Melting Points and Mixed Melting Points Experiment 1: Identify a compound by its melting point and mixed melting points.

Acetamide p-Aminobenzoic acid Camphoric Acid trans-Cinnamic Acid Malonic Acid p-Nitrophenol Resorcinol Succinic Acid Urea 113 - 115 o C 188 - 189 o C 183 - 186 o C 133 - 134 o C 135 - 137 o C 113 - 115 o C 110 - 113 o C 187 - 189 o C 133 - 135 o C

Melting Point Tube

A sample is put in the bottom of a melting point tube.

Filling 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.

Thiele Tube

Use a Thiele tube filled with mineral oil to heat your sample.

Using Thiele Tube

Attach the melting point tube to a thermometer.

Heating Rate

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

Salt on Roads

Why is salt put on snow covered roads?

Ice melts at 0 o C What happens to the melting point if salt is added?

Ice melts!

Impurities

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

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.

Eutectic Point

Plot of melting point vs. mole fraction water for mixtures of water and acetic acid.

Eutectic point

Cool

Cool a acetic acid - water solution with a mole fraction water of 0.9C

Heat

Heat a acetic acid - water solution with a mole fraction water of 0.9

Heat

Heat a acetic acid - water solution with a mole fraction water of 0.2

Pure - Impure

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.

Purity

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

Unknown

Two of these bottles contain benzoic acid and one m-nitrophenylacetic acid.

m.p. = 120 o - 122 o m.p. = 120 o - 122 o 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

Mixed 1 and 2 Mixed 1 and 3 Mixed 2 and 3

2 3

m.p. = 120 o - 122 o m.p. = 114 o - 117 o m.p. = 115 o - 118 o

Summary

1. Measure the melting point of your unknown 2. Run mixed melting points to confirm identification