Melting Point Determination and Re-crystallization Labs

What is the normal range for a pure substance?

1-2 degrees

What are the steps to re-crystallization?

1. Dissolve the sample in a minimal amount of a suitable hot solvent to make a saturated solution near the solvents boiling point. 2. Filter out the solid impurities using gravity filtration (be sure warm the gravity filtrator prior to filtration). 3. Cool the solution to room temperature and then place it in an ice bath until crystals form. 4. Leave the solution undisturbed until crystals stop forming. 5. Collect the crystals of the purified product by putting them through gravity filtration. 6. Rinse the beaker with 3-4 mL of cold water and add to filtration. 7. Wash the crystals with 2-3 mL of cold water and press them firmly into the filter paper with a clean spatula. 8. Air dry the crystals 9. Determine a melting point range

What are the characteristics of a good crystallization solvent?

1. Must dissolve impurities at all temperatures. Must dissolve the solute (substance being crystallized) only at hot temperature. 2. Must be inert 3. Volatile 4. Inexpensive 5. Safe

What should the temperature of the hot plate be for the re-crystallization lab?

130 degrees

How fast should the temperature change with the slow heating?

2 degrees/minute

How fast should the temperature change with the rapid heating?

8 degrees/minute

What is the best type of solvent for re-crystallization?

A solvent that is directly proportional to temperature and solubility.

What is the worst type of solvent for re-crystallization?

A solvent that is inversely proportional to temperature and solubility.

In the re-crystallization lab what is in the solution after it has been cooled to room temperature?

Dissolved impurities Vanillin

What is the composition of Vanquish?

Acetaminophen aspirin and caffeine.

What is the composition of Excedrin?

Acetaminophen, aspirin and caffeine

Suppose you used extra amount of solvent to crystallize vanillin, which makes the solid fail to precipitate out of the cold solvent. What would you do to recover the the material from the solution and promote crystal growth?

Evaporate the excess amount of solvent and cool it down to see if the vanillin recrystallizes. Then scratch the side of the beaker and see if any crystals start to form. If not, add a seed crystal for the molecules to sit on for the crystal lattice to grow.

Why does the melting point only suggest the identity of a solid while the mixed melting point proves the identity?

It can only suggest because you can have more than one solid with the same melting point. The mixed melting point proves the identity because you can see if the melting point changes or not by mixing it with a known substance. If the melting point changes then you have not guessed the right substance.

How are melting points usually determined?

Heating a small mount of the sample in a capillary tube that has been sealed at one end in a mel-temp.

At the end of the re-crystallization lab where is the crude vanillin?

In the paper towel

What is the effect on the melting point and melting point range of the presence of sand in the sample?

It will be a heterogeneous mixture so the sand will have no effect on the melting point. The melting point of the sand is much higher than the melting point of vanillin so the canillin will melt long before the sand.

In the re-crystallization lab why do we want slow cooling?

Larger crystals and smaller surface area for adherence of impurities. Larger crystals are easier to wash.

How should you treat a burn?

Place immediately under cold tap water and as soon as possible into ice water

What should you do if after chilling the filtrate crystals do not form?

Scratch the sides of the flask with a glass rod or inoculate the solution with a few tiny crystals of the product.

Why should the rate of cooling of the crystals be slow?

Slow growing crystals tend to exclude impurities

What was the purpose of washing the crystals of the vanillin with ice water? Why does the water have to be cold?

So the vanillin doesn't dissolve. If it were warm it could dissolve some of the vanillin.

What would be the effect of incomplete removal of water have on the melting point and melting point range?

The H2O will act on the vanillin as an impurity and widen the range of the melting point. The melting point becomes lower because then it it easier to break the bond when there are impurities in the way. The impurities disrupt inter-molecular forces.

What are the components of a Mel-Temp apparatus?

The components of the Mel-Temp are the on/off switch, voltage control, power cord, thermometer, light source, heating bath, and magnifying eye piece, observation window, and the sample slots.

What do the two numbers in the melting point mean?

The first number is the point when the sample first begins to liquefy. The second number is the point when all the sample has melted.

