Chem 237 Experiment 5: Simple Distillation & Gas Chromatography

How is fractional distillation better than simple distillation and when is it preferred?

provides more surface area for vapor to contact and increases # of theoretical plates; favored when bp diff. between 2 compounds is less than 20 degrees

It takes a temperature difference of ___________ between the distillate and the temp. of the ___________________ in order to get efficient condensation of the vapor.

at least 25 degrees; condenser

What are volatile compounds?

compounds that can be converted to gas phase at reasonable temperatures without decomposition

Distillation has been used for hundreds of years to:

concentrate alcohol and isolate fragrance of flowers for perfume

Increased vapor pressure = _______________ boiling point

decreased

Solubility of a dissolved gas ___________ with increasing temperature.

decreases

_____________ the diameter of the column achieves what? _________ inch inside diameter column is the minimum.

decreases distance that vapor molecules must travel before striking a condensing surface 0.5 inches

As boiling point difference increases, # of theoretical plates required _____________________. Why?

decreases; larger bp difference makes it easier to separate two liquids

All useful methods for increasing the number of plates in a column depends on:

decreasing the distance that vapor molecules must travel before striking a condensing surface

Resolution is (directly/indirectly) proportional to theoretical plates. What is the boil-up rate?

directly the volume of liquid that enters the column as vapor per minute

What happens if you heat your distillation apparatus too rapidly?

drives heated vapor up the column and prevents formation of an equilibrium that allows the lower boiling point component to separate and decreases # of theoretical plates

Each vaporization-condensation step is a ____________ __________________. That means that the _____________ part is being enriched in the lower-boiling point component and the _________________ is being enriched in the higher boiling component.

dynamic equilibrium vapor phase liquid in the distilling flask & liquid in lower reaches of the column

The ____________ the boil-up rate, the better the resolution.

slower

What does miscible mean?

soluble in each other

Between heptane & octane, which was the main component after taking 3 fractions and why?

octane; because it was the higher boiling material so it stayed in the distillation pot at the bottom while the heptane vaporized earlier

What are nonvolatile materials?

ones with extremely low vapor pressures

Vapor pressure definition:

the gaseous vapor in equilibrium with a liquid

Gas chromatography may be used to quantitatively determine:

the relative proportions of volatile compounds in each sample

Definition of boiling point:

the temperature at which the vapor pressure equals the atmospheric pressure and gaseous bubbles of the liquid will form below the surface of the liquid

The digital thermometer should be positioned so that the ______________________ is where?

thermocouple is just below the lip of the assembly that the condensate will be collected in

The best column produces _______ plate(s) per ________ cm of column length.

1 plate per 10 cm

Describe how distillation happens in the distillation apparatus. (3)

1. vapor from liquid in distillation flask travels up flask & column towards exit (away from heat source) 2. vapor condenses because it touches surfaces that are below temperature in distillation flask. 3. condensation heats up apparatus and condensation continues to go towards receiving flask & thermocouple.

It is safe to distill most organic compounds boiling up to about _________ degrees. If your compound is an ____________ or an ____________, you can distill at higher temperatures without risk of decomposition.

150 aromatics or esters

What is a theoretical plate?

Each separate, "mini-distillation" (condensation and vaporization)

What is distillation?

The process of heating a liquid to the boiling point, then collecting the condensed vapors from the boiling liquid in a different vessel

If the temperature difference is less than _______ degrees between the distillate and the temp. of the condenser, what happens?

a lot of vapor travels through the condenser and escapes through the outlet

Resolution is dependent on what 3 factors?

boil-up rate, reflux ratio, and number of theoretical plates

Because azeotropes are possible, what can you not say about the purity of your compound?

cannot say your compound is pure solely because it boils at a constant temperature

The most well known min. boiling point azeotropes is:

ethanol-water

Liquids with high vapor pressure _________________ easily and have ______ boiling points.

evaporate; low

Rate of __________ affects the number of _______________________ in the column.

heating; theoretical plates

The mixtures used in this experiment were:

hexane and octane vs. heptane and octane

Which was more efficiently separated, the hexane/octane or heptane/octane mixture? Why?

hexane/octane mixture because the boiling point difference between the 2 was greater than heptane/octane. We know that higher bp difference = less plates needed for efficient separation

Today the grease we use is: Never use ___________ to grease glass joints. However, use this substance to lubricate:

highly viscous silicone oil glycerol; hoses before attaching them to glassware

Vapor pressure __________ with increased temperature.

increases

Increasing the ______________ of the column produces ____________ theoretical plates.

length; more

Liquids with low vapor pressure have _______________ boiling points.

low

The condensed liquid that is in the collection vial of the distillation apparatus is enriched in the _________________________________ component of the mixture.

lower boiling point

Each vaporization and condensation step enriches the: What does this mean?

lower boiling point of the vapor that, the more time you distill the mixture, the more % of the lower boiling point material you will have

To do simple distillation, the compounds must be ______________ and ____________.

miscible and volatile

Simple distillation provides acceptable separation for 2 __________ liquids with a boiling point difference of ___________ or more.

miscible; 25 degrees

If you control the rate of heating, you can maximize the _________________________ ______________________ between the distilling pot and distilling head, meaning you'll maximize _________________________.

temperature difference; the number of theoretical plates in the column

The retention time tr is:

the amount of time it takes a component to pass through the GC column

The more theoretical plates that are present,:

the better the separation

What is resolution of mixtures?

the efficiency of the separation of components in the distillation of a mixture

What is a minimum boiling point azeotrope?

unique mixture boils at a temperature below that of either pure liquid

Simple distillation is used to separate what two types of liquids?

volatile liquid from a soluble nonvolatile impurity

What is the reflux ratio? _____________ values increase resolution.

volume of liquid that is returned to the pot/volume of liquid removed as distillate increase

When should you grease glassware? Why should we grease?

when 2 pieces of glass come together UNLESS told otherwise because glass expands and contracts during heating and without it, the joints may freeze

When do you first record the temperature for distillation? You should also record the temperature when:

when the first drop of distillate lands in the receiver or when you notice liquid beginning to collect you change the collection vessel (remove the distillate)