The Formation of a Solution

2 Describe the formation of a saturated solution from the time excess solid solute is first placed into a liquid solvent. 

3 Identify and explain the factors that determine the time required to dissolve a given amount of solute or to reach equilibrium. 

When a soluble ionic crystal is placed in water, the negatively charged ions at the crystal s surface are attracted by the positive region of the polar water molecules (Fig. 16.6). A mg of war for the negative ions begins. Water molecules tend to pull them from the crystal, while neighboring positive ions tend to hold them in the crystal. In a similar way, positive ions at the crystal s surface are attracted to the negative portion of the water molecules and are torn from the crystal. Once released, the ions are surrounded hy the polar water molecules. Such ions are said to be hydrated. 

The dissolving process is reversible. As the dissolved solute particles move randomly through the solution, they come into contact with the undissolved solute and crystallize, that is, return to the solid state. For NaCl this process may be represented by the reversible reaction equation 

Determination of molecular polarity (Section 13.5) follows from an analysis of bond polarity (Section 12.4) and molecular geometry (Section 13.3). 

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Note on the laboratory preparation of monoethylaniline. Although the laboratory preparation of monomethyl- or monoethyl-aniline is hardly worth whUe, the following experimental details may be useful to those who wish to prepare pure monoethylaniline directly from amline. In a flask, fitted with a double surface reflux condenser, place 50 g. (49 ml.) of aniline and 65 g. of ethyl bromide, and boU gently for 2 hours or until the mixture has almost entirely sohdified. Dissolve it in water and boil off the small quantity of unreacted ethyl bromide. Render the mixture alkaUne with concentrated sodium hydroxide solution, extract the precipitated bases with three 50 ml. portions of ether, and distil off the ether. The residual oil contains anihne, mono- and di-ethylaniline. Dissolve it in excess of dilute hydrochloric acid (say, 100 ml. of concentrated acid and 400 ml. of water), cool in ice, and add with stirring a solution of 37 g. of sodium nitrite in 100 ml. of water do not allow the temperature to rise above 10°. Tnis leads to the formation of a solution of phenyl diazonium chloride, of N-nitrosoethylaniline and of p-nitrosodiethylaniline. The nitrosoethylaniline separates as a dark coloured oil. Extract the oil with ether, distil off the ether, and reduce the nitrosoamine with tin and hydrochloric acid (see above). The yield of ethylaniline is 20 g. 

A fundamental property of a substance is the tendency for its atoms or molecules to spread into the surrounding space. A consequence of this property is the observed vapor pressure of liquids and solids. In order to understand the effects of the formation of a solution on this property, reference may be drawn to a solution consisting of two substances, A and B, with A being the solvent and B the solute. If the vapor pressure, PA, of the solvent over the solution is considered, it is clear that it must be proportional to the amount of A present in the solution. Thus,... 

The formation of a solution depends on many factors, such as the nature of the solvent, the nature of the solute, the temperature, and the pressure. 

As you know from Chapter 7, a change is favoured when AG is negative. When a salt dissolves, the entropy of the system always increases, because ions in solution are more disordered than ions in a solid crystal. An increase in entropy favours the formation of a solution because the term -TAS is negative. Most solids dissolve to a greater extent at higher solution temperatures, because the term -TAS becomes more negative. 

On the other hand, it can be shown that the energy of the solution is also greater, energy being required to form the solution. So at low temperatures, where the entropy is ineffective, solutions are not formed and the two compounds are completely immiscible. With increasing temperature, the solubility will increase, more rapidly the smaller the change in energy which impedes the formation of a solution. 

A special type of reaction is the formation of a solution from its components,... 

The change in entropy for the formation of a solution of this type is27... 

Several subjects that might be considered under the title of thermochemistry are discussed in previous chapters. Such subjects are the heat capacities of a single-phase system, the dependence of the enthalpy of a single-phase system on temperature and pressure, and the dependence of the enthalpy of a one-component, multiphase system on the temperature, volume, and mole numbers. Here we are concerned with heat capacities of multiphase systems, with changes of enthalpy for the formation of a solution and for a change of concentration of the solution, and with changes of enthalpy of systems in which chemical reactions occur. First the basic concepts of calorimetry are reviewed. 

