RAOULT'S LAW AND DISTILLATION
Mixture of 2 miscible liquids
ideal mixture of 2 miscible liquids: when intermolecular forces of attraction in mixture is the same as the forces in pure liquid A & pure liquid B
ideal mixture: no heat/vol change
[ideal mixture formed: especially bet successive members in homologous series (liquid org cpd)]
Raoult's law: the partial pressure of a component in an ideal mixture of miscible liquids is equal to the mole fraction of the component multiplied by the vapour pressure of the pure component liquid at the same temp
p_a = n_a/(n_a+n_b) × p_a⁰ [p_a: partial pressure of A in ideal mixture, n_a/(n_a+n_b): mole fraction of A, p_a⁰: vapour pressure of pure liquid A]
Vapour pressure/composition diagram for ideal mixture

Dalton's law: total vapour pressure = sum of partial pressures (pp) of components
p_t = p_a + p_b +..

Non-ideal mixtures of 2 miscible liquids- deviation from Raoult's law
+ve / -ve deviation
slight deviation: no max / min value of total vapour pressure
extreme deviation: max / min value of total vapour pressure present
+ve deviation
-actual partial pressure greater than that expected from Raoult's law
-due to intermolecular forces in mixture less than that of forces in pure liquids > less attraction (more repulsion) > more liquid molecules escape surface > greater partial pressure



-ve deviation
actual partial pressure less than that expected from Raoult's law
-due to intermolecular forces in mixture greater than that of forces in pure liquids > more attraction (less repulsion) > less liquid molecules escape surface > smaller partial pressure



Boiling-point/composition diagram
bp ∝ 1/vp
vp high > liquid more volatile > bp low
measure bpt of miscible liquids via distillation

at any bp, T_c, vapour & liquid in equilbm & vapour mixture has greater % of more volatile liquid > liquid mixture greater % in less volatile liquid (x% B in liquid, y% B in vapour)


distillatn: distillate = liquid w/ lower bp, liquid w/ higher bp left in distillatn flask

non-ideal liquid mixture w/ extreme deviation can form azeotrope / azeotropic mixture (const bp mixture) at Z
+ve deviation: max vp > min bp


distillatn: distillate = azeotrope, liquid A / B left in distillatn flask (dpd on compositn)
-ve deviation: min vp > max bp


distillatn: distillate = liquid A / B (dpd on compositn), azeotrope left in distillatn flask

Azeotrope / azeotropic mixture: non-ideal mixture of 2 miscible liquids that boils at const min temp to give a vapour having the same % composition has liquid mixture
azeotrope occurs only for extreme deviations


FRACTIONAL DISTILLATION
-separates mixture of miscible liquids of different bp by distilling at components' respective bp [liquid w/ lower bp always as distillate]

-for efficient distillation:
-long fractionating column (more fractionating steps- ie condnstn + vaprtn)
-more material (glass bead/chips) in column (more SA for condnstn & reboiling)
-insulation for fractionating column (reduce heat loss)
-distill slowly (better liquid/vapour eqlbm established)
Explanation:
-due to diff bp of components liquid/vapour equilibria set up at diff plates in column
-vapour comp: higher % of more volatile liquid (lower bp)
-repeated vaprtn & condnstn step occur in column > separtn of liquids w/ diff bp

# of theoretical plates = # of distillation steps (condstn & vaprtn) required to obtain a specified mixture
in practice, # of theoretical plates greater than # of steps needed
Fractionating column (steel tower) for distillation of crude oil


STEAM DISTILLATION

-used to separate org liquids (perfume, oils, medicine), immiscible in water, by passing steam through impure org liquid (which are easily decomposed on heatiing)
-due to liquid being immiscible, water & liquid each exert own vp independent of each other [total saturated vp = vp_liquid + vp_water]
-when mixture boils & distils total vp of mixture equal to atm pressure (101kPa)
p_x + p_water = 101kPa
-therefore bp of mixture less than bp of pure liquid X or pure water
Principle: steam carries vapour of X as mixture distils > distillate contains water + liquid X as 2 immiscible layers
mass compositn of liquid : water in distillate = (M_r × vp) of X / (M_r × vp) of water

Advantages	Disadvantages
decompostn less likely (distil at temp much lower than bp)	only small amt of liquid obtained c/o to large amt of water
only other sub used is water (cheap, safe, easily obtained)	org liq needs to be separated from water in distillate
process relatively simple, easily scaled up for industry	if org liquid partially miscible > emulsion formed > separatn difficult

high pressure steam distillation
advantages
-increase of bp of water (can distil org cpd w/ higher bp)
-increase amt of cpd distilled
disadvantages
-increase of bp of water (decompose delicate org cpd)
-cost of maintaining apparatus increases
low pressure steam distillation
advantages
-lowers bp of water (can distil org cpd that decompose at high temp)
disadvantages
-less amt of cpd distilled
-cost of maintaining apparatus increases


DISTRIBUTION OF A SOLUTE BETWEEN 2 IMMISCIBLE SOLVENTS: Partition law
Partition law: a solute [(s) or (l)] which is soluble in 2 immiscible solvents will distribute itself so that the ratio of the conc of solute in solvents is const at const temp
conc of X in solvent A/conc of X in solvent B = [X]_A/[X]_B = k
conc of X in solvent A/conc of X in solvent B = 1/(conc of X in solvent B/conc of X in solvent A)
k: partitn cons of solute X bet solvent A & B (varies w/ temp)
validity;
-temp const
-solute must not react, associate / dissociate in solvent
-solutns (or immiscible solvents & solutes) reasonably dilute
-solvents immiscible & don't react
Principle of solvent extraction
-dpds on diff solubility of solute in diff solvent
-solute dissolves if solvent has similar type of intermolecular forces of attraction ('like dissolves like') (polar solute dissolve in polar solvent)
[types of attractive forces: ion-permanent dipole attractn, H-bonds, van der Waals F]
-solute & 2 immiscible solvents shaken: solute dissolves in solvents (more in solvent w/ similar intermolecular forces of attractn > can be extracted & separated > purified)
[if solvent has identical F of attractn as solute > more solute dissolves in that solvent > conc of solute in solvent increases]
-at equilbm; [solute] in solvent A / [solute] in solvent B = const (at const temp)
Uses of solvent extraction
-remove caffeine > decaffeinated coffe
-obtain chlorophyll from green plant materials, essential oils from plants
-separate & purify pharmaceutical chemicals (penicillin)
Choosing suitable solvent
-immiscible w/ soltn to be purified
-doesn't react chemically w/ solute (only dissolves solute)
-low bp (easily removed by distillatn)
-safe, non-flammable, non-toxic
Ether used as:         -good solvent (most non-polar org cpds soluble in it)
-chemically unreactive (most org cpds don't react w/ it)
-low bp
-immiscible in water
disadvantages: very flammable, can form explosive peroxide on long keeping

Experimental procedure to extract solute from aq solution using org solvent
-aq sol containing solute to be extracted placed in separating funnel
-org solvent put into funnel
-shake sol > solute dissolves in solvents > attain dynamic equilbm
-wait for the 2 layers of solvent to form
-obtain aq sol and put into separating funnel > repeat process
-collect org solvent w/ solute needed
-purify solute in org solvent [remove impurities by rinsing w/: -NaCO₃ / NaOH if impurities acidic, -distil to retain org layer]
-org layer dried w/ anhydrous CaCl₂ / Na₂SO₄ (removes water)
-org layer distilled (solute liquid: at rtp) / evaporated (solute: solid at rtp)


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