Solutions

Occur in all phases u The solvent does the dissolving. u The solute is dissolved. u We will focus on aqueous solutions.

Ways of Measuring u Molarity = moles of solute Liters of solvent 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

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. u Read section 11.3 – structure effects! pg. 492

Soap PO-O- CH 3 CH 2 O-O- O-O-

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. C= kP

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. u Gas molecules can move fast enough to escape. u Thermal pollution.

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 Vapor pressure of a solution is directly proportional to the mole fraction of solvent present.

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

Aqueous Solution Pure water u Water evaporates faster from the water than solution

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

u Calculate the expected vapor pressure at 25oC for a solution prepared by dissolving 158.0g of common table sugar (molar mass = 342.3g/mol) in cm3 of water. At 25oC, the density of water is g/mL and a vapor pressure in torr. 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.