EXERCISE 3: Simple Distillation and Steam Distillation

VolatilityAbility of the substance to escape from the liquid into the vapor phase

strong IMFA, low volatility

Vapor Pressure the force exerted on the walls of the container (closed) containing the liquid at a given temperature a quantitative measure of volatility

higher vapor pressure, less volatile

Boiling PointTemperature at which the equilibrium vapor pressure (the vapor pressure at which there is no transfer of the liquid particles in the gas phase and vice versa) equals the atmospheric pressure

Normal boiling point boiling point at the standard atmospheric pressure (1 bar)

Distillation used for purification and boiling point determinationTYPES

SimpleSteamFractionalVacuumModified

NOTEDistillate liquid condensate from distillationPot residue / residue substance remaining in the distilling flask

Simple DistillationTakes advantage of the very wide difference in volatility of the components of the mixtureSeparating liquids boiling BELOW 150 C from nonvolatile impurities or another liquid boiling at least 70 C higher than the first (liquids should dissolve in each another)

Principle Behind Simple DistillationRaoults Law

distillation relies on the fact that the vapor above a liquid mixture is richer in the more volatile component in the liquidin an ideal solution the partial pressure (PA) of component A at a given temperature is equal to the vapor pressure PoA of pure A multiplied by the mole fraction of A (XA) in solution. now consider an ideal solution of A and B: XA = nA/ (nA + nB) XB = nB/ (nA + nB) XA + XB = 1 where nA and nB represent the number of moles of components A and B.

Principle Behind Simple DistillationSo that the total vapor pressure will be:PA = XAPoA PB = XBPoBPT (total vapor pressure) = PA + PB = XAPoA + XBPoB

Simple Distillation

Points to Remember in Simple DistillationPosition of the thermometer bulb should be at 5 mm below the side arm of the flask

For accurate determination of the vapor temperatureHigher than the 5 mm position: lower vapor temperature readingLower than the 5mm position: higher vapor temperature reading

Points to Remember in Simple DistillationVolume of distilling flask should be at least twice the volume of the liquid to be distilled

To allow room for boilingWater in should be lower than water out

In order for the condenser to be filled thoroughly with water

Points to Remember in Simple DistillationCondenser should be cold enough to ensure condensation of the vapor

If solvent to be recovered is volatile, put receiving flask into an ice bath

Use of boiling chips provide site for boiling and thus prevent bumping

Steam DistillationCodistillation of water with a substance which it is immiscible

Codistillation distillation of 2 or more liquidsUsed to separate slightly volatile water insoluble substance from non-volatile materials

Principle Behind Steam DistillationDaltons Law of Partial Pressure

When 2 or more gases or vapors which do not react chemically with one another are mixed at constant temperature,the sum of their individual vapor pressure is equal to the the total pressure of the system. PT = PT= P1 + P2 + P3 +where: PT = total pressure Pn = vapor pressure of the pure compound at the same temperature***PT > Pressure of the most volatile component

Principle Behind Steam DistillationThe total pressure is dependent only in the individual vapor pressures and not in the mole fraction () of each component of the mixture. The composition of vapor is constant at a particular temperature. The vapor pressure of a mixture is proportional to the number of molecules of each substance in the vapor phase.

Principle Behind Steam DistillationnorganicnH2OPorganicPH2O=

Sample ProblemsQuinoline has a boiling point of 237 C at 1 atm. At 99.6 C, the vapor pressure of water is 750 mmHg while that of quinoline is only 10 mmHg. What is the mole ratio of quinoline and water in the distillate.At 98 C, the vapor pressure of acetophenone (MM = 120 g/mol) is 53 mmHg and that of water is 707 mmHg. If steam distillation was employed, what will be the weight of acetophenone that will distill per gram of water?

AssignmentAt 80.0 C, the vapor pressure of substance carlkyne is 61 mmHg while that of water is 699 mmHg. Calculate for the molar mass of carlkyne if 50.00 g carlkyne was mixed with 200.0 g of water.

Applications of Steam DistillationEach substance in an immiscible mixture exerts vapor pressure independently of the others. Thus, when the combined vapor pressures of the immiscible substances equal the opposing atmospheric pressure, the mixture will boil.The mixture will boil at a temperature lower than of the boiling point of the lowest boiling component

Sample for Steam DistillationSuitable samples

Liquid at room temperatureSlightly volatileWater immiscible

Sample cut finely

Increase surface area of contact between oil and water

Role of SteamContributes a relatively high vapor pressure to the total pressure of the mixture to allow the mixture to boil at a lower temperature

Comparison of Simple and Steam Distillation

SIMPLESTEAMPrincipleRaoults LawDaltons LawSample usedSolution of nonvolatile solute in volatile solvent(Homogeneous)Immiscible Liquids are both volatile (Heterogeneous)

Comparison of Simple and Steam Distillation

SIMPLESTEAMDistillate collectedhomogeneousHeterogeneousBoiling pointNormal boiling pointLess than the normal boiling point of the lowest boiling component

Comparison of Simple and Steam Distillation

SIMPLESTEAMapplicationSubstances with wide difference in boiling point e.g. CHCl3 (61-65 C) and caffeine (238 C)Volatile and water immisciblePurification of substances that are heat sensitive (decompose at high T)Mixture contains large amounts of nonvolatile materials

Comparison of Simple and Steam Distillation

SIMPLESTEAMlimitations- mixture whose components have close BP- Compounds that decompose at their normal BPCompounds that react with water or decompose upon prolonged contact with waterNonvolatile compound (P 5 mmHg at 100 C)

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