Unit 12F - Colligative Properties - IMF Effects - AP Chemistry

colligative properties

properties of a solution that depend on the physical number of particles dissolve, but not the chemical properties of those particles (the solute)

freezing point depression

when a solute is added to a solvent, the solvent particles must “push” the solute particles out of the way in order to form a solid, which requires energy

to make a solution freeze, the temperature must be lower in order to increase the amount of energy given off when the solution forms a solid

freezing point depression formula

∆Tf = imKf

where:

i = van’s Hoff factor (# solute particles from each molecule, sometimes called the dissociation factor

m = morality of the solute (mol solute/kg solvent)

Kf = freezing point depression constant

van’t Hoff factor (i)

covalent compounds have i value of 1, ionic compounds vary as can split into 2 or more ions

if a compound is a weak electrolyte, the value of i must be measured empirically

boiling point elevation (gases and vapor pressure)

because liquid molecules are constantly forming and breaking bonds to other liquid molecules, a liquid molecule on the surface of the liquid can free itself from the other liquid molecules —> this makes it a gas molecule

any time a gas is in a sealed container that also contains a liquid, some molecules of that liquid will also be present in the gas phase

vapor pressure

the partial pressure of a chemical equilibrium in which the rate at which molecules evaporate is the same as the rate at which molecules condense

normal boiling point

the temperature when Pu = Patmosphere and and ALL of the liquid evaporates

boiling point elevation (solutions w/ solute)

solute particles attract solvent molecules as they boil and attempt to escape as a gas, solution needs extra energy (higher temp) in order to overcome extra attraction

liquids with solutes dissolved in them boil at higher tempe rates

boiling point elevation formula

∆Tb = imKb

where:

i = van’t Hoff factor (# of solute particles from each molecules, sometimes called the dissociation factor)

m = morality of the solute (mol solute/kg solvent)

Kb = boiling point elevation constant

Raoult’s Law (vapor pressure lowering)

states that the partial pressure of vapor “i” (Pi) equals the vapor pressure of (pure) “i” (Pv,i) times the mole fraction of liquid “i” (Xi) in the mixture

the “why” behind Raoult’s Law

solute particles attract solvent molecules and this attraction is strong enough to prevent some off those solvent molecules from escaping into the vapor phase

vapor pressure is the number of molecules a liquid that can escape into the gas phase at a given temp, expressed as a pressure —> the presence of solute particles lowers the vapor pressure of the solvent, lowering the BP, forming weaker IMFs

osmotic pressure (variable pi)

force of attraction measured as a pressure as high concentrations of solute molecules on one side of the membrane, the higher attraction to solvent molecules on the other side

the observed pressure difference across a semi-permeable membrane because of differences in solute concentration

can apply the ideal gas law because we assume molecules (because they are behaving like a gas) are obeying KMT

osmotic pressure formula

Pi(V) = inRT

where:

V = volume of solution

i = van’t Hoff factor

n = moles. of. solute.

R = gas constant

T = temperature (Kelvin)