Super Quiz 1 - Melting Point, Recrystallization, Distillation

Question 1

Why are solvent pairs used and what is the procedure of how to use solvent pairs?

Answer 1

1.) When using solvent pairs: - Adding the 2nd solvent to the 1st solvent results in a solvent mixture where the solute is less soluble. - Shows up cloudy which indicates initial crystal formation. 2.) If the 2nd solvent has a lower boiling point than the 1st: - Danger: Can cause sudden vigorous boiling of the mixture and hot solvent can spew from the apparatus. - Remedy: Lower the temperature slightly before putting in the 2nd Solvent. 3.) If you use too much of the 1st solvent, you will need to use extremely large amounts of the 2nd solvent in order to start crystallization.

Question 2

Comparison of the amount of heat needed for carrying out fractional distillation vs. simple distillation - why the difference needed

Answer 2

• Fractional distillation requires more heat since the vapors have to travel a further distance to the condenser than in simple distillation.

Question 3

Why large crystals preferred over small crystals?

Answer 3

• They can be easily separated by filtration and they can be easily washed free of adhering impure solvent.

Question 4

What are the precautions to take in all steps of recrystallization (especially dissolving, heating while dissolving, crystallization, drying) – what observations expected if precautions not adhered to, impact on % recovery and purity if precautions on adhered to

Answer 4

1.) Dissolving/Heating while Dissolving: - Dissolve in a minimum amount of solvent to make sure solution is saturated. - Between adding droplets, let them dissolve before adding more for complete saturation. - Do not heat crystal solution too strongly - solvent will boil away bringing the impure solid out of the solution. - If you add too much solvent, you should evaporate to the point of saturation. - % recovery would be affecting by not saturating solution correctly - precipitation effected. 2.) Crystallization: - Make sure boiling stick is removed when crystallizing solution so the crystals do not form on boiling stick = impacts % recovery. - Cool slowly - If cooled too fast, solid will come crashing out of solution as powder, precipitating impurities along with it = impacts purity. - Ensure slow cooling by insulating test tubes with paper towel. - Do not disturb crystallizing solution because if disturbed too many nuclei will form leading to small crystals that come out of solution along with impurities = impacts purity. - There will always be some solute dissolved in the cold solution = impacts % recovery. 3.) Drying: - If crystals are wet, then this would effect melting point and purity of crystals. - If crystals are wet then % recovery can be impacted because crystals may adhere to filter paper. - If crystals are removed from Hirsch funnel too quickly, then (a) Tiny particles will contaminate crystals when they are scraped off filter paper (impacts purity) (b) OR Crystals remain on filter paper (% recovery impacted).

Question 5

What is the reason for carrying out recrystallization?

Answer 5

• A method used to purify solids.

Question 6

Why perform a mixed melting point experiment? How does this work?

Answer 6

• To confirm the identity of an unknown. - Mix unknown with a small quantity of a known compound with a similar melting point. - If the melting point remains the same, then the unknown = known. - If the melting point is lower, then the unknown DOES NOT = known.

Question 7

What are the requirements for solvent-solvent pairs?

Answer 7

1.) Must be miscible in each other (homogeneous layer) 2.) One solvent has to be a poorer solvent for solute than the other 3.) Used when no one solvent has ideal properties with respect to solute.

Question 8

What are the criteria for impurity to depress melting point? What would one observe when taking the melting point of an impure sample?

Answer 8

1.) Moisture (wet) solvents like water or other solvents depress the melting point. 2.) Impure compounds melt over a wider range of temperatures and have a lower melting point than its pure component. - Energy is used to overcome the crystal lattice structure of the solid. - Impurities disrupt this crystal lattice structure so less energy (heat) is required to melt the solid.

Question 9

Know all precautions listed in lecture - why important and what may be observed/impacted if precautions not adhered to.

Answer 9

1.) Never distill in an airtight system (closed system). - Can cause the apparatus to rupture. 2.) Never distill to dryness. - Dry residue can be explosive. 3.) Do no fill flask more than 2/3 full. - Too much would cause liquid in the flask to bump and contaminate distillate.

Question 10

1.) Picking solvent/solvent pairs. -Why is it important?

Answer 10

1.) Ideal characteristics for the solvent with respect to solute. - Dissolves the solute completely when hot (at BP of solvent) - Precipitates solute completely when cold - Does not react with solute 2.) If solvent and solute are too similar in polarities then: - The solute will dissolve in solvent even when cold but will definitely dissolve completely when hot. 3.) If solvent and solute are too different in polarities then: - The solute will definitely precipitate when cold. - The solute may not completely dissolve when hot which means that this does not allow for impurities to be removed.

Question 11

How to control, increase, decrease heat to apparatus - impact on experiment/observation if heated too much or not enough

Answer 11

• Use the spatula to add or move sand from the bottom of the flask. 1.) A rate of distillation must be slow: - So the ascending vapors and descending liquid are in equilibrium. - Maintain a proper temperature gradient in the column to ensure separation of liquid - Overheating can lead to bumping of liquid in the flask - Underheating can lead to reflux (where vapors condense and return to distilling flask instead of distilling).

