distillation

when to use simple distillation

-to purify more than 60% -as the last step in purification of a liquid -to remove solids from a liquid -to remove dissolved, non-volatile components from a liquid -when two components of a system have a boiling point difference of 60 C or more

t/f: -distillation is physical separation -over time, the amount of the most volatile component in the distillation flask increases -simple distillation is effective at separating mixtures of liquids with very similar volatilities -over time, the amount of the most volatile component in the distillate increases

-true -false -false -true

how can injection time be indicated on a gas chromatograph?

-use pen to mark on recorder base line immediately after the injection -if GC has analysis software, it will automatically be shown on the print out -if detector is thermal conductivity detector, inject several microliters of air with the sample

[fractional distillation] ideal distillation rate

1 drop every 2-3 seconds

when is simple distillation appropriate?

1- when removing a low-boiling solvent from an organic compound with a high boiling point 2- as a last step in the purification of a liquid compound to obtain a pure product and determine its boiling point

Calculate the number of theoretical plates used in a distillation when the column is 1.5 m long, with a HETP (height equivalent to a theoretical plate) of 50 cm.

3 plates

A 40:60 (mole %) ratio of acetone:water is heated to 65.12 °C at 760 mmHg. What is the total vapor pressure of the system? The vapor pressures for acetone and water at 65.12 °C are 1019 mmHg and 291 mmHg respectively.

582.2 mmHg, liquid below boiling

simple distillation can effectively separate mixture of liquids whose boiling points differ by less than

60-70 degrees

[simple distillation] acceptable receiving vessels

Erlenmeyer flask and graduated cylinder

appropriate glassware for collecting distillate

Erlenmeyer flask, roundbottom flask, conical vial, graduated cylinder

intermolecular interaction responsible for attraction alkane molecules to their neighbors

London dispersion forces

pyrrole IR spectrum

N-H at 3550-3250, C=C at 1650-1600

What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?

The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.

[simple distillation] where should thermometer be?

align the top of the thermometer bulb with the bottom of the side arm on the distilling head

[simple distillation] how many receiving vessels should you keep?

at least 2

where is boiling point of azeotrope on minimum-boiling azeotropic system graph?

at meeting point of two graphs

[simple distillation] nonacceptable receiving vessels

beaker (bc wide opening readily allows vapors to escape)

[simple distillation] when should you lower heat source at the end of distillation?

before distillation flask reaches dryness

[fractional distillation] where is fractionating column placed?

between distillation flask and distilling head of distillation apparatus

[fractional distillation] cut off points

boiling points

when the boiling point of a substance is determined...

both the atmospheric pressure and the experimental boiling point need to be recorded

[fractional distillation] if too little heat is applied

column may lose heat faster than it can be warmed by the vapor, thus preventing vapor from reaching the top of the column so thermometer reading drops below boiling point of liquid (vapor does not reach thermometer bulb)

def. of hydrophobic effect

lack of solubility of greasy, nonpolar compounds in water

[fractional distillation] fractionating column provides ____ surface area

large

why does no separation occur when azeotropes are boiling/vaporizing as a gas?

liquid and vapor curves intersect

[simple distillation] what happens if distilling flask is too full?

liquid can easily bump over into the condenser

distillation is ____-___ separation in which vapor pressure differences are usedc to separate different compounds

liquid-gas

[fractional distillation] at the end of distillation, the vapor that reaches distilling head at the top of the column is composed entirely of the component with ___ boiling point

lower

lower atm = higher elevation =

lower boiling point

def. of distillation

method for separating two or more liquid compounds by taking advantage of their boiling point differences

what causes vapor pressure?

molecules leaving the surface of the liquid to become vapor

why do nonpolar organic compounds mix?

nonpolar compounds cannot participate in h-bonding and dipole-dipole interactions

[fractional distillation] efficiency of fractionating column is expressed as

number of theoretical plates

[fractional distillation] one theoretical plate corresponds to

one complete vaporization of the original solution followed by condensation of the vapor

how can efficiency of fractionating columns be improved?

