Melting Point Lab - Organic Chemistry

Chapter 3:3 Melting Point


Objectives

To show, by experimentation, that pure substances have a narrower range of melting while impure substances have a broader range, where the beginning and final temperatures of melting are below the melting point of the pure substance

To successfully determine melting point of compounds by using meting point apparatus

To identify unknown substances by comparing observed melting points with previously known substances



Background Information: MSDS



Compounds Melting Points (celcius) MSDS Overview

Acetanilide 114.3 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. Hazardous in case of ingestion, inhalation, eye contact. Slightly hazardous in case of skin contact. Flash point is open cup, 173 Celsius degrees.

Adipic Acid 152.1 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. Can cause skin irritation, eye irritation, respiratory tract if inhalation occurs. Flash point is closed cup, 196 Celsius degrees.

Benzamide 127-130 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E Can cause skin irritation, eye irritation, respiratory tract if inhalation occurs. Flash point not available.

Benzoic Acid 122.41 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. Hazardous in case of ingestion, inhalation, eye contact. Slightly hazardous in case of skin contact. Be sure to use under approved respirator. Flash point is closed cup, 121 Celsius degrees.

Salicylic Acid 159 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. Light and moisture sensitivity. Hazardous in case of ingestion, inhalation, eye contact. Slightly hazardous in case of skin contact. Be sure to use under approved respirator. Severe exposure can result in death. Flash point is closed cup, 157 Celsius degrees.

Succinic Acid 184 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. Hazardous in case of skin contact, ingestion, inhalation, eye contact. Flashpoint is closed cup, 160 Celsius degrees.

Trans-Cinnamic Acid 133 Flammability: 1 (combustible at high temp), Health: 1, Reactivity: 0 (normally stable), Personal Protection: E. Slightly hazardous in case of skin contact, ingestion, inhalation, eye contact. Flash point is closed cup, more than 100 Celsius degrees.

Urea 133-135 Flammability: 1 (combustible at high temp), Health: 2, Reactivity: 0 (normally stable), Personal Protection: E. May be combustible at high temperatures. In case of fire, for small samples, use dry powder and big fires use water spray and fog or foam. Hazardous in case of ingestion, inhalation, eye contact. Flash point is not available.



Materials Needed

One-end open capillary tubes

Long class column

Thermometer

Melting point apparatus

Spatula for moving substances

Pulverizing tools for powder samples

Goggles

Apron

Clamp

Disposal system: beaker

Procedures

Gather all needed materials for this experiment

Immediately wear safety goggles and apron and make sure to put all books away.

For Experiment 1: To find the melting point (MP) of one of the assigned compounds (Adipic Acid, Benzoic Acid, Succinic Acid), choose one compound

Make sure to use the spatula to pulverize the sample taken into powder form

Transfer the pulverized compound into a one-end open capillary tube and pack the tube until 2-3 mm high by dropping the closed end of the capillary tube down a long glass column, 2-3 times.

Obtain the melting point apparatus, switch on the power, and follow the machinery instructions for setting

As the temperature of the substance is close to the literature MP, start heating it slowly because a thermometer might fall behind and not capture the accurate temperature of the heating compound.

Acknowledge the first tiny droplet of liquids that form because this is the beginning value for the melting range

Acknowledge the last bit of solid that melts because this is the end value for the melting range

Note: Remember that the change of temperature (Celsius) can be .5, 1, or 2 away

If discoloration occurs with or without gas evolution, this means that the compound is decomposed so write “dec” for the MP temperature range

Use the first MP range as an idea of the actual temperature range the compound of the mixture melts

Cool the instrument about 10-15 Celsius below the first MP range before doing another trial. Remember to heat the other trials 2 Celsius degrees per minute.

Do steps 3-10 two more times to make total of 3 trials.

For Experiment 2: Choose one of the unknown compounds (B1, B6, or B8) and perform steps 3-10 to perform three trials to gather three data entries for the MP ranges

For Experiment 3A: Choose two known compounds with different MP and make a mixture by taking 10-15% of one of the compound and mixing it with the 85-90% of the other compound.

Perform steps 3-10 three times to gather 3 MP ranges. Average the values.

For Experiment 3B: Choose two compounds having about the same MP and prepare a 50/50 mixture of the two knowns.

Perform steps 3-10 three times to gather 3 MP ranges. Average the values.

Make sure to record all data and observations

Clean up the area appropriately and put away all equipment where they should be.

