Distillation

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

1.)significant contact between liquid and vapor phases in column -- fill column with packing material that has a large surface area. 2.) Maintenance of proper temperature gradient along column dependent on the rate of distillation. 3.) Sufficient length of column (trial and error) 4.) sufficient difference in boiling points of two liquids being separated 20-30 degrees difference. If smaller difference, then length of column and or type and amount of packing material must be adjusted.

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

A mixture of liquids of a certain definite composition that distills at a constant temperature without a change in composition and cannot be altered by simple or fractional distillation minimum- mixture boils below the pure boiling points of each liquid maximum-mixture boils above the pure boiling points of both liquids

Purpose of vacuum distillation

Alternative to steam distillation. Can work on compounds that are not volatile. Separates liquids - useful for compounds that decompose at their boiling points or are sensitive to oxidation - compounds must be miscible

What may occur if the distillation occurs too slowly - why, observation when it happens and how to remedy

Flooded column - characterized by column of liquid observed in the fractional column at the joint between column and reaction flask -The vapors of target liquid condense too early and thus fall back down into the column.

Definition of hold up

Hold up: unrecoverable distillate that wets column packing.

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

Less % recovery than expected. There may be holdup in column. there may be bumping of reaction flask. Broken Glassware Lower temperature on thermometer than expected.

How to control heat

Move sand around in sand bath under flask. Affects the number of drops per minute when not constant and controlled.

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

Simple -good at separating mixtures with large difference in BP (at least 75 degrees) -if one component is nonvolatile. -Also good if there is a difference in composition of mixtures (one liquid has a composition of less than 20%). Fractional (series of continuous distillation where a fraction gets vaporized, some compound gets condensed and re-distillated) -significant composition of each liquid (i.e. 50/50, 60/40) -Small difference in boiling points.

Insulation of apparatus

So that vapors can condense due to the cold temperature and seal of apparatus.

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

Steam distillation is used for compounds that are immiscible and decompose at their boiling point. So, you can complete distillation at a lower boiling point than the one these compounds decompose in through steam distillation. Only works if compound is volatile. There must be conditions so that the boiling point remains constant. How is this done? as long as there are adequate amounts of both H2O and organic material to saturate vapor space. Vacuum distillation does not need a volatile solution. Steam distillation does require a volatile solution.

What may occur if the distillation occurs too quickly - why, observation when it happens and how to remedy

There needs to be constant heat so that the temperature gradient can be constant. If this does not happen, then the ascending vapors and descending liquid will not attain equilibrium. If this happens, then all the vapors of both liquids may go all the way to the end, thus not separating. In short, the condensate will partially vaporize as it flows down column. Also, if the distillation occurs too quickly, bumping can occur.

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

Thermometer just at the bottom of the sidearm so that you record the temperature of the vapor. The thermometer should also not touch the glass.

Why does the insolubility of benzene and water in each other cause each liquid to exert its own vapor pressure when a mixture of benzene and water undergo steam distillation?

bc they are insoluble, the molecules of benzene are not diluted by water, so its vapor pressure behaves independently

In vacuum distillation, why is it important that heating of the flask is begun only after the system is evacuated to the desired pressure?

bc vacuum distillation is used when a substance boils at such high temps or decomposes bc its sensitive to oxidation. lower pressure= lower bp

Behavior of temperature during distillation of pure compound

bp liquid= temp vapor

Why would boiling stones not be able to control the bumping of liquids in vacuum distillation as they are able to for simple and fractional distillation

most boiling stones lose their activity in an evacuated system

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

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

the boiling point will be similar and thus distilling this mixture will be difficult (the closer the boiling points are, the higher the fractional column needs to be).

Precautions about distilling to dryness, distilling in closed system

-dry residues of liquids can be explosive -never distill in an airtight system (rupture) -never distill to dryness (explosive) -dont fill more than 3/4 flask (bumping= contamination)

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

-loose connectors- will cause vapor to escape -large surface area of apparatus-makes it easy to lose heat to the environment Thermometer not calibrated (put in ice, then in boiling water)--> incorrect reading. Incorrect clamping= broken apparatus.

Reasons for not recovering all liquid in distillation and why

-positioning of the side arm (if it touched the receiving vial) -hold up (fractional distillation) -significant amount of distillate left adhering to the surface of glass apparatus -loose connectors and adapters

Effect of changing atmospheric pressure on boiling point

-the lower the atmospheric pressure, the lower the boiling point

2 reasons why fractional distillation must be carrier out at a slow enough rate in order to ensure good separation of liquid mixtures

-to ensure vapor is same temp as boiling solutions -to prevent super heating -to allow the soln to vaporize & condense while ascending arm more times

What specifically causes the vaporization of the lower boiling liquid on the packing in a fractional distillation?

Condensing of higher bp vapor releases heat, which causes vaporization of the lower bp liquid on the packing so that the lower bp component moves on while higher bp moves down

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

When a series of simple distillations take place within a fractional column, it is important that complete equilibrium be attained between the ascending vapors and the descending liquid. heat should be transferred between them with no gain of heat or no net heat loss to the surrounding

Position of end of sidearm

end of sidearm must in inside the receiver vial to minimize release of vapors in the hood

Reason for carrying out

to purify/separate liquid mixtures by separating it from a nonvolatile component or from another liquid with a significant difference in boiling point

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

used in fractional distillation. It creates a high number of theoretical plates for distillation (cycles of distillation) A steel sponge is used: Criteria: -low hold up -good heat transfer -easy to insert -large surface area -doesn't come out of the column -should not be too dense because pressure changes can cause nonequilibrium conditions -should not have too large of a surface area because it will hold up too much of the material