Question 12

What are azeotropes?

Answer 12

• They are mixtures that act like pure compounds. - The boiling point remains constant throughout the distillation - The composition of the liquid = composition of the vapor - On boiling point composition graph- The liquid and vapor curves pinch together at the azeotropic composition - Can be minimum azeotrope (boiling point lower than both pure liquids) or maximum azeotrope (a boiling point higher than both pure liquids).

Question 13

How do you minimize loss of product in crystallization, collecting and washing steps and why does this product get lost in these steps?

Answer 13

• You must wash crystals with the ice-cold solvent to minimize the dissolving of crystals during the washing process. - Crystals will still be lost during the collection part because some crystals will adhere to the filter paper. - Some crystals will be lost in the washing step due to dissolving.

Question 14

How is calibration of the thermometer carried out for boiling point and distillation?

Answer 14

• Test the O degree Celsius point by using water and crushed ice. - Test 100 Degree boiling point by boiling a water with a boiling chip to prevent bumping. - See if the thermometer can distinguish superheating. - Make sure the thermometer bulb is not touching the glass.

Question 15

What are the reasons for carrying out the different kinds of distillation

Answer 15

1.) A way to purify liquids (separated based on boiling points). 2.) Distillation of pure liquid: - The boiling point of the liquid = temperature of the vapor - Temperature remains constant throughout the distillation 3.) Distillation of Mixtures: - Temperature does not remain constant. It increases throughout distillation. (a.) Because the composition of vapor that is distilling varies throughout. (b.) Some lower boiling point component comes first, then vapor is enriched with higher boiling point component. Boiling points are affected by both volatile and nonvolatile impurities.

Question 16

What is are the definitions of observed and real melting points? What characteristics are not considered melting?

Answer 16

1.) Observed Melting Point = the temperature where the first drop of liquid is seen to when the sample is completely liquified. 2.) Real Melting Point = From eutectic point to when the sample is completely liquified. - Sagging, shrinking, color change, texture change, and sweating (droplets are seen on the outside of the capillary tube due to insufficient drying) are not considered melting.

Question 17

What are the steps in recrystallization?

Answer 17

1.) Choosing solvent/solvent pairs. 2.) Dissolving solute. 3.) Decolorizing solution. 4.) Filter suspended solutions. 5.) Crystallizing solute. 6.) Collection and washing crystals. 7.) Drying.

Question 18

Where should the thermometer be positioned and why?

Answer 18

• The thermometer should be below the sidearm so it can accurately record the temperature of the vapor distilling through the sidearm. - It should not touch the glass

Question 19

1.) Picking solvent/solvent pairs. -Why is it important?

Answer 19

1.) Ideal characteristics for the solvent with respect to solute. - Dissolves the solute completely when hot (at BP of solvent) - Precipitates solute completely when cold - Does not react with solute 2.) If solvent and solute are too similar in polarities then: - The solute will dissolve in solvent even when cold but will definitely dissolve completely when hot. 3.) If solvent and solute are too different in polarities then: - The solute will definitely precipitate when cold. - The solute may not completely dissolve when hot which means that this does not allow for impurities to be removed. So there must be a balance in polarity. If you use a solvent that does not do this (wrong solvent), you will have poor recovery. If the solvent does neither of these than purity of crystals is suspect.

Question 20

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

Answer 20

1.) Simple distillation - Not a complete separation - Both liquids are purer than originally but not completely pure. -Best criteria: (a.) When both liquid boiling points are large (greater than 75 degrees Celcius) (b.) When 1 liquid composition is less than 10% 2.) Fractional distillation: - Best criteria: (a.) The boiling point difference between liquids is small (b.) Two liquid compositions are substantial.

Question 21

How is a sample prepared?

Answer 21

A sample must be dry, finely powdered, tightly packed, and no higher than 2-3 mm in the capillary tube.

Question 22

Filtering of suspended solids - when is it carried out, why vacuum filtration not used, how it should be carried out, a problem if not done rapidly?

Answer 22

• Vacuum filtration is not used because the hot solvent will cool during the process and the product will recrystallize in the filter. - This step should be carried out rapidly otherwise solution cools and premature crystallizing occurs.

Question 23

Vapor pressure - definition, how it relates to boiling point

Answer 23

• Vapor pressure measures how easily molecules escape the surface of a liquid. - Liquids with higher vapor pressures have lower boiling points compared to those with lower vapor pressures.

Question 24

Where should you position of the end of sidearm - importance to the aim of the experiment as well as counting drops?

Answer 24

• The end of the sidearm should be inside the receiver vial to minimize the release of vapors into the hood (loss of liquid).

Question 25

Why wrap the apparatus in tinfoil?

Answer 25

• To insulate the apparatus to minimize heat loss.

Question 26

Purpose of a packing material and criteria for choosing, why should the packing material not be too dense or have too large a surface area?