Write conclusion/possible errors and finish post lab



Data/Observations



Compound Literature MP (celcius) Trial #1 MP Range Observed Trial #2 MP Range Observed Trial #3 MP Range Observed

1st sight of droplet Melting of last bit of solid 1st sight of droplet Melting of last bit of solid 1st sight of droplet Melting of last bit of solid

Benzoic Acid 122.41 121 124 121 123 121 123

Unknown ____ N/A* N/A* N/A* N/A* N/A* N/A* N/A*

Mixture of known impure compound _____ (10-15%) into known compound ______ (85-90%) N/A* N/A* N/A* N/A* N/A* N/A* N/A*

Mixture of known compound Urea (50%) and known compound Transinnic Acid (50%) Should be less than 133-135 117 120 116 121 115 120

Urea 133-135 132 134 133 135 N/A* N/A*

Transinnic Acid 133 132 135 133 134 N/A* N/A*

*N/A = NOT AVAILABLE: NOT ENOUGH TIME; RESULTS WERE NOT EXPERIMENTED AND OBTAINED



Observations

When determining the melting point for Benzoic Acid, it is to be noted that the melting ranges were of 2-3 Celsius degrees. When compared to the literature melting point, the melting ranges obtained in lab were precise and also accurate. Determining melting points for an unknown compound did not take place due to not enough time in lab and as per instructed. Determining melting points for an impurity of a compound by approx. 15% of another compound also did not take place due to same reason. When determining the melting ranges for Urea and Transinnic Acid, it is to be noted that the melting ranges were of 2-3 Celsius degrees. When determining the melting points of Urea and Transinnic Acid individually, the melting ranges existed from 1 Celsius degree to 2 Celsius degrees. Afterwards, when a mixture of Urea and Transinnic Acid took place (approx. 50/50 mixture), the melting ranges differed by a larger gap, from 3 Celsius degrees to 5 Celsius degrees. The final melting points were also much lower than the final melting points of these two substances individually, resulting in a total difference of 15-16 Celsius degrees between the two different types of trials.

Conclusions W/ Mention of Possible Errors

In conclusion, when calculating melting points for pure substances, in an ideal situation, the most pure substance should not have a melting range but realistically, they can have a melting range that exists from .5 Celsius degrees to 2 Celsius degrees. Trials done in this lab showed a difference of 2-3 Celsius degrees on average. The multiple trials showed precision and were accurate to a large extent, considering the multiple errors that could have altered the results. When a mixture of Urea and Transinnic Acid took place, it was expected to observe lower final melting points and the ranges for the melting points were expected to be bigger in value. This is due to the fact that more impure a sample is, it displays a broad melting point range in which the beginning and final temperatures are both below the melting point of the individual samples themselves. The melting point of a substance decreases by the amount of impurity present in it. In this experiment, Transinnic acid and Urea had similar melting point ranges but when combined in one sample together, the impure sample melted at a much lower temperature, exhibiting a broader melting range as well. Some errors could have occurred during observation of the first sight of droplet and also the melting of the last bit of solid. This observation may be subjective since it depends on the observer’s perspective and eye vision accuracy. What one scientist can witness as the beginning melting point can differ from another’s due to these variables. Bias can also be a factor since the literature melting points for some of these trials were available beforehand. At least three trials should be done lab experiments of each sample, and due to not enough time, this was not done. More trials can lead to more observable data and greater accuracy and precision.

POST LAB



Given that an unknown solid was found to have a melting point range of 160-161.5, an experiment could be designed to determine whether it is Benzoic Acid, Transinnic Acid, or Urea, or perhaps not any of these substances at all. If a melting apparatus has been provided, specific steps can be taken in order to determine the answer.

Procedure:

Gather all needed materials for this experiment including the MSDS sheets on the three compounds being used in this lab

Immediately wear safety goggles and apron and make sure to put all books away.

For Experiment 1: Use spatula to pulverize the powder sample of Benzoic Acid and transfer the compound into a one-end open capillary tube. Pack the tube by dropping the closed end of the tube on a hard surface, such as the lab desk 2-3 times. Make sure to have it about 2-3 mm high.

Obtain the melting point apparatus, switch on the power, and follow the machinery instructions for setting

As the temperature of the substance is close to the literature MP, start heating it slowly because a thermometer might fall behind and not capture the accurate temperature of the heating compound.

Acknowledge the first tiny droplet of liquids that form because this is the beginning value for the melting range

Acknowledge the last bit of solid that melts because this is the end value for the melting range

Note: If discoloration occurs with or without gas evolution, this means that the compound is decomposed so write “dec” for the MP temperature range

Use the first MP range as an idea of the actual temperature range the compound of the mixture melts

Cool the instrument about 10-15 Celsius below the first MP range before doing another trial. Remember to heat the other trials 2 Celsius degrees per minute.

Do steps 3-9 two more times to make total of 3 trials.

For Experiment 2: Perform steps 3-10 for Transinnic Acid

For Experiment 3: Perform steps 3-10 for Urea.

Make sure to record all data and make observations whether any of the three compounds have similar melting point ranges to the unknown solid. From Experimentation done in our lab, it is expected that none of the 3 compounds will be identified as the unknown due to their low literature melting points.

Clean up the area appropriately and put away all equipment where they should be.

Write conclusion/possible errors and finish post lab