Solubility and Intermolecular Forces

• Solubility refers to the amount of a solute that dissolves in a specific amount of solvent at a given temperature.
• Increased water solubility allows medications to be absorbed into the bloodstream more readily.
• Factors that affect solubility include intermolecular interactions, temperature, and pressure.
• Medications forming hydrogen, dipole-dipole, or ion-dipole bonds are more water-soluble than those forming covalent bonds. Alkanes and alkenes do not form hydrogen bonds with water.
• A saturated solution contains the maximum amount of solute that can dissolve.
• A supersaturated solution contains more solute than its solubility allows.
• Supersaturated solutions are unstable and the excess solute precipitates out as a solid, liquid, or gas.
• Miscible liquids are soluble in each other in all proportions.

"Like Dissolves Like"

• Solubility is enhanced when intermolecular interactions between substances have similar electron configurations.
• Polar solutes dissolve best in polar solvents, while nonpolar solutes dissolve best in nonpolar solvents.
• Salt (NaCl) dissolving in water demonstrates this principle, due to the similar polarities of water and salt's components.
• Nonpolar substances like nitrogen, carbon dioxide, and oxygen are typically insoluble in polar solvents like water.

Effect of Temperature on Solubility

• The solubility of solid and liquid solutes in liquid solvents generally increases as temperature increases.
• Gas solubility in liquids is inversely related to temperature.
• As temperature rises, less gas dissolves in a liquid because increased kinetic energy enables gas molecules to escape, preventing further dissolving.
• Lower temperatures reduce the kinetic energy of gas molecules, allowing them to dissolve into liquids.
• An example of how temperature affects solubility is the slower emergence from anesthesia in hypothermic patients receiving volatile anesthetic agents.
• Hypothermic patients retain anesthetic gases in the blood, as increased solubility is related to temperature.

Energy Changes & the Solution Process

• When a solute dissolves, there is an associated energy change that often results in a temperature change.
• The heat of solution, or enthalpy of solution (ΔHsoln), represents the energy change when dissolving one mole of solute in a solvent.
• Enthalpy (H) equals heat (Q) when pressure is constant.
• The energy change can be endothermic or exothermic.
• An exothermic process releases energy from the system (solute and solvent) to the surroundings, increasing the solution temperature.
• An endothermic process absorbs energy from the surroundings into the system, decreasing the solution temperature.
• Whether the heat of solution is endothermic or exothermic depends on the relative magnitudes of the lattice energy and the heat of solvation.
• If more energy is needed to separate ions than released by solvation, ΔHsoln is positive (endothermic).
• If more energy is released by solvation than needed to separate ions, ΔHsoln is negative (exothermic), decreasing heat.
• Breaking bonds requires energy (endothermic, cooling effect), while forming new bonds releases energy (exothermic, warming effect).
• Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.
• If more heat energy is released than taken in, the reaction is exothermic; if more heat energy is taken in than released, the reaction is endothermic.
• In an endothermic reaction, an increase in temperature shifts the equilibrium towards the products.
• In an exothermic reaction, an increase in temperature shifts the equilibrium towards the reactants.

Le Châtelier’s Principle

• The solubility of a solute decreases with increasing temperature if ΔHsoln is negative (exothermic).
• The solubility of a solute increases with increasing temperature if ΔHsoln is positive (endothermic).
• A system will attempt to restore equilibrium after a change, according to Le Châtelier’s Principle.
• The position of equilibrium is influenced by changes in temperature, pressure, and concentration.
• More reactants (endothermic) favor a shift in the equilibrium to the right (products).
• More products (exothermic) will shift the equilibrium to the left (reactants).
• A decrease in the equilibrium constant indicates a shift towards the reactants.

Solubility is the amount of solute that dissolves in a solvent at a specific temperature. Intermolecular interactions, temperature, and pressure affect solubility. Polar solutes dissolve best in polar solvents and nonpolar solutes dissolve best in nonpolar solvents.