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Ch 12 notes - Ch 12 notes The spontaneous mixing of gases...

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Ch 12 notes The spontaneous mixing of gases illustrates one of nature's strong “driving forces” for change. A system, left to itself, will tend toward the most probable state . 1 At the instant we remove the partition, the container holds two separate gas samples, in contact but unmixed. This represents a highly improbable state because of the natural motions of the molecules. The vastly more probable distribution is one in which the molecules are thoroughly mixed, so formation of the gaseous solution involves a transition from a highly improbable state to a highly probable one. What limits the ability of most substances to mix completely, however, are intermolecular forces of attraction. For a liquid solution to form, there must be a balance among the attractive forces so the natural tendency of particles to intermingle can proceed. In other words, the attractive forces between molecules within the solvent and between molecules within the solute must be about as strong as attractions between solute and solvent molecules. immiscible , meaning they are mutually insoluble. like dissolves like” rule Strongly polar and ionic solutes tend to dissolve in polar solvents and nonpolar solutes tend to dissolve in nonpolar solvents. “like dissolves like” principle also applies to the solubility of solids in liquid solvents. As they become free, the ions become completely surrounded by water molecules The phenomenon is called the hydration of ions The general term for the surrounding of a solute particle by solvent molecules is solvation , so hydration is just a special case of salvation. Attractions between the solvent and solute dipoles help to dislodge molecules from the crystal and bring them into solution. The total energy absorbed or released when a solute dissolves in a solvent at constant pressure to make a solution is called the molar enthalpy of solution , or usually just the heat of solution, Δ H soln . Energy is required to separate the particles of solute and also those of the solvent and make them spread out to make room for each other. This step is endothermic , because we must overcome the attractions between molecules to spread the particles out. But once the particles come back together as a solution , the attractive forces between approaching solute and solvent particles yield a decrease in the system's potential energy, and this is an exothermic change. The enthalpy of solution, Δ H soln , is simply the net result of these two opposing enthalpy contributions. The heat of solution for a solid is the lattice energy plus the solvation energy For a solid dissolving in a liquid, it is convenient to imagine a two-step path. Step 1. Vaporize the soli substances and io Step 2. Bring the separa
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exothermic, and in the solvent is c also be called the Solution of a liquid in another liquid can be modeled as a three-step process Step 1.
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