Organic 1 Lab Exam 1
Organic 1 Lab Exam 1 CHEM 2011
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This 12 page Study Guide was uploaded by Kari Henkel on Saturday August 27, 2016. The Study Guide belongs to CHEM 2011 at East Tennessee State University taught by Kady in Fall 2016. Since its upload, it has received 4 views. For similar materials see Organic Chemistry Lab 1 in Chemistry at East Tennessee State University.
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Date Created: 08/27/16
OriginalAlphabetical Melting Points ... An impurity _______the melting point of an organic compound. decreases What is the eutectic point? Eutectic point is the percent composition of the mixtures that have a very sharp melting point. Does a eutectic mixture have a broad or narrow melting point? A eutectic mixture has a narrow melting point. At the melting point of a sample, the temperature should not be raised more than _______°C per _______. 1°C per minute Label the percent composition of the eutectic point and record the temperature that 40% silicon melts at. Hint this will be a single temp not a range. Eutectic point 20% silicon & 80% 40% silicon melts at 800°C The melting points of pure benzoic acid and pure 2napthol are 122.5°C and 123°C respectively. Given an unknown pure sample that is known to be either pure benzoic acid or pure 2napthol, describe a procedure you might use to determine the identity of the sample. 1. First I would mix the unknown with pure benzoic acid. 2. I would take the melting point (using the slow heating method). 3. I would compare the mixture melting point with the m.p. of pure benzoic acid. If the m.p. is decreased than I know it is not benzoic acid and I would need to mix the unknown with the second known pure sample. 4. After mixing the unknown w/ 2napthol, I would take the melting point of the sample. 5. Last, I would compare the m.p. mixture w/ that of the pure sample of 2napthol. If the m.p's are the same it is 2napthol. Define melting point The melting point of a substance is defined as the temperature at which the liquid and solid phases exist in equilibrium with one another w/o change in temperature. The melting point is expressed as... the temperature range over which the solid starts to melt and then is completely converted to liquid. Used for: Characterization (identification) of the compound technique called mixture melting point determination. Determination of purity (if pure it should melt over a normal range no more than 1°) Melting phase diagram ... What are the errors that would cause a higher than correct m.p? This error could be caused by the calibration of the thermometer being slightly off. What are the errors that would cause a broad range. A broad range could also be a result of an impurity in the sample. Vapor pressure as applied to melting When a solid has the same vapor pressure as the liquid that it is in equilibrium with the solid, the solid will melt. Melting point or meltingpoint range This is a temperature range that begins when the solid begins to melt until the solid has melted completely. Mixture or mixed melting point A mixed melting point occurs when an impurity is mixed into a pure compound and as a result will broaden and lower the range of the pure compounds melting point. Eutectic point The eutectic point occurs at which both compounds melt at exactly the same rate at equilibrium composition Eutectic mixture A eutectic mixture is an impure solid that consists of two compounds that will melt at exactly the same rate causing a sharp melting point range. Eutectic mixtures can sometimes be mistaken for pure compounds because of the sharp melting point. Compound A and compound B have approximately the same melting point. State two ways in which a mixed melting point of these two compounds would be different from the melting point of either pure A or pure B. The first way in which the mixed melting points of both pure compounds would be different is by the depression of the melting point, causing it to be lower than both of the pure compounds melting points (A and B). The second way in which the melting points of both pure compounds would be different is by a broader range in the melting point. Instead of a sharp melting point within .5 to 1°C the impurities would cause the melting point to be about 5 10°C range. The melting points of pure benzoic acid and pure 2napthol are 122.5°C and 123°C, respectively. Given a pure sample that is known to be either pure benzoic acid or 2napthol, describe a procedure you might use to determine the identity of the sample. The first step would be to mix the unknown pure sample with benzoic acid. If the melting point is lower than 122.5°C and has a broader range then the unknown sample is not benzoic acid. If the melting point is the same as benzoic acid, it could be assumed that the unknown pure sample is benzoic acid. If the first procedure does not identify that the sample is benzoic acid then it would be mixed with 2napthol and the melting point would be obtained. If the melting point is lower than 123°C and has a broader range then the unknown sample is not 2napthol. If the melting point is the same as 2napthol, it can be assumed that the unknown pure sample is 2napthol. 