Solids: Recrystallization and Melting Points

Topics: Chemical substance, Temperature, Mixture Pages: 5 (1441 words) Published: October 10, 2013

Solids: Recrystallization and Melting Points
Reference: Chapter 3. Solids: Recrystallization and Melting Points. Read pages 93-94 and 113-117. Experimental procedure, pages 118-119. Parts 1-3. Part 1: Melting points of Urea and Cinnamic Acid. Part 2: Melting point of unknown. Part 3: Melting point of Mixture- Urea and Cinnamic Acid (take melting points of mixtures in 1:4, 1:1 and 4:1 ratios).

Reactions
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

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
Known Melting Point Range of Pure Compounds
Experimental Melting Point Range (Slow)
Experimental Melting Point Range (Fast)
Urea
132.5-133 C
131.5-134.8 C
133.6-134 C
Cinnamic Acid
133-134.5 C
133.1-135.2 C
134-135 C
Unknown #40

209-214.8 C
210-215 C
4:1 ratio Urea to C.Acid

96.1-131.1 C
96-130.1 C
1:1 ratio Urea to C.Acid

97.7-114.8 C
98.9-111.2 C
1:4 ratio Urea to C.Acid

97.7-125.5 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. Molar mass of cinnimic acid (C9H8O2) is 148.16g/mol and 1g was used so converting it to moles take 1g/ 148.16g/mol = 0.00674946 moles of cinnimic 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 these melting point ranges to the pure Urea and Cinnamic Acid compounds from part a to see how impurities effect 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 were 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 Cinnimic 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...
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