What happens to the melting point when impurities are added to a substance?

The melting point gets lower and broader.

What would the effect on the melting point and melting point range be if there are inactive binders in a tablet of aspirin?

The melting point range will be lower because they are not pure, they are mixed with other active ingredients. This is a homogeneous mixture so the melting point will be lower and have a wider range.

An impure sample of vanillin is re-crystallized from a suitable solvent system by heating the crude sample and the solvent mixture together, then cooling, evaporating off the solvent and weighing the crystals. Should the melting point of these crystals be higher than, lower than, or similar to the melting point of the crude sample? Explain why.

The melting point should be similar to the melting point of the crude sample because you did not filter the crystals. Since it wasn't filtered the impurities are still mixed with the crystals which will keep the melting point the same.

Why do we need to heat the substance slow when finding the melting point?

The sample needs time to absorb the heat generated by the metal bath.

What does it mean if a substance becomes discolored upon melting?

The substance has decomposed when it melted

How are the solute and solvent bonded and why?

The sugar molecules and the water are attracted by hydrogen bonding because both of the molecules have an -OH

Melting Point

The temperature at which solid and liquid phases of the substance are at equilibrium. The temperature at which a solid changes to a liquid.

Why should the solid be rinsed with ice water instead of room temperature water?

The use of room temperature water will case dissolution of some product.

Why do impurities affect the melting point?

They disrupt the intermolecular forces holding the crystals together

What is the effect on the melting point and range is you mix the vanillin and table sugar?

They mix together so we expect that the melting point will be lower and the range will be wider.

What was the purpose of using water during the re-crystallization of vanillin?

To be a solvent for the re-crystallization of vanillin.

Why should a second batch of crystals be collected from the filtrate?

To ensure maximum recover of product.

Why should the sample be packed tightly in the melting point tube?

To ensure that there is uniform heat distribution. If it isn't packed tightly, air pockets are made which can cause the melting point to be altered. The air has different heat capacity than the solid.

What can the melting point of a compound be used for?

To identify a substance and to establish purity

An unknown compound melting at 136-138 degrees C was suspected to be either aspirin (mp 138-140 degrees C) or urea (mp 133-135). Explain how you could determine whether one of these two suspected compounds was identical to the unknown compound.

Use a mixed melting point.

In the re-crystallization lab what were the major components of the crude mixture?

Vanillin Sugar Impurities

What did you hypothesize in your pre-lab report about your experiment? Use your experimental results to support or discount your hypothesis.

We hypothesized that isolating the mixture would be best done by dissolving the mixture into room temperature water. But from our experimental results we found that it was best done by dissolving the mixture in hot water so both the vanillin and the sugar would dissolve.

When does actual melting begin?

When the first drop of liquid becomes visible.

What is hydrogen bonding?

When there are hydrogen between two electronegative elements.

What is the "mixed melting point" procedure?

When you mix an equal amount of the authentic compound with the unknown and find the melting point. Compare the mixed melting point with the pure unknown melting point and if the melting points are the same then you can assume the compounds are identical.

What can you conclude about water as a crystallization solvent for vanillin? Justify your conclusion.

You can conclude that water was a safe and inexpensive solvent for the re-crystallization of vanillin. It dissolved vanillin and the impurities at a hot temperature but it did not dissolve the vanillin at a cool temperature.

Why do you need heat for the re-crystallization lab?

You need heat to break the H bonds in the vanillin so they bond to the vanillin and dissolve it.

Why should you heat the metal bath of the mel-temp slowly and steadily instead of in vicinity of the melting point? At what rate should the sample be heated?

You should heat it at about 1-2 degrees per minute so that it has time to absorb the heat because there is a lag time between the rate of heating and heat absorption.

Which of the 12 Principles of Green Chemistry would apply to the re-crystallization/melting point experiment you have performed?

• Less Hazardous Chemical Synthesis • Designing safer Chemicals • Safer Solvents and Auxiliaries • Design for Degradation • Inherently Safer Chemistry for Accident Prevention