13) The meltingpointcomposition diagram for two substances, Q and R, is provided in Figure 3.2, which should be used to answer the following questions. a. What are the melting points of pure Q and R? b. What are the melting point and the composition of the eutectic mixture? c. Would a mixture of 20 mol % Q and 80 mol % R melt if heated to 120°C? d. A mixture of Q and R was observed to melt at 105110 °C. What can be said about the composition of this mixture? Explain briefly. a. Q = 180°C, R = 155°C b. 35% mol R/ 65% mol Q at a temperature of 80°C c. No, to 160°C? Yes, to 75°C? No d. The composition of the mixture would have been either 50% mol R and 50% mol Q or it could have been 30% mol R and 70 % mol Q. Both Q and R in different compositions are acting as impurities to the other and lowering and broadening the melting point of the pure compounds. Recrystallization ... WHat are the five criteria for selecting a solvent for recrystallization? 1. The compound (being purified) needs to be soluble in a hot solvent and insoluble in a cold solvent. 2. The impurities need to be insoluble at all temperatures or mostly soluble in cold temperatures. 3. In order for the solute to crystallize, the b.p. of the solvent needs to be low. 4. The b.p. of the solvent needs to be lower than the m.p of the compound being purified. 5. The solvent should not react chemically with the compound being purified. What are the 7 steps of recrystallization? 1. Selection 2. Dissolution 3. Decoloration 4. Hot filtration 5. Formation 6. Isolation 7. Drying Why was hot filtration used? Hot filtration was used because impurities were present. In order to separate the insoluble impurities from the pure compound the solution needed to be filtered through filter paper. List the steps in the systematic procedure for miniscale recrystallization briefly explaining the purpose of each step 1. Selection is used to select the appropriate solvent. In selecting a solvent, it is important that the compound (that is being recrystallized) is soluble in the hot solvent and insoluble in the cold solvent. The impurities should be opposite of this and be insoluble in all temperatures of a solvent or slightly soluble in a cold solvent. Another factor that affects selection of appropriate solvent is the boiling point of the solvent, which needs to be low so that the crystals can be removed. The boiling point of the solvent also needs to be lower than the melting point of the solid. It is also important that the solvent does not react chemically with the solid. 2. Dissolution to dissolve the solid, the mixture is heated to the boiling point of the solvent. This method completely dissolves the solid (if the correct solvent is used), while the impurities remain insoluble in the hot solvent. 3. Decoloration if the solution is colored, there could be impurities present, so it would need decolorizing carbon to remove any colored impurities. 4. Hot filtration is used when there are impurities present. This process separates the impurities from the pure solid by pouring the solution through a filter. 5. Formation allows the hot solution to cool and begin to form crystals. 6. Isolating using a vacuum filtration removes the excess solvent trapped within the crystals. 7. Drying the crystals airdrying the crystals helps remove any last traces of solvent from the crystal. In performing a hot filtration at the miniscale level, what might happen if the filter funnel is not preheated before the solution is poured through it? If the filter funnel is not preheated, the solution would begin to cool and some of the dissolved pure compound would cool down as it was poured through the filter. As you filter out the impurities, having a cooled filter would also filter out some of the pure compound. Briefly explain how a colored solution may be decolorized. In order to decolorize a solution (as a result of impurities present,) adding a decolorizing carbon to the solution is the first step. The next steps would be to heat the solution to a boil, allow it to cool before adding a filteraid (celite) to absorb the carbon, and then reheat the solution to a boil. Then, using hot filtration, filter out the impurities and the decolorizer from the pure solid. Briefly explain how insoluble particles can be removed from a hot solution. The insoluble impurities (particles) are insoluble in the solvent and the pure compound is soluble in the solvent. This results in easy separation of the two using the hot filtration process. After heating the solvent to its boiling point and allowing the pure soluble compound to dissolve, it is cooled. Using a funnel filter and filter paper, the solution is poured through the filter paper. The filter paper catches the impurities and separates them from the solution that now contains the pure compound and the solvent. List five criteria that should be used in selecting a solvent for a recrystallization. 1. The compound needs to be soluble in a hot solvent and insoluble in a cold solvent. 2. The impurities should be insoluble in all temperatures of solvent or slightly soluble in a cold solvent. 3. In order for the pure solid to be easily removed from the crystals, the boiling point of the solvent needs to be low. 4. The solvent also needs to have a boiling point that is lower than the melting point of the solid. 