Term
| When to use a burner (Bunsen) |
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Definition
| To heat aqueous, nonflammable solutions and higher-boiling flammable liquids in a round-bottomed flask with a reflux condenser |
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Term
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Definition
| Provides more uniform heating |
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Term
| Precautions of heating mantle |
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Definition
| Never be plugged directly into a wall outlet. Must not come in contact with liquids. If all contents in flask are boiled out, the mantle must be turned off. |
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Term
| Problems with heating mantle |
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Definition
| Heats up slowly. Hotter than the contents of the flask. High heat capacity. Hard to maintain a constant temperature. |
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Term
| Advantages of heating bath |
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Definition
| Temperature can be determined with a thermometer in the liquid. Heat is transferred uniformly to the full surface of the immersed flask (no hot spots). |
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Term
| Problems with heating bath |
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Definition
| Limitation in the maximum temperature. Takes time to reach desired bath temperature. High heat capacity. Cleaning the lab equipment can be difficult with a water-insoluble oil. |
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Term
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Definition
| With flat-bottomed containers heating liquids such as water and nonflammable organic solvents underneath a hood. To heat a round-bottomed flask, a bath must be used. |
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Term
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Definition
| Steam valve is connected to a steam bath or steam cone, which both have an outlet to drain the condensed water. Valve should be adjusted to provide a slow, steady stream. Heat transfer can be increased by wrapping towels around the flask and having them in contact with the steam. Heating a flask with a volatile solvent requires a vacuum. |
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Term
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Definition
| burners, electrical heating, heating mantles, oil baths, hot plates, and steam heating |
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Term
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Definition
| swirling, magnetic stirring, mechanical stirring |
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Term
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Definition
| Flask should be centered on the magnetic stirrer. Revolution rate should be adjusted to rotate at a reasonable speed, without wobbling. The stirring bar should be placed gently in the flask before adding any reagents to it. Not effective with viscous liquids. |
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Term
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Definition
| Electric motor with a paddle at the end stirs the contents directly. Used for heterogeneous or very viscous mixtures. |
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Term
| Determining the type of paddle to use in mechanical stirring |
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Definition
| If there are active metals in the reaction mixture, a glass paddle must be used. |
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Term
| Stirring bearing with mechanical stirring |
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Definition
| Underneath the stirring shaft. It contains drops of silicone or mineral oil at the top to lubricate the shaft and provide an effective seal. |
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Term
| When to heat under reflux |
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Definition
| With reactions that occurs more rapidly at higher temperature but take considerable time for completion |
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Term
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Definition
| Drying agents that are used to remove small traces of moisture before distillation. |
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Term
| 2 requirements in choosing a drying agent |
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Definition
| (1) neither it nor its hydrolysis product will react with the organic liquid being dried (2) it can be completely removed from the dry liquid |
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Term
| How to remove a drying agent that forms a hydrate |
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Definition
| By gravity filtration or decantation |
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Term
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Definition
| Start by adding a small amount and swirling the flask. If liquid is cloudy, add more drying agent. Repeat until the liquid is clear. Remove by gravity filtration if liquid is high-boiling and nonvolatile. Remove by decantation if it is low-boiling and volatile. |
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Term
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Definition
| Solids that have been recrystallized using a volatile organic solvent can be air dried. If it was recrystallized with water or a high-boiling solvent, then it can be dried with an oven at a temperature below its melting point. |
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Term
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Definition
| Used to protect a reaction mixture from moisture. Place a loose plug of glasswool at the bottom of the tube and fill the tube with a granular drying agent, such as anhydrous calcium chloride. |
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Term
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Definition
| Gently warming the apparatus with a Bunsen burner from the most remote from the opening and work toward the opening. Insert drying tube while it cools |
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Term
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Definition
| Used when there are colored impurities in the liquid or solution. Has a large surface area. |
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Term
| When to decant a solution |
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Definition
| If working with a very volatile organic liquid. Can be used with drying agents, but not decolorizing carbon. |
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Term
| Purpose of vacuum filtration trap |
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Definition
| Prevents water from aspirator from backing up into the filter flask. Protects the vacuum system. Collects solvent that may overflow the filter flask. |
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Term
| Steps of vacuum filtration |
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Definition
| Cut proper size filter paper for Buchner funnel and wet with the solvent. Swirl the flask containing the crystals or use a spatula and pour into the funnel with adjoining filter. Last of crystals can be transferred by washing them from the flask with some of the "mother liquor" (filtrate). Crystals are washed by adding a cold, pure solvent to cover the crystals. |
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Term
| How to speed up drying of crystals |
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Definition
| Use a vacuum desiccator or an oven, making sure they don't get hotter than 20 degrees below the melting point. |
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Term
| Vacuum filtration with finely divided solids |
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Definition
