Organic Chemistry Lab- Superquiz 1

Sources of error that can cause mp to widen:

- Moisture in air absorbed by sample - Determination made too fast - Sample not dry, packed tightly, finely powdered, or more than 2-3mm high in capillary - Contamination of sample

Sources of error that cause mp to depress:

- Moisture in air absorbed by sample - Thermometer not calibrated - Sample not dry - Contamination of sample

What does HETP stand for? How is it related to efficiency in distillation?

- Stands for Height Equivalent of a Theoretical Plate. - Having more theoretical plates increases the efficiency of the separation process.

What is vapor pressure?

The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.

Sources of error that cause mp to elevate:

- Thermometer not calibrated

Sources of error when taking melting point (5):

1. Moisture in air absorbed by sample 2. Thermometer in Mel-Temp not calibrated 3. Determination made too fast 4. Criteria for packing sample not adhered to a. Sample not dry b. Sample not packed tightly c. Sample not powdered d. Sample more than 2-3mm high in capillary 5. Contamination of sample

Sources of error during a distillation and their effects:

1. Loose connectors --> will cause vapor to escape (loss of liquid) 2. Airflow in hoods --> cannot avoid, will cause temperature variations 3. Large surface area of microscale apparatus --> cannot avoid, becomes easy to lose heat to the environment 4. Thermometer not calibrated --> inaccuracy of temperature recordings 5. Positioning of sidearm not adhered to --> loss of liquid

Reasons for not recovering all liquid to be purified/separated:

1. Positioning of sidearm into receiver vial 2. Holdup (fractional sidearm) 3. Significant amount of distillate adhering to glass surface of apparatus 4. Loose connectors

How should the sample be heated and why?

1. Rapidly heat the sample to within 20 degrees C below the literature melting point. 2. Then adjust the heat setting so that the temperature increases by 1 degree per minute (so that the determination is slow). Why --> So that there is enough time for the heat to be transferred from the heating block to sample to thermometer.

Why should you make 2 samples of the pure unknown when taking the melting point of the unknown?

1. Run a very fast determination on the first sample to ascertain the approximate melting point. 2. Cool the melting point bath to just below the melting point and make a slow, careful determination using the other capillary.

Three things you can do to a sample so that there is no external effect on the mp:

1. Sample must be finely powdered. 2. Sample must be packed tightly. 3. Sample must not be more than 2-3mm high in capillary.

What is the criteria for using simple distillation?

1. The boiling point difference between the 2 liquids is large (at least 75 degrees C). 2. When one liquid composition is less than 10% (90/10, 95/5). 3. The liquid has a non volatile component.

What is the criteria for using fractional distillation?

1. The boiling point difference between the 2 liquids is small. 2. The two liquids' compositions are substantial (40/60, 50/50).

Why are melting point experiments carried out? (3 answers)

1. To characterize a known compound. 2. Recording a physical characteristic for a new compound. 3. To establish purity of a known compound.

What is a minimum azeotrope, maximum azeotrope, and what is the difference between non-azeotropic and azeotropic graphs?

A minimum boiling azeotrope is one in which the BP of the mixture is below that of either compound, so the mixture gets boiled out before either one of the compounds can. A maximum boiling azeotrope is one in which the BP of the mixture is larger than that of either compound. The BP composition graph looks different because in non-azeotropic mixtures, the vapor and liquid curve only touch at the 100% composition values at either end of the graph. An azeotropic BP composition graph will have it's vapor and liquid curves touch more than twice.

Why should you wait 10 minutes before disassembling the apparatus after a distillation?

Because the glassware will be hot and you can burn yourself or accidentally drop the glassware onto the floor and break it.

How do you control the heat to the apparatus? (Distillation)

By moving the sand of the heating mantle around or away from the flask.

How does packing material cause multiple condensations and vaporizations to occur?

Condensing of the higher-boiling vapor releases heat, which causes vaporization of the lower-boiling liquid on the packing so that the lower-boiling component moves up while the higher-boiling component moves down. Some of the lower-boiling component will run back into the distilling flask.

Why do impure compounds melt over a wider melting range?

Energy is used to overcome the crystal lattice and impurities disrupt the crystal lattice so there is less energy (heat) required to break the forces.

What is an azeotrope?

It is a mixture of two liquids that has a constant boiling point and composition throughout distillation.

What is entrainment in distillation?

It is the physical carryover of liquid by vapors caused by a rapid distillation.

Why should you never distill in an airtight (closed) system?

It can cause the apparatus to rupture.