5. The solvent should not chemically react with the solid. A 10g sample containing 9g of compound A and 1g of compound B is dissolved in 100 mL of solvent at 60°C. When the solution is cooled to 21°C it becomes supersaturated in compound A but is still unsaturated in compound B. Crystallization of compound A takes place. After crystallization and filtration obtains: (PIC) 6g of compound A (crystals) 3g of compound A and 1g of compound B in solution % recover of A = 6/3 x 100 = 67% Distillation and Boiling Points ... Define Boiling Point The temperature at which the vapor pressure of a liquid just equals 760m of Hg called "standard boiling point of liquid" The temp at which the vapor pressure of a liquid just equals the applied pressure is the boiling point of the the liquid. Example: The b.p. of water in Denver, Colorado Denver is 1609 meters above sea level. The average atmospheric pressure is 630 mm of Hg. The boiling point of water at 630 mm of Hg is 95°C Raoult's Law PA = P°AXA (for liquid A) PA = partial vapor pressure of pure A P°A = vapor pressure of pure A XA = mole fraction of A PB = P°BXB (liquid B) PTotal = PA + PB (Only valid for Raoult's law if it is homogeneous mixture) A mixture of 75 mole % toluene and 25 mole% cycloexane is distilled through a simple distillation apparatus. The boiling temperature is found to be 100°C as the first amount of distillate is collected. The standard vapor pressurs of toluene and cyclohexane are known to be 436 mm Hg and 1732 mm Hg, respectively at 100°C. Calculate the percentages of each of the two components in the first few drops of distillate. PT = P°TXT PT = 436 mm Hg x 0.75 PT = 327 mmHg Pc = P°cXc Pc = 1732 mmHg x 0.25 Pc = 433 mmHg Ptotal = 327 + 433 = 760 mmHg Mole Fraction is found by: % Cyclohexane = 433/760 x 100 = 57% % toluene = 327/760 x 100 = 43% Boilingpointcomposition curves for cyclohexanetoluene mixture ... Vapor pressure vs. temperature for cyclohexane and toluene ... Refer to figure 4.1 and answer the following: a) What total pressure would be required in a system in order for the liquid to boil at 45°C? b) At about what temperature would the liquid boil when the total pressure in the system is 300 torr? a. 400 torr b. 40°C Using Dalton's law, explain why a fresh cup of tea made with boiling water is not as hot at higher altitudes as it is at sea level. Dalton's law is defined by Ptotal = Psample + Pair The molecules of the vapor are mixed with air, at higher altitudes the air pressure is lowered. This lowers the total pressure in the system. The boiling point is dependent upon the total pressures. (reduction in the total pressure of the system reduces the boiling point of the sample to a temperature at which it no longer decomposes) At a given temperature, liquid A has a higher vapor pressure than liquid B. Which liquid has the higher boiling point? Liquid B will have a higher boiling point. Explain why a packed fractional distillation column is more efficient than an unpacked column for separating two closely boiling liquids. A packed fractional distillation column is more efficient than an unpacked column because it has more theoretical plates. Each of the theoretical plates is where distillations can occur as vapors move up the column. These mini distillations continue enriching the vapors so it becomes more pure as it goes higher and higher in the column, which allows for better separation. A mixture of 60 mol % npropylcyclohexane and 40 mol % n propylbenzene is distilled through a simple distillation apparatus; assume that no fractionation occurs during distillation. The boiling temperature is found to be 157°C (760 torrs) as the first small amount of distillate is collected. The standard vapor pressure of n propylcyclohexane and npropylbenzene are known to be 769 torr and 725 torr, respectively at 157.3°C. Calculate the percentage of each of the 2 compounds in the first few drops of distillate. PC = P°CXC PC = 769 torr x .60 PC = 461.4 torr PB = P°BXB PB = 725 torr x .40 PB = 290 torr Ptotal = 461.4 + 290 torr = 751.4 torr XC = 461.4/751.4 = 61% XB = 290/751.4 = 39% Steam Distillation ... Define Steam distillation Steam distillation is a technique that is very useful for the separation of slightly volatile, waterinsoluble substances from nonvolatile substances. It is especially useful in cases where a relatively small amount of volatile material is to be separated from a large amount of nonvolatile material. This technique is often used in the isolation of natural products. What are the principles of steam distillation? (PIC) Heterogeneous distillation This method requires that the two components are immisible (two phases). If the two compound are completely insoluble in each other, each component vaporizes independently of the other. In steam distillation the vapor pressure of the mixture is the sum of the vapor pressure of water and the insoluble organic compound Important points concerning steam distillation 1. The b.p. of the mixture is below the b.p. of either component. 2. Since the vapor pressure of each substance and of the mixture is independent of concentration, it follows that the b.p. of the mixture will remain constant as long as there is some of each component present. If either component becomes exhausted the b.p. will suddenly change to that of the remaining component. 