Lab 3: Melting Points and Recrystallization

What is melting point?

temperature at which a solid is at equilibrium with its liquid

What is the melting point range?

span of temperature when crystals first being to liquefy to complete liquid

What is the melting point range of a pure compound?

1-2 degrees Celsius

What will happen with impure compounds?

decrease melting point and broaden range

What is the eutectic temperature?

lowest possible melting point for a mixture of 2 compounds

When preparing the sample to obtain a melting point, what should you ensure?

the sample is a fine powder

How much of the sample should you obtain in the capillary tubes for melting point observation?

1-2 mm of the bottom of the tube

What will happen if you have too much sample in the capillary tube or if it is not firmly packed?

you'll get an inaccurate melting point

What was used in Part I of the melting point portion to practice how to use the Digimelt?

sample of benzoic acid

What should you set the start temperature at when given a known compound?

10-15°C below the expected melting point

What is the expected melting point for benzoic acid?

~250°C

What should you set the ramp rate at when given a known compound?

2°C/min

For Part III of the lab, we measured the melting point of an unknown (orientation). What is the start temperature set at?

50°C

For Part III of the lab, we measured the melting point of an unknown (orientation). What is the ramp rate set at?

20°C/min

If there is an error in reading the melting point of a sample, what should you not do?

never remelt the sample; always make a new one

What do you do after you obtain the melting point of your unknown?

find the 2 compounds with similar melting temperatures in manual, pick one, and mix 1:1 mixture on watchglass, take the melting point

What does it mean if the 1:1 mixture melts at the same temperature?

the compound picked was the unknown

What is the recrystallization, purification process, used for?

remove impurities from organic compounds, which are solid at room temperature

What rule does recrystallization take advantage of?

as temperature increases, so does solubility

What does an increased temperature after recrystallization indicate?

compound more soluble in solvent

What does a decreased temperature after recrystallization indicate?

compound less soluble in solvent

What is the solvent criteria for this lab?

compound must be insoluble at room temperature and soluble at high; solvent BP lower than compound's MP; numerous crystal recovered at cooled temperature; soluble impurities at all temperatures

What are the four solvents that are tested in this lab for recrystallization?

water, acetone, ethanol, petroleum ether

What is added to the four test tubes to test recrystallization?

2 mL of respective solvent

What does it mean if the solvent dissolves at room temperature?

bad solvent

What do you do next if the solvent does not dissolve at room temperature?

place the solvents in hot water bath

What does it mean if the solvent dissolves in the hot water bath?

you can continue to test the solvent

What does it mean if the solvent does not dissolve in the hot water bath?

bad solvent

What do you do next if the solvent does dissolve in the hot water bath?

place in ice-water bath

What does it mean if crystals do form in the ice-water bath?

good solvent

What does it mean if crystals do not form in the ice-water bath?

bad solvent

What is the next step after obtaining the crystallized solvent?

heat solvent to boiling in an E. flask; in another flask, weight out acetanilide and slowly pipet the boiling solvent into the E. flask

Once you've obtained ~3 mL of the boiling solvent into the E. flask of acetanilide, what do you do next?

place flask on the hot plate and add minimum amount of boiling solvent; once dissolved, add 5% excess

What might occur if you place the E. flask of acetanilide onto the hot plate too early?

charring of acetanilide

Why do you add 5% excess to the flask once it is dissolved?

prevent premature crystallization

What do you do after adding the 5% excess into the E. flask?

remove dissolved solution from heat and add 60 mg of activated carbon after finishing boiling

What is the purpose of adding activated carbon in this lab?

decolorizes solution and the surface area absorbs dissolved organic substances

What will hot gravity filtration do?

remove the activated carbon and majority of crystals should be in solution

What are the steps to perform hot gravity filtration?

20 mL of selected solvent and 2 boiling chips into E. flask on a hot plate → make fluted filter paper and place in stemless funnel on top of a beaker → add the acetanilide activated carbon back onto hot plate until boiling → pour heated solvent into filter paper → quickly remove stemless funnel to a clean beaker → place back on hot plate → pour acetanilide activated carbon

What do you use when you are pouring a volatile solvent?

stemless funnel into an E. flask

What do you use when you are pouring a nonvolatile solvent?

stemless funnel into a beaker

Why is it better to quickly pour the acetanilide activated carbon once the stemless funnel is on a new beaker?

the better the percent recovery

What do you do after you have finished the hot gravity filtration process?

allow filtrate to reach room temperature and place in ice-water bath

What happens if the filtrate is cooled too quickly?

the crystals may form with impurities

What should you do if crystals do not form?

induce by scratching the inside of flask with glass stir rod

What is the last step of recrystallization?

isolate crystals and separate them from another liquid by vacuum filtration

How do you set up vacuum filtration?

clamp filter flask to the stand → connect vacuum tube to yellow outlet → place funnel on top of flask with filter paper → wet paper with solvent → turn vacuum on and pour the crystal solution on top of filter paper

How should the filter paper be arranged to get the best results for crystallization?

trimmed to size and lay flat on funnel

What do you do the following week, once the crystals obtained have dried?

weigh and record melting point

What is the equation for percent recovery?

(mass of recrystallized compound/mass of crude compound) x 100%

What are the hazards for benzoic acid used in this lab?

irritant

What are the hazards for the unknowns used in this lab?

flammable, toxic, irritant

What are the hazards for acetanilide used in this lab?

toxic and irritant

What are the hazards for acetone and ethanol used in this lab?

flammable and irritant

What are the hazards for petroleum ether used in this lab?

flammable and toxic

Where do we dispose of the activated carbon in this lab?

trash can

Where do we dispose of all liquids used in this lab?

liquid waste container

Where do we dispose of the capillary tubes used in this lab?

blue and white cardboard box (for broken glassware)