| Form a filter-aid by adding 0.5 to 1 gram of filter-aid to a few milliliters of the solvent, then pouring it through the funnel with the vacuum on. Only desired when keeping the solution, NOT the solids. |
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Term
| Purpose of recrystallization |
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Definition
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Term
| Solution recrystallization |
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Definition
(1) Select appropriate solvent
(2) Dissolution of solid to be purified in the solvent near or at its boiling point
(3) Decolorization to remove colored impurities and filtration of hot solution to remove insoluble impurities and decolorizing carbon
(4) Formation of crystals from the solution as it cools
(5) Filtration of the purified crystals from the cooled solution and removal of solvent
(6) Drying the crystals |
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Term
| Alternative to solution recrystallization |
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Definition
| Melting the solid in the absence of a solvent and allowing the crystals to regrow with the impurities left in the melt |
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Term
| How to determine if the crystals are pure |
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Definition
| Find its melting point and compare with its actual melting point |
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Term
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Definition
(1) Must be insoluble or nearly insoluble in the cold solvent
(2) Must be soluble in the hot solvent
(3) Solvent should not react chemically with the substance being purified
(4) The boiling point of the solvent should be lower than the melting point of the solid |
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Term
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Definition
| Used when a chemist can't find just one solvent for recrystallization. Uses 2 solvents: one when the compound is insoluble when the solvent is cold and one when the compound is soluble when the solvent is cold. The compound is placed in the insoluble solvent and brought to the solvent's boiling point. The second solvent is added in with small portions until the solid dissolves. The insoluble and soluble solvents must be soluble with each other. |
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Term
| Steps of decolorizing carbon in recrystallization |
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Definition
| Put a little bit of decolorizing carbon in the hot, not boiling, solution. Heat the solution to boiling for a few minutes while stirring. Remove with gravity filtration. Boil again and refilter. |
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Term
| Why do we use gravity filtration instead of vacuum filtration during recrystallization? |
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Definition
| Because vacuum filtration will both cool and concentrate the solution and may result in premature crystallization |
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Term
| Why not crystallize by immersing the solution in cold water? |
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Definition
| The crystals formed will tend to be small, and therefore, will result in a large surface area and may cause adsorption of impurities. |
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Term
| What went wrong if recrystallization did not occur? |
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Definition
| Too much solvent or the solution is supersaturated |
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Term
| How can you induce crystallization in a supersaturated solution? |
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Definition
| Either by "seeding," which is putting a tiny amount of the original solid into the cooled solution, or by scratching the surface of the flask just above the surface of the solution with a glass stirring rod |
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Term
| Cause of oil in a recrystallization process |
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Definition
| There wasn't enough solvent in the solution |
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Term
| 2 types of oils and their remedies |
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Definition
(1) oils persist on full cooling with no evidence of crystallization. scratch the oil against the side of the flask with the glass rod. if that fails, place several small seed crystals and allow to stand. if that fails, separate the oil from the mother liquor and crystallize it from a different solvent.
(2) oil and solids form. oil consists of solvent, solute, and other impurities with a lower freezing point than that which the oil formed. reheat the mixture with additional solvent to fix the problem. |
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Term
| Vacuum filtration in recrystallization |
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Definition
| Filter the crystals with a Buchner funnel. Wash with pure, cold solvent. A second crop of crystals can be obtained by cooling the filtrate in an ice-water bath or by boiling away some of the solvent and cooling again. |
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Term
| Selection of solvent for compounds |
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Definition
| Place 20 mg of substance in 0.5 mL of water. Stir and determine its solubility. If it's not soluble at room temperature, stir in a hot-water bath. Try same procedure for ethanol and petroleum ether. If it dissolves, allow crystals to form and compare their quantity, size, color, and form |
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Term
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Definition
| melting point, boiling point, density, refraction index, and specific rotation |
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Term
| Melting range of pure compound |
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Definition
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Term
| Raoult's law for A + B (impurity) |
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Definition
| The addition of the impurity to a pure solid results in a decrease of its melting point |
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Term
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Definition
| The composition and temperature of both A + B where they can cocrystallize. |
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Term
| Why can an impure compound be mistaken for a pure compound using melting points? |
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Definition
| If a compound with impurities is at the composition of the eutectic point, it will have a sharp melting point, like that of a pure compound. |
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Term
| How to tell between a pure and impure compound using melting points |
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Definition
| Add the known compound to the mixture in question and redetermine the melting point. If the melting point is the same, the sample is pure. The sample is impure if the melting point increases. |
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Term
| Temperature rate to determine melting range of capillary tube sample |
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Definition
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Term
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Definition
| (1) impurities must have vapor pressures substantially lower than that of the compound to be obtained (2) the compound to be purified must have a high vapor pressure |
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Term
| What substances can sublime? |
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Definition
| Nonpolar substances with fairly symmetrical structures |
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Term
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Definition
| temperature at which the vapor pressure of a liquid equals the external pressure |
