Flashcards in super quiz Deck (57):
Both steam and vacuum distillations allow liquids to boil below the actual boiling points. What differentiates these two types of distillations?
Steam is for immiscible mixtures and vacuum is for miscible mixtures
In steam distillation, what requirement is necessary in order for the boiling point of the mixture to remain constant?
Adequate amounts of both water and organic component are present to saturate vapor space
what is purpose of vacuum distillation
miscible mixes carried out under reduced pressure (for compounds that decompose at their boiling points)
what is purpose of steam distillation?
used with immiscible liquids, usually water. Bp of the mix is below the bp of each pure compound. Used when substances decompose before reaching their boiling point.
what is azeotrope?
mixtures with a fixed composition that cannot be altered by simple or fractional distillation
what is maximum azeotrope?
has a bp above pure bps of each liquid
What are some sources of error in a distillation
1. loose connectors which would cause vapor to escape
2. varying temperatures which are because of airflow in hoods
3. large SA of microscale apparatus which is easy to lose heat to the environment
4. thermometer calibration
What are reasons for not recovering all liquid to be purified and seperated?
1. positioning sidearm
2. hold up in fract. dist
3. alot of distillate left sticking to glass surface of apparatus
What is HTEP?
Height equivalent to a theoretical plate (HETP) which signifies each successive condensation-vaporization cycle, also called a theoretical plate
How does HTEP relate to efficiency?
The smaller the HETP, the more plates the column will have and the more efficient it will be.
WHat is the 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
o Packing cannot be so dense that pressure changes take place within the column to cause non-equilibrium conditions.
o The column packing has a very large surface area, it will absorb (hold up) much of the material being distilled
what is hold up?
unrecoverable distillate that wets the column packing
What is the difference between fract and simple is respect to heat?
need to supply more heat to fractional distillation than for a comparable simple distillation
• The vapor has to travel a greater distance before reaching the condenser
describe the Rate of carrying out distillation (simple and fractional) and why this is important - too slow or too fast and the impact of either on fractional distillation
1. must be slow, so that ascending vapors and descending liquid attain equilibrium
2. Also to maintain a proper temperature gradient in the column (to ensure separation of liquids)
3. Overheating can lead to bumping of liquid in flask
Describe the Behavior of temperature during a simple or fractional distillation of a mixture, comparison of temperature of boiling liquid mixture and vapor temperature, why the difference.
Simple more linear, fractional is more sigmoidal= graph of temperature change (on y-axis) vs. volume of distillate
In simple distillation is the product purer or less purer than the initial?
What is the best criteria for simple dist.?
• When boiling point difference between 2 liquids is large (at least 75 °C)
• When 1 liquid composition
What is the best criteria for fract dist.
• Boiling point difference between 2 liquids is small
• The 2 liquids compositions are substantial
How is the position of the sidearm? WHat is the importance of it?
End of sidearm must be insider receiver vial to minimize release of vapors into the hood (loss of liquid)
WHy do we wrap a sidearm with wet paper towel?
o Use wet paper towel to cover condenser to ensure vapor pressure condense in the side arm
What is the point of insulation of apparatus?
o Wrap apparatus with foil to insulate so as to minimize heat loss
What is vapor pressure and how does it relate to boiling point?
o Vapor pressure measures how easily molecules escape the surface of a liquid
o Liquids with higher vapor pressure have lower boiling point when compared with liquid with lower vapor pressure
What are some reasons for carryingout distillation?
o A way to purify liquids (separated based on boiling points)
what is the difference between distillation of mixture and pure liquid?
o Distillation of a Pure Liquid – capture gas boiled off, condense and collect it
• Boiling point of liquid = temperature of vapor
• Temperature remains constant throughout distillation \
o Distillation of Mixtures
• Temperature does not remain constant
• Increases throughout distillation
• Boiling points are affected by BOTH
what are some precautions of dist.
o NEVER distill in an airtight system (closed system) because it can cause the apparatus to break
o NEVER distill to dryness because dry residue of the liquids can be explosive
o Use boiling chip to avoid superheating
o DO NOT fill flaks more than 2/3 full because Too much would cause liquid to bump and contaminate distillate
list the steps of recrystallization
o 1) Choosing solvent/solvent pairs
o 2) Dissolving solute
o 3) Decolorizing solution
o 4) Filter suspended solutions
o 5) Crystallizing solute
o 6) Collecting and washing crystals
o 7) Drying
what are expected observations in recrystlalization?