Answer 26

• Packing material usually a steel sponge - Criteria: 1.) Easy to insert into the column 2.) Does not come out of column-as beads do 3.) Large surface area 4.) Good heat transfer conductors 5.) Low holdup - Packing material cannot be packed so dense that pressure changes take place within the column to cause non-equilibrium conditions. - If the column packing has too large a surface area then it will absorb much of the material being distilled (hold up).

Question 27

Where and why do you need to wrap a wet paper towel around?

Answer 27

• Wrap the paper towel around the condenser to ensure vapors get condensed.

Question 28

Impact of too much or too little in reaction flask (pot) during distillation?

Answer 28

• Too much may lead to bumping while too little may lead to dryness and explosive properties.

Question 29

Sources of error in a distillation - what happens and why it happens and how you can minimize effects

Answer 29

1.) Loose connectors - Will cause vapor to escape (loss of liquid). 2.) Temperature variations - Due to airflow in hoods. 3.) LArge SA of microscale apparatus - Easy to lose heat to the environment. 4.) Thermometer calibration 5.) Reasons for not recovering all liquid to purify/separated - Positioning of sidearm - Hold up (fractional distillation) - A significant amount of distillate left adhering to the glass surface of the apparatus.

Question 30

Definition of hold up - the impact of hold up on the distillate collected

Answer 30

• Hold up is unrecoverable distillate that wets the column packing. - Distillate collection is less.

Question 31

How to carry out an effective fractional distillation - requirements, why important, how it is fulfilled?

Answer 31

1.) Significant contact between the liquid and vapors in the column - Achieved by filling the column with packing material that has a large surface area. 2.) Maintenance of the proper temperature gradient along the column - dependent on the rate of distillation - Don’t distill too fast. - Problem: Flooding = a column of liquid observed in the fractional distillation column at the joint between column and reaction flask. 3.) Sufficient length of the column - Figured out by trial and error. 4.) Sufficient difference in boiling point (20-30 Degrees) - If smaller difference column length and/or type of packing material may be adjusted.

Question 32

Why are melting points carried out?

Answer 32

1.) To identify a known compound. 2.) To record a physical characteristic for a new compound. 3.) To establish the purity of a known compound.

Question 33

Collecting crystals - why gravity filtration not used, items necessary in apparatus for this step and why necessary. Impact on % recovery and purity if not done properly or missing parts on apparatus

Answer 33

• Vacuum filtration is more efficient in recovering solvent from crystals. - Need filter paper to wash the crystals to remove adhering

Question 34

What are other examples that depress the melting point of solids, not including impurities?

Answer 34

1.) Need to fill the capillary tube with finely powdered solid, not chunks. - Air pockets will form causing slow heat transfer and the sample will appear to have a large melting range. Air pockets are not good conductors for heat transfer. 2.) The sample must be packed tightly. - Again air pockets will form depressing the melting point and creating a larger range. 3.) The sample must not be more than 2-3 mm high. - If too much solid is in the capillary than there will be different temperatures throughout. The bottom will be hotter than the top. - Bottom melts first; wider range; temperature doesn’t register on the thermometer. If these criteria are not adhered to but the sample is pure and dry, then the melting point will have a wider range and the upper range will eventually match the true melting point.

Question 35

Boiling stick vs. boiling chip - why one preferred over the other for recrystallization. How boiling chip and boiling stick prevent superheating.

Answer 35

• The boiling stick is preferred because you can remove the boiling stick from the solution while the boiling chip will remain in solution and may become crystallized impacting results. - Boiling chips and sticks prevent superheating through

Question 36

What are the ideal characteristics of the solvent with respect to impurities?

Answer 36

1.) Does not dissolve the impurities at all. - So impurities can be filtered out from hot solution. - Solvent MUST be able to dissolve the solute and NOT impurities when hot. 2.) Dissolves the impurities very well (even when cold). -So impurities along with the cold solvent can be removed from the crystals via filtration. - Solvent MUST precipitate solute while keeping impurities in cold solution. If the solvent does neither of these than purity of crystals is suspect.

Question 37

What are the sources of error for melting point experiment?

Answer 37

1.) Moisture in the air absorbed by the sample. 2.) Contamination of the sample. 3.) A thermometer in Mel-Temp. not calibrated (only instance where the melting point could be higher than expected). 4.) Determination moved too fast. - If too fast, the temperature of the sample and the reading of the thermometer are totally different and the sample appears to have a wider range. - Noting the heat reaches the sample BEFORE the thermometer, so you would expect the observed temperature to be lower than the expected temperature. 5.) Criteria for not filling capillary tubes not adhered too.

Question 38

What are the Definition and Remedies for an unsaturated solution, supersaturation, and oiling out?

Answer 38

1.) Supersaturated solution = when saturated solution cools and no crystals form. - Scrape the bottom of the test tube with glass rode to induce crystallization or use seed crystals. 2.) Oiling out = crystals come out of the solution as oil. - Occurs when (a) Saturated solution temperature OR BP of solvent is higher than a melting point of solute (b) If melting point if the solute is depressed to a point such that a low melting eutectic mixture of the solute and solvent comes out of solution. Remedies: Lower the temp at which the solution becomes saturated w/the solute by adding more solvent. If it persists, use another solvent.