We observe that the standard change of enthalpy for the formation of a solute in an infinitely dilute solution is the sum of the standard change of enthalpy for the formation of the pure compound and the difference between its partial molar enthalpy in the infinitely dilute solution and its molar enthalpy when pure. 

As mentioned, for a solution to behave ideally the solute-solute, solvent-solvent, and solute-solvent interactions would have to be identical. This would correspond to a situation where AHw n = 0. On the other hand, when a solute and solvent release large quantities of energy in the formation of a solution— that is, when AHSO n is large and negative—we can assume that strong interactions exist between the solute and the solvent. In this case we expect a negative deviation from Raoult s law. 

Adsorption is the adhesion of molecules of a gas, liquid, or dissolved substance or of particles to the surface of a solid substance. Absorption is the assimilation of molecule into a solid or liquid subsunce, with the formation of a solution or a compound. Sometimes the word sorption is used to indude both of these phenomena. We say that a heated glass vessel adsorbs water vapor from the air on cooling, and becomes coated with a very thin layer pf water a dehydrating agent such as concentrated sulfuric add absorbs water, forming hydrates. Activated alumina sorbs water vapor, probably both by adsorption (the adhesion of a layer of water vapor to the surface of rhe particles.) and by absorptimi (the formation ipf aluimnunt hydroxMe). 

The enthalpy change associated with the formation of a solution from the solute elements and the solvent at 25°C is called the standard heat of formation of the solution. If a solution contains n moles of solvent per mole of solute, then... 

Available literature solubility data of pure lipids belonging to major (fatty acids, mono-, di- and triacylglycerols, and fatty acid esters) and minor lipid classes (pigments, sterols, vitamins, and hydrocarbons) in SCCO2 were compiled (26, 27). These references (26, 27) contain exhaustive bibliography on lipid + SCCO2 binary systems. Literature data were correlated using Chrastil s equation, which is an empirical model used quite commonly to correlate the solubility of lipid components (30). This model is based on the formation of a solute-solvent complex on association of the solute and solvent molecules and establishes a linear relationship between In(solubility) and In(density) as follows ... 

Polar compounds will dissolve in polar solvents because the latter will solvate the compound and thereby overcome the electrostatic forces which hold the crystal together. It is for this reason that polar compounds will not dissolve in nonpolar solvents. Similarly, nonpolar compounds -will not dissolve in polar solvents because the relatively strong intermolecular forces in the liquid would be decreased if a solution were formed. This makes the formation of a solution energetically unfavorable. 

In the formation of a solution the attractions among the particles in the original phases (solvent-to-solvent and solute-to-solute attractions) are broken up and replaced, at least in part, by new solvent-to-solute attractions. Unlike a compound, a solution has its components present in variable proportions and cannot be represented by a chemical formula. Equations for dissolution reactions do not include the solvent as a reactant. They indicate the original state of the solute in parentheses on the left side of the equation and identify the solvent in parentheses on the right side. For example, solid (s) sucrose dissolves in water to give an aqueous (aq) solution of sucrose ... 

Describe the formation of a solution in molecular terms by comparing intermolecular forces in the pure phases and in the solution (Section 11.2). 

The formation of a solution is a physical reaction. It involves three steps ... 

Since the combined mixture is more disordered than the separated pure substances, most of the time, the formation of a solution involves an increase in entropy. In fact, positional entropy always increases in the formation of a solution, so, on the MCAT, solution formation has positive entropy. 

Heat of solution (molar) The amount of heat absorbed in the formation of a solution that contains one mole of solute the value is positive if heat is absorbed (endothermic) and negative if heat is released (exothermic). 

Problem 3.4 Starting from Flory-Huggins equation (3.50) show that for the formation of a solution from a monodisperse polymer the partial molar Gibbs free energy of mixing, AGi, for the solvent is given by Eq. (3.53). 

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