Distillation

Sources of error in a distillation - what happens and why it happens

1) Loose connectors cause vapors to escape (loss of liquid) 2) Temperature variations due to airflow in hoods 3) Large surface area of microscale apparatus - heat lost to env 4) Thermometer calibration not done properly 5) Fast distillation rate 6) Position of thermometer and sidearm not adhered to

Precautions about distilling to dryness, distilling in closed system

1) Never distill in airtight system - apparatus will rupture 2) Never distill to dryness - dry liquid residues can be explosive 3) Do not fill flask more than 2/3 full - too much causes liquid to bump and contaminates distillate

Requirements for a successful fractional distillation -including problems and how to solve

1) significant contact between liquid and vapor phases in the column -achieved by filling the column with packing material that has a large surface area 2)maintenance of the proper temperature gradient along the column - dependent on the rate of distillation (hotter at bottom, cold at top) 3) sufficient length of column (figured out by trial and error) 4) sufficient difference in boiling points of the two liquids being separate (prefer 20-30 degrees difference) Problem: flooded column (don't go too slow) -characterized by a column of liquid observed in the fractional column at the joint between column and reaction flask -avoided by having rate of distillate collected adjusted so that the column does not get flooded

What requirement is necessary for boiling point in steam distillation to be constant?

Adequate amounts of water and organic component (must be volatile) present to saturate the vapor space

What may occur if the distillation occurs too slowly why

Flooded column - characterized by column of liquid observed in the fractional column at the joint between column and reaction flask

What kind of compounds need to be purified by vacuum distillation

For compounds that decompose at boiling pts or are sensitive to oxidation

Use of a packing material and criteria for choosing, why should the packing material not be too dense or have too large a surface area

Packing material (only for fractional distillation) purpose: to exchange heat btwn ascending vapor and descending liquid Criteria: 1) Easily insertable into column 2) Doesn't fall out of column like glass beads do 3) Large SA for heat exchange 4) Good heat transfer characteristics 5) Low holdup 6) Can be used in microscale and macroscale experiments Not too dense: Pressure may change within column, causing non-equilibrium conditions Not too wide a SA: will holdup much of material to be distilled (also will lose heat to environment)

Expected observations and impact on result from sources of error and when precautions not adhered to

Precautions: -never distill in an airtight system (can cause apparatus to rupture) -never distill to dryness (dry residue of liquids can be explosive) -do not fill flask more than 2/3 full (too much would cause liquid in flask to bump and contaminate distillate) Sources of error: -loose connectors (will cause vapor to escape - loss of liquid) -temperature variations (due to airflow in hoods) -large surface area of micro scale apparatus (easy to lose heat to environment) -thermometer calibration -positioning of thermometer and sidearm of apparatus not adhered to

Uses of simple and fractional distillation - what kinds of mixtures are suitable for each kind of distillation

Simple distillation: - When bp difference btwn two liquids is LARGE (at least 75 deg C difference) - When one liquid composition is < 10% (ex. One liquid = 95%, other liquid = 5%) - simple distillation: not a complete separation (both liquids purer than before but not completely pure) Fractional distillation: - When bp difference btwn two liquids is small (prefer 20-30 deg C diff) - When both liquids are present in substantial amounts - Fractional distillation: series of continuous distillations (after a fraction vaporizes, some compound condenses and gets redistillied)

Position of the thermometer

just below the level of the sidearm - So it accurately records the temperature of the vapor distilling through sidearm - Bulb must not touch glass of apparatus. do not want to record temp. of glass

How to control heat

scrape away sand from flask to cool down / pile up sand around flask to heat up using spatula

Behavior of temperature during a simple or fractional distillation of a mixture.

temperature increases throughout distillation because composition of vapor that is distilling varies throughout -some lower boiling point component comes of first -then vapor is enriched with higher boiling point component

Reasons for not recovering all liquid in distillation and why

-positioning of sidearm -hold up in fractional distillation -significant amount of distillate left adhering to glass surface of apparatus -loose connectors/adaptors

Definition of azeotrope (minimum and maximum) - recognize azeotrope on boiling point composition graph, behavior of azeotrope

Azeotrope: mixture of 2 liquids with a fixed boiling pt and fixed composition that cannot be altered by simple or fractional distillation - Minimum boiling azeotrope: bp of mixture is below pure bp's of both liquids - Maximum boiling azeotrope: bp of mixture is above pure bp's of both liquids

Effect of changing atmospheric pressure on boiling point

Bp = When vapor pressure of liquids = atmospheric pressure as the liquids are heated The lower the atmospheric pressure, the lower the boiling pt. The higher the atmospheric pressure, the higher the boiling pt

Boiling point of mixture of a homogeneous mixture and a heterogeneous mixture

Bp of MISCIBLE mixture: bp will be between bp's of pure compounds

Definition of hold up

Definition of holdup: unrecoverable distillate that wets the column packing (only for fractional distillation)

Purpose of vacuum distillation

Distillation carried out under reduced pressured - used when neither ordinary distillation nor steam distillation is practical - For compounds that decompose at boiling pts or are sensitive to oxidation - LIQUIDS MUST BE MISCIBLE

Steam distillation - uses, what kind of mixture used in steam distillation - difference between vacuum distillation and steam distillation

Uses: can volatilize liquids and solids below their boiling pts by passing steam into boiling flask Mixtures: high-boiling substances that decompose before bp is reached - Used with immisicible liquids (one of which is water) - Bp of mixture is below bp of each pure component - Only works on VOLATILE COMPOUNDS (b/c the pt. of steam distillation is to volatilize the liquid) Diff btwn vacuum and steam dist: Vacuum - used on miscible liquids; good for compounds decomposing at their bp Steam - used on immiscible liquids; good for compounds decomposing before their bp reached

Vacuum distillation - uses, what kind of mixture used in vacuum distillation

Uses: to separate miscible mixtures under reduced pressure for compounds that decompose at their bp's or are sensitive to oxidation

Wrapping with a wet paper towel - why?

Want to keep sidearm as cool as possible - wrap with paper towel to make sure vapor condenses.

What may occur if the distillation occurs too quickly - why

Will not get equilibrium of descending liquid and ascending vapor Will not maintain the proper temperature gradient in the column (to ensure liquid separation)

Insulation of apparatus

Wrap sides of flask and distillation head (up to level of sidearm) with foil to make process as adiabatic as possible - want to keep this area as warm as possible so that liquid vaporizes.

Reason for carrying out

a way to purify liquids based on different bp's

Position of end of sidearm

inside receiver vial to minimize release of vapors into the hood (loss of liquid)