CHEM 2364 Midterm Study Guide
Safety and Good Lab Practices
- Review Safety and Good Lab Practices information found in the syllabus and as discussed on the first day of lab.
o It is never acceptable to wear gloves in the hallway
o The last thing you should do: Take off PPE
o Put away glassware after washing them
o Weak bases: NaHCO3
Pour down the drain
o Strong acids:
Pour down the drain with sodium bicarbonate to neutralize and LOTS of running water
Exp. 1: Recrystallization of Aspirin Topics
- Be able to rank solvents by polarity.
o Acetylsalicylic acid (aspirin): non-polar, slightly soluble in water at room temperature
Dipole-dipole, and OH segment
o Sucrose: polar, soluble in water
o Food dye: polar (water based), highly soluble in water
o Ethanol: polar, lower BP and is better at drying
o Water: very polar
o Ex: DMSO>acetone>methylene chloride>ethyl acetate>toluene>hexane
- Know the criteria for selecting a recrystallization solvent.
o The compound of interest should be insoluble at cold temperatures, but soluble at high temperatures.
The impurities should be insoluble in the solvent at all temperatures, or soluble at all temperatures.
If all solvents are equal, select the one that is safe to use and minimizes environmental impact
o The solvent should be easily removed and have a low BP.
- Understand how recrystallization works to purify a compound.
o Recrystallization: separation of the substance of interest from impurities, which includes any solvents
The aspirin solid contains 10% impurities (sugar) as well as food dye
The filtration will allow for separation between aspirin and the impurities on the basis of their
solubility in water
- Understand the relationship between solubility and temperature.
o Temperature usually increases solubility
o Water should be cold to avoid collected crystals from becoming melted again
- Understand the relationship between solubility and IMFs.
o Van-der Waals: weakest, decreased solubility
o Dipole-Dipole: middle
Opposite charges on molecules, permanent dipoles
o Hydrogen bonding: strongest, increased solubility of the solute in the solvent
H bonded to a very EN atom like N, O, or F
o IMF that are the same are miscible with one another
o Polarity increases with IMF and BP/MP
- Understand the relationship between melting point temperature and purity.
o The purity of a solid will be determined by comparing its experimental MP to the MP found in literature.
o Melting Point: measures how much energy it takes to break the IMF in the solid
o Impurities lower the MP by interrupting the IMF present, resulting in a lower amount of energy required
to break the IMF (MP range will be broader)
, Exception: if the impurity is present in a large quantity and has a significantly higher melting
point
o The freezing point temperature is identical to the melting point temperature
- Calculate percent recovery (know formula).
- Understand the purpose of each step involved in vacuum filtration.
o Filter paper: prevents any small solid particles from passing through holes in the Buchner funnel
o Vacuum needs to be started before the solution is poured to maintain a seal between the filter paper
and funnel. Often, the solvent is volatile, so do not run the vacuum for very long before pouring the
solution, otherwise it could evaporate and the seal would be lost. Remove tubing from filter flask barb
before turning off the water
o Vacuum filtration vs. gravity filtration benefits
Faster speed, more quickly removes the filtrate, therefore has a purer product
Allowing air to flow through the solid helps dry both the filter paper and the solid (But if water is
the solvent, the paper will take significantly longer to dry
- Identify what is happening in each filtration (e.g., what is in the filtrate and precipitate for each step).
o In Part 2, separation of aspirin and sucrose
Filtrate: water, sucrose, food dye?
Precipitate: Aspirin, maybe water
o In Part 3, filtration/ethanol rinse
Filtrate: water and ethanol
Precipitate: Aspirin, maybe ethanol
*Watch glass is needed since the solvent is ethanol, which evaporates quicker than water
Ethanol-water mixture rinse removes water droplets and the solvent without much loss from
dissolution
Since water has a high BP and cannot be removed easily from the recrystallized solid, ethanol is
used
- Understand why each reagent was used in the order that it was in a recrystallization.