mass decreases and solute is smaller in quantity after process since solid is still dissolved in solvent even when it is cold
what are the Requirements for solvents with respect to solute and impurities—if solvent does not have ideal properties (impact on % recovery and purity)
1. it dissolves the solute when the solvent is hot and in its boiling point
2. when the solventis cold the solute precipitates
3. does not react with solutes
o Back to like dissolves like → if solute and solvent are too similar in polarities solute will dissolve even in cold solvent
o If solute and solvent have different polarities solute will not dissolve at all
o Thus there MUST be a balance in polarity
o If you use solvent that does not do this (wrong solvent) you will have poor recovery
• Why solvent pairs are used and procedure of how to use solvent pairs?
o When using solvent pairs:
• Adding 2nd solvent to first solvent results in a solvent mixture in which the solute is less soluble ‘cloudiness’; indicated initial crystal formation
• If 2nd solvent has lower BP than first:
• Danger: Can cause sudden vigor boiling of mixture and hot solvent might spew from apparatus
• Remedy: Lower temperature of solution slightly before adding 2nd solvent
o If you use to much of the first solvent, you will have to use extreme large amounts of 2nd solvent to start crystallization
Boiling stick vs. Boiling chip- why one is preferred over the other for recrystallization. How boiling chip and boiling stick prevent superheating.
in a boiling stick you can take it out easily as thesolutions cools while the chip stays put in the solution. Because of this the crystals will form off the chip.
They both allow for surfaces for the gases to collect and begin boiling.
• How to minimize loss of product in crystallization, collecting, and washing steps—why does product get lost in these steps?
o You MUST wash crystals with ice-cold solvent to minimize crystals dissolving during the washing process
• You still will lose some crystals because crystals adhere to filter paper (at collecting step) and some crystals dissolve during the washing process
• Precautions to take in all steps of recrystallization (especially dissolving, crystallization, collecting, washing, drying)—what observations expected if precaution not adhered to, impact on % recovery and purity if precautions not adhered to)
• Don’t heat crystal solution too strongly at this step → solvent will boil away bringing impure solid out of solution
• Prevent super heating you must add boiling stick or boiling chip
• All steps and precaution to ensure a saturated crystallization solution—all necessary steps to be taken while dissolved solute
o Dissolve in minimum amount of hot solvent → to make sure solution is saturated
• Requirements when cooling—precautions, why, impact on % recovery and purity if precautions not adhered
o Cool SLOWLY
• If cooled too fast, solid will come crashing out of solution as powder, precipitating impurities along with it
• To ensure slow cooling, you must use paper towels to insulate the test tube
o Don’t move or disturb crystallizing solution
• If disturbed, too many nuclei for crystallization form leading to small crystals that come out of solution along with impurities
• Definition and remedies for unsaturated solution, supersaturating
o Solution get supersaturated → when saturated solution cools and no crystals form
• To induce crystallization – sue seed crystals (it is advised to save some impure crystals as seeds before recrystallization)
• Scratch bottom of test tube with a glass rod
• Why large crystals preferred over small crystals?
o What are 2 reasons why large crystals are preferable in the crystallization step?
• On small scale, it is difficult to obtain crystals that are too large and occlude impurities
• On large scale, if flask is moved during recrystallization many nuclei will form and crystals will be small and have large SA. They will be difficult to filter and wash clean of the mother liquor
• Filtering of suspended solids - when is it carried out, why vacuum filtration not used, how it should be carried out, problem if not done rapidly.
o Vacuum filtration is not used because the hot solvent will cool during the process, and the product will recrystallize in the filter
o This step would have to be done rapidly otherwise solution cools and premature crystallization occurs
o Microscale – filtration suing Pasteur pipette; centrifugation; pressure filtration with Micro Buchner Funnel
o Macroscale – decantation; gravity filtration
o It is carried out when to remove impurities & charcoal, via gravity filtration
o Don’t use vacuum filtration because hot solvent will cool, causing compound to recrystallize in filter
• 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.