What is the purpose of packing material in fractional distillation?

It provides a large surface area for heat exchange between the ascending vapor and the descending liquid. This causes multiple condensations and vaporizations to occur as the vapor ascends the column.

What happens if the sample is coarse (not finely powdered)?

It will cause air pockets to form in the sample which cause slow heat transfer and therefore a wider melting range.

What happens if the sample is not packed tightly?

It will cause air pockets to form in the sample which cause slow heat transfer and therefore a wider melting range.

What is the impact of having too little in reaction flask during distillation?

Leads to dry residue in flask which can be explosive.

Why should the sample by finely powdered and packed tightly?

Otherwise air pockets form and will cause slow heat transfer and therefore a wider melting range.

What is holdup in distillation and how is it caused?

Refers to the down-coming liquid that travels down the column but gets trapped by the packing material.

What are the different types of distillation?

Simple distillation, Fractional distillation, Steam distillation, Vacuum distillation

How do you calibrate the thermometer for a melting point experiment and why?

Take samples of pure known compounds (of which you know the melting points for) and conduct the melting point experiment. They should melt at the literature melting point, but if they don't, you know that your thermometer is off by x number of degrees, so when you conduct the other melting points, you know to add or subtract x number of degrees. You should do this otherwise you will get an inaccurate reading of the melting points.

Why should you use your heavy duty dishwashing gloves to handle the steel sponge?

Some steel sponges have sharp edges and you can cut yourself.

What happens when a sample "sweats"? Why?

Sweat: droplets on the outside of the surface of the sample. Why--> Traces of solvent may be be present due to insufficient drying.

What happens if you heat the sample too quickly?

Temperature of the sample will be very different from temperature of the thermometer and so the sample will appear to have a wide range. Also, heat will reach the sample before the thermometer and the observed mp temp will be lower than expected temp.

How do you calibrate the thermometer for boiling point and distillation?

Test the 0°C point of your thermometer with a well-stirred mixture of crushed ice and distilled water. To check the 100°C point, put 2 mL of water in a test tube with a boiling chip to prevent bumping, and boil the water gently over a hot sand bath with the thermometer in the vapor from the boiling water. Make sure that the thermometer does not touch the side of the test tube. Then immerse the bulb of the thermometer into the liquid and see if you can observe superheating. Check the atmospheric pressure to determine the true boiling point of the water.

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

The boiling point remains constant during a steam distillation as long as adequate amounts of both water and the organic component are present to saturate the vapor space.

Why should you never distill to dryness?

The dry residue of liquids can be explosive.

Why is the rate of distillation important? What happens if you carry it out too fast?

The rate is important because a slow rate will result in a purer product. With a rapid distillation, the flow of gaseous material into the condenser in faster and may cause entrainment.

What is the impact of rate on taking the melting point?

The temperature increase should not be greater than 1 degree C per minute. Melting points will not be accurate if they are heating too fast as you are likely to miss the end MP range.

Comparison of temperature of the boiling liquid temperature vs. that of the vapor:

The vapor temperature is lower than the boiling liquid mixture temperature because it has a different composition, which has a lower boiling point.

What is the position of the thermometer in distillation apparatus and why?

Thermometer should be positioned below sidearm so that it can accurately record the temperature of the vapor distilling through the sidearm.

Why should you wrap the flask with foil during distillation?

To insulate the flask so as to minimize heat loss.

Why do we wrap the the sidearm and receiver vial with wet paper towel?

To keep the sidearm and vial as cool as possible as to ensure that the vapor condenses and that it stays condensed.

What is the reason for carrying out recrystallization?

To purify compounds/chemicals.

Why shouldn't you fill the flask more than 2/3 full?

Too much solution would cause the liquid to bump and contaminate the distillate.

What is steam distillation used for?

Used for the purification of mixtures in which the components are temperature or heat sensitive.

What is vacuum distillation used for?

Used when the boiling point of the compound is too high (over 150) in order to distill the compound without significant decomposition.

When do liquids boil?

When vapor pressure of liquid = atmospheric pressure as liquid is heated

Why is there a difference in heat needed between simple and fractional distillation?

You may need to supply more heat to fractional distillation than for a comparable simple distillation because the vapor has to travel a greater distance before reaching the condenser than it does in simple distillation.

What would happen if you record the start of your melting point range when you observe the list of behaviors that are considered not melting?

You will end up with a wide melting range.

What happens if you have too much sample?

You will have caused a differential in temperature throughout sample (temperature of the sample will not be same in all parts of the sample --> inefficient transfer of heat).