Organic Chemistry I Lab Midterm – Flashcards
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Two useful functions served by knowing the melting point of an organic compound
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1. confirming presence of an unknown compound 2. testing purity of a substance
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Explain why a finely powdered sample should be used in a melting point measurement
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finely ground particles of the compound are necessary for good heat transfer
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Explain why it is important to heat a sample slowly to obtain an accurate melting point
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heating a sample too rapidly may cause the thermometer reading to differ from the actual temperature of the heat source
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Identify two reasons why it is sometimes difficult to measure the temperature at which the crystals first begin to liquefy.
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1. Sweating can be confused with melting. 2. Shrinking may seem like melting.
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Identify what two effects a soluble impurity usually has on the melting point of a compound.
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1. affects the melting point by 5 degrees C or more 2. melting point range will widen
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Identify what occurs when crystals begin to disappear form the bottom of the capillary tube rather than turning to a liquid.
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sublimation
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A sample has an experimental melting point of 100-101 degrees C. Can you conclude that sample is pure?
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Yes, it is narrow enough
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What error in volume occurs if a micropipet plunger is pressed all the way down before filling the pipet?
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an incorrect amount of the substance will be dispensed
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How can a refractive index measurement be used to indicate the purity of an organic compound?
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the more pure a compound is, the closer its refractive index will be to the literature value
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What causes a liquid to enter a capillary bell during a boiling point measurement?
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atmospheric pressure forces liquid into the bell when the vapor temperature cools to the boiling point of the liquid
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When compared to macroscale experiments, what safety advantage does the use of microscale experiments in organic chemistry offer students and laboratory instructors?
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significantly reduces the exposure of students and instructors to large volumes of hazardous organic chemicals
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Briefly explain why sand baths are commonly used in microscale organic laboratories.
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sand is a poor conductor of heat
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What is meant by "tare"?
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mass of an empty container
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Why should solids be weighed on glazed paper instead of on filter paper?
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filter paper and other coarse papers trap solids in their fibers
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What is the purpose of the cotton plug in a Pasteur filter pipet?
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slows liquid flow, making leakage less likely
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Layout of the laboratory
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STUDY THIS
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Descripe the steps to be taken in the event 10 mL of ethanol in a 50 mL beaker ignites in the lab
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1. Caution neighbors of fire and alert instructor 2. Move flammable materials away 3. Turn equipment off/remove equipment from vicinity 4. Shut off gas 5. Smother burning material or let it burn out
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Identify two reasons for notifying the laboratory instructor of any incidents that occur in the lab
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1. the lab instructor will most likely know how to deal with the incident 2. Information you provide can help the instructor with avoiding similar problems with other students
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Why is unauthorized experimenting by a student in the laboratory not allowed?
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Unauthorized experimenting will waste time and may expose you and others to unreasonable risk
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According to the information in the MSDS, which hazardous category is of greatest concern for 1-propanol?
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dangerous fire hazard
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What are the first and second actions to be taken in the event a person spills some reagent on himself or herself?
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1. Wash with large volumes of water 2. Get in safety shower and remove clothes to wash with soap and water
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Identify three precautions to be taken before beginning the distillation of an organic liquid.
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1. Obtain approval from lab instructor 2. Make sure apparatus has an opening 3. Use a fresh boiling chip
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The index of refraction is given in tables with a superscript 20 and a subscript D. What change in light is being measure?
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refraction index change in velocity of light
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Glassware components used in a simple microscale distillation
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thermometer hickman still conical vial
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What precautions are necessary when working the GC injection port?
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avoid touching the injection port with hand or with the glass barrel of the syringe. It can burn your hands or crack the syringe barrel.
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What problems result if the temperature is set too low for the injector?
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the compound will not instantly vaporize, moving slowly into the column, making separation and quantification difficult
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What problems result if the temperature is set too low for the column
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compounds with lower boiling points spend more time in the mobile phase than compounds with higher boiling points. The lower boiling point comes off the column faster and produces broad peaks.
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What problems result if the temperature is set too low for the detector?
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a low boiling temperature could condense and clog the detector.
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Will injector temperature have an effect on retention time?
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No
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Will column temperature have an effect on retention time?
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Yes, lower boiling points come off the condenser quicker
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Will detector temperature have an effect on retention time?
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no
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Will length or type of column have an effect on retention time?
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Yes, short column = faster retention time long column = slower retention time
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Will size of injected sample have an effect on retention time?
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no, it's not in the table
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Will detector attenuation have an effect on retention time?
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No, it's reading data, doesn't have an actual effect on the substance in the column
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Names of two compounds that are separated by distillation
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cyclohexane and toluene
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What is the name of the technique that is discussed in the CER lab for determining the composition of the distillate fractions collected?
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Refractive Index
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Main glassware components used in distillation
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thermometer fractionating column hickman still spinner 5 ml conical vial
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Explain why you should not add boiling chips to a boiling liquid
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the large surface of the boiling chip can cause the hot liquid to foam out
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Explain how and why you should position the thermometer in the distillation head during a distillation.
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Insert the thermometer so it's even with the distilling head so it's submerged with vapors and can measure temperature correctly
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What is the purpose of the outer jacket on a fractionating column?
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it insulates to protect against heat loss from the inner jacket
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How is the rate of heating adjusted when using a sand bath as a heat source?
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it is increased 2 degrees C per minute
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How is the distillate collected in a test tube microscale distillation?
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use of a bent-tip pasteur-pipet
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what effect does an increase in the heating rate have on the boiling temperature during distillation?
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boiling temperature doesn't change
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As molecules move up a fractional distillation column, they condense and then revaporize. During which of these steps is the concentration of the more volatile compound of the mixture increased? Explain.
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When molecules condense and revaporize, it increases the more volatile compound while the less volatile compound decreases.
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Composition of a cyclohexane-toluene distillate collected at a) 85 b) 95 c) 105
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a) 75-25 b) 35-65 c) 10-90
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What are the two main types of gas chromatography detectors discussed?
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Flame ionization detector (FID) Thermal Conductivity Detector (TCD)
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How will raising column temperature in a gas chromatograph affect the retention time and separation of the sample components?
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Raising column temp will increase retention time and speed up separation of the sample components
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What are the two main types of gas chromatography columns discussed?
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Packed and capillary
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What if Rf?
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Retention factor = distance traveled by compound / distance traveled by eluent front
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What are the two methods used for visualizing spot positions in TLC?
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iodine and ultraviolet light
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Which spot would move fastest in a TLC experiment, a polar or nonpolar compound? Why ? Explain in terms of intermolecular forces.
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A nonpolar compound: less intermolecular forces, meaning it can move faster. A polar compound has strong intermolecular forces which would slow it down.
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List TLC structures in order of increasing polarity.
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biphenyl < benzophenone < benzhydrol
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How to fix decoloration during recrystallization
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fix with activated charcoal
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When does oiling out occur?
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Occurs when compound is above the melting point when it begins to separate form the solution, forms oil instead of crystals
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What are the best conditions for solvent selection?
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1. insoluble at room temperature 2. soluble in boiling solvent