o Vacuum filtration is more efficient in recovering solvent from crystals than gravity filtration
o Need to use filter paper
• The frit will not become clogged with insoluble impurities if don’t use filter paper
o You will need to wash the crystals to remove adhering impurities
• Washing crystals - why important and precautions to take, impact on % recovery and purity if precautions not adhered to
o You MUST wash crystals with ice-cold solvent to minimize crystals dissolving during the washing process
• Drying crystals - why important, impact on % recovery and purity if precautions not adhered to
o Necessary since solvent can act as impurity (crystals are wet → lower mp)
o Material left behind after each transfer (in test tube, on spatula, on weighing paper)
o Tiny paper particles will contaminate crystals when they are scraped off filter paper
• Sources of error in recrystallization and expected observations and impact on result due to those
o Always some solute soluble in cold water (lower % recovery)
o Material left behind after each transfer (lower % recovery)
o Sample NOT fully dried (higher % recovery)
o Crystals dissolve during washing (lower % recovery)
o Solution gets supersaturated (NO crystals form after cooled)
o Forget to insulate test tube with paper towels (solutions comes crashing out as powder)
• Which sources of error (or precautions not adhered to) affect purity and which affect % recovery
o Those affections purity: forget to insulate test tube with paper towels
o Those affecting % recovery: always some solute soluble in cold water, material left behind after each transfer, sample not fully dried, crystals dissolve during washing, solution gets supersaturated, foget to insulate test tube with paper towles
• The impact of rate on taking mp—how to take mp accuratelywith criteria for packing melting point ?
determination must be slow to give enough time for heat to be transferred equally form heating block to sample to the thermometer, if you go fast, the temp at the sample will be very different from the temp at the thermometer and the sample appears to have a wide range
• How is sample prepared? Why sample should be prepared in a specific manner?
o Sample MUST be dry, finely powdered, tightly packed, column height 2-3mm
o If in chunks, it will form air pockets which slow heat transfer which are not good heat conductors and have a large melting point range and with too much in capillary it will have differential temperature and bottom will be hotter than top
Describe the range and melting point for impure compounds
they are mixture which have lower mp and wider ranges
• How should sample be heated? Why? What may happen if you heat sample too quickly?
o If you go too fast temperature at sample is very different than temperature at thermometer (sample feels heat first, then thermometer = wide range)
o Determination MUST be slow
• To give enough time for heat to be transferred equally from heating block → to sample → thermometer
o When you heat a sample too quickly → the observed initial mp temperature to be lower
what is real melting point range
From eutectic point to when sample is completely melted
what is observed melting point range?
Temperature from when 1st drop of liquid is observed to temperature when sample is completely liquefied
• How to label and interpret data from a melting point composition diagram-know all parts of a melting point composition diagram (from Continuation of Melting Point Lecture)
x axis: composition of mixture of 2 comp.
y axis: temp of different mp.
two lines of cest fit drawn from pure mp of each comp. and the point of intersection is eutectic point, the lowest point where a mixture has a narrow melting range. temp is eutectic temp
• Definition and observations when sample sweats-why it happens
sample sweating happens when sample expels some amounts of recrystallizing solvents. its not melting. sweating is when drops appear outside the surface of sample because of improper drying methods
• Observation when sample decomposes, what to do and kinds of compounds that decompose
o Ionic molecules often decompose before melting as do compounds having strong hydrogen bonds.
o Due to crystal structure changes
sample sublimes and remedy is to use sealed capillaries
• What would happen if you record the start of your melting point range when you observe the lisr of behaviors that are considered not melting.`
we will get wider range than expected. higher mp than actual
• When you take melting points of unknowns in the procedure , it is suggested that you make two samples of the pure unknown- why is this?
o first determination is to obtain an approximate range of when the sample melts, second is to match the range with the known pure compound
• What happens if you have too much sample?
o If too much in capillary, there will be differential in temperature throughout sample; bottom will be hotter than the top
• What happened if a sample is coarse (not powdered)?
o If sample is in chunks, sample will not pack well causing air pockets
• Sources of error when taking mp
o Moisture in air may have been absorbed by sample
o Contamination of sample by impurities
o Thermometer calibration
o Determination made too quickly
o Criteria for filling capillaries not adhered to