Transcript Chapter 13
Chapter 13 Properties of Solutions The Solution Process A solution is formed when one substance disperses uniformly throughout another. The ability of substances to form solutions depends on two factors: 1. The types of intermolecular interactions involved in the solution process 2. The tendency of substances to spread into larger volumes The Effect of Intermolecular Forces Any type of intermolecular force can operate between solute and solvent particles in a solution. A major factor determining whether a solution forms is the strengths of the intermolecular forces between and among the solute and solvent particles. Review of Intermolecular Forces • Dipole-dipole –occurs when polar molecules attract each other • London dispersion- temporary dipole on one atom induces a temporary dipole in another atom • Hydrogen bonding- a special type of intermolecular attraction between the hydrogen atom in a polar bond and a nearby small electronegative ion or atom, usually F,O, or N • Ion-dipole- exists between an ion and the partial charge on the end of a polar molecule(see p. 440) Why doesn’t NaCl dissolve in nonpolar solvents such as hexane, C6H14? There must be enough energy to separate the Na+ and Cl- ions and disperse them throughout a solvent. C6H14 is nonpolar, and ions are not attracted to nonpolar molecules. Therefore, the energy required to separate the ions in NaCl is not recovered by solute-solvent interactions. Three kinds of intermolecular interactions are involved in solution formation: 1. Solute-Solute interactions must be overcome to disperse the solute particles throughout the solvent 2. Solvent-Solvent interactions must be overcome to make room for the solute particles in the solvent 3. Solvent-solute interactions occur as the particles mix Energy Changes and Solution Formation There are three components in the energy changes that occur when solutions are formed: 1. Breaking the solute-solute interactions /\H1 2. Breaking the solvent-solvent interactions /\H2 3. Forming the solute-solvent interactions/\H3 /\Hsoln = /\H1 + /\H2 + /\H3 Quick Enthalpy Review H is the symbol for enthalpy. An enthalpy change (/\H) is the amount of energy absorbed by a system as heat during a process at constant pressure. Breaking the Solute-Solute Interactions Regardless of the solute being considered, separation of the solute particles from one another requires an input of energy. This process is always endothermic. Breaking the Solvent-Solvent Interactions Breaking the solvent-solvent interactions also requires energy and is an endothermic process. Forming the Solute-Solvent Interactions This energy change arises from the interactions between the solute and solvent and is an exothermic process. Depending on the magnitude of each of the three terms, the formation of a solution can be an endothermic or exothermic process. Processes that are exothermic tend to proceed spontaneously, so in order for a solution to form the solvent-solute interaction must be strong enough for /\ H 3 to be comparable in size to /\H1 + /\H2. Solution Formation, Spontaneity, and Entropy When two substances are mixed, dissolving occurs spontaneously– that means it occurs without any extra input of energy from the outside system. The first basic principle identifying spontaneous processes and the direction they take is: Processes in which the energy content of the system decreases tend to occur spontaneously. Some spontaneous processes do not follow this principle. In fact , even some endothermic processes occur spontaneously. In Figure 13.6 , the CCl4 and the C6H14 combine to form a solution , resulting in the molecules of each being more dispersed after a solution was formed. Formation of the solution has increased the degree of randomness, because the molecules of each substance are now mixed and distributed in a volume twice as large as before. The degree of randomness of the system, sometimes called disorder, is termed entropy. This illustrates our second basic principle: Processes occurring at a constant temperature in which the randomness or dispersal in space(entropy) of the system increases tend to occur spontaneously. Summary The solution process involves two factors: a change in enthalpy and a change in entropy. In most cases , the formation of solutions is favored by the increase in entropy that accompanies mixing.