Solutions Ch 15 & 16

What is a solution? A solution is uniform mixture that may contain solids, liquids, or gases. Known as a homogenous mixture Solution = solute + solvent Solvent – The substance in greater abundance in the solution A solvent dissolves the solute. (dissolving medium) Solute – The substance dissolved in the solvent Solutions: Solid-solid (alloy) Solid-liquid (kool-aid) Liquid-liquid (alcoholic beverages; ethanol and water) Liquid-gas (carbonated beverages) Gas-gas (air; oxygen, hydrogen, carbon dioxide)

Characteristics of Solutions Soluble – The solute’s ability to dissolve in a solvent Insoluble – The solute is not able to dissolve in a solvent Immiscible – Two liquids that can be mixed together, but separate shortly after you stop mixing them Miscible – Two liquids that are soluble in each other

Solvation in Aqueous Solutions Solvation – The process of surrounding solute particles with solvent particles to form a solution Aqueous solution – A solute dissolved in water Caused by constant random molecular movement of solvent and solute particles (KMT)

Rules for Solvation Like dissolves like Polar molecules dissolve polar molecules Non-polar molecules dissolve non-polar molecules Non-polar Molecule Polar Molecule CO2 O H2O H H Equal distribution of electrons Unequal distribution of electrons

1 2 3 4 The water is polar so it dissolves the polar color and sugar. The alcohol has a polar area but a larger non-polar area. It is not a good dissolver of color or sugar. The oil is non-polar and does not dissolve the color or sugar. The polar parts of the glucose molecule are attracted to and interact with water and cause glucose to dissolve.

Factors that Affect Rate of Solvation Need to increase collisions between solute and solvent molecules Agitating the mixture (Stirring) New collisions between solute and solvent will occur Increasing the surface area of the solute (Crushing the solute) Greater surface area allows more collisions to occur Increasing the temperature of the solvent Increases kinetic energy of molecules and more frequent collisions occur Increasing the pressure (For gases) Increases the rate of collision between particles. “how fast something dissolves”

Solubility Solubility – The maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure. Solubility is usually expressed in grams of solute per 100 g of solvent or in grams/liter. How much is dissolved

Saturated solution contains the maximum amount of dissolved solute for a given amount of solvent at a specific pressure and temperature. Unsaturated solution contains less dissolved solute for a given temperature and pressure than a saturated solution Supersaturated solution contains more dissolved solute than a saturated solution at the same temperature

Factors That Affect Solubility Temperature Most substances as temperature increases solubility increases Gases are the exception and solubility tends to decrease as temperature increases, because they are moving quickly escaping the solvent Pressure The solubility of a gas in any solvent increases as the pressure above the solution increases, keeping the gas from escaping in the solvent

Exception

Example 1 If the solubility of KNO3 at 20˚C is 79 grams in 250 grams of water, what is the solubility in grams per liter?

Example 2 If the solubility of NaCl at 25˚C is 36.2 g/100 g H2O, what is the maximum mass of NaCl that can be dissolved in 200.0 g of H2O at the same temperature?

Henry’s Law Henry’s Law states that at a given temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure (P)

Example 3 The solubility of a gas is 0.35 g/L at 25.0 kPa of pressure. What is the solubility when the pressure is increased to 115 kPa?

Electrolytes  

Electrolytes dissociate into individual ions and conduct an electric current

Strong Electrolyte Weak Electrolyte Nonelectrolyte Conducts a strong current. NaCl Conducts a weak current. HgCl2 Does not conduct a current. Sugar water

Strong and Weak Electrolytes A strong electrolyte conducts a strong current and the compound has completely dissociated into ions NaCl A weak electrolyte conducts a poor current because only part of the solute exists as ions. NH3 (Ammonia) A nonelectrolyte does not dissociate or form ions, thus does not conduct a current. Most molecular compounds are nonelectrolytes Sucrose (sugar)