Solutions

Occur in all phases u The solvent does the dissolving. u The solute is dissolved. u There are examples of all types of solvents dissolving all types of solutes. u We will focus on aqueous solutions.

Ways of Measuring u Molarity = moles of solute Liters of solution u % mass = Mass of solute x 100 Mass of solution  Mole fraction of component A  A = N A N A + N B

u Molality = moles of solute Kilograms of solvent  Molality is abbreviated m Ways of Measuring

Energy of Making Solutions  Heat of solution (  H soln ) is the energy change for making a solution. u Most easily understood if broken into steps. u 1.Break apart solvent u 2.Break apart solute u 3. Mixing solvent and solute

1. Break apart Solvent  Have to overcome attractive forces.  H 1 >0 2. Break apart Solute.  Have to overcome attractive forces.  H 2 >0

3. Mixing solvent and solute   H 3 depends on what you are mixing.  Molecules can attract each other  H 3 is large and negative.  Molecules can’t attract-  H 3 is small and negative. u This explains the rule “Like dissolves Like”

EnergyEnergy Reactants Solution H1H1 H2H2 H3H3 Solvent Solute and Solvent  Size of  H 3 determines whether a solution will form H3H3 Solution

Types of Solvent and solutes  If  H soln is small and positive, a solution will still form because of entropy. u There are many more ways for them to become mixed than there is for them to stay separate.

Structure and Solubility u Water soluble molecules must have dipole moments -polar bonds. u To be soluble in non polar solvents the molecules must be non polar.

Soap u Hydrophobic non- polar end PO-O- CH 3 CH 2 O-O- O-O-

Soap u Hydrophilic polar end PO-O- CH 3 CH 2 O-O- O-O-

u A drop of grease in water u Grease is non-polar u Water is polar u Soap lets you dissolve the non-polar in the polar.

Hydrophobic ends dissolve in grease

Hydrophilic ends dissolve in water

u Water molecules can surround and dissolve grease. u Helps get grease out of your way.

Pressure effects u Changing the pressure doesn’t effect the amount of solid or liquid that dissolves u They are incompressible. u It does effect gases.

Dissolving Gases u Pressure effects the amount of gas that can dissolve in a liquid. u The dissolved gas is at equilibrium with the gas above the liquid.

u The gas is at equilibrium with the dissolved gas in this solution. u The equilibrium is dynamic.

u If you increase the pressure the gas molecules dissolve faster. u The equilibrium is disturbed.

u The system reaches a new equilibrium with more gas dissolved. u Henry’s Law. P= kC Pressure = constant x Concentration of gas

Temperature Effects u Increased temperature increases the rate at which a solid dissolves. u We can’t predict whether it will increase the amount of solid that dissolves. u We must read it from a graph of experimental data.

Gases are predictable u As temperature increases, solubility decreases. Why… u Gas molecules can move fast enough to escape.

Vapor Pressure of Solutions u A nonvolatile solvent lowers the vapor pressure of the solution. u The molecules of the solvent must overcome the force of both the other solvent molecules and the solute molecules.

Raoult’s Law:  P soln =  solvent x P solvent u Vapor pressure of the solution = mole fraction of solvent x vapor pressure of the pure solvent u Applies only to an ideal solution where the solute doesn’t contribute to the vapor pressure.

Aqueous Solution Pure water u PURE water has a higher vapor pressure than the solution

Aqueous Solution Pure water u So, water evaporates faster from the water beaker than solution

u The water condenses faster in the solution so it should all end up in that beaker. Aqueous Solution Pure water

Osmotic Pressure  ) u Is also a colligative property u Is equal to MRT (molarity x r x Kelvin) u Is the pressure just sufficient to prevent osmosis

u The vapor pressure of ethanol at 20 o C is mm Hg. u What is the vapor pressure of ethanol in a solution containing 5.0% by mass of iodine.  (assume 100g of solution and find  ) Review Question

Colligative Properties u Because dissolved particles affect vapor pressure - they affect phase changes. u Colligative properties depend only on the number - not the kind of solute particles present u Useful for determining molar mass

Boiling point Elevation u Because a non-volatile solute lowers the vapor pressure it raises the boiling point.  The equation is:  T = K b m solute   T is the change in the boiling point u K b is a constant determined by the solvent.  m solute is the molality of the solute

Freezing point Depression u Because a non-volatile solute lowers the vapor pressure of the solution it lowers the freezing point.  The equation is:  T = K f m solute   T is the change in the freezing point u K f is a constant determined by the solvent  m solute is the molality of the solute

1 atm Vapor Pressure of solution Vapor Pressure of pure water

1 atm Freezing and boiling points of water

1 atm Freezing and boiling points of solution

1 atm TfTf TbTb

Electrolytes in solution u Since colligative properties only depend on the number of molecules. u Ionic compounds should have a bigger effect. u When they dissolve they dissociate. u Individual Na and Cl ions fall apart. u 1 mole of NaCl makes 2 moles of ions. u 1mole Al(NO 3 ) 3 makes 4 moles ions.

u Electrolytes have a bigger impact on on melting and freezing points per mole because they make more pieces.  Relationship is expressed using the van’t Hoff factor i i = Moles of particles in solution Moles of solute dissolved u The expected value can be determined from the formula.

u The actual value is usually less because u At any given instant some of the ions in solution will be paired. u Ion pairing increases with concentration.  i decreases with ion concentration. u We can change our formulas to  T = iKm

u Liquid-liquid solutions where both are volatile. u Modify Raoult’s Law to  P total = P A + P B =  A P A 0 +  A P B 0 u P total = vapor pressure of mixture u P A 0 = vapor pressure of pure A u If this equation works then the solution is ideal. u Solvent and solute are alike. Ideal solutions

Deviations u If Solvent has a strong affinity for solute (H bonding). u Lowers solvents ability to escape. u Lower vapor pressure than expected. u Negative deviation from Raoult’s law.   H soln is large and negative exothermic.  Endothermic  H soln indicates positive deviation.