Distillation

criteria for choosing packing material

1. Easy to insert in the column 2. Does not easily fall out 3. Large surface area 4. Good heat transfer 5. Low hold-up

Sources of error

1. Loose connectors a. Vapor escapes -> loss of liquid 2. Temperature variations a. Air flow in hood 3. Large surface area of apparatus a. Heat is easily lost to environment 4. Thermometer calibration 5. Positioning of thermometer and sidearm are not adhered to

What are the reasons for not recovering all liquid in distillation and why?

1. Positioning of side arm 2. Hold up- fractional distillation 3. Loose connectors and adapters 4. Significant amount of distillate left adhering to glass surface

Requirements for a successful fractional distillation

1. Significant contact between liquid and vapor phases a. Achieved by the packing material which has a large surface area 2. Proper temperature gradient along the column- dependent on rate of distillation a. Problem: flooded column i. How to avoid: adjust the rate of distillation 3. Length of the column a. Trial and error 4. Difference in boiling points of the two liquids separated

What kind of mixture is suitable for fractional distillation?

1. The boiling point difference is small (20-30 C) 2. The liquid composition must be substantial (large)

What kind of mixture is suitable for simple distillation?

1. The boiling point difference must be large at least 75 C 2. One liquid composition must be <10% 3. The liquid has a non-volatile component (like a solid contaminant)

When carrying fractional distillation: What is the ideal boiling point difference for the 2 components of the mixture you are distilling? What needs to be adjusted if the bp difference is smaller than above?

20-30 C length of column and/or packing material

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

Adequate amounts of both water and organic compound are present to saturate the vapor space

What may occur if the distillation occurs too slowly and why? observation and remedy

Flooded column - characterized by column of liquid observed in the fractional column at the joint between column and reaction flask remedy: adjust the rate of distillate collected

What are the precautions about distilling in closed system?

If you distill in a closed system, this may cause the apparatus to rupture.

What are the precautions about distilling to dryness?

If you distill to dryness, the dry residue may be explosive.

How to position the end of the sidearm?

Place the end of the sidearm inside the receiver vial to minimize loss of vapors into the hood

Position of the thermometer and why it must not touch the glass

Place the thermometer below the side arm to measure the vapor temperature accurately throughout the distillation do not want to record temperature of the glass

What is the behavior of temperature during a simple or fractional distillation of a mixture?

Temperature increases at a constant rate, however more heat must be supplied in fractional distillation.

boiling point of mixture of a homogeneous mixture and a heterogeneous mixture

The boiling point of a homogeneous mixture is between the boiling points of the pure compounds, while the boiling point of a heterogeneous mixture is below the boiling points of the pure compounds

What are the components of a boiling point composition graph?

Two phases: liquid and gas. From a specific composition, you can find the boiling point by going straight up and hitting the solid line. Then to find the composition of the vapor after 1st distillation, make a straight line from the boiling point then a straight line down. To find the temperature from the 1st distillation, make a horizontal line from the point on the solid line.

What is the behavior of temperature during distillation of a pure compound?

bp of liquid = temperature of vapor temperature remains constant throughout distillation

Why should the packing material not be too dense?

it will cause pressure changes, which will result in non-equilibrium conditions

What aspect of water makes it favorable to use in steam distillation?

its low molecular weight

How to control the heat to the apparatus?

pile up or scrape away the hot sand from the flask

What may occur if the distillation occurs too quickly and why? observation and remedy

poor separation: Will not get equilibrium of descending liquid and ascending vapor Will not maintain the proper temperature gradient in the column (to ensure liquid separation) observation: Overheating -> bumping and potential contamination remedy: adjust rate of distillation

Purpose of a packing material

provides a large surface area for heat exchange btwn ascending vapor and descending liquid

One of the ways used to control bumping in a vacuum distillation is to allow a fine stream of air bubbles drawn through a very fine capillary into the boiling liquid. What is the requirement for how fine the capillary should be?

so fine that even under vacuum only a few bubbles of air are drawn in each second

Effect of changing atmospheric pressure on boiling point

the lower the atmospheric pressure, the lower the bp (direct relationship)

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 (the lower pressure allows for the solution to boil below its bp) - LIQUIDS MUST BE MISCIBLE

Steam distillation

Usually used with immiscible liquids (one of the liquids is usually water) advantage: -volatilizing liquids & solids below their bps by passing steam into a boiling flask -good for high boiling substances that decompose before bp is reached

insulation of apparatus

Wrap the apparatus with foil in order to minimize heat loss The system needs to be adiabatic (no net change in heat) -> important to maintain the rate of distillation to get good and efficient separation

azeotrope

a mixture with a fixed composition that cannot be altered by simple or fractional distillation, A mixture of liquids of a certain definite composition that distills at a constant temperature without a change in composition -> this is because the vapor phase has the same proportions of the constituents as the unboiled liquid phase • Has a constant boiling point • Behaves like a pure substance • Can be minimum: below the bp of the pure compounds • Can be maximum: above the bp of the pure compounds

Explain the adiabatic process as it specifically pertains to a fractional distillation.

complete equilibrium btw the ascending vapors and descending liquid is important --> an adiabatic process: heat should be transferred from the ascending vapors to the descending liquid with no gain of heat or net heat loss to the surroundings

In vacuum distillation, how you do ensure that the liquid does not boil too suddenly upon reduction of pressure?

heating of the flask is begun only after the system has been evacuated to the desired pressure

Why should the packing material not have a too large surface area?

it will absorb the material being distilled (hold up)

Why wrap the apparatus with a wet paper towel?

to ensure vapor condenses in side arm, to promote condensation

Reason for carrying out distillation

to purify and separate liquids based on the principle of different boiling points

Besides azeotropes, what is another instance where the constancy of a boiling point does not mean the liquid being boiled consists of only one compound?

two miscible liquids of similar chemical structure that boil at the same temperature individually will have nearly the same boiling point as a mixture

hold up

unrecoverable distillate that wets the column packing

Difference between vacuum and steam distillation

vacuum: miscible liquids, works with compounds that are sensitive to oxidation steam: immiscible liquids, only works with volatile compounds