Chapter 13 – Solutions

13.1 – The Solution Process Know these terms: solution, solvent, solute The ability of substances to form solutions depends on natural tendency toward mixing. intermolecular forces.

Natural tendency toward mixing: The formation of solutions is favored by the increase in entropy that accompanies mixing. Intermolecular forces: Any of the intermolecular forces of attraction can be the attraction between solute and solvent molecules. In order for a solution to form, the following must occur regarding the intermolecular forces: 1. Solute–solute interactions must be overcome to disperse these particles when making a solution. 2. Solvent–solvent interactions must be overcome to make room for the solute. 3. Solvent–solute interactions occur as the particles mix.

Energetics of Solution Formation Enthalpy of Solution (Hsoln) = the sum of 3 enthalpies: the enthalpy to separate solvent molecules (always positive (endothermic)) the enthalpy to separate solute molecules (always positive (endothermic)) the enthalpy of bringing together the solute and solvent (always negative (exothermic))

Solution vs. Chemical Reaction? Just because a substance disappears when it comes in contact with a solvent, it does not mean the substance dissolved. It may have reacted, like nickel with hydrochloric acid. Since nickel changed from neutral to an ion, a chemical reaction occurred. Dissolving does NOT involve a chemical reaction – it is a physical process.

13.2 – Saturated Solutions and Solubility The solution-making process and crystallization are opposing processes. When the rate of the opposing processes is equal (i.e. at equilibrium), the maximum solute has been dissolved, and additional solute will not dissolve unless some crystallizes from solution. This is a saturated solution. If we have not yet reached the amount that will result in crystallization, we have an unsaturated solution.

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a given temperature. Saturated solutions have that amount of solute dissolved. Unsaturated solutions have any amount of solute less than the maximum amount dissolved in solution.

Supersaturated Solutions In supersaturated solutions, the solvent holds more solute than is normally possible at that temperature. They are formed at a high temperature, and then slowly cooled. These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

13.3 – Factors Affecting Solubility The extent to which one substance dissolves in another depends on: The nature of both substances (Solute–solvent Interactions) Pressure (for gaseous solutes) Temperature

Solute – Solvent Interactions In general, the stronger the attractions between solute and solvent molecules, the greater the solubility of the solute in that solvent. General rule: “like dissolves like” (substances with similar intermolecular attractive forces tend to be soluble in one another) Liquids that mix in all proportions (i.e. ethanol in water) are called miscible. Liquids that don’t dissolve in one another (i.e. gasoline and water) are called immiscible. solubility of alcohols

Solubility and Biological Importance Fat-soluble vitamins (like vitamin A) are nonpolar; they are readily stored in fatty tissue in the body. Water-soluble vitamins (like vitamin C) need to be included in the daily diet, as they are not stored in the body.

Pressure Effects The solubility of solids and liquids are not appreciably affected by pressure. Gas solubility is affected by pressure.

Henry’s Law The solubility of a gas is proportional to the partial pressure of the gas above the solution. or S1/P1 = S2/P2

Temperature Effects For most solids, as temperature increases, solubility increases. For all gases, as temperature increases, solubility decreases. Cold rivers have higher oxygen content than warm rivers. Cold soda is more carbonated than warm soda.

13.4 – Expressing Solution Concentration Qualitative ways of expressing solution concentration: Dilute – relatively small concentration of solute Concentrated – large concentration of solute Quantitative ways of expressing solution concentration: Molarity (M) = moles of solute / liters of solution Mole fraction(i) = moles of component / total moles of all components Mole fractions have no units. The sum of all the mole fractions of a solution equals 1.