Last Updated 28 Jan 2021

Solids: Recrystallization and Melting Points

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Main Reaction: Not a chemical reaction but a physical reaction. Urea in solid mixed with water melts when heated. Then through filtration and drying, the liquid mixture yields the pure Urea. This is the same main reaction for Cinnamic Acid, mixtures, and the unknown compound.

Mechanism: Recrystallization.

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Side Reactions: The impurities are being dissolved wither at room temperature with the addition of water and/or with heating. They remain in liquid form during the recrystallization process.

Chart

Compound Urea Cinnamic Acid Unknown #40 4:1 ratio Urea to C.Acid 1:1 ratio Urea to C.Acid 1:4 ratio Urea to C.Acid
Known Melting Point Range of Pure Compounds 132.5-133 C 133-134.5 C * * * *
Experimental Melting Point Range (Slow) 131.5-134.8 C 133.1-135.2 C 209-214.8 C 96.1-131.1 C 97.7-114.8 C 97.7-125.5 C
Experimental Melting Point Range (Fast) 133.6-134 C 134-135 C 210-215 C 96-130.1 C 98.9-111.2 C 98.9-118.8

Calculations

Molar mass of urea (CO(NH2)2) = 60.05526 g/mol and 1g of urea was used so converting it to moles take 1g / 60.05526g/mol = 0.0166513 moles of urea.

The molar mass of cinnamic acid (C9H8O2) is 148.16g/mol and 1g was used so converting it to moles take 1g/ 148.16g/mol = 0.00674946 moles of cinnamic acid.

Procedure

A. Calibrate Thermometer

a. Use Table 3.2 of known melting point temperatures for a series of standard substances.

b. Note the deviations from the given temperature given in Table 3.2 to that given by the thermometer. This deviation value will be applied to all temperature measurements taken.

B. Determine Melting Points

a. Pure Compound

a.i. Determine melting point range of Urea. Compare with the known melting point range given to make sure your results are accurate.

a.ii. Determine melting point range of Cinnamic Acid. Compare with the known melting point range given to make sure your results are accurate.

b. Unknown Compound

b.i. You will be given an unknown compound. Determine the melting point range of the unknown pure compound.

c. Mixed Melting Points

c.i. Using Urea, prepare 3 mixed samples by adding Cinnamic Acid in ratios of (1:4, 1:1, and 4:1) and mix them with a watch glass and stirring rod or spatula.

c.ii. Determine the melting point range of these mixed samples.

c.iii. Compare this melting point ranges to the pure Urea and Cinnamic Acid compounds from part to see how impurities affect the melting range.

Observations/Conclusions

The purpose of this experiment was to explore the process of recrystallization in regards to compound purity and to determine an unknown compound. Using the chart of known melting points, the unknown compound #40 was determined to be p-terphenyl because the known melting point range of 210-211 C was similar to the experimental melting point range obtained at 210-215 C. Observations noted during this experiment was that heating at a fast rate of 20 degrees per minute starting from 65 C until 150 C (except in the case of the unknown which was heated until 220 C) resulted in higher temperature ranges than those obtained using the slow rate of 5 degrees per minute using the starting and ending temperature of 145 C and 220 C respectively. This is most likely due to the heating rate being too fast so that the thermometer lagged behind the actual temperature of the solution. Also, the mixture of Urea to Cinnamic Acid with the ratio of 4:1 is observed to be the eutectic point because it has the lowest melting point starting at 96.1 C and although the range is broad (96.1-131.1 C) it is most likely due to using too much of the mixture requiring a loner amount of time to melt completely. The two other mixtures show a depressed melting point range compared to the pure Urea and Cinnamic Acid because they acted as impurities upon each other.

Questions

1. Describe errors in a procedure that may cause an observed capillary melting point of a pure compound

a. To be lower than the correct melting point

For a melting point of a pure compound to be lower than the correct melting point, the capillary tubes could not be completely clean causing impurities to contaminate the pure compound and depress the melting point.

b. To be higher than the correct melting point

For a melting point of a pure compound to be higher than the correct melting point the error could be because of heating the compound too fast instead of using the suggested 1-2 degree per minute heating rate. This is because heating too fast causes the substance to heat faster than the thermometer can keep up with.

c. To be broad in range (over several degrees)

For a melting point of a pure compound to be broad in the range rather than at the correct melting point range could be a result of using too much of the compound in the sample since more heat is required to melt all of the samples.

2. Briefly define the following terms:

a. Vapor pressure as applied to melting

The vapor pressure is an indication of a liquid's evaporation rate which happens during melting.

b. Melting point or melting-point range

The temperature or temperature range where the solid and liquid phases exist in equilibrium with each other. c. The mixture or mixed melting point.

A mixed melting point would be used to determine or eliminate an unknown substance because a mixture of a known substance with a known melting point was mixed with something besides itself then the melting point would depress.

d. Eutectic point

The melting point temperatures of a eutectic mixture.

e. Eutectic mixture

A mixture of more than one component, which melts sharply below the melting point of any individual component because the solids of the mixture are at equilibrium. 6. 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.

A mixture of A and B that results in a different melting point of either pure A or pure B would differ because one is acting as an impurity to the other depressing the melting point. Another possibility is that A and B have been mixed to the eutectic point where the melting point would be significantly lower than both pure A and pure B. 11. The melting points of pure benzoic acid and pure 2-naphthol are 122.5 C and 123 C, respectively.
Given a pure sample that is known to be either pure benzoic acid or 2-naphthol, describe a procedure you might use to determine the identity of the sample.

The melting point of a mixture containing the unknown sample and benzoic acid compared to the melting point of a mixture consisting of the unknown sample and 2-naphthol would be the best approach to determine if the sample is either pure benzoic acid or 2-naphthol. If the unknown sample is benzoic acid then when mixed with benzoic acid the melting point would remain the same (122.5 C) and when mixed with 2-naphthol the melting point would depress. 13. The melting-point-composition 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?

The melting point of Q= 157 C and the melting point of R= 180 C

b. What are the melting point and the composition of the eutectic mixture? The melting point of the eutectic mixture would fall right under 80 C so approximately 78 C. The composition would consist of 35 mol % R and 65 % Q.

c. Would a mixture of 20 mol % Q and 80 mol % R melt if heated to 120 C? to 160 C? to 75 C?
The mixture would not melt when heated to 120 C or 75 C, but it would melt at 160 C.

d. A mixture of Q and R was observed to melt at 105-110 C. What can be said about the composition of this mixture? Explain briefly.
This mixture has two possible compositions. Either consisting of 22 mol % R and 78 mol % Q, or 50 mol % R and 50 mol % Q. In these two possible mixtures, this melting point would be reached when the liquid + solid phases of both R and Q were in equilibrium with each other. This can be explained by using the table 3.2 to see where the mixtures of Q and R reached the melting point of 105-110 C and from that seeing the mol % of R and Q along the X-axis corresponding to that melting point.

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