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Melting Points and Mixed Melting Points Experiment 1: Identify a compound by its melting point and mixed melting points. Acetamide 113 - 115 o. C p-Aminobenzoic acid 188 - 189 o. C Camphoric Acid 183 - 186 o. C trans-Cinnamic Acid 133 - 134 o. C Malonic Acid 135 - 137 o. C p-Nitrophenol 113 - 115 o. C Resorcinol 110 - 113 o. C Succinic Acid 187 - 189 o. C Urea 133 - 135 o. C
A sample is put in the bottom of a melting point tube.
Put a small amount of the compound in the open end of the melting point tube. Turn over and tape the closed end on the desk top until the compound falls to the bottom.
Sample in the melting point tube.
Use a Thiele tube filled with mineral oil to heat your sample.
Attach the melting point tube to a thermometer.
Heat about 5 o per minute until within about 10 o of the melting point Near the melting point heat at 1 - 2 o per minute
Mel-Temp in the lab
Temperature Starts to melt Finished melting m. p. = Start - Finish
Why is salt put on snow covered roads?
Ice melts at o 0 C What happens to the melting point if salt is added? Ice melts!
Impurities such as salt lowers the melting point of water. Putting salt on icy roads causes the ice to melt because it lowers the melting point of water. Impure compounds usually melt lower than pure compounds so the melting point may be used as a measure of the purity of a compound
Acetic Acid, CH 3 COOH, is a colorless liquid that melts at 16. 6 o. C. Let’s look at the melting point of mixtures of water and acetic acid.
Plot of melting point vs. mole fraction water for mixtures of water and acetic acid. Eutectic point
Cool a acetic acid - water solution with a mole fraction water of 0. 9
Heat a acetic acid - water solution with a mole fraction water of 0. 9
Heat a acetic acid - water solution with a mole fraction water of 0. 2
Pure compounds usually melt over a narrow temperature range, often 1 o or less. Impure compounds melt lower than pure compounds and over a wider temperature range.
Melting points are a measure of purity m. p. = 115 o - 119 o m. p. = 118 o - 120 o m. p. = 121 o - 122 o
Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid. m. p. = 120 o - 122 o How do you tell what is in each bottle?
Mixed Melting Points Grind samples together to be sure they are mixed and then measure the melting point.
Results 1 2 3 Mixed 1 and 2 m. p. = 120 o - 122 o Mixed 1 and 3 m. p. = 114 o - 117 o Mixed 2 and 3 m. p. = 115 o - 118 o
1. Measure the melting point of your unknown 2. Run mixed melting points to confirm identification