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Term
| Types of distillation used to purify liquids and their purposes |
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Definition
Simple distillation - used to remove a pure liquid from nonvolatile impurities or to separate a mixture of liquids
Fracitonal Distillation - used to separate mixtures of liquids with boiling points between 20-30 degrees of each other
Vacuum Distillation - used when substances decompose, oxidize, or undergo undesirable reactions at their normal boiling point
Steam Distillation - used to separate volatile organic compounds that are immiscible with water |
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Term
| How fractional distillation works |
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Definition
| The uncondensed vapor rises higher and higher up the column and undergoes a series of repeated condensations and revaporizations. The vapor phase becomes rich in the more volatile component. The efficiency of the set-up is determined by the number of plates in the column. |
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Term
| Apparatus requirements for fractional distillation |
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Definition
| (1) intimate and extensive contact between the liquid and vapor phases in the column (2) maintenance of proper temperature gradient along the column (3) sufficient length of column (4) sufficient different in boiling points |
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Term
| Temperature gradient in fractional distillation column |
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Definition
| The bottom of the column should be the temperature of the boiling point of the mixture in the pot and the top of the column should be the temperature of the boiling point of the more volatile component. |
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Term
| Rate the distillate is removed from the stillhead in fractional distillation |
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Definition
| 1 drop every 2 to 3 seconds |
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Term
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Definition
| Amount of vapor that condenses and returns to the pot to the amount of vapor taken off as a distillate. higher = better |
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Term
| Approximations of the effect of lowered pressure on boiling points |
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Definition
| Reduce the pressure from atmospheric to 25 torrs lowers the boiling point from 250-300 by about 100-125 degrees. After that, each time the pressure is cut in half, it lowers the temperature about 10 degrees. |
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Term
| Steps of vacuum distillation |
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Definition
(1) Never use glassware with cracks or flat-bottomed flask. Examine the glassware carefully.
(2) Lubricate and seal all joints carefully
(3) Place liquid in distilling flask
(4) Turn on vacuum with release valve open
(5) Close release valve
(6) Discontinue heating
(7) Release vacuum then turn off the vacuum source |
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Term
| distribution coefficient (K) |
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Definition
in liquid-liquid extraction, a solute (A) in contact with a mixture of two solvents (S, S') will be distributed between the liquids so that at equilibrium, the ratio of concentrations of A in each phase will be constant, at constant T.
K = grams A in S/ grams of A in S' x mL of S'/mL of S
If the volume of S is increased by a facot of two, then the amount of grams of A in S increases by two, as well |
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Term
| Most efficient method of extraction with distribution law |
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Definition
| Performing more extractions with less volume of S results in more of the desired substance extracted. The larger the distribution coefficient, the least number of extractions needed. |
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Term
| Principles in choosing an extracting solvent |
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Definition
(1) Solvent must be immiscible with the solvent of the solution
(2) The solvent chosen must have the most favorable distribution coefficient for the component that is to be separated and unfavorable for impurities
(3) Solvent cannot react chemically with the mixture
(4) Solvent should be readily separated from the solute (distillation - use volatile solvent) |
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Term
| Purpose of separatory funnels |
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Definition
| To separate mixtures in two immiscible liquid phases or to wash organic layers |
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Term
| When to use a spherical separatory funnel |
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Definition
| When the liquid phases are similar densities |
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Term
| How to determine layers in a separatory funnel |
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Definition
| Take a small sample from one of the layers. Add about 0.5 mL of water. If the layer is aqueous, it will dissolve in water. |
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Term
| How to hold a separatory funnel |
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Definition
| With your left hand over the stopper and your right hand's first two fingers and thumb curled around the handle of the stopcock |
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Term
| What to do if the liquids in the separatory funnel form an emulsion |
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Definition
(1) leave unattended for an extended period of time to allow separation
(2) add a few milliliters of brine which increases the ionic strength of water
(3) filter the heterogeneous mixture by gravity filtration and return the filtrate to the separatory funnel
(4) add a small quantity of water-soluble detergent and reshake |
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Term
| Equipment in simple distillation |
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Definition
| heating mantle/heating bath -> distillation flask w/ boiling chips -> stillhead w/ thermometer at bottom of side outlet -> condenser w/ inlet at bottom-> vacuum adapter -> distillate flask |
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Term
| fractional distillation equipment |
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Definition
| heating bath/heating mantle -> distillation flask w/ boiling chips -> Hempel column -> stillhead w/ thermometer at bottom of sidearm -> condenser w/ inlet at bottom ->vacuum adapter -> receiver |
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Term
| rotary vacuum evaporator equipment |
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Definition
| water bath -> rotating evaporator flask -> vapor duct -> motor -> condenser w/ vacuum connection at bottom and outlet tap -> receiver flask connected to condenser |
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Term
| steam distillation equipment |
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Definition
| distilling flask -> claisen head -> steam inlet with tube down to bottom of distilling flask at the straight arm -> stillhead at the sidearm of claisen head -> condenser w/ water inlet at bottom -> vacuum adapter -> receiver |
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Term
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Definition
A flat plate is coated with a thin layer of adsorbent (silica gel, alumina). The mixture is separated and dissolved in an appropriate solvent. It is spotted onto the plate and the solvent evaporates. The plate is placed in a developing jar containing a little developing solvent. The solvent rises through the adsorbent layer. When it reaches the top, the compounds should be well separated.
