Experiment 1: Identify a compound by its meltingpoint and mixed melting points.

Acetamide

p-Aminobenzoic acid

Camphoric Acid

trans-Cinnamic Acid

Malonic Acid

p-Nitrophenol

Resorcinol

Succinic Acid

Urea

113 - 115 oC

188 - 189 oC

183 - 186 oC

133 - 134 oC

135 - 137 oC

113 - 115 oC

110 - 113 oC

187 - 189 oC

133 - 135 oC

Melting Points and Mixed Melting Points

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

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.

Filling Tube

Sample in the melting point tube.

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

Thiele Tube

Attach the melting point tube to a thermometer.

Using Thiele Tube

Heat about 5o per minute until within about 10o of the melting point.

Near the melting point heat at 1 - 2o per minute

Heating Rate

Mel-Temp in the lab

Starts to melt Finished melting

m.p. = Start - Finish

Temperature

Why is salt put on snow covered roads?

Salt on Roads

What happens to the melting point if salt is added?

Ice melts!

Ice melts at 0oC

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

Impurities

Acetic Acid, CH3COOH, is a colorless liquid that melts at 16.6oC.

Let’s look at the melting point of mixtures of water and acetic acid.

Acetic Acid

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

Eutectic point

Eutectic Point

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

Cool

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

Heat

Pure compounds usually melt over a narrow temperature range, often 1o or less.

Impure compounds melt lower than pure compounds and over a wider temperature range.

Pure - Impure

m.p. = 115o - 119o m.p. = 118o - 120o m.p. = 121o - 122o

Melting points are a measure of purity

Purity

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

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

How do you tell what is in each bottle?

Unknown

Grind samples together to be sure they are mixed and then measure the melting point.

Mixed Melting Points

m.p. = 114o - 117o

m.p. = 120o - 122o

m.p. = 115o - 118o

Mixed 1 and 2

Mixed 1 and 3

Mixed 2 and 3

1 2 3

Results

1. Measure the melting point of your unknown

2. Run mixed melting points to confirm identification

Summary