Properties of Solutions Chapter 11

Composition of Solutions  Solutions = homogeneous mixtures, any state of matter

(mol solute*cation charge) L solution Mass Percent Molarity How to Calculate Concentration Molality Normality Mole Fraction

Energies of Solutions  “Like Dissolves Like” Polar solvents dissolve polar solutesPolar solvents dissolve polar solutes Non-Polar solvents dissolve non-polar solutesNon-Polar solvents dissolve non-polar solutesWHY? ENTHALPY OF SOLUTION  3 part process 1. Solute components separate (endo) 2. Overcome IMF’s in solvent (endo) 3. Solvent and Solute interact (exo)

Enthalpy of Solution   H soln =  H 1 +  H 2 +  H 3 Heat of solution can either be + or – Heat of solution can either be + or –   is expected to be small for non-polar molecules except for when they are very large molecules.  H soln is large and + This rarely occurs  H soln is large and - This usually occurs  H soln is close to zero Exceptions, because Spontaneity does not Depend solely on enthalpy

Try Me!  Decide whether liquid hexane (C 6 H 14 ) or liquid methanol (CH 3 OH) is more appropriate solvent for substances, grease (C 20 H 42 ) and potassium iodide (KI).

Factors Affecting Solubility  Structure---Polarity Non polar are hydrophobic Non polar are hydrophobic Polar are hydrophillic Polar are hydrophillic Some are a combo with different ends. Some are a combo with different ends. Vitamin C and scurvy Vitamin C and scurvy  Pressure---works only on gases C = kP (Henry’s Law) C = kP (Henry’s Law) [gas]  P vapor [gas]  P vapor  Temperature---not always a direct relationship Gas in water; as T increases, solubility decreases Gas in water; as T increases, solubility decreases See solubility curves for individual compound reactions to temperature. See solubility curves for individual compound reactions to temperature.

Colligative Properties  A property of a solution that depends only on the number, and not on the identity of the solute particles present. Vapor Pressure Vapor Pressure Boiling Point Elevation Boiling Point Elevation Freezing Point Depression Freezing Point Depression Osmotic Pressure Osmotic Pressure

Vapor Pressure  Non-volatile Solute added to solvent Reduces the tendency of the solvent molecules to escape (evaporate) Reduces the tendency of the solvent molecules to escape (evaporate) Equilibrium pressure of the solvent is higher than equilibrium pressure of solution. More of the solvent (pure) has to evaporate to be at equilibrium than is the case in a beaker full of solution. Equilibrium pressure of the solvent is higher than equilibrium pressure of solution. More of the solvent (pure) has to evaporate to be at equilibrium than is the case in a beaker full of solution.

François Raoult and his Law  Vapor pressure of a solution is directly proportional to the mole fraction of the solvent present. P soln = X solvent P o solvent Just what I was thinking!! Me Too! I agree!

Try Me!  Calculate the expected vapor pressure at 25 o C for a solution prepared by dissolving g of table sugar (sucrose molar mass = g/mol) in cm 3 of water. The vapor pressure of water is torr at this temperature.

Answer  torr

More solute, lower vapor pressure  Predict the vapor pressure of a solution prepared by mixing 35.0 g solid Na2SO4 with 175 g water at 25oC. The vapor pressure of water is still torr.

Volatile solutes  Raoult’s Law is modified to account for the vapor pressure of both solute.  P total = P a + P b = X a P a o + X b P b o  Always some degree of deviation from ideal…more similar the solute is to the solvent the more ideal it behaves.

Boiling Point Elevation  When a solute is added, vapor pressure is lowered.  Must be heated to higher temperature to give reach normal boiling point.  Boiling point is higher than usual.   T = K b m solute

Freezing Point Depression  When a solute is added, the vapor pressure is lowered.  Water in the solution has a lower vapor pressure than that of pure ice.  Ice will not form.  Freezing point has been lowered.   T = K f m solute

Try Me!  A chemist is trying to identify a human hormone, which controls metabolism by determining its molar mass. A sample weighing g was dissolved in 15.0 g benzene, and the freezing point depression was determined was determined to be o C. Calculate the molar mass of the hormone.

Osmotic Pressure  Solvent, but not solute can pass through a semi-permeable membrane via osmosis.  When pure solvent and solution volumes are stabilized (the system is at equilibrium) there is a greater pressure on the solution side.  The difference in pressure is the Osmotic Pressure.

 = MRT  Used to determine osmotic pressure. P is osmotic pressureP is osmotic pressure M is molarity of solutionM is molarity of solution R is the gas law constant ( Latm/Kmol)R is the gas law constant ( Latm/Kmol) T is the temperature in KelvinT is the temperature in Kelvin

Colligative Properties of Electrolyte Solutions  The type of solute DOES matter sometimes  Only in ability to dissociate  Compare: 0.1 m glucose to 0.1m NaCl

Van’t Hoff Factor  Van’t Hoff Factor expresses the relationship between moles of solute and solution:  This is a multiplier number that describes the ratio of elevation or depression. i = moles particles moles solute  T = iK b m  T = iK f m  = iMRT

Ion Pairing  Van’t Hoff factor is fallible.  Ion Pairing: at a certain instant, some ions may not be dissociated, and thus count as a single particle.  Still use equations, but keep in mind that there can be some deviation.

Colloids  Suspension of tiny particles in a medium caused by electrostatic repulsion.  Tyndall Effect: Scattering of light effect by suspended particles.