Solid-liquid chromatography |
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Term
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Definition
| Made of thick glass and can be coated with adsorbent using special spreading equipment |
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Term
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Definition
| Ready-coated with a layer of adsorbent on thin glass, plastic sheet, or aluminum foil. Gives better resolving power. |
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Term
| application of sample in TLC chromatography |
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Definition
| 1-2% solution with a volatile solvent from a capillary dropper onto the plate 1 cm from the bottom |
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Term
| developing the chromatogram |
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Definition
| Spots should be right above the developing liquid. The jar should be lined with a piece of filter paper that dips into the solvent to keep the atmosphere saturated with the solvent vapor. Jar should be capped during development. When it comes close to reaching the top of the plate, remove and mark with a pencil. Allow solvent to evaporate |
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Term
| Examining a colorless chromatogram |
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Definition
(1) Use an inorganic fluorescent agent in the adsorbent layer. Shine UV light on the plate. The adsorbent layer glows green or blue and the spots are dark.
(2) Dip plate in a developing jar with a few crystals of iodine. The organic spots are stained brown |
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Term
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Definition
| Run the mixture and the "authentic" sample side by side on the same plate. Add a little of the "authentic" to a separate sample of the mixture to see whether the spots coincide. |
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Term
| Distance moved up plate depends on |
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Definition
| Its affinity for the adsorbent and the strength (polarity) of the developing solvent |
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Term
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Definition
| Larger plates with thicker adsorbent. The sample solution is applied as a band along the origin using a dropper or syringe using a straight-edged guide. A mechanical streaker can be used. |
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Term
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Definition
| Separation using a column of adsorbent in glass tube. At top of glass tube is the weighed out mixture. The eluting solvent is above that. When the eluent is released, the mixture is carried down the adsorbent at a rate depending on its affinity for it. |
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Term
| How compounds move down the adsorbent tube in column chromatography |
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Definition
| Polar compounds move slowly (alcohols, amines, carboxylic acids) and nonpolar compounds move fast (aldehydes, ketones, ethers, hydrocarbons) |
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Term
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Definition
| R_f difference of 0.15+. Eluting solvent driven down column by air or nitrogen pressure. Best results with solvent falls at 2 inch/min. Packing is high grade silica gel. Takes about 15 minutes. |
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Term
| Packing column for flash chromatography |
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Definition
| Pack small plug of glass wool at base of tube. 1/8 inch of sand. Silica gel. 1/8 inch of sand. Turn nitrogen/oxygen on and adjust flow to 7 psi with flow control valve. Pipette a 25% sample in eluting solvent on top of adsorbent bed and pressurize the column to push it down the silica, NOT the packing bed. |
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Term
| Dry-column flash chromatography |
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Definition
| Fill sintered glass funnel to lip with TLC-grade silica and apply suction. Carry out TLC experiments to find best solvent mixture. Pre-elute the column then apply suction. Open tap A, pour 25% sample in solvent, close tap A, and suction. Make varying strengths of eluting solvent in test tubes. Pour on silica gel, suck dry, open tap, remove flask, then pour back into test tube. |
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Term
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Definition
| TLC, preparative TLC, flash chromatography, dry-column flask chromatography, column chromatography, medium-pressure liquid chromatography, gravity elution chromatography |
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