# Unit 11 Solutions Essential Questions: What factors determine the rate at which a solute dissolves?

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Unit 11 Solutions Essential Questions: What factors determine the rate at which a solute dissolves?

Recall the Terms Solute ◦ The substance being dissolved (the one that changes phase) Solvent ◦ The substance doing the dissolving Universal solvent is water If it is a solution of 2 liquids or 2 solids, the solvent is the one in large quantity

Solution formation Nature of the solute and the solvent ◦ Whether a substance will dissolve ◦ How much will dissolve Factors determining rate of solution... ◦ stirred or shaken (agitation) ◦ particles are made smaller ◦ temperature is increased Why?

Making solutions In order to dissolve, the solvent molecules must come in contact with the solute. Stirring moves fresh solvent next to the solute. The solvent touches the surface of the solute. Smaller pieces increase the amount of surface area of the solute.

Temperature and Solutions Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often. ◦ Speeds up dissolving. Usually increases the amount that will dissolve (exception is gases)

Gases In Liquids Henry’s Law - says the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid ◦ Think of a bottle of soda ◦ Removing the lid releases pres. Equation: S 1 S 2 P 1 P 2 =

Liquids Recall from Unit 9 Miscible means that two liquids can dissolve in each other ◦ water and antifreeze, water and ethanol Immiscible means they can’t ◦ oil and vinegar

How Much? Solubility-The maximum amount of substance that will dissolve at a specific temperature (g solute/100 g solvent) Saturated solution ◦ Contains the maximum amount of solute dissolved Unsaturated solution ◦ Can still dissolve more solute Supersaturated ◦ Solution that is holding more than it theoretically can

Solubility Summary For solids in liquids, as the temperature goes up-the solubility usually goes up For gases in a liquid, as the temperature goes up-the solubility goes down For gases in a liquid, as the pressure goes up-the solubility goes up

Concentration Is… A measure of the amount of solute dissolved in a given quantity of solvent A concentrated solution has a large amount of solute A dilute solution has a small amount of solute – Thus, only qualitative descriptions But, there are ways to express solution concentration quantitatively

Concentration of Solutions ◦ Recall from Unit #4  Molarity  Molality  Mole fraction  % by weight  % by volume  Recall how to make solutions

What is equal at Equilibrium? Rates are equal Concentrations are not. Rates are determined by concentrations and activation energy. The concentrations do not change at equilibrium. or if the reaction is verrrry slooooow.

Law of Mass Action For any reaction jA + kB lC + mD K = [C] l [D] m PRODUCTS power [A] j [B] k REACTANTS power K is called the equilibrium constant. is how we indicate a reversible reaction Ignore pure solids and liquids

Law of Mass Action Write the equilibrium constant expression for the reaction equation: NH 3 (aq) + HCl (aq) NH 4 + (aq) + Cl - (aq) What is K when [NH 3 ] = 0.100M, [HCl] = 1.00M, [NH 4 + ] = 0.200M, [Cl - ] = 0.100M

Ksp Solubility product constant Write dissociation equation NaCl (s) Na + (aq) + Cl - (aq) Ksp= [Na + ] [Cl - ] No denominator since it will always be a solid

Ksp If your Ksp calculated value is = to actual value the solution is saturated If your Ksp calculated value is greater than the actual value the solution is supersaturated If your Ksp calculated value is less than the actual value the solution is unsaturated

Colligative Properties Depend only on the number of dissolved particles Not on what kind of particle

Vapor Pressure decreased The bonds between molecules keep molecules from escaping. In a solution, some of the solvent is busy keeping the solute dissolved. Lowers the vapor pressure Electrolytes form ions when they are dissolved = more pieces. NaCl  Na + + Cl - (= 2 pieces) More pieces = bigger effect

Boiling Point Elevation The vapor pressure determines the boiling point. Lower vapor pressure = higher boiling point. Salt water boils above 100ºC The number of dissolved particles determines how much, as well as the solvent itself.

Freezing Point Depression Solids form when molecules make an orderly pattern. The solute molecules break up the orderly pattern. Makes the freezing point lower. Salt water freezes below 0ºC How much depends on the number of solute particles dissolved.

Why Molality? The size of the change in boiling point is determined by the molality.  T b = K b x m x n  T b is the change in the boiling point K b is a constant determined by the solvent m is the molality of the solution. n is the number of pieces it falls into when it dissolves.

What about Freezing? The size of the change in freezing point is also determined by molality.  T f = -K f x m x n  T f is the change in freezing point K f is a constant determined by the solvent m is the molality of the solution. n is the number of pieces it falls into when it dissolves.

Molar Mass We can use changes in boiling and freezing to calculate the molar mass of a substance Find: 1) molality 2) moles, and then 3) molar mass

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