Unit 11— Solutions Factors affecting solvation The Solvation Process Attractive forces exist among the particles of all substances London Dispersion Dipole-dipole Hydrogen bonding If the attractive forces between the solute and the solvent are greater than the attractive forces holding the solute particles together, the solute dissolves in the solvent The Solvation Process Solvation is the process of surrounding solute particles with solvent particles to form a solution. Solvation in water is called hydration. For ionic compounds that dissolve in water, the attraction between dipoles of a water molecule and the ions of a crystal are greater than the attraction among ions of a crystal. How do you know if a substance will dissolve? The general rule is “like dissolves like.” Polar will dissolve polar (or ionic) Nonpolar will dissolve nonpolar Is oil polar or nonpolar? Since it does not dissolve in water, and water is polar, oil is nonpolar Sucrose (sugar) molecules have several O–H bonds, which become sites for hydrogen bonding with water molecules. That’s why sugar dissolves in water. During solvation, the solute must separate into particles and move apart, which requires energy The overall energy change that occurs during solution formation is called the heat of solution. When energy is released, it is an exothermic process and feels warm or hot. When energy is absorbed, it is an endothermic process and feels cool or cold. Factors that affect rate of dissolving of solid in liquid 1. Agitation Stirring or shaking moves dissolved particles away from the contact surfaces more quickly and allows new collisions to occur. This is why you stir your tea after you put sugar in it. 2. Surface area Breaking the solute into small pieces increases surface area and allows more collisions to occur. This is why you use granulated sugar instead of sugar cubes Factors that affect rate of dissolving of solid in liquid 3. Temperature As temperature increases, rate of solvation increases. Particles are moving faster. This is why sugar dissolves faster in hot tea than in iced tea. Solubility Solubility depends on the nature of the solute and solvent. Solubility is affected by increasing the temperature of the solvent because the kinetic energy of the particles increases. Solubility of solids in liquids Generally increases with an increase in temperature (some exceptions) Not affected by pressure Solubility of gases in liquids Increases with an increase in pressure (Henry’s Law) This is how carbonated beverages are made and why they go flat if left open As the diver goes deeper in the water, pressure increases, allowing more gases (N2) to dissolve in his blood If the diver goes to the surface too quickly, the gases will un- dissolve in his blood vessels instead of his lungs This results in the bends (decompression sickness, caisson’s disease. .) Solubility of gases in liquids How is the solubility of gases affected by temperature? Solubility of gases decreases with an increase in temperature This is why soda at room temperature goes flat faster than soda in the refrigerator Thermal pollution What do fish breathe? O2 dissolved in water If these cooling towers dump hot water into the lake, what happens to the temperature of the water? Increases What happens to the O2 dissolved in the water? Decreases What happens to the fish? They could suffocate There are now regulations on how much the temperature of water can be affected by dumping hot water used for cooling Summary of factors that affect solubility Solubility of Solubility of solids in liquids gases in liquids Increase temperature Increase pressure Summary of factors that affect solubility Solubility of Solubility of solids in liquids gases in liquids Increase Generally increases temperature Increase pressure Summary of factors that affect solubility Solubility of Solubility of solids in liquids gases in liquids Increase Generally increases Decreases temperature Increase pressure Summary of factors that affect solubility Solubility of Solubility of solids in liquids gases in liquids Increase Generally increases Decreases temperature Increase No effect pressure Summary of factors that affect solubility Solubility of Solubility of solids in liquids gases in liquids Increase Generally increases Decreases temperature Increase No effect Increase pressure p. 511 127-129, 86-88, 93, 94 .