pack column with glass helives, metal sponge, wire spiral, thin metal strips (not with ice)

def. of fractional distillation

process of repeated vaporizations and condensations that allows you to separate liquid components of a mixture by exploiting the vapor pressure differences of the components

The carrier gas in a GC separation is usually an inert gas such as nitrogen, helium, or argon, but can also be hydrogen. What are the pros and cons of using hydrogen as a carrier gas in a GC separation?

pros: cheaper than helium, can be generated at GC instead of from the gas tanks, greater difference in mass between carrier gas and sample than nitrogen cons: explosion hazard

[fractional distillation] ideal receiving containers

round-bottomed flask, vial, Erlenmeyer flask

why do you need boiling stones in distillation?

some form of agitation of the liquid in the distilling flask is needed to provide even, controlled boiling and to prevent bumping-- must be added before turning on the heat

what does the magnitude of van der Waals interactions depend on

surface area of the interacting molecules

def. of boiling point

temperature at which the vapor pressure of the liquid exactly equals the pressure exerted on it by the atmosphere

[simple distillation] what does thermometer bulb measure?

temperature of the vapor pressure, not of the boiling liquid

[simple distillation] what happens if distilling flask is too empty?

a substantial fraction of the material will be needed just to fill the flask and distilling head with vapor

def. of azeotrope

constant-boiling mixtures that cannot be further purified by distillation, caused by intermolecular interactions in the liquid state

[miniscale] boiling temperature has been reached when

distillate is condensing steadily and temperature stabilizes

[fractional distillation] applying too much heat causes...

distillation to occur so quickly that the repeated liquid-vapor equilibria required to bring about maximum separation cannot occur

the larger the column surface area on which liquid-vapor equilibria can occur, the more ______ the column will be

efficient

abrupt temperature increase in distillation curve signifies

efficient fractional distillation

procedures that can be used to identify a compound in gas chromatography?

enhancing peaks, measuring and analyzing retention times, analyzing with other spectroscopic methods

when vapor pressure equals the ___, a liquid will boil

external pressure

why is a flask never distilled to dryness?

flask could crack or break, flask could become charred, flask could explode because of residue

A mixture of acetone and benzene is heated at 60.2 °C and at 760 mmHg. The vapor pressures for acetone and benzene at 60.2 °C are 868 mmHg and 394 mmHg respectively. Calculate the fractions of acetone and benzene in the liquid phase.

fractions of acetone and benzene in liquid phase: 0.772, 0.228 fractions of acetone and benzene in vapor phase: 0.882, 0.118

mobile phase in GC is

gas

mole fraction of component with lower boiling point is ___ in vapor than in liquid

greater

strongest intermolecular force

h-bonding

van der waals includes all intermolecular bonding forces except

h-bonding

higher boiling points caused by

h-bonding, dipole-dipole interactions, polar compounds, increased molecular weight (increased surface area and greater van der Waals)

for gas chromatograph, last peak has ___ boiling point

highest

as distillation increases, boiling point of the solution ___

increases

what does a temperature drop in the middle of a distillation mean?

indication of a break between fractions: not enough vapor of the higher-boiling compound reaching thermometer bulb to register on thermometer; increase rate of heating

solubility of one compound in another is governed by

intermolecular forces (enthalpy) and entropy

During a fractional distillation of acetone and toluene, a student notices that their data shows a drop in temperature from 56 °C to 51 °C. What does the student conclude?

there is no more acetone vapor and the toluene has not started to distill

elements that make up wire in TCDs

tungsten, rhenium

[simple distillation] what does it mean if temperature begins to drop

vapor is no longer reaching thermometer bulb and distillation should be discontinued

boiling point of liquid mixture depends on

vapor pressure of its components

[fractional distillation] each vaporization and condensation cycle causes vapor to be enriched in more ___ compound

volatile

[simple distillation] how should condenser be arranged?

water flows in at the bottom and out at the top

when is azeotropic distillation useful?

when removing a product such as water from a reaction mixture by codistillation with an immiscible organic liquid

when is fractional distillation appropriate?

when separating a mixture of two or more liquid copounds whose boiling points differ by less than 60-75 C