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)