3. Since the molar concentration of each substance in the vapor is proportional to its vapor pressure, the composition of the distillate (which remains constant throughout the distillation) Determined by the following ratio (PIC) Why is it important that a drop of condensate be suspended from the thermometer during a distillation? It ensures that it is not heating up too quickly A drop of liquid needs to be seen suspended from the end of the thermometer, this ensures steady distillation and a constant temperature. The top of the mercury bulb of the thermometer placed at the head of a distillaion apparatus should be adjacent to the exit opening in the condenser. Explain the effect on the observed temperature reading if the bulb is placed: a. below the opening to the condensor. This would cause a head temperature reading that was too high. b. above the opening to the condenser. This would cause a head temperature reading that was too low. Using the temperaturecomposition diagram below, answer the following questions for a mixture that contains 25 mole % cyclohexane and 75 mole % Toluene. a. At what temp will it boil? b. What is the composition, in mole % of the first small amount of vapor that forms? c. If the vapor in b) is condensed and the resulting liquid a. 100°C b. 40% Toluene & 60% cyclohexane (boils first line, next line vaporizes stair step) c. 11% Toluene & 89% cyclohexane Using the temperaturecomposition diagram below, answer the following questions for a mixture that contains 25 mole % cyclohexane and 75 mole % Toluene. a. At what temp will it boil? b. What is the composition, in mole % of the first small amount of vapor that forms? c. If the vapor in b) is condensed and the resulting liquid a. 100°C b. 40% Toluene & 60% cyclohexane (boils first line, next line vaporizes stair step) c. 11% Toluene & 89% cyclohexane Why was a steam distillation rather than a simple distillation performed in the isolation of citral from lemon grass oil? Simple distillation is used to isolate a pure liquid that is nonvolatile from another substance that is also nonvolatile. Steam distillation is used to separate liquid and solid components that are moderately volatile and are immiscible in water. Citral is moderately volatile, so steam distillation was chosen. Why was a steam distillation rather than a simple distillation performed in the isolation of citral from lemon grass oil? Simple distillation is used to isolate a pure liquid that is nonvolatile from another substance that is also nonvolatile. Steam distillation is used to separate liquid and solid components that are moderately volatile and are immiscible in water. Citral is moderately volatile, so steam distillation was chosen. What type of product is expected from the reaction of citral with Br2/CH2Cl2 With chromic acid? ... What type of product is expected from the reaction of citral with Br2/CH2Cl2 With chromic acid? ... Both geranial and vitamin A are members of the class of natural products called terpenes. This group of compounds has a common characteristic of being biosynthesized by linkage of the appropriate number of fivecarbon units having the skeletal structure shown below. Determine the number of such units present in each of these terpenes and indicate the bonds linking the various individual units. CCCCC Geranial = 2 fivecarbon units Vitamin A = 4 fivecarbon units Both geranial and vitamin A are members of the class of natural products called terpenes. This group of compounds has a common characteristic of being biosynthesized by linkage of the appropriate number of fivecarbon units having the skeletal structure shown below. Determine the number of such units present in each of these terpenes and indicate the bonds linking the various individual units. CCCCC Geranial = 2 fivecarbon units Vitamin A = 4 fivecarbon units Anhydrous (drying agent) always goes in solid organic. (True/False) False After shaking and venting during extractions, the stopper is always removed once ready to separate layers (True/False) True Using equation 5.9, show that three extractions with 5mL portions of a solvent give a better recovery than a single extraction with 15 mL of solvent when K = 0.5. FA = Cf/Ci = (Vo/KVx+Vo)n Extraction 1: 15 mL 5mL/(0.5*15mL + 5mL) = 40% remains in solution and 60% is extracted from solution Extraction 2: 3 extractions of 5mL each (5mL/(0.5*5mL + 5mL))³ = 29.6% remains in solution and 70.4% is total extracted from solution. Balancing redox equations using the halfreaction method. 1. Remove spectator ions and separate the equation into two half reactions. 2. Balance all atoms other than oxygen and hydrogen 3. Balance oxygen atoms with water molecules 4. Balance hydrogen atoms with hydrogen ions (H+) 5. Balance the charge with electrons (e) 6. Multiply each halfreaction by a whole number so that the electrons will cancel out when the two halfreactions are added together. 7. Add the two halfreactions; cancel water and H+ 8. Add hydroxide ion (OH) to neutralize H+ when necessary 9. Add spectator ions to the equation
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