Solutions C-16 Properties of solutions 2015

Solutions … Mixture (but special)  Solute + solvent Homogeneous (molecular level) Do not disperse light Do not settle out

Colloids do disperse light

Colloids Have larger particles Do not settle out Disperse light (Tyndall effect) Ex. Milk (homogenized)

Suspensions Have large particles Will settle out on standing Can be filtered out Particles are not dissolved, just held in the mixture until they “fall out” Ex. Clay in water

Making solutions What the solute and the solvent are determines  Whether a substance will dissolve.  How much will dissolve.

SOLVATION - solvent particles attract solute particles and cause dissolving Here an ionic crystal is being pulled apart by water molecules.

Vocab. check Soluble – the solute will dissolve Insoluble – the solute will not dissolve Electrolyte – a solute that conducts electricity when dissolved Nonelectrolyte – a solute that does not conduct electricity when dissolved Miscible – two liquids which dissolve Immiscible – two liquids which do not dissolve

Rate of dissolving A substance dissolves faster if-  It is stirred or shaken.  The particles are made smaller. (surface area)  The temperature is increased. Know why

Solutes dissolve until equilibrium is reached.

How Much? Solubility- The maximum amount of substance that will dissolve at that temperature (usually g/L). Saturated solution- Contains the maximum amount of solute dissolved. Unsaturated solution- Has less than the maximum dissolved. (Can dissolve more solute.) Supersaturated- A solution that is temporarily holding more than it can, a seed crystal will make excess come out

Heat of Solution  Another ∆H value that shows the energy change during the dissolving process.  If ∆H is negative, energy is released  If ∆H is positive, energy is absorbed Three steps 1) Separate solute 2) Separate solvent 3) Combine

Heat of Solution diagram

Solubility can be shown in graphs The line in a graph represents the maximum amount of solute that can dissolve at a particular temperature.

Temp. Solubility g/ 100ml

Liquids Miscible means that two liquids can dissolve in each other. Ex. Water and Alcohol Immiscible means they can’t. Forms layers  Ex. Water and oil (Depends on the bonding)

What affects solubility? For solids in liquids as the temperature goes up the solubility 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.  (Henry’s Law)

Pressure increase forces gas into solution

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.

Measuring Solutions

Concentration A measure of the amount of solute dissolved in a certain amount of solvent. Concentrated solution has a large amount of solute. Dilute solution has a small amount of solute. Sometimes g/l or g/mL or g/100 mL. But chemical reactions don’t happen in grams

Molarity The number of moles of solute in 1 Liter of the solution. M = moles/Liter What is the molarity of a solution with 2.0 moles of NaCl in 4.0 Liters of solution? What is the molarity of a solution with 3.0 moles dissolved in 250 mL of solution?

Making solutions Pour in a small amount of solvent. Then add the solute and dissolve it Fill to final volume. M x V (L) = moles

Dilution Adding water to a solution

Dilution The number of moles of solute doesn’t change if you add more solvent.  The moles before = the moles after  M 1 x V 1 = M 2 x V 2  M 1 and V 1 are the starting concentration and volume.  M 2 and V 2 are the final concentration and volume. Stock solutions are pre-made to known M

Colligative Properties Depends only on the number of dissolved particles Not the type of particle

Vapor Pressure The IMF 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 dissolved → more pieces. NaCl  Na + + Cl - 2 pieces More pieces →bigger effect.

Boiling Point Elevation The vapor pressure determines the boiling point. Lowered vapor pressure →causes raised boiling point. Salt water boils above 100ºC

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

Molality a new unit for concentration m = Moles of solute kilogram of solvent What is the molality of a solution with 9.3 mole of NaCl in 450 g of water? molality = 9.3 mol / 0.45 kg = 20.7m

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 determined by the molality.  T f = K f x m x n  T f is the change in the 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.