between the precipitant and the solvent in the case of aqueous salt solutions, this conclusion seems to confirm some predictions of Flory s. "... 

FI6URE 12.2 A molecular view of the solution process portrayed as taking place in three steps First the solvent and solute molecules are separated (steps I and 2). Then the solvent and solute molecules mix (step 3). 

A MOLECULAR VIEW OF THE SOLUTION PROCESS Review Questions... 

The presence of a solute causes vapor-pressure lowering of a solvent. If the solute is nonvolatile (nonevaporating), the solution has a lower vapor pressure than the pure solvent does. (Review vapor pressure in Chapter 7.) From a molecular view, the solute particles at the surface of the liquid inhibit the movement of solvent molecules from going into the vapor phase, but do not inhibit solvent molecules in the vapor phase from returning to the liquid phase, so the rate of evaporation is lower than the rate of condensation until there are fewer solvent molecules in the vapor phase. For solving problems, the vapor pressure of any component (call it A) in the solution. Pa, is related to the vapor pressure of the pure substance, P, by Raoult s law ... 

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