- Procedure
o Part 1: Preparation
DI water + impure aspirin solid is heated
o Part 2: Separation of Aspirin and Sucrose
Solution is dissolved and Aspirin solid is allowed to crystallize (precipitated solid)
First filtration with water
o Part 3: Removal of water
Safety and Good Lab Practices
- Review Safety and Good Lab Practices information found in the syllabus and as discussed on the first day of lab.
o It is never acceptable to wear gloves in the hallway
o The last thing you should do: Take off PPE
o Put away glassware after washing them
o Weak bases: NaHCO3
Pour down the drain
o Strong acids:
Pour down the drain with sodium bicarbonate to neutralize and LOTS of running water
Exp. 1: Recrystallization of Aspirin Topics
- Be able to rank solvents by polarity.
o Acetylsalicylic acid (aspirin): non-polar, slightly soluble in water at room temperature
Dipole-dipole, and OH segment
o Sucrose: polar, soluble in water
o Food dye: polar (water based), highly soluble in water
o Ethanol: polar, lower BP and is better at drying
o Water: very polar
o Ex: DMSO>acetone>methylene chloride>ethyl acetate>toluene>hexane
- Know the criteria for selecting a recrystallization solvent.
o The compound of interest should be insoluble at cold temperatures, but soluble at high temperatures.
The impurities should be insoluble in the solvent at all temperatures, or soluble at all temperatures.
If all solvents are equal, select the one that is safe to use and minimizes environmental impact
o The solvent should be easily removed and have a low BP.
- Understand how recrystallization works to purify a compound.
o Recrystallization: separation of the substance of interest from impurities, which includes any solvents
The aspirin solid contains 10% impurities (sugar) as well as food dye
The filtration will allow for separation between aspirin and the impurities on the basis of their
solubility in water
- Understand the relationship between solubility and temperature.
o Temperature usually increases solubility
o Water should be cold to avoid collected crystals from becoming melted again
- Understand the relationship between solubility and IMFs.
o Van-der Waals: weakest, decreased solubility
o Dipole-Dipole: middle
Opposite charges on molecules, permanent dipoles
o Hydrogen bonding: strongest, increased solubility of the solute in the solvent
H bonded to a very EN atom like N, O, or F
o IMF that are the same are miscible with one another
o Polarity increases with IMF and BP/MP
- Understand the relationship between melting point temperature and purity.
o The purity of a solid will be determined by comparing its experimental MP to the MP found in literature.
o Melting Point: measures how much energy it takes to break the IMF in the solid
o Impurities lower the MP by interrupting the IMF present, resulting in a lower amount of energy required
to break the IMF (MP range will be broader)
, Exception: if the impurity is present in a large quantity and has a significantly higher melting
point
o The freezing point temperature is identical to the melting point temperature
- Calculate percent recovery (know formula).
- Understand the purpose of each step involved in vacuum filtration.
o Filter paper: prevents any small solid particles from passing through holes in the Buchner funnel
o Vacuum needs to be started before the solution is poured to maintain a seal between the filter paper
and funnel. Often, the solvent is volatile, so do not run the vacuum for very long before pouring the
solution, otherwise it could evaporate and the seal would be lost. Remove tubing from filter flask barb
before turning off the water
o Vacuum filtration vs. gravity filtration benefits
Faster speed, more quickly removes the filtrate, therefore has a purer product
Allowing air to flow through the solid helps dry both the filter paper and the solid (But if water is
the solvent, the paper will take significantly longer to dry
- Identify what is happening in each filtration (e.g., what is in the filtrate and precipitate for each step).
o In Part 2, separation of aspirin and sucrose
Filtrate: water, sucrose, food dye?
Precipitate: Aspirin, maybe water
o In Part 3, filtration/ethanol rinse
Filtrate: water and ethanol
Precipitate: Aspirin, maybe ethanol
*Watch glass is needed since the solvent is ethanol, which evaporates quicker than water
Ethanol-water mixture rinse removes water droplets and the solvent without much loss from
dissolution
Since water has a high BP and cannot be removed easily from the recrystallized solid, ethanol is
used
- Understand why each reagent was used in the order that it was in a recrystallization.
- Procedure
o Part 1: Preparation
DI water + impure aspirin solid is heated
o Part 2: Separation of Aspirin and Sucrose
Solution is dissolved and Aspirin solid is allowed to crystallize (precipitated solid)
First filtration with water
o Part 3: Removal of water
